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Accreting millisecond X-ray pulsars are known to provide a wealth of physical information during their successive states of outburst and quiescence. Based on the observed spin-up and spin-down rates of these objects it is possible, among other things, to infer the stellar magnetic field strength and test models of accretion disc flow. In this paper we consider the three accreting X-ray pulsars (XTE J1751-305, IGR J00291+5934, and SAX J1808.4-3658) with the best available timing data, and model their observed spin-up rates with the help of a collection of standard torque models that describe a magnetically-threaded accretion disc truncated at the magnetospheric radius. Whilst none of these models are able to explain the observational data, we find that the inclusion of the physically motivated phenomenological parameter $\xi$, which controls the uncertainty in the location of the magnetospheric radius, leads to an enhanced disc-integrated accretion torque. These 'new' torque models are compatible with the observed spin-up rates as well as the inferred magnetic fields of these objects provided that $\xi \approx 0.1-0.5$. Our results are supplemented with a discussion of the relevance of additional physics effects that include the presence of a multipolar magnetic field and general-relativistic gravity., Comment: 15 pages, 9 figures. Accepted for publication in MNRAS

Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks. The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion model's prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed., published in Nature; open access link: https://www.nature.com/articles/s41586-021-03751-5

Using the vector magnetograms observed by SDO (Solar Dynamics Observatory)/HMI (Helioseismic and Magnetic Imager), the current distribution and evolution associated with an X9.3 flare occurred in the active region AR12673 on Sept. 6, 2017 are studied in detail. The results show that there is a pair of electric current ribbons with opposite directions and the magnitude of 0.4 A/m 2 , which can be called a pair of conjugate electric current ribbons. This pair of conjugate electric current ribbons exist before, during, and after the flare; their positions are almost coincident with the two bright ribbons of the flare, and their shapes are also extremely similar with them. The light curve of the total electric current intensity on Sept. 6 shows that the electric current intensity enhanced sharply during the strong X9.3 flare eruption, and this phenomenon persisted for several hours. The results of this study strongly support the model of QSL (Quasi-Separatrix Layer) 3D magnetic reconnection.

In this paper, an efficient algorithm is developed to automatically detect and extract coronal loops. First of all, in the algorithm, three characteristics associated with coronal loops are used to construct a match filter able to enhance the loops. Secondly, the method combining a high-pass filter (unsharp-mask enhancement) with a global threshold is used to further enhance and segment the loops. Thirdly, to extract every individual coronal loop and obtain their parameters (the 2D projected space coordinates and lengths) from the segmented loops, a clustering method of the pixels with approximate local direction and connected domain is further used. Fourthly, to evaluate the performance of the developed algorithm, images observed by the Transition Region and Coronal Explorer (TRACE), the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) and the High-Resolution Coronal Imager (Hi-C) are used, and comparison experiments between the existing algorithms and the developed algorithm are performed. Finally, it is found that the developed algorithm is commensurate with the two most promising algorithms, oriented coronal curved loop tracing (OCCULT) and its improved version, OCCULT-2, in performance. Therefore, for scientific applications associated with coronal loops, the developed algorithm will be a powerful tool.

The hot Jupiter HD189733b is expected to be a source of strong radio emission, due to its close proximity to its magnetically active host star. Here, we model the stellar wind of its host star, based on reconstructed surface stellar magnetic field maps. We use the local stellar wind properties at the planetary orbit obtained from our models to compute the expected radio emission from the planet. Our findings show that the planet emits with a peak flux density within the detection capabilities of LOFAR. However, due to absorption by the stellar wind itself, this emission may be attenuated significantly. We show that the best time to observe the system is when the planet is near primary transit of the host star, as the attenuation from the stellar wind is lowest in this region., Comment: 4 pages, 4 figures. Proceedings of the IAU Symposium 354, "Solar and Stellar Magnetic Fields: Origins and Manifestations", June 2019

We study the evolution of the decaying active region NOAA 12708, from the photosphere up to the corona using high resolution, multi-wavelength GREGOR observations taken on May 9, 2018. We utilize spectropolarimetric scans of the 10830 Å spectral range by the GREGOR Infrared Spectrograph (GRIS), spectral imaging time-series in the Na ID2 spectral line by the GREGOR Fabry-Pérot Interferometer (GFPI) and context imaging in the Ca IIH and blue continuum by the High-resolution Fast Imager (HiFI). Context imaging in the UV/EUV from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) complements our dataset. The region under study contains one pore with a light-bridge, a few micro-pores and extended clusters of magnetic bright points. We study the magnetic structure from the photosphere up to the upper chromosphere through the spectropolarimetric observations in He II and Si I and through the magnetograms provided by the Helioseismic and Magnetic Imager (HMI). The high-resolution photospheric images reveal the complex interaction between granular-scale convective motions and a range of scales of magnetic field concentrations in unprecedented detail. The pore itself shows a strong interaction with the convective motions, which eventually leads to its decay, while, under the influence of the photospheric flow field, micro-pores appear and disappear. Compressible waves are generated, which are guided towards the upper atmosphere along the magnetic field lines of the various magnetic structures within the field-of-view. Modelling of the He i absorption profiles reveals high velocity components, mostly associated with magnetic bright points at the periphery of the active region, many of which correspond to asymmetric Si I Stokes-V profiles revealing a coupling between upper photospheric and upper chromospheric dynamics. Time-series of Na ID2 spectral images reveal episodic high velocity components at the same locations. State-of-the-art multi-wavelength GREGOR observations allow us to track and understand the mechanisms at work during the decay phase of the active region.

Stellar magnetic field is the driver of activity in stars and can trigger spots, energetic flares, coronal plasma ejections and ionized winds. These phenomena play a crucial role in understanding the internal mechanisms of the star, but can also have potential effects in orbiting planets. During the transit of a planet, spots can be occulted producing features imprinted in the transit light curve. Here, we modelled these features to characterize the physical properties of the spots (radius, intensity, and location). In addition, we monitor spots signatures on multiple transits to estimate magnetic cycles length of Kepler stars. Flares have also been observed during transits in active stars. We derive the properties of the flares and analyse their UV impact on possible living organisms in planets orbiting in the habitable zone.

During the final stages of a neutron-star binary coalescence, stellar quasi-normal modes can become resonantly excited by tidal fields. If the strain exerted by the excited modes exceeds the extent to which the crust can respond linearly, localised crustal failures may occur. In this work, we re-examine resonant $g$-mode excitations of relativistic neutron stars in the last $\sim$ 10 seconds of an inspiral. We adopt realistic equations of state that pass constraints from GW170817, include 3rd order post-Newtonian terms for the conservation orbital motion, and employ a 2.5 post-Newtonian scheme for gravitational back-reaction. Frequency modulations of the modes due to tidal fields, Lorentz forces, and (slow) rotation are also considered to investigate the maximal strain achievable by resonantly-excited $g$-modes. Depending on the equation of state, degree of stratification, and stellar magnetic field, we find that certain $g$-modes excitations may be able to break the crust some seconds prior to coalescence., 15 pages, 11 figures, 2 tables. Matches published version in MNRAS

Context. The observation of numerous magnetic switchbacks and associated plasma jets in Parker Solar Probe (PSP) during its first five orbits, particularly near the Sun, has attracted considerable attention. Switchbacks have been found to be systematically associated with correlated reversals in the direction of the propagation of Alfvénic fluctuations, as well as similar reversals of the electron strahl. Aims. Here we aim to see whether the energetic particles change direction at the magnetic field switchbacks. Methods. We use magnetic field data from the MAG suite’s fluxgate magnetometer instrument to identify switchback regions. We examine the radial anisotropy of the energetic particles measured by the EPI-Lo instrument of the IS⊙IS suite. Results. We find that energetic particles measured by EPI-Lo generally do not preferentially change their directionality from that of the background magnetic field to that of the switchbacks. Conclusions. A reasonable hypothesis is that particles with smaller gyroradii, such as strahl electrons, can reverse direction by following the magnetic field in switchbacks, but that larger gyroradii particles cannot. This provides the possibility of setting a constraint on the radius of the curvature of the magnetic field in switchbacks, a property not otherwise observed by PSP. We expect that particles at higher energies than those detectable by EPI-Lo will also not respond to switchbacks. The observed reversals of radial energetic particle flux are separate phenomena, likely associated with source locations or other propagation effects occurring at greater radial distances.

In this review, we explain recent progress made in the Babcock-Leighton dynamo models for the Sun, which have been most successful to explain various properties of the solar cycle. In general, these models are 2D axisymmetric and the mean-field dynamo equations are solved in the meriodional plane of the Sun. Various physical processes (e.g., magnetic buoyancy and Babcock-Leighton mechanism) involved in these models are inherently 3D process and could not be modeled properly in a 2D framework. After pointing out limitations of 2D models (e.g., Mean-field Babcock-Leighton dynamo models and Surface Flux Transport models), we describe recently developed next-generation 3D dynamo models that implement more sophisticated flux emergence algorithm of buoyant flux tube rise through the convection zone and capture Babcock-Leighton process more realistically than previous 2D models. The detailed results from these 3D dynamo models including surface flux transport counterpart are presented. We explain the cycle irregularities that are reproduced in 3D dynamo models by introducing scattering around the tilt angle only. Some results by assimilating observed photospheric convective velocity fields into the 3D models are also discussed, pointing out the wide opportunity that these 3D models hold to deliver., Comment: Invited review submitted to the Journal of Astrophysics and Astronomy

The rotational evolution of accreting pre-main-sequence stars is influenced by its magnetic interaction with its surrounding circumstellar disk. Using the PLUTO code, we perform 2.5D magnetohydrodynamic, axisymmetric, time-dependent simulations of star-disk interaction---with an initial dipolar magnetic field structure, and a viscous and resistive accretion disk---in order to model the three mechanisms that contribute to the net stellar torque: accretion flow, stellar wind, and magnetospheric ejections (periodic inflation and reconnection events). We investigate how changes in the stellar magnetic field strength, rotation rate, and mass accretion rate (changing the initial disk density) affect the net stellar torque. All simulations are in a net spin-up regime. We fit semi-analytic functions for the three stellar torque contributions, allowing for the prediction of the net stellar torque for our parameter regime, and the possibility of investigating spin-evolution using 1D stellar evolution codes. The presence of an accretion disk appears to increase the efficiency of stellar torques compared to isolated stars, for cases with outflow rates much smaller than accretion rates, because the star-disk interaction opens more of the stellar magnetic flux compared to that from isolated stars. In our parameter regime, a stellar wind with a mass loss rate of $\approx 1 \%$ of the mass accretion rate is capable of extracting $\lesssim 50 \%$ of the accreting angular momentum. These simulations suggest that achieving spin-equilibrium in a representative T Tauri case within our parameter regime, e.g., BP Tau, would require a wind mass loss rate of $\approx 25\%$ of the mass accretion rate., 26 pages, 14 figures

The paper presents magnetic field measurements for 15 chemically peculiar (CP) stars of subgroup~1b in the OrionOB1 association. We have found that the proportion of stars with strong magnetic fields among these 15 CP stars is almost twice as large as in subgroup 1a. Along with this, the age of subgroup 1b is estimated as 2 Myr, and the age of subgroup~1a is in the order of 10 Myr. The average root-mean-square magnetic field Be for stars in subgroup 1b is 2.3 times higher than that for stars in subgroup 1a. The conclusions obtained fall within the concept of the fossil origin of large-scale magnetic fields in B and A stars, but the rate of field weakening with age appears anomalously high. We present our results as an important observational test for calibrating the theory of stellar magnetic field formation and evolution., Comment: Submitted to Astrophysical Bulletin

The astropause (heliopause for the Sun) is the tangential discontinuity separating the stellar wind from the interstellar plasma. The global shape of the heliopause is a matter of debates. Two types of the shape are under discussion: comet-like and tube-like. In the second type the two-jets oriented toward the stellar rotation axis are formed by the action of azimuthal component of the stellar magnetic field. We explore a simplified global astrosphere in which: (1) the surrounding and moving with respect to the star circumstellar medium is fully ionized, (2) the interstellar magnetic field is neglected, (3) the radial component of the stellar magnetic field is neglected as compared with the azimuthal component, (4) the stellar wind outflow is spherically symmetric and supersonic. We present the results of numerical 3D MHD modelling and explore how the global structure depends on the gas-dynamic Mach number of the interstellar flow, $M_\infty$, and the Alfvenic Mach number in the stellar wind. It is shown that the astropause has a tube-like shape for small values of $M_\infty$. The wings of the tube are distorted toward the tail as larger as larger the Mach number is. The new (to our knowledge) result is the reverse interstellar flow in the vicinity of the astropause in the tail. The larger the interstellar Mach number is the narrower the reverse flow is. At some values of the Mach number the stellar wind overcomes the reverse interstellar flow and moves out in downwind. In this regime the astropause changes its topology from tube-like to sheet-like., Comment: Typos have been corrected compared to the previous version. Added some clarifications and links to other authors. Results have not been changed

In this paper, we report the observed temporal correlation between extreme-violet (EUV) emission and magneto-acoustic oscillations in a EUV moss region, which is the footpoint region only connected by magnetic loops with million-degree plasma. The result is obtained from a detailed multi-wavelength data analysis to the region with the purpose of resolving fine-scale mass and energy flows that come from the photosphere, pass through the chromosphere and finally heat solar transition region or the corona. The data set covers three atmospheric levels on the Sun, consisting of high-resolution broad-band imaging at TiO 7057 \AA\ and the line of sight magnetograms for the photosphere, high-resolution narrow-band images at Helium \textsc{i} 10830 \AA\ for the chromosphere and EUV images at 171 \AA\ for the corona. We report following new phenomena: 1) Repeated injections of chromospheric material shown as 10830 \AA\ absorption are squirted out from inter-granular lanes with the period of $\sim$ 5 minutes. 2) EUV emissions are found to be periodically modulated with the similar periods of $\sim$ 5 minutes. 3) Around the injection area where 10830 \AA\ absorption is enhanced, both EUV emissions and the strength of magnetic field are remarkably stronger. 4) The peaks on the time profile of the EUV emissions are found to be in sync with oscillatory peaks of the stronger magnetic field in the region. These findings may give a series of strong evidences supporting the scenario that coronal heating is powered by magneto-acoustic waves., Comment: 8 pages, 4 figures

Measuring the mean densities of close-in planets orbiting pre-main-sequence (PMS) stars is crucially needed by planet formation and evolution models. This requires to measure both planet radii, from the depth of their transit light curves, and masses, from the radial velocity (RV) wobbles induced by the planet on its host star. However, PMS stars exhibit intense magnetic activity responsible for fluctuations in both photometric and RV curves that are much stronger than planet signatures. As a result, no close-in planet younger than 25 Myr has a well-constrained bulk density.AU Microscopii (AU Mic) is a nearby active 22-Myr old M1 star around which a close-in transiting planet was recently detected from TESS and Spitzer light-curves (Plavchan et al., 2020). Despite velocimetric follow-ups in the optical domain, the authors reported no more than an upper limit for the planet mass due to the large dispersion induced by stellar activity in their RV time-series. The high stellar brightness, and the expected decrease in the amplitude of stellar activity RV signals from the optical to the near-infrared domains, makes the nIR (YJHK bands) spectropolarimeter SPIRou (Canada-France-Hawaii Telescope, atop Mauna Kea) the ideal instrument to measure the mass of AU Mic b. In this study, we present a spectropolarimetric and velocimetric analysis of 27 observations of AU Mic collected with SPIRou from September to November 2019. The dispersion of our RV time-series is about 45 m/s, ~2.5 times lower than that obtained in the optical domain. We jointly model the planet and stellar activity components of the RV data set, resulting in a 3.5σ detection a close-in transiting planet AU Mic b, with an estimated mass of 16.7 +/- 4.9 Earth mass, implying a Neptune-like bulk density of 1.3 +/- 0.4 g/cm³. A consistent detection of the planet is independently obtained by simultaneously reconstructing the surface brightness of the star and estimating the planet parameters using Zeeman-Doppler imaging (ZDI). Using ZDI, we invert our intensity and circularly-polarized spectra into surface brightness and large-scale magnetic field, resulting in a mainly poloidal and axisymmetric field of 475 G, dominated by a 450 G dipole tilted at 19° to the rotation axis towards phase 0.2. Moreover, we find that the large-scale magnetic field is sheared by solar-like differential rotation of 0.167 rad/d, twice as large as that shearing the spot/plage distribution. Finally, we compute various indicators of the stellar activity and study their rotational modulation and correlation with RVs. We find that the bisector inverse slope and small-scale magnetic field correlate best with the stellar activity RV signal. Surprisingly, chromospheric indices based on Helium I (HeI, 1083 nm) and Paschen Beta (PaB, 1282 nm) probe different regions of the stellar disc, HeI being mostly emitted around the magnetic equator while PaB emission is linked to the magnetic pole.AU Mic b already appears as a prime target for constraining planet formation and evolution models. Moreover, the interactions between the planet and the debris disk surrounding the system could give rise to promising synergies between photometric, spectroscopic and imaging techniques. Finally, AU Mic b is a primary candidate for an atmosphere characterization and, potentially, the detection of an extended H/He exosphere around AU Mic b with upcoming space- and ground-based missions.

longitudinal magnetic field often suffers the saturation effect in strong magnetic field region when the measurement performs in a single-wavelength point and linear calibration is adopted. In this study, we develop a method that can judge the threshold of saturation in Stokes $V/I$ observed by the Solar Magnetic Field Telescope (SMFT) and correct for it automatically. The procedure is that first perform the second-order polynomial fit to the Stokes $V/I$ \textit{vs} $I/I_{m}$ ($I_{m}$ is the maximum value of Stokes $I$) curve to estimate the threshold of saturation, then reconstruct Stokes $V/I$ in strong field region to correct for saturation. The algorithm is proved to be effective by comparing with the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). The accurate rate of detection and correction for saturation is $\sim$99.4\% and $\sim$88\% respectively among 175 active regions. The advantages and disadvantages of the algorithm are discussed., 13 pages, 8 figures, accepted by RAA

Stars form by accreting material from their surrounding disks. There is a consensus that matter flowing through the disk is channelled onto the stellar surface by the stellar magnetic field. This is thought to be strong enough to truncate the disk close to the so-called corotation radius where the disk rotates at the same rate as the star. Spectro-interferometric studies in young stellar objects show that Hydrogen is mostly emitted in a region of a few milliarcseconds across, usually located within the dust sublimation radius. Its origin is still a matter of debate and it can be interpreted as coming from the stellar magnetosphere, a rotating wind or a disk. In the case of intermediate-mass Herbig AeBe stars, the fact that the Br gamma emission is spatially resolved rules out that most of the emission comes from the magnetosphere. This is due to the weak magnetic fields (some tenths of G) detected in these sources, resulting in very compact magnetospheres. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve. However, the small angular size of the magnetosphere (a few tenths of milliarcseconds), along with the presence of winds emitting in Hydrogen make the observations interpretation challenging. Here, we present direct evidence of magnetospheric accretion by spatially resolving the inner disk of the 60 pc T Tauri star TW Hydrae through optical long baseline interferometry. We find that the hydrogen near-infrared emission comes from a region approximately 3.5 stellar radii (R*) across. This region is within the continuum dusty disk emitting region (Rcont = 7 R*) and smaller than the corotation radius which is twice as big. This indicates that the hydrogen emission originates at the accretion columns, as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance (>1au)., Publish in Nature

HD 133880 is one of the six hot magnetic stars known to produce coherent pulsed radio emission by the process of electron cyclotron maser emission (ECME). In this paper, we present observations of ECME from this star over a wide frequency range, covering nearly 300 - 4000 MHz with the Giant Metrewave Radio Telescope (GMRT) and the Karl G. Jansky Very Large Array (VLA). This study, which is the first of its kind, has led to the discovery of several interesting characteristics of the phenomenon and also of the host star. We find that the observable properties of ECME pulses, e.g. the time lag between right and left circularly polarized pulses, the amplitudes of the pulses, and their upper cut-off frequencies appear to be dependent on the stellar orientation with respect to the line of sight. We suggest that all these phenomena, which are beyond the ideal picture, can be attributed to a highly azimuthally asymmetric matter distribution in the magnetosphere about the magnetic field axis, which is a consequence of both the high obliquity (the angle between rotation axis and the magnetic field axis) of the star and the deviation of the stellar magnetic field from a dipolar topology., Accepted for publication in MNRAS

Initial results are presented from 3D MHD modelling of stellar-wind bubbles around O stars moving supersonically through the ISM. We describe algorithm updates that enable high-resolution 3D MHD simulations at reasonable computational cost. We apply the methods to the simulation of the astrosphere of a rotating massive star moving with 30 km/s through the diffuse interstellar medium, for two different stellar magnetic field strengths, 10 G and 100 G. Features in the flow are described and compared with similar models for the Heliosphere. The shocked interstellar medium becomes asymmetric with the inclusion of a magnetic field misaligned with the star's direction of motion, with observable consequences. When the Alfvenic Mach number of the wind is $\leq$10 then the stellar magnetic field begins to affect the structure of the wind bubble and features related to the magnetic axis of the star become visible at parsec scales. Prospects for predicting and measuring non-thermal radiation are discussed., Accepted for publication in Journal of Physics Conference Series, proceedings of the 19th Annual International Astrophysics Conference (Santa Fe, New Mexico, 9-13 March 2020)

We present spectral analysis of Algol stellar flare data. The 2-day observation of Algol in our Galaxy was done by Suzaku Satellite. Algol binary system consists of two component stars that eclipse one another periodically. The stellar magnetic field activities and Roche Lobe overflow/mass transfer mechanisms during rapid rotation of the component stars generate an X-ray emission. The variation in brightness of the stellar flare from epoch to epoch provides useful information of stellar properties of the component stars. The atomic physics of the Kα line emission process in the vicinities of chromospherically active and X-ray flare stars, and binaries is well understood. The photo ionization/collisional excitation in the Algol’s corona produces 6.7 keV line emission. The accumulated spectra in X-ray Imaging Spectrometer (XIS) sensors fitted combined with bremsstrahlung model and Gaussian line show a strong fixed X-ray energy of 6.7 keV.

The recent discovery of hot dust grains in the vicinity of main-sequence stars has become a hot issue among the scientific community of debris disks. Hot grains must have been enormously accumulated near their sublimation zones, but it is a mystery how such a high concentration of hot grains is sustained. The most difficult conundrum is that the size of hot dust grains is estimated to lie in the submicrometer range, while submicrometer-sized grains are instantly swept away from near-stellar environments by stellar radiation pressure. One and only mechanism proposed for prolonging the residence time of hot grains in the near-stellar environments is trapping of charged nanoparticles by stellar magnetic fields. We revisit the model of magnetic grain trapping around main-sequence stars of various spectral classes by taking into account sublimation and electric charging of the grains. The model of magnetic grain trapping predicts that hot dust grains are present in the vicinity of main-sequence stars with high rotation velocities and intermediate magnetic-field strengths. On the contrary, we find that the detection of hot dust grains has no correlation with the rotation velocities of central stars nor the magnetic field strengths of the stars. Our numerical evaluation of electric grain charging indicates that the surface potential of submicrometer-sized grains in the vicinity of main-sequence stars is typically 4-5 V, which is one order of magnitude smaller than the value assumed by the model of magnetic grain trapping. On the basis of our numerical simulation on sublimation of dust grains in the vicinity of a star, it turns out that their lives end due to sublimation in a timescale much shorter than the period of one revolution at the gyroradius. It is, therefore, infeasible to dynamically extend the dwell time of hot grains inside the sublimation zone by magnetic trapping, ..., 30 pages, 8 figures, 3 tables, to appear in Planetary and Space Science

Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H $${\alpha }$$ (6562.8 A) and Fe i (6569.22 A) is presented in this paper. The corresponding active region numbers are NOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe i under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H $${\alpha }$$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and then decreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.

Stellar and galactic cosmic rays (SCR and GCR) are the primary factors influencing the radiation conditions near exoplanets. The GCR spectrum and its time variations are shaped by modulation processes in the astrosphere (the parameters of stellar wind and the local interstellar medium). The estimation of the velocity and density of hot stellar wind in the Parker model demonstrates that their values may change several fold, while the observations reveal that the stellar magnetic fields (and, consequently, the magnetic field of stellar wind) vary by one or two orders of magnitude. Galactic CR may be virtually non-existent near exoplanets of stars with strong magnetic field as a result of the modulation. The radiation conditions are then determined by SCR: the stellar activity, the energy of stellar flares, and the parameters of exoplanet orbits. The estimates of GCR and SCR fluxes near the exoplanets in the habitable zone of the TRAPPIST-1 system, which were obtained using the Parker model and the available data on the stellar magnetic field and activity, are presented below.

We developed a data base of pre-calculated flare images and spectra exploring a set of parameters which describe the physical characteristics of coronal loops and accelerated electron distribution. Due to the large number of parameters involved in describing the geometry and the flaring atmosphere in the model used, we built a large data base of models (∼250 000) to facilitate the flare analysis. The geometry and characteristics of non-thermal electrons are defined on a discrete grid with spatial resolution greater than 4 arcsec. The data base was constructed based on general properties of known solar flares and convolved with instrumental resolution to replicate the observations from the Nobeyama radio polarimeter spectra and Nobeyama radioheliograph (NoRH) brightness maps. Observed spectra and brightness distribution maps are easily compared with the modelled spectra and images in the data base, indicating a possible range of solutions. The parameter search efficiency in this finite data base is discussed. 8 out of 10 parameters analysed for 1000 simulated flare searches were recovered with a relative error of less than 20 per cent on average. In addition, from the analysis of the observed correlation between NoRH flare sizes and intensities at 17 GHz, some statistical properties were derived. From these statistics, the energy spectral index was found to be δ ∼ 3, with non-thermal electron densities showing a peak distribution ⪅107 cm−3, and Bphotosphere ⪆ 2000 G. Some bias for larger loops with heights as great as ∼2.6 × 109 cm, and looptop events were noted. An excellent match of the spectrum and the brightness distribution at 17 and 34 GHz of the 2002 May 31 flare is presented as well.

The long-term monitoring of magnetic cycles is a key diagnostic in understanding how dynamo generation and amplification of magnetic fields occur in solar-like stars. One of the current key problems is the establishment of the magnetic field behavior during the activity cycles for stars of different ages and evolutionary statuses. We present the experience of using own long-term datasets for study of activity cycles in selected stars at the Crimean Astrophysical Observatory.

We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modeling of the radio emission for HR5907. The wavelength-dependent radio light-curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light-curves leads us to conclude that the stellar magnetic field of HR5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907., 18 pages, 13 figures; accepted to MNRAS

Aims. We conduct simulations of the inner regions of protoplanetary disks (PPDs) to investigate the effects of protostellar magnetic fields on their long-term evolution. We use an inner boundary model that incorporates the influence of a stellar magnetic field. The position of the inner disk is dependent on the mass accretion rate as well as the magnetic field strength. We use this model to study the response of a magnetically truncated inner disk to an episodic accretion event. Additionally, we vary the protostellar magnetic field strength and investigate the consequences of the magnetic field on the long-term behavior of PPDs. Methods. We use the fully implicit 1+1D TAPIR code which solves the axisymmetric hydrodynamic equations self-consistently. Our model allows us to investigate disk dynamics close to the star and to conduct long-term evolution simulations simultaneously. We assume a hydrostatic vertical configuration described via an energy equation which accounts for the radiative transport in the vertical direction in the optically thick limit and the equation of state. Moreover, our model includes the radial radiation transport in the stationary diffusion limit and takes protostellar irradiation into account. Results. We include stellar magnetic torques, the influence of a pressure gradient, and a variable inner disk radius in the TAPIR code to describe the innermost disk region in a more self-consistent manner. We can show that this approach alters the disk dynamics considerably compared to a simplified diffusive evolution equation, especially during outbursts. During a single outburst, the angular velocity deviates significantly from the Keplerian velocity because of the influence of stellar magnetic torques. The disk pressure gradient switches sign several times and the inner disk radius is pushed towards the star, approaching < 1.2 R⋆. Additionally, by varying the stellar magnetic field strength, we can demonstrate several previously unseen effects. The number, duration, and the accreted disk mass of an outburst as well as the disk mass at the end of the disk phase (after several million years) depend on the stellar field strength. Furthermore, we can define a range of stellar magnetic field strengths, in which outbursts are completely suppressed. The robustness of this result is confirmed by varying different disk parameters. Conclusions. The influences of a prescribed stellar magnetic field, local pressure gradients, and a variable inner disk radius result in a more consistent description of the gas dynamics in the innermost regions of PPDs. Combining magnetic torques acting on the innermost disk regions with the long-term evolution of PPDs yields previously unseen results, whereby the whole disk structure is affected over its entire lifetime. Additionally, we want to emphasize that a combination of our 1+1D model with more sophisticated multi-dimensional codes could improve the understanding of PPDs even further.

In this work, we study the effects of temperature on magnetic white dwarfs. We model their interior as a nuclei lattice surrounded by a relativistic free Fermi gas of electrons, accounting for effects from temperature, Landau levels and anomalous magnetic moment. We find that, at low densities (corresponding to the outer regions of star), both temperature and magnetic field effects play an important role in the calculation of microscopic thermodynamical quantities. To study macroscopic stellar structures within a general-relativistic approach, we solve numerically the coupled Einstein's-Maxwell's equations for fixed entropy per particle configurations and discuss how temperature affects stellar magnetic field profiles, masses and radii.

Thermally emitting neutron stars represent a promising environment for probing the properties of axion-like particles. Due to the strong magnetic fields of these sources, surface photons may partially convert into such particles in the large magnetospheric region surrounding the stars, which will result in distinctive signatures in their spectra. However, the interaction depends on the polarization state of the radiation and is rather weak due to the low experimentally allowed values of the coupling constant $g_{\gamma a}$. In this work, we compute the degree of photon-axion transition in the case of 100% O-mode polarization and spectral energy distribution of an isotropic blackbody with uniform surface temperature. The stellar magnetic field is assumed to be dipolar. We show that with the maximum effect reached for the magnetic fields $\sim10^{13}$ - $10^{14}$ G (typical for X-ray dim isolated neutron stars) and $g_{\gamma a} = 2 \times 10^{-11}$ GeV$^{-1}$, the optical flux is reduced by 30 - 40%, while the high-energy part of the spectrum is not affected. The low-energy decrease exceeds 5% at $g_{\gamma a} \geq 2 \times 10^{-12}$ GeV$^{-1}$ and $m_a \leq 2\times 10^{-6}$ eV, which is below the present experimental and astrophysical limits on axion parameters. To obtain the actual observational constraints, rigorous treatment of the radiative surface layers is required., Comment: 8 pages, 4 figures

In a microlensing event, at caustic crossing, flux of a spot can be magnified several times with respect of the source star. This magnification contrast provide a unique opportunity to measure the magnetic field over a source star. In this work we investigate possibility of magnetic field detection through the Zeeman effect when a source crosses a caustic line. Using Fourier analysis, one can separate other broadening agents from actual Zeeman broadening. At next stage we use microlensing formalism to measure the magnification contrast of spot and source and finally using two reliable strategy, we find the delectability of magnetic field as a function of maximum magnification for spots with different sizes. The resolution and the signal to noise in each strategy determine which magnetic fields are detectable.&nbsp

In this contribution we review the current state-of-the-art of coronal magnetometry, in both optical and radio domains. We address the achievable objectives and the challenges of present measurement techniques and interpretation tools. In particular, we focus on the role that these observations can play for constraining and validating numerical models of the global coronal magnetic field. With regard to optical techniques, we mainly focus on the use of M1 diagnostics, further developing the theory of the formation of their polarization signatures in the magnetized corona.

We revisit the nature of large dips in flux from extinction by dusty circumstellar material that is observed by Kepler for many young stars in the Upper Sco and $\rho$ Oph star formation regions. These young, low-mass "dipper" stars are known to have low accretion rates and primarily host moderately evolved dusty circumstellar disks. Young low mass stars often exhibit rotating star spots that cause quasi-periodic photometric variations. We found no evidence for periods associated with the dips that are different from the star spot rotation period in spectrograms constructed from the light curves. The material causing the dips in most of these light curves must be approximately corotating with the star. We find that disk temperatures computed at the disk corotation radius are cool enough that dust should not sublime. Crude estimates for stellar magnetic field strengths and accretion rates are consistent with magnetospheric truncation near the corotation radius. Magnetospheric truncation models can explain why the dips are associated with material near corotation and how dusty material is lifted out of the midplane to obscure the star which would account for the large fraction of young low mass stars that are dippers. We propose that variations in disk orientation angle, stellar magnetic field dipole tilt axis, and disk accretion rate are underlying parameters accounting for differences in the dipper light curves.

The interaction between a YSO stellar magnetic field and its protostellar disc can result in stellar accretional flows and outflows from the inner disc rim. Gas flows with a velocity component perpendicular to disc midplane subject particles to centrifugal acceleration away from the protostar, resulting in particles being catapulted across the face of the disc. The ejected material can produce a "dust fan", which may be dense enough to mimic the appearance of a "puffed-up" inner disc rim. We derive analytic equations for the time dependent disc toroidal field, the disc magnetic twist, the size of the stable toroidal disc region, the jet speed and the disc region of maximal jet flow speed. We show how the observed infrared variability of the pre-transition disc system LRLL~31 can be modelled by a dust ejecta fan from the inner-most regions of the disc whose height is partially dependent on the jet flow speed. The greater the jet flow speed, the higher is the potential dust fan scale height. An increase in mass accretion onto the star tends to increase the height and optical depth of the dust ejection fan, increasing the amount of 1--8~$\mu$m radiation. The subsequent shadow reduces the amount of light falling on the outer disc and decreases the 8-- 40~$\mu$m radiation. A decrease in the accretion rate reverses this scenario, thereby producing the observed "see-saw" infrared variability., Comment: 18 pages, 21 figures

One of the most striking observations made by Parker Solar Probe during its first solar encounter is the omnipresence of rapid polarity reversals in a magnetic field that is otherwise mostly radial. These so-called switchbacks strongly affect the dynamics of the magnetic field. We concentrate here on their macroscopic properties. First, we find that these structures are self-similar, and have neither a characteristic magnitude, nor a characteristic duration. Their waiting time statistics shows evidence for aggregation. The associated long memory resides in their occurrence rate, and is not inherent to the background fluctuations. Interestingly, the spectral properties of inertial range turbulence differ inside and outside of switchback structures; in the latter the $1/f$ range extends to higher frequencies. These results suggest that outside of these structures we are in the presence of lower amplitude fluctuations with a shorter turbulent inertial range. We conjecture that these correspond to a pristine solar wind., 10 figures, to appear in The Astrophysical Journal Supplement Series (2020)

While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($\langle B_z\rangle=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $\langle B_z\rangle=100$ G. From these setups we compute the dependence of the outgoing radiative energy flux on the mean vertical magnetic field strength in the photosphere at continuum optical depth $\tau=1$ ($\langle \vert B_z\vert\rangle_{\tau=1}$). We find that a quiet Sun setup with a mean vertical field strength of $\langle \vert B_z\vert\rangle_{\tau=1}=69$ G is about $0.6~\%$ brighter than a non-magnetic reference case. We find a linear dependence of the outgoing radiative energy flux on the mean field strength $\langle \vert B_z\vert\rangle_{\tau=1}$ with a relative slope of $1.4\cdot 10^{-4}$ G$^{-1}$. With this sensitivity, only a moderate change of the quiet Sun field strength by $10\%$ would lead to a total solar irradiance variation comparable to the observed solar cycle variation. While this does provide strong indirect constraints on possible quiet Sun variations during a regular solar cycle, it also emphasizes that potential variability over longer time scales could make a significant contribution to longer-term solar irradiance variations., Comment: 15 pages, 10 figures, accepted for publication in ApJ

We present new ATCA multi-wavelength radio measurements (range 2.1-21.2 GHz) of the early-type magnetic star rho Oph A, performed in March 2019 during 3 different observing sessions. These new ATCA observations evidence a clear rotational modulation of the stellar radio emission and the detection of coherent auroral radio emission from rho Oph A at 2.1 GHz. We collected high-resolution optical spectra of rho Oph A acquired by several instruments over a time span of about ten years. We also report new magnetic field measurements of rho Oph A that, together with the radio light curves and the temporal variation of the equivalent width of the HeI line (lambda=5015 Angstrom), were used to constrain the rotation period and the stellar magnetic field geometry. The above results have been used to model the stellar radio emission, modelling that allowed us to constrain the physical condition of rho Oph A's magnetosphere. Past XMM measurements showed periodic X-ray pulses from rho Oph A. We correlate the X-ray light curve with the magnetic field geometry of rho Oph A. The already published XMM data have been re-analyzed showing that the X-ray spectra of rho Oph A are compatible with the presence of a non-thermal X-ray component. We discuss a scenario where the emission phenomena occurring at the extremes of the electromagnetic spectrum, radio and X-ray, are directly induced by the same plasma process. We interpret the observed X-ray and radio features of rho Oph A as having an auroral origin., Comment: 20 pages, 13 figures; accepted to MNRAS