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Stellar winds of cool and pulsating asymptotic giant branch (AGB) stars enrich the interstellar medium with large amounts of processed elements and various types of dust. We present the first study on the influence of gas-to-dust drift on ab initio simulations of stellar winds of M-type stars driven by radiation pressure on forsterite particles. Our study is based on our radiation hydrodynamic model code T-800 that includes frequency-dependent radiative transfer, dust extinction based on Mie scattering, grain growth and ablation, gas-to-dust drift using one mean grain size, a piston that simulates stellar pulsations, and an accurate high spatial resolution numerical scheme. To enable this study, we calculated new gas opacities based on the EXOMOL database, and we extended the model code to handle the formation of minerals that may form in M-type stars. We determine the effects of drift by comparing drift models to our new and extant non-drift models. Three out of four new drift models show high drift velocities, 87- 310 km s-1. Our new drift model mass-loss rates are 1.7- 13 per cent of the corresponding values of our non-drift models, but compared to the results of two extant non-drift models that use the same stellar parameters, these same values are 0.33- 1.5 per cent. Meanwhile, a comparison of other properties such as the expansion velocity and grain size show similar values. Our results, which are based on single-component forsterite particles, show that the inclusion of gas-to-drift is of fundamental importance in stellar wind models driven by such transparent grains. Assuming that the drift velocity is insignificant, properties such as the mass-loss rate may be off from more realistic values by a factor of 50 or more., QC 20231115

Context. The properties and the evolution of asymptotic giant branch (AGB) stars are strongly influenced by their mass loss through a stellar wind. This, in turn, is believed to be caused by radiation pressure due to the absorption and scattering of the stellar radiation by the dust grains formed in the atmosphere. The optical properties of dust are often estimated using the small particle limit (SPL) approximation, and it has been used frequently in modelling AGB stellar winds when performing radiation-hydrodynamics (RHD) simulations. Aims. We aim to investigate the effects of replacing the SPL approximation by detailed Mie calculations of the size-dependent opacities for grains of amorphous carbon forming in C-rich AGB star atmospheres. Methods. We performed RHD simulations for a large grid of carbon star atmosphere+wind models with different effective temperatures, luminosities, stellar masses, carbon excesses, and pulsation properties. Also, a posteriori radiative transfer calculations for many radial structures (snapshots) of these models were done, resulting in spectra and filter magnitudes. Results. We find that, when giving up the SPL approximation, the wind models become more strongly variable and more dominated by gusts, although the average mass-loss rates and outflow speeds do not change significantly; the increased radiative pressure on the dust throughout its formation zone does, however, result in smaller grains and lower condensation fractions (and thus higher gas-to-dust ratios). The photometric K magnitudes are generally brighter, but at V the effects of using size-dependent dust opacities are more complex: brighter for low mass-loss rates and dimmer for massive stellar winds. Conclusions. Given the large effects on spectra and photometric properties, it is necessary to use the detailed dust optical data instead of the simple SPL approximation in stellar atmosphere+wind modelling where dust is formed.

Based on observations obtained with the Nordic Optical Telescope (NOT) we investigate the spectral variability of the Herbig Ae star RR Tau. This star belongs to the UX Ori family, characterized by very deep fadings caused by the screening of the star with opaque fragments (clouds) of the protoplanetary discs. At the moments of such minima one observes strong spectral variability due to the fact that the dust cloud occults, for an observer, not only the star but also a part of the region where the emission spectrum originates. We calculated a series of obscuration models to interpret the observed variability of the H a line parameters. We consider two main obscuration scenarios: (1) the dust screen rises vertically above the circumstellar disc, and (2) the screen intersects the line-of-sight moving azimuthally with the disc. In both cases, the model of the emission region consists of a compact magnetosphere and a magnetocentrifugal disc wind. Comparison with observations shows that the first scenario explains well the variability of the radiation flux, the equivalent width, as well as the asymmetry of the H a line during eclipses, while the second scenario explains them only partly. This permits us to suggest that in the case of RR Tau, the main causes of the eclipses are either a structured disc wind, or the charged dust lifted along the field lines of the poloidal component of the magnetic field of the circumstellar disc.

Context: Young stellar systems actively accrete from their circumstellar disk and simultaneously launch outflows. The physical link between accretion and ejection processes remains to be fully understood. Aims: We investigate the structure and dynamics of magnetospheric accretion and associated outflows on a scale smaller than 0.1 au around the young transitional disk system GM Aur. Methods: We devised a coordinated observing campaign to monitor the variability of the system on timescales ranging from days to months, including partly simultaneous high-resolution optical and near-infrared spectroscopy, multiwavelength photometry, and low-resolution near-infrared spectroscopy, over a total duration of six months, covering 30 rotational cycles. We analyzed the photometric and line profile variability to characterize the accretion and ejection processes. Results: The optical and near-infrared light curves indicate that the luminosity of the system is modulated by surface spots at the stellar rotation period of 6.04 +/- 0.15 days. Part of the Balmer, Paschen, and Brackett hydrogen line profiles as well as the HeI 5876 angstrom and HeI 10830 angstrom line profiles are modulated on the same period. The Pa beta line flux correlates with the photometric excess in the u ' band, which suggests that most of the line emission originates from the accretion process. High-velocity redshifted absorptions reaching below the continuum periodically appear in the near-infrared line profiles at the rotational phase in which the veiling and line fluxes are the largest. These are signatures of a stable accretion funnel flow and associated accretion shock at the stellar surface. This large-scale magnetospheric accretion structure appears fairly stable over at least 15 and possibly up to 30 rotational periods. In contrast, outflow signatures randomly appear as blueshifted absorption components in the Balmer and HeI 10830 angstrom line profiles. They are not rotationally modulated and disappe

Context. X-Ray bursts (XRBs) are powerful thermonuclear events on the surface of accreting neutron stars (NSs) where nucleosynthesis of intermediate-mass elements occurs. The high surface gravity of an NS prevents the ejection of material by the XRB thermonuclear explosion. However, the predicted and observed XRB luminosities sometimes exceed Eddington’s value, and some of the material may escape by means of stellar wind. Aims. This work aims to determine the mass-loss and chemical composition of the material ejected through radiation-driven winds and its significance for Galactic abundances. It also reports on the evolution of observational quantities during the wind phase, which can help constrain the mass-radius relation in NSs. Methods. A non-relativistic radiative wind model was implemented, with modern opacity tables and treatment of the critical point, and linked through a new technique to a series of XRB hydrodynamic simulations that include over 300 isotopes. This allowed us to construct a quasi-stationary time evolution of the wind during the XRB. Results. In the models we studied, the total mass ejected by the wind was about 6 × 10^19 g; the average ejected mass per burst represented 2.6% of the accreted mass between bursts, with 0.1% of the envelope mass ejected per burst; and approximately 90% of the ejecta was composed by 60Ni, 64Zn, 68Ge, and 58Ni. The ejected material also contained a small fraction (10^-4 - 10^-5) of some light p-nuclei, but not enough to account for their Galactic abundances. Additionally, the observable magnitudes during the wind phase showed remarkable correlations, partly due to the fact that photospheric luminosity stays close to the Eddington limit. Some of these correlations involve wind parameters, such as energy and mass outflows, that are determined by the conditions at the base of the wind envelope. Conclusions. The simulations resulted in the first realistic quantification of mass-loss for each isotope synthesized in the, Peer Reviewed, Postprint (author's final draft)

GGD 34 is a protostellar jet with wiggles which are accompanied by "sine-like" variations in the radial velocity of the emitting material by as much as 60 km s(-1). Thus GGD 34 is an interesting object to understand the physical mechanisms involved in the generation of wiggles in protostellar jets. In this work we present high resolution images obtained with the Canada-France-Hawaii Telescope (CFHT) which shows that GCD 34 consists of a narrow (unresolved) jet roughly bisecting an extended faint envelope. The [S II] emission from the working surface has an arrow shaped morphology; the body of the jet is clearly distinguished as well as two backtails disposed in an approximately symmetric manner with respect to the jet axis. The H alpha emission is concentrated at the head of the jet indicating that the gas is significantly more excited at this location tin particular at the so-called Knot 5); we suggest that Knot 5 traces the location of the Mach disk since spectra of GGD 34 indicate that it is a light beam of gas. The high resolution images also show that the envelope around GGD 34 connects smoothly with the back tails at the head of the jet. We speculate whether it traces the backflow; the expected backflow velocity is shown to be similar to 32 km s(-1) which is consistent with the degree of excitation of the envelope. However, an accurate determination of the proper motion of the head is necessary to check whether this interpretation is correct. We also present radiocontinuum (3.6 and 6 cm) VLA observations and report the detection of a radio source close to the apex of the cavity from which the jet emerges. This radio source has a spectral index of 0.7+/-0.5, consistent within error with the value of 0.6 expected for a thermal jet. We suggest that this radio object is associated with the source of the outflow. Additional (12)CO(3-2) observations obtained with the JCMT show molecular gas redshifted by similar to 2.5 km s(-1) with respect to the cloud at this loca, Unidad Deptal. de Astronomía y Geodesia, Fac. de Ciencias Matemáticas, TRUE, pub

Analytical and numerical analysis of the interaction between the magnetosphere of the Pre-Main-Sequence (PMS) stars and accretion disks are converging to a robust, fundamental mechanism for jet formation. They provide a reasonably solid base-line to explain the current observations on the formation of solar-like stars. In this summary, the connection between observations and this paradigm are reviewed critically and some observational tests proposed., Unidad Deptal. de Astronomía y Geodesia, Fac. de Ciencias Matemáticas, TRUE, pub

Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO’s Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope spectra of the first stellar generations, the individual spectra of low-Z stars need to be understood, which is exactly where XShootU can deliver.

Context. The mass-loss mechanisms in M-type asymptotic giant branch (AGB) stars are still not well understood; these include, in particular, the formation of dust-driven winds from the innermost gaseous layers around these stars. One way to understand the gas-dust interaction in these regions and its impact on the mass-loss mechanisms is through the analysis of high-resolution observations of the stellar surface and its closest environment. Aims. We aim to characterize the inner circumstellar environment (~3 R*) of the M-type Mira star R Car in the near-infrared at different phases of a pulsation period. Methods. We used GRAVITY interferometric observations in the K band obtained during two different epochs over 2018. Those data were analyzed using parametric models and image reconstruction of both the pseudo-continuum and the CO band heads observed. The reported data are the highest angular resolution observations on the source in the K band. Results. We determined sizes of R Car’s stellar disk of 16.67 ± 0.05 mas (3.03 au) in January 2018 and 14.84 ± 0.06 mas (2.70 au) in February, 2018, respectively. From our physical model, we determined temperatures and size ranges for the innermost CO layer detected around R Car. The derived column density of the CO is in the ~9.18×1018–1×1019 cm−2 range, which is sufficient to permit dust nucleation and the formation of stable dust-driven winds. We find that magnesium composites, Mg2SiO4 and MgSiO3, have temperatures and condensation distances consistent with the ones obtained for the CO layer model and pure-line reconstructed images, which are the dust types most likely to be responsible for wind formation. Our reconstructed images show evidence of asymmetrical and inhomogeneous structures, which might trace a complex and perhaps clumpy structure of the CO molecule distribution. Conclusions. Our work demonstrates that the conditions for dust nucleation and thus for initialising dust-driven winds in M-type AGB stars are met i

The transient X-ray source MAXI J1810−222 was discovered in 2018 and has been active ever since. A long combined radio and X-ray monitoring campaign was performed with Australia Telescope Compact Array and Swift, respectively. It has been proposed that MAXI J1810−222 is a relatively distant black hole X-ray binary, albeit showing a very peculiar outburst behaviour. Here, we report on the spectral study of this source making use of a large sample of NICER observations performed between 2019 February and 2020 September. We detected a strong spectral absorption feature at ∼1 keV, which we have characterized with a physical photoionization model. Via a deep scan of the parameters space, we obtained evidence for a spectral-state dependent outflow, with mildly relativistic speeds. In particular, the soft and intermediate states point to a hot plasma outflowing at 0.05–0.15c. This speeds rule-out thermal winds and hence, they suggest that such outflows could be radiation pressure or (most likely) magnetically driven winds. Our results are crucial to test current theoretical models of wind formation in X-ray binaries.

[Context] Massive colliding-wind binaries (CWBs) can be non-thermal sources. The emission produced in their wind-collision region (WCR) encodes information of both the shock properties and the relativistic electrons accelerated in them. The recently discovered system Apep, a unique massive system hosting two Wolf-Rayet stars, is the most powerful synchrotron radio emitter among the known CWBs. It is an exciting candidate in which to investigate the non-thermal processes associated with stellar wind shocks., [Aims] We intend to break the degeneracy between the relativistic particle population and the magnetic field strength in the WCR of Apep by probing its hard X-ray spectrum, where inverse-Compton (IC) emission is expected to dominate., [Methods] We observed Apep with NuSTAR for 60 ks and combined this with a reanalysis of a deep archival XMM-Newton observation to better constrain the X-ray spectrum. We used a non-thermal emission model to derive physical parameters from the results., [Results] We detect hard X-ray emission consistent with a power-law component from Apep. This is compatible with IC emission produced in the WCR for a magnetic field of ≈105–190 mG, corresponding to a magnetic-to-thermal pressure ratio in the shocks of ≈0.007–0.021, and a fraction of ∼1.5 × 10−4 of the total wind kinetic power being transferred to relativistic electrons., [Conclusions] The non-thermal emission from a CWB is detected for the first time in radio and at high energies. This allows us to derive the most robust constraints so far for the particle acceleration efficiency and magnetic field intensity in a CWB, reducing the typical uncertainty of a few orders of magnitude to just within a factor of a few. This constitutes an important step forward in our characterisation of the physical properties of CWBs.

V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution Ultraviolet and Visual Echelle spectrograph spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line SHAPE model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionized environment. Temperature, density, and abundance analyses of the evolving nov a shell are presented.

We present Stratospheric Observatory For Infrared Astronomy (SOFIA) + Atacama Large Millimeter/submillimeter Array (ALMA) continuum and spectral-line polarization data on the massive molecular cloud BYF 73, revealing important details about the magnetic field morphology, gas structures, and energetics in this unusual massive star formation laboratory. The 154 mu m HAWC+ polarization map finds a highly organized magnetic field in the densest, inner 0.55 x 0.40 pc portion of the cloud, compared to an unremarkable morphology in the cloud's outer layers. The 3 mm continuum ALMA polarization data reveal several more structures in the inner domain, including a parsec-long, similar to 500 M (circle dot) "Streamer" around the central massive protostellar object MIR 2, with magnetic fields mostly parallel to the east-west Streamer but oriented north-south across MIR 2. The magnetic field orientation changes from mostly parallel to the column density structures to mostly perpendicular, at thresholds N (crit) = 6.6 x 10(26) m(-2), n (crit) = 2.5 x 10(11) m(-3), and B (crit) = 42 +/- 7 nT. ALMA also mapped Goldreich-Kylafis polarization in (CO)-C-12 across the cloud, which traces, in both total intensity and polarized flux, a powerful bipolar outflow from MIR 2 that interacts strongly with the Streamer. The magnetic field is also strongly aligned along the outflow direction; energetically, it may dominate the outflow near MIR 2, comprising rare evidence for a magnetocentrifugal origin to such outflows. A portion of the Streamer may be in Keplerian rotation around MIR 2, implying a gravitating mass 1350 +/- 50 M (circle dot) for the protostar+disk+envelope; alternatively, these kinematics can be explained by gas in free-fall toward a 950 +/- 35 M (circle dot) object. The high accretion rate onto MIR 2 apparently occurs through the Streamer/disk, and could account for similar to 33% of MIR 2's total luminosity via gravitational energy release.

[Context] X-Ray bursts (XRBs) are powerful thermonuclear events on the surface of accreting neutron stars (NSs) where nucleosynthesis of intermediate-mass elements occurs. The high surface gravity of an NS prevents the ejection of material by the XRB thermonuclear explosion. However, the predicted and observed XRB luminosities sometimes exceed Eddington’s value, and some of the material may escape by means of stellar wind., [Aims] This work aims to determine the mass-loss and chemical composition of the material ejected through radiation-driven winds and its significance for Galactic abundances. It also reports on the evolution of observational quantities during the wind phase, which can help constrain the mass-radius relation in NSs., [Methods] A non-relativistic radiative wind model was implemented, with modern opacity tables and treatment of the critical point, and linked through a new technique to a series of XRB hydrodynamic simulations that include over 300 isotopes. This allowed us to construct a quasi-stationary time evolution of the wind during the XRB., [Results] In the models we studied, the total mass ejected by the wind was about 6 × 1019 g; the average ejected mass per burst represented 2.6% of the accreted mass between bursts, with 0.1% of the envelope mass ejected per burst; and approximately 90% of the ejecta was composed by 60Ni, 64Zn, 68Ge, and 58Ni. The ejected material also contained a small fraction (10−4 − 10−5) of some light p-nuclei, but not enough to account for their Galactic abundances. Additionally, the observable magnitudes during the wind phase showed remarkable correlations, partly due to the fact that photospheric luminosity stays close to the Eddington limit. Some of these correlations involve wind parameters, such as energy and mass outflows, that are determined by the conditions at the base of the wind envelope., [Conclusions] The simulations resulted in the first realistic quantification of mass-loss for each isotope synthesized in the XRB. The photospheric correlations we found could be used to link observable magnitudes to the physics of the innermost parts of the envelope, close to its interface with the NS crust. This is a promising result regarding the issue of NS radius determination.

We present an observational study of the luminous red nova (LRN) AT 2021biy in the nearby galaxy NGC 4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from ∼231 days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT 2021biy shows a short-duration blue peak, with a bolometric luminosity of ∼1.6 × 1041 erg s−1, followed by the longest plateau among LRNe to date, with a duration of 210 days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT 2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum (TBB ≈ 2050 K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT 2021biy has local dust properties similar to those of V838 Mon in the Milky Way Galaxy. Inspection of archival Hubble Space Telescope data taken on 2003 August 3 reveals a ∼20 M⊙ progenitor candidate with log (L/L⊙) = 5.0 dex and Teff = 5900 K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17–24 M⊙ primary component.

Low- and intermediate-mass stars evolve into asymptotic giant branch (AGB) stars near the end of their lives, losing mass through slow and massive winds. The ejected material creates a chemically-rich expanding envelope around the star, namely the circumstellar envelope (CSE). Investigating the anisotropy of the mass-loss phenomenon on the AGB is crucial in gaining a better understanding of the shaping of the CSE during the transition from AGB star to planetary nebula (PN). We investigate possible signs of deviation from spherical symmetry in the CO-emitting CSEs of 70 AGB stars by analysing their emission maps in CO J = 2 - 1 and 3 - 2 observed with the Atacama Compact Array, as part of the DEATHSTAR project. We find that about one third of the sources are likely aspherical, as they exhibit large-scale asymmetries that are unlikely to have been created by a smooth wind. Further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures.

Context. The winds observed around asymptotic giant branch (AGB) stars are generally attributed to radiation pressure on dust formed in the extended dynamical atmospheres of these long-period variables. The composition of wind-driving grains is affected by a feedback between their optical properties and the resulting heating due to stellar radiation. Aims. We explore the gradual Fe enrichment of wind-driving silicate grains in M-type AGB stars to derive typical values for Fe/Mg and to test the effects on wind properties and synthetic spectra. Methods. We present new radiation-hydrodynamical DARWIN models that allow for the growth of silicate grains with a variable Fe/Mg ratio and predict mass-loss rates, wind velocities, and grain properties. Synthetic spectra and other observables are computed a posteriori with the COMA code. Results. The self-regulating feedback between grain composition and radiative heating, in combination with quickly falling densities in the stellar wind, leads to low values of Fe/Mg, typically a few percent. Nevertheless, the new models show distinct silicate features around 10 and 18 microns. Fe enrichment affects visual and near-IR photometry moderately, and the new DARWIN models agree well with observations in (J - K) versus (V - K) and Spitzer color-color diagrams. The enrichment of the silicate dust with Fe is a secondary process, taking place in the stellar wind on the surface of large Fe-free grains that have initiated the outflow. Therefore, the mass-loss rates are basically unaffected, while the wind velocities tend to be slightly higher than in corresponding models with Fe-free silicate dust. Conclusions. The gradual Fe enrichment of silicate grains in the inner wind region should produce signatures observable in mid-IR spectro-interferometrical measurements. Mass-loss rates derived from existing DARWIN models, based on Fe-free silicates, can be applied to stellar evolution models since the mass-loss rates are not significantly affe

High-mass microquasar jets, produced in an accreting compact object in orbit around a massive star, must cross a region filled with stellar wind. The combined effects of the wind and orbital motion can strongly affect the jet properties on binary scales and beyond. The study of such effects can shed light on how high-mass microquasar jets propagate and terminate in the interstellar medium. We study for the first time, using relativistic hydrodynamical simulations, the combined impact of the stellar wind and orbital motion on the properties of high-mass microquasar jets on binary scales and beyond. We have performed 3-dimensional relativistic hydrodynamic simulations, using the PLUTO code, of a microquasar scenario in which a strong weakly relativistic wind from a star interacts with a relativistic jet under the effect of the binary orbital motion. The parameters of the orbit are chosen such that the results can provide insight on the jet-wind interaction in compact systems like for instance Cyg X-1 or Cyg X-3. The wind and jet momentum rates are set to values that may be realistic for these sources and lead to moderate jet bending, which together with the close orbit and jet instabilities could trigger significant jet precession and disruption. For high-mass microquasars with orbit size a ∼ 0.1 AU, and (relativistic) jet power Lj∼1037(M˙w/10−6M⊙yr−1) erg s−1, where M˙w is the stellar wind mass rate, the combined effects of the stellar wind and orbital motion can induce relativistic jet disruption on scales ∼1 AU.

The clumpy stellar wind from the companion star in high-mass X-ray binaries causes variable, partial absorption of the emission from the X-ray source. We studied XMM–Newton observations from a 7.22¿d long monitoring campaign, in order to constrain the effects of the stellar wind on the short-time-scale X-ray spectral-timing properties of Cygnus X-1. We find these properties to change significantly in the presence of the wind. In particular, the longest sampled time-scales (corresponding to temporal frequencies of ¿ ~ 0.1–1 Hz) reveal an enhancement of the fractional variability power, while on the shortest sampled time-scales (¿ ~ 1–10 Hz), the variability is suppressed. In addition, we observe a reduction (by up to a factor of ~ 1.8) of the otherwise high coherence between soft and hard band light curves, as well as of the amplitude of the hard X-ray lags intrinsic to the X-ray continuum. The observed increase of low-frequency variability power can be explained in terms of variations of the wind column density as a consequence of motions of the intervening clumps. In this scenario (and assuming a terminal velocity of v8=2400km s-1¿), we obtain an estimate of l ~ 0.5–1.5 × 10-4R* for the average radial size of a clump. On the other hand, we suggest the behaviour at high frequencies to be due to scattering in an optically thicker medium, possibly formed by collision of the stellar wind with the edge of the disc., Peer Reviewed, Postprint (author's final draft)

M33 X-7 is the only known eclipsing black hole high mass X-ray binary. The system is reported to contain a very massive O supergiant donor and a massive black hole in a short orbit. The high X-ray luminosity and its location in the metal-poor galaxy M33 make it a unique laboratory for studying the winds of metal-poor donor stars with black hole companions and it helps us to understand the potential progenitors of black hole mergers. Using phase-resolved simultaneous HST- and XMM-Newton-observations, we traced the interaction of the stellar wind with the black hole. We observed a strong Hatchett-McCray effect in M33 X-7 for the full range of wind velocities. Our comprehensive spectroscopic investigation of the donor star (X-ray+UV+optical) yields new stellar and wind parameters for the system that differ significantly from previous estimates. In particular, the masses of the components are considerably reduced to ≈38 M⊙ for the O-star donor and ≈11.4 M⊙ for the black hole. The O giant is overfilling its Roche lobe and shows surface He enrichment. The donor shows a densely clumped wind with a mass-loss rate that matches theoretical predictions. An extended ionization zone is even present during the eclipse due to scattered X-ray photons. The X-ray ionization zone extends close to the photosphere of the donor during inferior conjunction. We investigated the wind-driving contributions from different ions and the changes in the ionization structure due to X-ray illumination. Toward the black hole, the wind is strongly quenched due to strong X-ray illumination. For this system, the standard wind-fed accretion scenario alone cannot explain the observed X-ray luminosity, pointing toward an additional mass overflow, which is in line with our acceleration calculations. The X-ray photoionization creates an He II emission region around the system emitting ∼1047 ph s−1. We computed binary evolutionary tracks for the system using MESA. Currently, the system is transitioning towar

[Context] Relativistic jets are ubiquitous in the Universe. In microquasars, especially in high-mass X-ray binaries, the interaction of jets with the strong winds driven by the massive and hot companion star in the vicinity of the compact object is fundamental for understanding the jet dynamics, nonthermal emission, and long-term stability. However, the role of the jet magnetic field in this process is unclear. In particular, it is still debated whether the magnetic field favors jet collimation or triggers more instabilities that can jeopardize the jet evolution outside the binary. [Aims] We study the dynamical role of weak and moderate to strong toroidal magnetic fields during the first several hundred seconds of jet propagation through the stellar wind, focusing on the magnetized flow dynamics and the mechanisms of energy conversion. [Methods] We developed the code Lóstrego v1.0, a new 3D relativistic magnetohydrodynamics code to simulate astrophysical plasmas in Cartesian coordinates. Using this tool, we performed the first 3D relativistic magnetohydrodynamics numerical simulations of relativistic magnetized jets propagating through the clumpy stellar wind in a high-mass X-ray binary. To highlight the effect of the magnetic field in the jet dynamics, we compared the results of our analysis with those of previous hydrodynamical simulations. [Result] The overall morphology and dynamics of weakly magnetized jet models is similar to previous hydrodynamical simulations, where the jet head generates a strong shock in the ambient medium and the initial overpressure with respect to the stellar wind drives one or more recollimation shocks. On the timescales of our simulations (i.e., ta

Therecently discovered massive binary system Apep is the most powerful synchrotron emitter among the known Galactic colliding-wind binaries. This makes this particular system of great interest to investigate stellar winds and the non-thermal processes associated with their shocks. This source was detected at various radio bands, and in addition the wind-collision region was resolved by means of very-long baseline interferometric observations. We use a non-thermal emission model for colliding-wind binaries to derive physical properties of this system. The observed morphology in the resolved maps allows us to estimate the system projection angle on the sky to be. The observed radio flux densities also allow us to characterise both the intrinsic synchrotron spectrum of the source and its modifications due to free-free absorption in the stellar winds at low frequencies; from this, we derive mass-loss rates of the stars of and. Finally, the broadband spectral energy distribution is calculated for different combinations of the remaining free parameters, namely the intensity of the magnetic field and the injected power in non-thermal particles. We show that the degeneracy of these two parameters can be solved with observations in the high-energy domain, most likely in the hard X-rays but also possibly in -rays under favourable conditions.

We present a multiwavelength characterization of the planetary nebula (PN) NGC 6905 and its [Wolf–Rayet]-type ([WR]) central star (CSPN) HD 193949. Our Nordic Optical Telescope Alhambra Faint Object Spectrograph and Camera (ALFOSC) spectra and images unveil in unprecedented detail the high-ionization structure of NGC 6905. The high-quality spectra of HD 193949 allowed us to detect more than 20 WR features including the characteristic O-bump, blue bump, and red bump, which suggests a spectral type no later than a [WO2]-subtype. Moreover we detect the Ne VII and Ne VIII broad emission lines, rendering HD 193949 yet another CSPN with Teff ≲ 150 kK exhibiting such stellar emission lines. We studied the physical properties (Te and ne) and chemical abundances of different regions within NGC 6905 including its low-ionization clumps; abundances are found to be homogeneous. We used the PoWR stellar atmosphere code to model the spectrum of HD 193949, which is afterwards used in a photoionization model performed with CLOUDY that reproduces the nebular and dust properties for a total mass in the 0.31–0.47 M⊙ range and a mass of C-rich dust of ∼2 × 10−3 M⊙. Adopting a current stellar mass of 0.6 M⊙, our model suggests an initial mass ∼1 M⊙ for HD 193949, consistent with the observations. © 2021 The Author(s).

We present the analysis of Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of the planetary nebula (PN) IC 4406. MUSE images in key emission lines are used to unveil the presence of at least five ring-like structures north and south of the main nebula of IC 4406. MUSE spectra are extracted from the rings to unambiguously assess for the first time in a PN their physical conditions, electron density (ne), and temperature (Te). The rings are found to have similar Te as the rim of the main nebula, but smaller ne. Ratios between different ionic species suggest that the rings of IC 4406 have a lower ionization state than the main cavity, in contrast to what was suggested for the rings in NGC 6543, the Cat’s Eye Nebula. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

Context. Eta Carinae is a highly eccentric, massive binary system (semimajor axis similar to 15.5 au) with powerful stellar winds and a phase-dependent wind-wind collision (WWC) zone. The primary star, eta Car A, is a luminous blue variable (LBV); the secondary, eta Car B, is a Wolf-Rayet or O star with a faster but less dense wind. Aperture-synthesis imaging allows us to study the mass loss from the enigmatic LBV eta Car. Understanding LBVs is a crucial step toward improving our knowledge about massive stars and their evolution. Aims. Our aim is to study the intensity distribution and kinematics of eta Car's WWC zone. Methods. Using the VLTI-MATISSE mid-infrared interferometry instrument, we perform Br alpha imaging of eta Car's distorted wind. Results. We present the first VLTI-MATISSE aperture-synthesis images of eta Car A's stellar windin several spectral channels distributed across the Br alpha 4.052 mu m line (spectral resolving power R similar to 960). Our observations were performed close to periastron passage in February 2020 (orbital phase similar to 14.0022). The reconstructed iso-velocity images show the dependence of the primary stellar wind on wavelength or line-of-sight (LOS) velocity with a spatial resolution of 6 mas (similar to 14 au). The radius of the faintest outer wind regions is similar to 26 mas (similar to 60 au). At several negative LOS velocities, the primary stellar wind is less extended to the northwest than in other directions. This asymmetry is most likely caused by the WWC. Therefore, we see both the velocity field of the undisturbed primary wind and the WWC cavity. In continuum spectral channels, the primary star wind is more compact than in line channels. A fit of the observed continuum visibilities with the visibilities of a stellar wind CMFGEN model (CMFGEN is an atmosphere code developed to model the spectra of a variety of objects) provides a full width at half maximum fit diameter of the primary stellar wind of 2.84 +/- 0.06 ma

Aims: We aim to constrain the sizes of, and investigate deviations from spherical symmetry in, the CO circumstellar envelopes (CSEs) of 16 S-type stars, along with an additional 7 and 4 CSEs of C-type and M-type AGB stars, respectively. Methods: We map the emission from the CO J = 2-1 and 3-2 lines observed with the Atacama Compact Array (ACA) and its total power (TP) antennas, and fit with a Gaussian distribution in the uv- and image planes for ACA-only and TP observations, respectively. The major axis of the fitted Gaussian for the CO(2-1) line data gives a first estimate of the size of the CO-line-emitting CSE. We investigate possible signs of deviation from spherical symmetry by analysing the line profiles and the minor-to-major axis ratio obtained from visibility fitting, and by investigating the deconvolved images. Results: The sizes of the CO-line-emitting CSEs of low-mass-loss-rate (low-MLR) S-type stars fall between the sizes of the CSEs of Cstars, which are larger, and those of M-stars, which are smaller, as expected because of the differences in their respective CO abundances and the dependence of the photodissociation rate on this quantity. The sizes of the low-MLR S-type stars show no dependence on circumstellar density, as measured by the ratio of the MLR to terminal outflow velocity, irrespective of variability type. The density dependence steepens for S-stars with higher MLRs. While the CO(2-1) brightness distribution size of the low-density S-stars is in general smaller than the predicted photodissociation radius (assuming the standard interstellar radiation field), the measured size of a few of the high-density sources is of the same order as the expected photodissociation radius. Furthermore, our results show that the CO CSEs of most of the S-stars in our sample are consistent with a spherically symmetric and smooth outflow. For some of the sources, clear and prominent asymmetric features are observed which are indicative of intrinsic circumstella

Context. Carbon stars have been, and still are, extensively studied. Given their complex internal structure and their peculiar chemical composition, they are living laboratories in which we can test stellar structure and evolution theories of evolved stars. Furthermore, they are the most relevant dust manufacturers, thus playing a crucial role in the evolution of galaxies. Aims. We aim to study the dust mineralogy of the circumstellar envelope of carbon stars in the Large Magellanic Cloud (LMC) to achieve a better understanding of the dust formation process in the outflow of these objects. We intend to investigate the expected distribution of carbon stars in the observational planes built with the filters of the Mid-Infrared Instrument (MIRI) mounted onboard the James Webb Space Telescope (JWST) to select the best planes allowing an exhaustive characterisation of the stars. Methods. We compared the synthetic spectral energy distributions, obtained by modelling asymptotic giant branch stars and the dust formation process in the wind, with the spectra of carbon stars in the LMC, taken with the Infrared Spectrograph onboard the Spitzer Space Telescope. From the detailed comparison between synthetic modelling and observation we characterise the individual sources and derive the detailed mineralogy of the dust in the circumstellar envelope. Results. The sample of stars considered here is composed of stars of diverse mass, formation epoch, degree of obscuration, and metallicity. We find that precipitation of MgS on SiC seeds is common to all non-metal-poor carbon stars. Solid carbon is the dominant dust component, with percentages above 80% in all cases; a percentage between 10% and 20% of carbon dust is under the form of graphite, the remaining being amorphous carbon. Regarding the observational planes based on the MIRI filters, the colour-magnitude ([F770W]-[F1800W], [F1800W]) plane allows the best understanding of the degree of obscuration of the stars, while the ([F18

We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at similar to 7 x 10(40) erg s(-1), while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type TM supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km s(-1), along with an Ha emission with a full-width at half-maximum velocity of 250 km s(-1). For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of M-F606W = -3.33 +/- 0.09 mag and a colour of F606W - F814W = 1.14 +/- 0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature.

We present the follow-up campaign of the luminous red nova (LRN) AT 2019zhd, the third event of this class observed in M 31. The object was followed by several sky surveys for about five months before the outburst, during which it showed a slow luminosity rise. In this phase, the absolute magnitude ranged from M-r=-2.8 +/- 0.2 mag to M-r=-5.6 +/- 0.1 mag. Then, over a four to five day period, AT 2019zhd experienced a major brightening, reaching a peak of M-r=-9.61 +/- 0.08 mag and an optical luminosity of 1.4x10(39) erg s(-1). After a fast decline, the light curve settled onto a short-duration plateau in the red bands. Although less pronounced, this feature is reminiscent of the second red maximum observed in other LRNe. This phase was followed by a rapid linear decline in all bands. At maximum, the spectra show a blue continuum with prominent Balmer emission lines. The post-maximum spectra show a much redder continuum, resembling that of an intermediate-type star. In this phase, H alpha becomes very weak, H beta is no longer detectable, and a forest of narrow absorption metal lines now dominate the spectrum. The latest spectra, obtained during the post-plateau decline, show a very red continuum (T-eff approximate to 3000 K) with broad molecular bands of TiO, similar to those of M-type stars. The long-lasting, slow photometric rise observed before the peak resembles that of LRN V1309 Sco, which was interpreted as the signature of the common-envelope ejection. The subsequent outburst is likely due to the gas outflow following a stellar merging event. The inspection of archival HST images taken 22 years before the LRN discovery reveals a faint red source (M-F555W=0.21 +/- 0.14 mag, with F555W-F814W=2.96 +/- 0.12 mag) at the position of AT 2019zhd, which is the most likely quiescent precursor. The source is consistent with expectations for a binary system including a predominant M5-type star.

The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray burst (sGRB) arising from a neutron star merger, has challenged our understanding of the angular structure of relativistic jets. Studies of the jet propagation usually assume that the jet is ejected from the central engine with a top-hat structure and its final structure, which determines the observed light curve and spectra, is primarily regulated by the interaction with the nearby environment. However, jets are expected to be produced with a structure that is more complex than a simple top-hat, as shown by global accretion simulations. We present numerical simulations of sGRBs launched with a wide range of initial structures, durations, and luminosities. We follow the jet interaction with the merger remnant wind and compute its final structure at distances greater than or similar to 10(11) cm from the central engine. We show that the final jet structure, as well as the resulting afterglow emission, depends strongly on the initial structure of the jet, its luminosity, and duration. While the initial structure of the jet is preserved for long-lasting sGRBs, it is strongly modified for jets barely making their way through the wind. This illustrates the importance of combining the results of global simulations with propagation studies in order to better predict the expected afterglow signatures from neutron star mergers. Structured jets provide a reasonable description of the GRB 170817A afterglow emission with an off-axis angle theta(obs) approximate to 22.5 degrees.

We derive a distance of D = 16.6 +/- 0.3 Mpc (mu = 31.10 +/- 0.04 mag) to the archetypal narrow-line Seyfert 1 galaxy NGC 4051 based on Cepheid period-luminosity relations and new Hubble Space Telescope multiband imaging. We identify 419 Cepheid candidates and estimate the distance at both optical and near-infrared wavelengths using subsamples of precisely photometered variables (123 and 47 in the optical and near-infrared subsamples, respectively). We compare our independent photometric procedures and distance-estimation methods to those used by the Supernovae, H0, for the Equation of State team and find agreement to 0.01 mag. The distance we obtain suggests an Eddington ratio of m. approximate to 0.2 for NGC 4051, typical of narrow-line Seyfert 1 galaxies, unlike the seemingly odd value implied by previous distance estimates. We derive a peculiar velocity of -490 +/- 34 km s(-1) for NGC 4051, consistent with the overall motion of the Ursa Major Cluster in which it resides. We also revisit the energetics of the NGC 4051 nucleus, including its outflow and mass accretion rates.

Binary systems that host a massive star and a non-accreting pulsar can be powerful non-thermal emitters. The relativistic pulsar wind and the non-relativistic stellar outflows interact along the orbit, producing ultrarelativistic particles that radiate from radio to gamma rays. To properly characterize the physics of these sources, and better understand their emission and impact on the environment, careful modeling of the outflow interactions, spanning a broad range of spatial and temporal scales, is needed. Full three-dimensional approaches are very computationally expensive, but simpler approximate approaches, while still realistic at the semi-quantitative level, are available. We present here the results of calculations done with a quasi three-dimensional scheme to compute the evolution of the interacting flows in a region spanning in size up to a thousand times the size of the binary. In particular, we analyze for the first time the role of different eccentricities in the large scale evolution of the shocked flows. We find that the higher the eccentricity, the closer the flows behave like a one-side outflow, which becomes rather collimated for eccentricity values >0.75. The simulations also unveil that the pulsar and the stellar winds become fully mixed within the grid for low eccentricity systems, presenting a more stochastic behavior at large scales than in the highly eccentric systems.

We present a comprehensive analysis of the compact planetary nebula M 2-31 investigating its spectral properties, spatio-kinematical structure, and chemical composition using Gran Telescopio Canarias (GTC) Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía (MEGARA) integral field spectroscopic observations and Nordic Optical Telescope(NOT) Alhambra Faint Object Spectrograph and Camera (ALFOSC) medium-resolution spectra and narrow-band images. The GTC MEGARA high-dispersion observations have remarkable tomographic capabilities, producing an unprecedented view of the morphology and kinematics of M 2-31 that discloses a fast spectroscopic bipolar outflow along position angles 50° and 230°, an extended shell, and a toroidal structure or waist surrounding the central star perpendicularly aligned with the fast outflows. These observations also show that the C ii emission is confined in the central region and enclosed by the [N ii] emission. This is the first time that the spatial segregation revealed by a two-dimensional map of the C ii line implies the presence of multiple plasma components. The deep NOT ALFOSC observations allowed us to detect broad Wolf-Rayet (WR) features from the central star of M 2-31, including previously undetected broad O vi lines that suggest a reclassification as a [WO4]-type star. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

Binary systems of Population III can evolve to microquasars when one of the stars collapses into a black hole. When the compact object accretes matter at a rate greater than the Eddington rate, powerful jets and winds driven by strong radiation pressure should form. We investigate the structure of the jet-wind system for a model of Population III microquasar on scales beyond the jet-wind formation region. Using relativistic hydrodynamic simulations we find that the ratio of kinetic power between the jet and the disk wind determines the configuration of the system. When the power is dominated by the wind, the jet fills a narrow channel, collimated by the dense outflow. When the jet dominates the power of the system, part of its energy is diverted turning the wind into a quasi-equatorial flow, while the jet widens. From the results of our simulations, we implement semi-analytical calculations of the impact of the quasi-equatorial wind on scales of the order of the size of the binary system. Our results indicate that Population III microquasars might inject gamma rays and relativistic particles into the early intergalactic medium, contributing to its reionization at large distances from the binary system.

We have modelled the 3D distribution of OH/IR stars in the Galactic plane, traced by 1612 MHz OH maser sources with classic double horned spectral profiles. We statistically analysed over 700 maser sources detected by the HI/OH/Recombination line survey of the Milky Way (THOR) and the Australia Telescope Compact Array interferometric follow-up observations of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). With a simple model constructed from a classical density distribution of stars and luminosity functions of OH maser sources in the Galaxy, we estimate the scale height, or the half thickness of the OH/IR star distribution along the Galactic disc to be 90–290 pc. The simple model also implies that there are ∼4000 OH/IR stars hosting 1612 MHz OH masers along the Galactic Plane. Therefore, next generation telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP) and SKA Phase 1 will detect about 80 per cent of such OH/IR stars in the Galaxy at a 10 mJy detection limit. Comparing the data of previously detected circumstellar 1612 MHz OH maser sources with those of THOR and SPLASH, the maser source lifetime is estimated to be ∼300 yr. This is likely a lower limit, since non-detections of masers in some cases could be affected by the flux variation of the maser source. © 2021 The Author(s).

© 2020. The Authors. The authors declare no conflict of interest. The funding sponsors have no participation in the execution of the experiment, the decision to publish the results, nor the writing of the manuscript. This work is supported by the Interdisciplinary Mathematics Institute of the Complutense University of Madrid and funded by the Spanish Ministry of Economy and Enterprise under the following research projects: PCIN‐2014‐013‐C07‐04, PCIN‐2016‐080 (UE ERANET Plus NEWA Project), CGL2016‐81828‐REDT, FEI‐EU‐17‐16, SAFEFLIGHT (CGL2016‐78702‐C2‐1‐R and CGL2016‐78702‐C2‐2‐R), PID2019‐105306RB‐I00. This work is also supported by the ECMWF special projects SPESMART and SPESVALE., Microbursts are meteorological phenomena in the lower troposphere which can produce damaging surface winds and pose a severe risk to aircraft flying close to the ground. As these events usually span less than 4 km and 15 min, the spatiotemporal resolution is a challenge for numerical simulations. Although research of microburst using operative mesoscale models is scarce, the Weather Research and Forecasting (WRF) model has been used in the diagnosis of this phenomenon. In this paper, such model is used to simulate several microburst conducive days using two different boundary conditions. The energy spectra of the simulations are computed to evaluate the effective resolution of the model. The results are in line with previous studies and produce no notable differences among the boundary conditions. Nonetheless, the energy spectra show an overenergetic troposphere at microscale resolutions, rendering the effective resolution inadequate for microburst forecasting using the simulated physics variables. Thus, mesoscale indices are analyzed as a prognostic tool. The wind index, the wet microburst severity index and the microburst windspeed potential index do not show high forecasting performances, even though improving the results of climatology. Also, notable differences among the boundary conditions can be seen. The most consistent results are achieved by the wet microburst severity index., Ministerio de Economía y Competitividad (MINECO), the Interdisciplinary Mathematics Institute of the Complutense University of Madrid, the ECMWF, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

© 2020. The Authors. The authors declare no conflict of interest. The founding sponsors have no participation in the execution of the experiment, the decision to publish the results, nor in the writing of the manuscript. This work is supported by the Interdisciplinary Mathematics Institute of the Complutense University of Madrid and the following research projects: METEORISK (RTC‐2014‐ 1872‐5), PCIN‐2014‐013‐C07‐04, PCIN‐ 2016‐080 (UE ERANET Plus NEWA Project), ESP2013‐47816‐C4‐4‐P, CGL2010‐15930, CGL2016‐81828‐ REDT, FEI‐EU‐17‐16, and SAFEFLIGHT (CGL2016‐78702‐C2‐1‐R and CGL2016‐78702‐C2‐2‐R). This research is founded by the Spanish Ministry of Economy and Enterprise under the framework of the SAFEFLIGHT research project (CGL2016‐78702‐C2‐1‐R and CGL2016‐ 78702‐C2‐2‐R)., Atmospheric microbursts are low‐level meteorological events that can produce significant damage on the surface and pose a major risk to aircraft flying close to the ground. Studies and ad hoc numerical models have been developed to understand the origin and dynamics of the microburst; nevertheless, there are few researches of the phenomenon using global and mesoscale models. This is mainly due to the limitations in resolution, as microbursts normally span for less than 4 km and 20 min. In this paper, the Weather Research and Forecasting model is used at resolutions of 400 m and 3 min to test if it can properly capture the variables and dynamics of high‐reflectivity microbursts. Several microphysics and planetary boundary layer parametrizations are tested to find the best model configuration for the simulation of this kind of episodes. General conditions are evaluated by using thermodynamic diagrams. Surface and vertical wind speed, reflectivity, precipitation, and other variables for each simulated event are compared with observations, and the model's sensitivity to the variables is assessed. The dynamics and evolution of the microburst is evaluated using different plots of a chosen event. The results show that the model is able to reproduce high‐reflectivity microbursts in accordance with observations, although there is a tendency to underestimate the intensity of variables, most markedly on the wind vertical velocity. Regarding the microphysics schemes, the Morrison parametrization performs better than the WRF single‐moment 6‐class scheme. No major differences are found between the Mellor‐Yamada‐Janjic and the Mellor‐Yamada‐Nakanishi‐Niino planetary boundary layer parametrizations., Ministerio de Ciencia e Innovación (MICCIN), Ministerio de Economía y Competitividad (MINECO), Interdisciplinary Mathematics Institute of the Complutense University of Madrid, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

Context.The observed orbital characteristics of post-asymptotic giant branch and post-red giant branch (post-RGB) binaries are not understood. We suspect that the missing ingredients needed to explain them probably lie in the continuous interaction of the central binary with its circumstellar environment. Aims.We aim at studying the circumbinary material in these complex systems by investigating the connection between the innermost structures and large-scale structures. Methods.We perform high-angular resolution observations of HD 101584 in the near-infrared continuum. HD 101584 has a complex structure as seen at millimeter wavelengths, with a disk-like morphology and a bipolar outflow due to an episode of a strong binary interaction. To account for the complexity of the target, we first perform an image reconstruction and use this result to fit a geometrical model to extract the morphological and thermal features of the environment. Results.The image reveals an unexpected double ring structure. We interpret the inner ring as having been produced by emission from dust located in the plane of the disk, and the outer ring having been produced by emission from dust that is located 1.6 [D/1kpc] au above the disk plane. The inner ring diameter (3.94 [D/1kpc] au) and temperature (T= 1540 +/- 10 K) are compatible with the dust sublimation front of the disk. The origin of the out-of-plane ring (with a diameter of 7.39 [D/1kpc] au and a temperature of 1014 +/- 10 K) could be episodic ejection or a dust condensation front in the outflow. Conclusions.The observed outer ring is possibly linked with the blue-shifted side of the large-scale outflow seen by the Atacama Large Millimeter/submillimeter Array and may trace its launching location to the central star. Such observations place morphological constraints on the ejection mechanism. Additional observations are needed to constrain the origin of the out-of-plane structure.

Context. This is the first publication from the DEATHSTAR project. The overall goal of the project is to reduce the uncertainties of the observational estimates of mass-loss rates from evolved stars on the Asymptotic Giant Branch (AGB). Aim. The aim in this first publication is to constrain the sizes of the (CO)-C-12 emitting region from the circumstellar envelopes around 42 mostly southern AGB stars, of which 21 are M-type and 21 are C-type, using the Atacama Compact Array (ACA) at the Atacama Large Millimeter/submillimeter Array. The symmetry of the outflows is also investigated. Methods. Line emission from (CO)-C-12 J = 2 -> 1 and 3 -> 2 from all of the sources were mapped using the ACA. In this initial analysis, the emission distribution was fit to a Gaussian distribution in the uv-plane. A detailed radiative transfer analysis will be presented in a future publication. The major and minor axis of the best-fit Gaussian at the line center velocity of the (CO)-C-12 J = 2 -> 1 emission gives a first indication of the size of the emitting region. Furthermore, the fitting results, such as the Gaussian major and minor axis, center position, and the goodness of fit across both lines, constrain the symmetry of the emission distribution. For a subsample of sources, the measured emission distribution is compared to predictions from previous best-fit radiative transfer modeling results. Results. We find that the CO envelope sizes are, in general, larger for C-type than for M-type AGB stars, which is as expected if the CO/H-2 ratio is larger in C-type stars. Furthermore, the measurements show a relation between the measured (Gaussian) (CO)-C-12 J = 2 -> 1 size and circumstellar density that, while in broad agreement with photodissociation calculations, reveals large scatter and some systematic differences between the different stellar types. For lower mass-loss-rate irregular and semi-regular variables of both M- and C-type AGB stars, the (CO)-C-12 J = 2 -> 1

The recurrent fast radio burst FRB 180916 was recently shown to exhibit a 16-d period (with possible aliasing) in its bursting activity. Given magnetars as widely considered FRB sources, this period has been attributed to precession of the magnetar spin axis or the orbit of a binary companion. Here, we make the simpler connection to a rotational period, an idea observationally motivated by the 6.7-h period of the Galactic magnetar candidate, 1E 161348–5055. We explore three physical mechanisms that could lead to the creation of ultralong period magnetars: (i) enhanced spin-down due to episodic mass-loaded charged particle winds (e.g. as may accompany giant flares), (ii) angular momentum kicks from giant flares, and (iii) fallback leading to long-lasting accretion discs. We show that particle winds and fallback accretion can potentially lead to a sub-set of the magnetar population with ultralong periods, sufficiently long to accommodate FRB 180916 or 1E 161348–5055. If confirmed, such periods implicate magnetars in relatively mature states (ages 1−10 kyr) and which possessed large internal magnetic fields at birth Bint ≳ 1016 G. In the low-twist magnetar model for FRBs, such long period magnetars may dominate FRB production for repeaters at lower isotropic-equivalent energies and broaden the energy distribution beyond that expected for a canonical population of magnetars, which terminate their magnetic activity at shorter periods P ≲ 10 s

Context. Photospheric radius expansion during X-ray bursts can be used to measure neutron star radii and help constrain the equation of state of neutron star matter. Understanding the stellar wind dynamics is important for interpreting observations, and therefore stellar wind models, though studied in past decades, have regained interest and need to be revisited with updated data and methods. Aims. Here, we study the radiative wind model in the context of X-ray bursts with modern techniques and physics input. We focus on characterization of the solutions and the study of observable magnitudes as a function of free model parameters. Methods. We implemented a spherically symmetric nonrelativistic wind model in a stationary regime, with updated opacity tables and modern numerical techniques. Total mass and energy outflows (¿, E) were treated as free parameters. Results. A high-resolution parameter-space exploration was performed to allow better characterization of observable magnitudes. High correlation was found between different photospheric magnitudes and free parameters. For instance, the photospheric ratio of gravitational energy outflow to radiative luminosity is directly proportional to the photospheric wind velocity. Conclusions. The correlations found here could help determine the physical conditions of the inner layers, where nuclear reactions take place, by means of observable photospheric values. Further studies are needed to determine the range of physical conditions in which the correlations are valid., Peer Reviewed, Postprint (published version)

Context. Episodic accretion has been observed in short-period binaries, where bursts of accretion occur at periastron. The binary trigger hypothesis has also been suggested as a driver for accretion during protostellar stages.Aims. Our goal is to investigate how the strength of episodic accretion bursts depends on eccentricity.Methods. We investigate the binary trigger hypothesis in longer-period (> 20 yr) binaries by carrying out three-dimensional magnetohydrodynamical simulations of the formation of low-mass binary stars down to final separations of similar to 10 AU, including the effects of gas turbulence and magnetic fields. We ran two simulations with an initial turbulent gas core of one solar mass each and two different initial turbulent Mach numbers, M=sigma(v)/c(s)=0.1 and M=0.2, for 6500 yr after protostar formation.Results. We observe bursts of accretion at periastron during the early stages when the eccentricity of the binary system is still high. We find that this correlation between bursts of accretion and passing periastron breaks down at later stages because of the gradual circularisation of the orbits. For eccentricities greater than e=0.2, we observe episodic accretion triggered near periastron. However, we do not find any strong correlation between the strength of episodic accretion and eccentricity. The strength of accretion is defined as the ratio of the burst accretion rate to the quiescent accretion rate. We determine that accretion events are likely triggered by torques between the rotation of the circumstellar disc and the approaching binary stars. We compare our results with observational data of episodic accretion in short-period binaries and find good agreement between our simulations and the observations.Conclusions. We conclude that episodic accretion is a universal mechanism operating in eccentric young binary-star systems, independent of separation, and it should be observable in long-period binaries a

Wind-driven outflows are observed around a broad range of accreting objects throughout the universe, ranging from forming low-mass stars to supermassive black holes. We study the interaction between a central isotropic wind and an infalling, rotating envelope, which determines the steady-state cavity shape formed at their interface under the assumption of weak mixing. The shape of the resulting wind-blown cavity is elongated and self-similar, with a physical size determined by the ratio between wind ram pressure and envelope thermal pressure. We compute the growth of a warm turbulent mixing layer between the shocked wind and the deflected envelope, and calculate the resultant broad-line profile, under the assumption of a linear (Couette-type) velocity profile across the layer. We then test our model against the warm broad velocity component observed in COJ = 16-15 by Herschel/HIFI in the protostar Serpens-Main SMM1. Given independent observational constraints on the temperature and density of the dust envelope around SMM1, we find an excellent match to all its observed properties (line profile, momentum, temperature) and to the SMM1 outflow cavity width for a physically reasonable set of parameters: a ratio of wind to infall mass flux of 4%, a wind speed ofv(w)30 km s(-1), an interstellar abundance of CO and H-2, and a turbulent entrainment efficiency consistent with laboratory experiments. The inferred ratio of ejection to disk accretion rate, 6%-20%, is in agreement with current disk wind theories. Thus, the model provides a new framework to reconcile the modest outflow cavity widths in protostars with large observed flow velocities. Being self-similar, it is applicable over a broader range of astrophysical contexts as well.

[Context]: Gamma-ray binaries are systems that emit nonthermal radiation peaking at energies above 1 MeV. One proposed scenario to explain their emission consists of a pulsar orbiting a massive star, with particle acceleration taking place in shocks produced by the interaction of the stellar and pulsar winds. [Aims]: We develop a semi-Analytical model of the nonthermal emission of the colliding-wind structure, which includes the dynamical effects of orbital motion. We apply the model to a general case and to LS 5039. [Methods]: The model consists of a one-dimensional emitter, the geometry of which is affected by Coriolis forces owing to orbital motion. Two particle accelerators are considered: one at the two-wind standoff location and the other one at the turnover produced by the Coriolis force. Synchrotron and inverse Compton emission is studied taking into account Doppler boosting and absorption processes associated to the massive star. [Results]: If both accelerators are provided with the same energy budget, most of the radiation comes from the region of the Coriolis turnover and beyond, up to a few orbital separations from the binary system. Significant orbital changes of the nonthermal emission are predicted in all energy bands. The model allows us to reproduce some of the LS 5039 emission features, but not all of them. In particular, the MeV radiation is probably too high to be explained by our model alone, the GeV flux is recovered but not its modulation, and the radio emission beyond the Coriolis turnover is too low. The predicted system inclination is consistent with the presence of a pulsar in the binary. [Conclusions]: The model is quite successful in reproducing the overall nonthermal behavior of LS 5039. Some improvements are suggested to better explain the phenomenology observed in this source, such as accounting for particle reacceleration beyond the Coriolis turnover, unshocked pulsar wind emission, and the three-dimensional extension of the emitter.

We present a detailed comparative study of the arcs and fragmented ring-like features in the haloes of the planetary nebulae (PNe) NGC 6543, NGC 7009, and NGC 7027 and the spiral pattern around the carbon star AFGL 3068 using high-quality multi-epoch HST images. This comparison allows us to investigate the connection and possible evolution between the regular patterns surrounding AGB stars and the irregular concentric patterns around PNe. The radial proper motion of these features, ≃15 km s-1, are found to be consistent with the AGB wind and their linear sizes and inter-lapse times (500 - 1900 yr) also agree with those found around AGB stars, suggesting a common origin. We find evidence using radiative-hydrodynamic simulations that regular patterns produced at the end of the AGB phase become highly distorted by their interactions with the expanding PN and the anisotropic illumination and ionization patterns caused by shadow instabilities. These processes will disrupt the regular (mostly spiral) patterns around AGB stars, plausibly becoming the arcs and fragmented rings observed in the haloes of PNe.© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society

Recent space- and ground-based studies of the circumgalactic medium (CGM) around nearby galaxies have revealed the dynamic interplay between the galaxy ecosystem and surrounding CGM using bright background quasars. In this thesis, we extend this investigation to higher redshifts by using the bright afterglows of gamma-ray bursts (GRBs) as background sources probing the CGM of their own host galaxies. We compiled a sample of 27 high-resolution (R $>$ 6000) rest-frame UV spectra of GRB afterglows in a redshift range (2 $\lesssim$ z $\lesssim$ 6); we call this the `CGM-GRB sample'. We find stronger blue wings in high-ionization species (Si IV, C IV) compared to the low-ionization species (Si II, Fe II), indicative of the presence of ubiquitous warm outflows in the GRB hosts at high redshifts. Using kinematic models, we estimated typical values of CGM properties (for the sample) such as CGM mass (10$^{9.8}$ $\mathrm{M_{\odot}}$) and outflow launch velocity (300 km s$^{-1}$). Further, by comparing our results with previous C IV absorption studies, we find a possible CGM-galaxy co-evolution. Over the course of evolution of present-day galaxies with $\mathrm{M_{*}}$ $>$ $10^{10}$ $\mathrm{M_{\odot}}$, the ratio of C IV mass in the CGM to the stellar mass remains fairly uniform, such that log$\mathrm{(M_{C IV}/M_{*})} \sim -4.5$ within $\pm$0.5 dex from $z\sim4$ to $z\sim0$, suggesting CGM-galaxy co-evolution. Next, we embarked on a search for possible relations between the outflow properties and those of the host galaxies such as $\mathrm{M_*}$, star formation rate (SFR), and specific SFR (= SFR/$\mathrm{M_*}$). To estimate the total SFR, we first investigated the degree of dust obscuration in the massive GRB hosts in our sample by comparing radio- and UV-based star-formation rates. We inferred that the GRB hosts in our sample are not heavily dust obscured, and hence, their SFRs can be estimated reliably using the established dust-correction methods. For the outflow-galaxy

Context The heavy mass loss observed in evolved stars on the asymptotic giant branch (AGB) is usually attributed to dust-driven winds, but it is still an open question how much AGB stars contribute to the dust production in the interstellar medium, especially at lower metallicities. In the case of C-type AGB stars, where the wind is thought to be driven by radiation pressure on amorphous carbon grains, there should be significant dust production even in metal-poor environments. Carbon stars can manufacture the building blocks needed to form the wind-driving dust species themselves, irrespective of the chemical composition they have, by dredging up carbon from the stellar interior during thermal pulses. Aims We investigate how the mass loss in carbon stars is affected by a low-metallicity environment, similar to the Large and Small Magellanic Clouds (LMC and SMC). Methods The atmospheres and winds of C-type AGB stars are modeled with the 1D spherically symmetric radiation-hydrodynamical code Dynamic Atmosphere and Radiation-driven Wind models based on Implicit Numerics (DARWIN). The models include a time-dependent description for nucleation, growth, and evaporation of amorphous carbon grains directly out of the gas phase. To explore the metallicity-dependence of mass loss we calculate model grids at three different chemical abundances (solar, LMC, and SMC). Since carbon may be dredged up during the thermal pulses as AGB stars evolve, we keep the carbon abundance as a free parameter. The models in these three different grids all have a current mass of one solar mass; effective temperatures of 2600, 2800, 3000, or 3200 K; and stellar luminosities equal to log L-*/L-circle dot = 3.70, 3.85, or 4.00. Results The DARWIN models show that mass loss in carbon stars is facilitated by high luminosities, low effective temperatures, and a high carbon excess (C-O) at both solar and subsolar metallicities Similar combinations of effective temperature, luminosity, and carbon excess

We present X-ray spectra spanning 18 yr of evolution for SN 1996cr, one of the five nearest SNe detected in the modern era. Chandra HETG exposures in 2000, 2004, and 2009 allow us to resolve spectrally the velocity profiles of Ne, Mg, Si, S, and Fe emission lines and monitor their evolution as tracers of the ejecta-circumstellar medium interaction. To explain the diversity of X-ray line profiles, we explore several possible geometrical models. Based on the highest signal-to-noise 2009 epoch, we find that a polar geometry with two distinct opening angle configurations and internal obscuration can successfully reproduce all of the observed line profiles. The best-fitting model consists of two plasma components: (1) a mildly absorbed (2x10(21) cm(-2)), cooler (approximate to 2 keV) with high Ne, Mg, Si, and S abundances associated with a wide polar interaction region (half-opening angle approximate to 58 degrees); (2) a moderately absorbed (2 x 10(22) cm(-2)), hotter (greater than or similar to 20 keV) plasma with high Fe abundances and strong internal obscuration associated with a narrow polar interaction region (half-opening angle approximate to 20 degrees). We extend this model to seven further epochs with lower signal-to-noise ratio and/or lower spectral-resolution between 2000 and 2018, yielding several interesting trends in absorption, flux, geometry, and expansion velocity. We argue that the hotter and colder components are associated with reflected and forward shocks, respectively, at least at later epochs. We discuss the physical implications of our results and plausible explosion scenarios to understand the X-ray data of SN1996cr., QC 20200131