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Spectroscopically, tidal disruption events (TDEs) are characterized by broad (∼104 km s−1) emission lines and show a large diversity as well as different line profiles. After carefully and consistently performing a series of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical and UV TDEs. We study a number of emission lines prominent among TDEs including Hydrogen, Helium, and Bowen lines and we quantify their evolution with time in terms of line luminosities, velocity widths, and velocity offsets. We report a time lag between the peaks of the optical light curves and the peak luminosity of Hα spanning between ∼7 and 45 days. If interpreted as light echoes, these lags correspond to distances of ∼2 − 12 × 1016 cm, which are one to two orders of magnitudes larger than the estimated blackbody radii (RBB) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of the He I 5876 Å line, which are smaller than the ones of Hα for H TDEs and similar or larger for N III Bowen TDEs. We report that N III Bowen TDEs have lower Hα velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii (RBB) and temperatures (TBB). We find a linear relationship between Hα luminosity and the RBB (Lline ∝ RBB) and potentially an inverse power-law relation with TBB (Lline ∝ TBB−β), leading to weaker Hα emission for TBB ≥ 25 000 K. The He II/He I ratio becomes large at the same temperatures, possibly pointing to an ionization effect. The He II/Hα ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties can potentially be attributed to viewing angle effects., We thank Tom Holoien for providing us with the spectra of AT2018zr. P.C., G.L., D.B.M. and M.P. are supported by a research grant (19054) from VILLUM FONDEN. IA is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that program, from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement number 852097), from the Israel Science Foundation (grant number 2752/19), from the United States – Israel Binational Science Foundation (BSF), and from the Israeli Council for Higher Education Alon Fellowship. M.N. is supported by a Royal Astronomical Society Research Fellowship and by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 948381). T.-W.C. acknowledges the EU Funding under Marie Skłodowska-Curie grant H2020-MSCA-IF-2018-842471. L.G. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal program RYC2019-027683 and from the Spanish MICIU project PID2020-115253GA-I00. M.G. is supported by the EU Horizon 2020 research and innovation programme under grant agreement No 101004719. T.M.B. was funded by the CONICYT PFCHA/DOCTORADOBECAS CHILE/2017-72180113. This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of PESSTO/ePESSTO/ePESSTO+ (the extended Public ESO Spectroscopic Survey for Transient Objects Survey). PESSTO/ePESSTO/ePESSTO+ observations were obtained under ESO program IDs 188.D-3003, 191.D-0935, 199.D-0143, 1103.D-0328. This work is based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias.

The photospheric emission in the prompt phase is the natural prediction of the original fireball model for gamma-ray burst (GRB) due to the large optical depth ($\tau >1$) at the base of the outflow, which is supported by the quasi-thermal components detected in several Fermi GRBs. However, which radiation mechanism (photosphere or synchrotron) dominates in most GRB spectra is still under hot debate. The shape of the observed photosphere spectrum from a pure hot fireball or a pure Poynting-flux-dominated outflow has been investigated before. In this work, we further study the photosphere spectrum from a hybrid outflow containing both a thermal component and a magnetic component with moderate magnetization ($\sigma_{0}=L_{P}/L_{\text{Th}}\sim 1-10$), by invoking the probability photosphere model. The high-energy spectrum from such a hybrid outflow is a power law rather than an exponential cutoff, which is compatible with the observed Band function in large amounts of GRBs. Also, the distribution of the low-energy indices (corresponding to the peak-flux spectra) is found to be quite consistent with the statistical result for the peak-flux spectra of GRBs best-fitted by the Band function, with similar angular profiles of structured jet in our previous works. Finally, the observed distribution of the high-energy indices can be well understood after considering the different magnetic acceleration (due to magnetic reconnection and kink instability) and the angular profiles of dimensionless entropy with the narrower core., Comment: 14 pages, 10 figures, published in MNRAS

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder ($kT_{\rm bb}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{\rm disk}$ $\sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and the flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect., 13 pages, accepted by MNRAS on 2021/08/16

We use the general relativistic radiation magnetohydrodynamics code \verb=KORAL= to simulate the early stages of accretion disk formation resulting from the tidal disruption of a solar mass star around a super massive black hole (BH) of mass $10^6\,M_\odot$. We simulate the disruption of artificially more bound stars with orbital eccentricity $e\leq0.99$ (compared to the more realistic case of parabolic orbits with $e=1$) on close orbits with impact parameter $\beta\geq3$. We use a novel method of injecting the tidal stream into the domain. For two simulations, we choose $e=0.99$ and inject mass at a rate that is similar to realistic TDEs. We find that the disk only becomes mildly circularized with eccentricity $e\approx0.6$ within the $3.5$ days that we simulate. The rate of circularization is faster for pericenter radii that come closer to the BH. The emitted radiation is mildly super-Eddington with $L_{\rm{bol}}\approx3-5\,L_{\rm{Edd}}$ and the photosphere is highly asymmetric with the photosphere being significantly closer to the inner accretion disk for viewing angles near pericenter. We find that soft X-ray radiation with $T_{\rm{rad}} \approx 3-5\times 10^5$ K may be visible for chance viewing angles. Our simulations predict that TDEs should be radiatively inefficient with $\eta\approx0.009-0.014$. These are the first simulations which simultaneously capture the stream, disk formation, and emitted radiation., Comment: 21 pages, 20 figures

We present simultaneous magnetic field measurements for the limb solar flare of 1981 July 17 using of the Ca II K, Hδ, He I 4471.5 A and Hβ lines. For two moments during the flare, which differ in time by 16 min, we analyzed Stokes I ± V profiles of these lines from observations made on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko National University of Kiev. At the time step that was close to the peak phase of the flare, all the spectral lines under study showed very wide emissions, with a full width at half maximum (FWHM) of 3.5–4 A. An interesting feature was observed in the blue wings of these lines, namely, narrow emission peaks with a FWHM of only 0.25–0.35 A. For heights of 10–18 Mm above the level of the photosphere, we found that (a) very strong kG magnetic fields (up to about 3 kG) existed at both moments of the flare, (b) the locations with strongest fields, in general, do not coincide for different spectral lines, (c) the polarities of the magnetic field for different spectral lines are in most cases identical, but sometimes they do not coincide. The data obtained indicate a significant inhomogeneity of the magnetic field in the flaring corona and the probable presence of the conditions necessary for magnetic reconnection of field lines.

Motivated by the controversy over the surface metallicity of the Sun, we present a re-analysis of the solar photospheric oxygen (O) abundance. New atomic models of O and Ni are used to perform Non-Local Thermodynamic Equilibrium (NLTE) calculations with 1D hydrostatic (MARCS) and 3D hydrodynamical (Stagger and Bifrost) models. The Bifrost 3D MHD simulations are used to quantify the influence of the chromosphere. We compare the 3D NLTE line profiles with new high-resolution, R = 700 000, spatially-resolved spectra of the Sun obtained using the IAG FTS instrument. We find that the O I lines at 777 nm yield the abundance of log A(O) = 8.74 +/- 0.03 dex, which depends on the choice of the H-impact collisional data and oscillator strengths. The forbidden [O I] line at 630 nm is less model-dependent, as it forms nearly in LTE and is only weakly sensitive to convection. However, the oscillator strength for this transition is more uncertain than for the 777 nm lines. Modelled in 3D NLTE with the Ni I blend, the 630 nm line yields an abundance of log A(O) = 8.77 +/- 0.05 dex. We compare our results with previous estimates in the literature and draw a conclusion on the most likely value of the solar photospheric O abundance, which we estimate at log A(O) = 8.75 +/- 0.03 dex., 22 pages, accepted for publication in MNRAS

The prompt emission of gamma-ray bursts (GRBs) is a long-standing problem. The spectral-lag phenomenon can provide us with some clues about the emission mechanism. By analogy with the spectral lag of blazars at radio frequencies, we propose using the photosphere in the jet model to interpret the spectral lag of GRBs. Assuming a canonical jet and respecting the scaling laws of both the magnetic field and the number density of radiative particles, the radius of the photosphere and the magnetic field can be predicted. Five long bursts with known redshifts are studied. The prompt emitting regions of these bursts are located at about one or several astronomical units from the jet base. Without strictly constrained parameters, the magnetic fields at the photosphere radius are not well constrained. Taking the median values, the magnetic field is distributed in the range of 104∼107 G, which has no conflicts with the constraints obtained from the synchrotron cooling time.

Alfven waves have proven to be important in a range of physical systems due to their ability to transport non-thermal energy over long distances in a magnetized plasma. This property is of specific interest in solar physics, where the extreme heating of the atmosphere of the Sun remains unexplained. In an inhomogeneous plasma such as a flux tube in the solar atmosphere, they manifest as incompressible torsional perturbations. However, despite evidence in the upper atmosphere, they have not been directly observed in the photosphere. Here, we report the detection of antiphase incompressible torsional oscillations observed in a magnetic pore in the photosphere by the Interferometric Bidimensional Spectropolarimeter. State-of-the-art numerical simulations suggest that a kink mode is a possible excitation mechanism of these waves. The excitation of torsional waves in photospheric magnetic structures can substantially contribute to the energy transport in the solar atmosphere and the acceleration of the solar wind, especially if such signatures will be ubiquitously detected in even smaller structures with the forthcoming next generation of solar telescopes. Spectropolarimetric observations of a solar pore at high temporal and spatial resolution identify the presence of magnetic field torsional oscillations. Simulations suggest that such oscillations are triggered by a photospheric kink mode, which can contribute substantially to upward energy transport within the solar atmosphere.

Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of time-scales – from ∼ ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e. unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper, we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site is σ ∼ 0.1, the initial magnetization is amplified by shocks formed in the flow to the point where it strongly suppresses baryon loading. We estimate that a significant contamination can be avoided if the magnetic energy at injection constitutes at least a few per cent of the jet energy. The variability time-scales of the jet after it breaks out of the star are then governed by the injection cycles rather than by the mixing process, suggesting that in practice jet injection should fluctuate on timescales as short as ∼10 ms in order to account for the observed light curves. Better stability is found for jets with shorter modulations. We conclude that for sufficiently hot jets, the Lorentz factor near the photosphere can be high enough to allow efficient photospheric emission. Our results imply that jets with 10−2 < σ < 1 injected by a variable engine with ∼10 ms duty cycle are plausible sources of long GRBs.

Model of a partial current-carrying torus loop anchored to the photosphere is analyzed. Conditions of the catastrophic loss of equilibrium are considered and corresponding value of the critical decay index of external magnetic field is found. Taking into account line-tying conditions leads to non-monotonous dependence of the critical decay index on the height of the apex and length of the flux rope (its endpoints separation). For relatively short flux ropes, the critical decay index is significantly lower than unity, which is in contrast to widespread models with the typical critical decay index above unity. The steep decrease of the critical index with height at low heights is due to the sharp increase of the curvature of the flux-rope axis that transforms from a nearly straight line to a crescent., Comment: 11 pages, 4 figures. Accepted for publication in MNRAS

Solar flares are a transient phenomenon occurred in the active region (AR) on the Sun’s surface, producing intense emissions in EUV and soft X-ray that can wreak havoc in the near-Earth space mission and satellite as well as radio-based communication and navigation. The ARs are accompanied with strong magnetic fields and manifested as dark spots on the photosphere. To understand the photospheric magnetic field properties of the ARs that produce intense flares, two ARs associated with X-class flares, namely AR 12192 and AR 12297, occurred respectively on 25 October 2014 and 11 March 2015, are studied in terms of magnetic classification and various physical magnetic parameters. Solar images from the Langkawi National Observatory (LNO) and physical magnetic parameters from the Space-weather HMI Active Region Patches (SHARP) are used in this study. A total of seven SHARP magnetic parameters are examined which are calculated as sums of various magnetic quantities and have been identified as useful predictors for flare forecast. These two ARs are classified as βγδ sunspots whereas their formation and size are quite different from each other. Our results showed that the intensity of a flare has little relationship with the area of an AR and the magnetic free energy; and the temporal variation of individual magnetic parameter has no obvious and consistent pre-flare feature. It is concluded that the temporal variation of individual magnetic parameter may not be useful for predicting the onset time of a flare.

The discovery of optical/UV tidal disruption events (TDEs) was surprising. The expectation was that, upon returning to the pericenter, the stellar-debris stream will form a compact disk that will emit soft X-rays. Indeed the first TDEs were discovered in this energy band. A common explanation for the optical/UV events is that surrounding optically-thick matter reprocesses the disk's X-ray emission and emits it from a large photosphere. If accretion follows the super-Eddington mass infall rate it would inevitably result in an energetic outflow, providing naturally the reprocessing matter. We describe here a new method to estimate, using the observed luminosity and temperature, the mass and energy of outflows from optical transients. When applying this method to a sample of supernovae our estimates are consistent with a more detailed hydrodynamic modeling. For the current sample of a few dozen optical TDEs the observed luminosity and temperature imply outflows that are significantly more massive than typical stellar masses, posing a problem to this common reprocessing picture., Comment: 9 pages, 8 figures, 1 table, accepted for publication in MNRAS

A study on the photosphere and disc of the Be star β Psc is presented. We recover almost 40 yr of high-resolution spectroscopic observations and additional data gathered from the BeSS data base. We evaluate the photospheric parameters from the spectral energy distribution (SED) and fittings of state-of-the-art non-LTE model atmospheres to observed helium, carbon, silicon, and magnesium line profiles. Our models include the stellar geometric deformation as well as the co-latitude dependence of temperature and gravity, aiming to derive the effects of rotation on the stellar parameters. We estimate the circumstellar disc parameters from the fitting of models assuming different disc properties, namely its radius and gas density profile. The disc inclination angle i is constrained from the fittings of He i 4471 Å, Mg ii 4481 Å, C ii 4267 Å, and Si ii 4128, 4132 Å lines with gravity darkened models. Our findings, based on model fittings, suggest that during the last 40 yr, the disc radius changed within the interval 5.5 ≤ Rd ≤ 7.8 $R/R_{*}\,$, the disc base gas density within 5 × 10−13 ≤ ρ ≤ 1 × 10−12 g cm−3, while the radial power-law density index m assumed values between 2.0 and 2.3. These results are in agreement with recent works dealing with spectroscopic and interferometric measurements of this object.

We have studied two regions located at the base of a coronal hole. For the K₁ intensity minima and K₂ peaks, which form between the upper photosphere and the lower chromosphere and in the lower chromosphere respectively, a number of Ca II line parameters have been computed. We have improved the determination technique for ∆λᴋ₁ᵥ and ∆λᴋ₁ᵣ, ∆λᴋ₂ᵥ and ∆λᴋ₂ᵣ line profile shifts, including certain cases when their direct determination was complicated. We have determined Iᴋ₁ᵥ, Iᴋ₁ᵣ, Iᴋ₂ᵥ, Iᴋ₂ᵣ intensities, K₁ minima and K₂ peaks separations SEPᴋ₁ = ∆λᴋ₁ᵣ – ∆λᴋ₁ᵥ, SEPᴋ₂ = ∆λᴋ₂ᵣ – ∆λᴋ₂ᵥ, respectively. We have constructed scatter plots and have computed correlation relationships between parameters relating to different levels of atmosphere. We have obtained the following results. The intensities observed in the lower and middle chromosphere are connected closer than intensities related to the upper photosphere and middle chromosphere. The structures with a stronger magnetic field are brighter at the upper photosphere and lower chromosphere levels as compared to the structures with a weaker magnetic field. K₁ minima separations are of greater value for the structures with a stronger magnetic field relative to the structures with a weaker magnetic field, whereas K₂ peaks separations demonstrate the opposite behavior. They are lower for the structures with a stronger magnetic field. It is true not only for the chosen structures belonging to quiet regions but also for the plage, though we need additional statistics for plages. The relation between shifts of K₁ minima and K₂ peak intensities for violet and red wings appeared to be weak. This may be due to the considerable contribution of random movements to the velocity field at the upper photosphere and lower chromosphere levels or due to different forming levels for the profile violet and red wings.

We use the Catalogue of Solar Activity (CSA) to study the latitudinal variations of tilt of solar active regions. The tilt angles β are computed taking into account changes of the heliographic grid with latitude φ. We show that when sunspot groups of different sizes and lifetimes are included, a classical representation of the Joy’s law as a linear function of latitude (β ∝ φ) is only the first approximation valid within a limited range of latitudes (−25° ≤ φ ≤ +25°). Outside this range, the functional dependence β = f(φ) becomes non-linear. Separating the data set on large long-living groups (LLG) and small short-living groups (SSG) reveals two quite different dependencies in β = f(φ): non-linearity in tilt is only present in LLGs and the steepness of linear section of β = f(φ) fit is higher for LLGs. This suggests a difference in the physical properties of two populations of solar groups, which could be hypothesized as an indication of different localization of subsurface zones of their formation in the framework of a distributed dynamo. However, since CSA contains the coordinates of sunspots averaged over the lifetime (or disc passage) of each group, one cannot rule out that the difference in tilts of SSG and LLG groups may be affected by the evolution of tilt angles during the lifetime/disc passage of the groups.

Two-dimensional distributions of the relative color index (RCI) of the continuous corona spectrum of 1991 and 2006 reveal that the energy distributions in the continuums of the corona and photosphere differ, as do the RCI values for different coronal structures. The approximate symmetry of RCI distributions relative to the HCS projection on the sky plane excludes the contribution of the dust component. The revealed reddening with distance over the entire corona at all position angles is explained by the propagation of electron fluxes in the entire corona volume. It is noted that scattering by bound ion electrons causes bluing at distances of

The situation with the reliability of calculations of the magnetic field of the solar corona remains a topic of discussion. There is, in particular, the problem of significantly lower calculated values of the magnetic field than those recorded near the Earth. The authors propose an explanation in which the magnetic fields are underestimated due to the existence of magnetic sources at different distances from the photosphere. Sources that are farther away from the photosphere create local zones of weaker photospheric fields, which are not accurately measured against strong fields from sources of another level. Model estimates of the contributions of sources showed that sources located closer to the photosphere give only 30% of the total contribution of sources of both levels. This value corresponds to the observed ratio of the calculated values of the magnetic field and recorded in the solar wind. This fact confirms the assumption that the magnetic fields of sources farther from the photosphere are not really taken into account in the calculations of the magnetic field of the solar corona.

In this paper, we search for the relationship between the Vlin linear projection velocity and the Vss velocity in the 3D space of coronal mass ejections (CMEs) recorded by LASCO coronagraphs with an “n” decay rate of the transverse component of the Bt magnetic field above the polarity inversion line (PIL) of the photospheric magnetic field in the active region (AR) in which the CME appeared. The Bt decay rate at an altitude h measured from the photosphere was calculated with the formula n(h) = –(h/Bt)(dBt/dh). Three-dimensional calculations of the magnetic field were performed in the potential approximation. The positive trend of the dependences 〈n〉(Vlin) and 〈n〉(Vss), however, does not have sufficient statistical significance: on average, the larger is 〈n〉, the higher are the determined Vlin and Vss CME velocities. Here, 〈n〉 includes the averaging of n over both the altitude and along the extended section of the PIL identified within each studied AR. The relationship between the quality of 〈n〉(Vlin) and 〈n〉(Vss) is shown to depend on the PIL section in the AR within which n is averaged. Based on the example of a fast CME on March 7, 2012, and a slow CME on October 22, 2013, it is shown that the difference in 〈n〉 for CMEs with different velocities is greater if the averaging of n along the PIL is performed within the CME initiation region, not along the entire extended PIL section in the AR.

The results of the measurements of the tilt of the magnetic-field axis of sunspots for active regions (ARs) with different morphological structures are presented. The observations are taken from the data archives of the RATAN-600 radio telescope and the Nobeyama Radioheliograph (NoRH) and processed with a new measurement method proposed by us earlier (Topchilo and Peterova, 2018). The method is based on the analysis of features of the microwave cyclotron source image above the sunspot and its dynamics with respect to the angle of view when the heliographic longitude of the sunspot changes due to the Sun’s rotation. We tested ten cases of observations and showed that the axis in most of them (~90%) is tilted toward the eastern limb and that the angle does not depend on the AR morphology. The degree of the tilt of the magnetic field axis of sunspots at the height of the lower part of the chromosphere–corona transition region (the boundary between them is at the height of ~2000 km) is ~1°–10° and increases with an increase in the height of the radiation source above the photosphere. Deviations from this rule are interpreted as the influence of neighboring ARs, as a result of which part of the magnetic flux of the main sunspot is closed, not on the tail of the AR as is the usual case, but on the neighboring, leading AR.

Identifying the heating mechanisms of the solar corona and the driving mechanisms of solar wind are key challenges in understanding solar physics. A full three-dimensional compressible magnetohydrodynamic (MHD) simulation was conducted to distinguish between the heating mechanisms in the fast solar wind above the open field region. Our simulation describes the evolution of the Alfvénic waves, which includes the compressible effects from the photosphere to the heliospheric distance s of 27 solar radii (R⊙). The hot corona and fast solar wind were reproduced simultaneously due to the dissipation of the Alfvén waves. The inclusion of the transition region and lower atmosphere enabled us to derive the solar mass-loss rate for the first time by performing a full three-dimensional compressible MHD simulation. The Alfvén turbulence was determined to be the dominant heating mechanism in the solar wind acceleration region (s > 1.3 R⊙), as suggested by previous solar wind models. In addition, shock formation and phase mixing are important below the lower transition region (s < 1.03 R⊙) as well.

We present the data and analysis of SN 2018gjx, an unusual low-luminosity transient with three distinct spectroscopic phases. Phase I shows a hot blue spectrum with signatures of ionised circumstellar material (CSM), Phase II has the appearance of broad SN features, consistent with those seen in a Type IIb supernova at maximum light, and Phase III is that of a supernova interacting with helium-rich CSM, similar to a Type Ibn supernova. This event provides an apparently rare opportunity to view the inner workings of an interacting supernova. The observed properties can be explained by the explosion of a star in an aspherical CSM. The initial light is emitted from an extended CSM (~ 4000 Rsun), which ionises the exterior unshocked material. Some days after, the SN photosphere envelops this region, leading to the appearance of a SN IIb. Over time, the photosphere recedes in velocity space, revealing interaction between the supernova ejecta and the CSM that partially obscures the supernova nebular phase. Modelling of the initial spectrum reveals a surface composition consistent with compact H-deficient Wolf-Rayet and LBV stars. Such configurations may not be unusual, with SNe IIb being known to have signs of interaction so at least some SNe IIb and SNe Ibn may be the same phenomena viewed from different angles or, possibly with differing CSM configurations., Accepted for publication in MNRAS

The physical conditions in the chromosphere of polar and equatorial coronal holes in the decay phase of the 24-th solar activity cycle are studied. Models of the chromosphere are constructed based on the intensities of the HeI 10830 A and Hα lines and using a non-LTE program. The calculations showed that the temperature in equatorial coronal holes is similar to that of polar coronal holes. The temperature in coronal holes is lower than that of the unperturbed chromosphere and increases depending on height in the chromosphere. The difference between the atmospheres of coronal holes and the quiet Sun begins at the level of the upper photosphere- lower chromosphere.

High resolution spectroscopic observation of V1719 Cyg were made at 1.8 meter telescope of Bohyunsan Optical Astronomy observatory in Korea. Spectral resolving power was R=45,000, signal to noise ratio S/N>100. The abundances of 28 chemical elements from carbon to dysprosium were found with the spectrum synthesis method. The abundances of oxygen, titanium, vanadium and elements with Z>30 are overabundant by 0.2–0.9 dex with respect to the solar values. Correlations of derived abundances with condensation temperatures and second ionization potentials of these elements are discussed. The possible influence of accretion from interstellar environment is not so strong as for ρ Pup and other stars with similar temperatures. The signs of accretion are absent. The comparison of chemical composition with solar system r- & s-process abundance patterns shows the enhancement of the photosphere by s-process elements.

Isolated brown dwarfs provide remarkable laboratories for understanding atmospheric physics in the low-irradiation regime, and can be observed more precisely than exoplanets. As such, they provide a glimpse into the future of high-SNR observations of exoplanets. In this work, we investigate several new considerations that are important for atmospheric retrievals of high-quality thermal emission spectra of sub-stellar objects. We pursue this using an adaptation of the HyDRA atmospheric retrieval code. We propose a parametric pressure-temperature (P-T) profile for brown dwarfs consisting of multiple atmospheric layers, parameterised by the temperature change across each layer. This model allows the steep temperature gradient of brown dwarf atmospheres to be accurately retrieved while avoiding commonly-encountered numerical artefacts. The P-T model is especially flexible in the photosphere, which can reach a few tens of bar for T-dwarfs. We demonstrate an approach to include model uncertainties in the retrieval, focusing on uncertainties introduced by finite spectral and vertical resolution in the atmospheric model used for retrieval (~8\% in the present case). We validate our retrieval framework by applying it to a simulated data set and then apply it to the HST/WFC3 spectrum of the T-dwarf 2MASS J2339+1352. We retrieve sub-solar abundances of H2O and CH4 in the object at ~0.1 dex precision. Additionally, we constrain the temperature structure to within ~100 K in the photosphere. Our results demonstrate the promise of high-SNR spectra to provide high-precision abundance estimates of sub-stellar objects., Accepted for publication in MNRAS, 21 pages, 15 figures

The wealth of detections of millisecond pulsars (MSPs) in $\gamma$-rays by {\em Fermi} has spurred searches for these objects among the several unidentified $\gamma$-ray sources. Interesting targets are a sub-class of binary MSPs, dubbed "Black Widows" (BWs) and "Redbacks" (RBs), which are in orbit with low-mass non-degenerate companions fully or partially ablated by irradiation from the MSP wind. These systems can be easily missed in radio pulsar surveys owing to the eclipse of the radio signal by the intra-binary plasma from the ablated companion star photosphere, making them better targets for multi-wavelength observations. We used optical and X-ray data from public databases to carry out a systematic investigation of all the unidentified $\gamma$-ray sources from the Fermi Large Area Telescope (LAT) Third Source Catalog (3FGL), which have been pre-selected as likely MSP candidates according to a machine-learning technique analysis. We tested our procedure by recovering known binary BW/RB identifications and searched for new ones, finding possible candidates. At the same time, we investigated previously proposed BW/RB identifications and we ruled out one of them based upon the updated $\gamma$-ray source coordinates., Comment: 22 pages, 14 figures, 21 pages - This manuscript is based on the MSc Thesis work of C. Braglia, defended on April 2020 at the University of Milan, accepted for publication in MNRAS

Type II supernovae (SNe) interacting with disklike circumstellar matter (CSM) have been suggested as an explanation of some unusual Type II SNe, e.g., the so-called "impossible" SN, iPTF14hls. There are some radiation hydrodynamics simulations for such SNe interacting with a CSM disk. However, such disk interaction models so far have not included the effect of the ionization and recombination processes in the SN ejecta, i.e., the fact that the photosphere of Type IIP SNe between $\sim 10$-$\sim 100$ days is regulated by the hydrogen recombination front. We calculate light curves for Type IIP SNe interacting with a CSM disk viewed from the polar direction, and examine the effects of the disk density and opening angle on their bolometric light curves. This work embeds the shock interaction model of Moriya, et al. (2013) within the Type IIP SN model of Kasen & Woosley (2009), for taking into account the effects of the ionization and recombination in the SN ejecta. We demonstrate that such interacting SNe show three phases with different photometric and spectroscopic properties, following the change in the energy source: First few tens days after explosion (Phase 1), $\sim 10 - \sim 100$ days (Phase 2) and days after that (Phase 3). From the calculations, we conclude that such hidden CSM disk cannot account for overluminous Type IIP SNe. We find that the luminosity ratio between Phase 1 and Phase 2 has information on the opening angle of the CSM disk. We thus encourage early photometric and spectroscopic observations of interacting SNe for investigating their CSM geometry., 12 pages, 9 figures, accepted for publication in MNRAS

Solar abundances have been historically assumed to be representative of cosmic abundances. However, our knowledge of the solar abundance of helium, the second most abundant element, relies mainly on models1 and indirect measurements through helioseismic observations2, because actual measurements of helium in the solar atmosphere are very scarce. Helium cannot be directly measured in the photosphere because of its high first ionization potential, and measurements of its abundance in the inner corona have been sporadic3,4. In this Letter, we present simultaneous global images of the helium (out to a heliocentric distance of 3R⊙ (solar radii)) and hydrogen emission in the solar corona during the minimum of solar activity of cycle 23 and directly derive the helium abundance in the streamer region and surrounding corona (out to 2.2R⊙). The morphology of the He+ corona is markedly different from that of the H corona, owing to significant spatial variations in helium abundance. The observations show that the helium abundance is shaped according to and modulated by the structure of the large-scale coronal magnetic field and that helium is almost completely depleted in the equatorial regions during the quiet Sun. This measurement provides a trace back to the coronal source of the anomalously slow solar wind observed in the heliosphere at the Sun–Earth Lagrangian point L1 in 2009, during the exceptionally long-lasting minimum of solar activity cycle 23. Global images of helium and hydrogen emission are used to directly derive the helium abundance out to 2.2R⊙. The helium abundance is shaped by the large-scale coronal magnetic field. Helium is almost completely depleted near the equator in the quiet Sun.

The spectroscopic features of white dwarfs are formed in the thin upper layer of their stellar photosphere. These features carry information about the white dwarf's surface temperature, surface gravity, and chemical composition (hereafter 'labels'). Existing methods to determine these labels rely on complex ab-initio theoretical models which are not always publicly available. Here we present two techniques to determine atmospheric labels from white dwarf spectra: a generative fitting pipeline that interpolates theoretical spectra with artificial neural networks, and a random forest regression model using parameters derived from absorption line features. We test and compare our methods using a large catalog of white dwarfs from the Sloan Digital Sky Survey (SDSS), achieving the same accuracy and negligible bias compared to previous studies. We package our techniques into an open-source Python module 'wdtools' that provides a computationally inexpensive way to determine stellar labels from white dwarf spectra observed from any facility. We will actively develop and update our tool as more theoretical models become publicly available. We discuss applications of our tool in its present form to identify interesting outlier white dwarf systems including those with magnetic fields, helium-rich atmospheres, and double-degenerate binaries., 11 pages, 7 figures. Accepted for publication in MNRAS; updated references

Determinations of the solar abundance of praseodymium (Pr) depend critically on the local thermodynamical equilibrium (LTE) and non-local thermodynamical equilibrium (NLTE) techniques beyond the capabilities of a classical one-dimensional model atmosphere. Here, in this analysis, we adopt an atomic model atom of Pr consisting of 105 energy levels and 14 bound–bound transitions of singly ionized praseodymium (Pr ii) and the ground state of the Pr iii continuum limit. We briefly analyse the solar abundance of Pr taking the solar model atmospheres of Holweger & Müller (1974, Solar Physics, 39, 19) with the measured equivalent linewidths and invoking a microturbulent velocity treatment. We succeed in accurately selecting nearby clear sections of the spectrum for 14 spectral lines of Pr ii with the improved atomic data of high-quality oscillator strengths available from the laboratory measurements of several possible sources as well as accurate damping constants successfully determined from the literature. We find a Pr abundance revised to be downwards log ϵPr(NLTE) = 0.75 ± 0.09, which is in good agreement with the meteoritic value (log ϵPr = 0.76 ± 0.03). A comparison of the NLTE abundance corrections with the standard LTE analysis, log ϵPr(LTE) = 0.74 ± 0.08, reveals a positive correction of +0.01 dex, estimated from the selected solar Pr ii lines. The Pr abundance value is clearly superior following the classical one-dimensional model atmospheres of Holweger & Müller, the absolute scales of gf-values, the microturbulent velocity and the adopted equivalent linewidths.

Solar flares are known to generate seismic waves in the Sun. We present a detailed analysis of seismic emission in sunspots accompanying M- and X-class solar flares. For this purpose, we have used high-resolution Dopplergrams and line-of-sight magnetograms at a cadence of 45 s, along with vector magnetograms at a cadence of 135 s obtained from Helioseismic and Magnetic Imager (HMI) instrument aboard the Solar Dynamic Observatory (SDO) space mission. In order to identify the location of flare ribbons and hard X-ray foot-points, we have also used H-alpha chromospheric intensity observations obtained from Global Oscillation Network Group (GONG) instruments and hard X-ray images in 12-25 KeV band from the Reuvan Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. The Fast Fourier Transform (FFT) technique is applied to construct the acoustic velocity power map in 2.5-4 mHz band for pre-flare, spanning flare, and post flare epochs for the identification of seismic emission locations in the sunspots. In the power maps, we have selected only those locations which are away from the flare ribbons and hard X-rays foot-points. These regions are believed to be free from any flare related artefacts in the observational data. We have identified concentrated locations of acoustic power enhancements in sunspots accompanying major flares. Our investigation provides evidence that abrupt changes in the magnetic fields and associated impulsive changes in the Lorentz force could be the driving source for these seismic emissions in the sunspots during solar flares., Comment: 27 pages, 42 figures, 2 tables, Accepted for publication in MNRAS

The observations of the solar facula in the Ba II λ 455.403 A line are used to construct a 3D model of the facula area by solving the inverse nonequilibrium radiative transfer problem and to investigate the fine structure of the field of unresolved velocities (microturbulence). New turbulent structures are formed in the layers of the upper photosphere. They are localized mainly between upward and downward flows with the formation of ring-shaped structures of increased turbulence around these flows. The mechanism of magnetic anisotropy of microturbulent velocity is proposed (small-scale eddy-type plasma motions mainly occur in the planes perpendicular to the magnetic field), which explains the height dependence of the field of unresolved velocities. Anisotropy of microturbulence begins to manifest itself in the lower photospheric layers outside the upward and downward flows, while it manifests itself in the higher layers inside these flows. The increase of microturbulence in the layers of the upper photosphere and the lower chromosphere in the areas between matter flows indicates the presence of inclined magnetic fields, which, along with the blurring of its spatial structure, indicates the existence of a magnetic canopy region. Microturbulence can be used as an additional tool for diagnostics of inclined magnetic fields.

The propagation of linear Alfven waves with periods of 10–200 s from the photosphere to the solar chromosphere under the conditions of an isothermal atmosphere taking into account the amplitude-phase relationships are considered. It was found that the amplitude of the wave velocity perturbation increases and the magnetic field perturbation decreases with increasing height, while the phase difference between the perturbations tends to π/2. The so-called turning points cannot adequately characterize the energy flux of Alfven waves. This conclusion indicates that results based on the analysis of oscillation theorems must be revised. It is shown that the efficiency of wave-energy transfer increases with increasing frequency of Alfven modes (when dissipative processes are disregarded).

A specificity of the boundary between the photosphere and the chromosphere of the Sun consists in considerable acceleration of plasma spicules above this boundary and appearance of wave motions (predominantly Alfven waves). Analytical solution to the equations of magnetic hydrodynamics is constructed in the vicinity of this boundary from the chromosphere side. Expressions describing the growth of velocity of plasma spicules and the Alfven waves in this vicinity are derived. It is demonstrated that the vertical motion of the Alfven waves and spicules is accompanied by horizontal motion of plasma in the presence of a vertical component of magnetic induction. The obtained expressions characterize the diversity of directions and complexity of motions in the lower chromosphere. They demonstrate that the horizontal component of magnetic induction considerably increases near the lower boundary of the chromosphere. The vertical velocity of spicules among which fast spicules II are dominant and the velocity of the Alfven waves substantially increase in a very thin layer of the lower chromosphere, by a factor of 1.8 and 1.5, respectively. Estimates of the horizontal motion of spicules and the vertical component of magnetic induction are obtained.

Data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) on the components of the magnetic field vector in the photosphere for 39 active regions (AR) of the 24th solar activity cycle are used to calculate the parameters of the electric current. The time variations in the electric current parameters in the AR over the time the region is within ±35° of the central meridian are studied. An attempt is made to relate the parameters of the current in the photosphere and their dynamics to the level of flare activity of the region. These studies have yielded the following results: (1) The change in the total unsigned current in an AR is synchronous or quasi-synchronous with changes in the magnetic flux; (2) A relation between the total unsigned current and the flare index (FI) exists, but is weak (correlation k = 0.48); (3) The imbalance of the vertical electric currents for all 39 of the AR studied here was low and did not exceed a few percent, which indicates closure of the current structures on all scales within an AR; (4) The highest correlation (k = 0.72) with the flare index is observed for a time averaged absolute value of the density of the vertical electric current; (5) Statistical studies yield a critical level of equal to 2.7 mA/m2: when this level is exceeded in an AR, flares with higher x-ray classes (M and X) are observed and when the current density falls below the threshold value, there is a reduction in the flare productivity of the region; (6) For the example of two regions with an emerging magnetic flux it is shown that pumping magnetic energy into the corona requires some time; the time interval between the jump in and the onset of the development of the first powerful flares in x-ray classes M and X is at least 12-20 hours; (7) The character of the relation between the time variations in the average value of the density of the horizontal current and the imbalance in the vertical current ρjz with the level of flare activity of an AR is more complicated and requires separate, more detailed studies.

I explore the origin of an asymmetry of the H$\alpha$ emission from a circumstellar (CS) shell around type IIP supernova SN 2013fs in the spectrum taken 10.3\,h after the shock breakout. A spherical model of the H$\alpha$ emission from the CS shell that takes into account a preshock gas acceleration by the supernova radiation permits us to successfully reproduce the \ha H$\alpha$ profile. Principal factors responsible for the H$\alpha$ asymmetry are the high velocity of the accelerated CS preshock gas ($\sim 3000$\,km s$^{-1}$) and a low H$\alpha$ Sobolev optical depth in a combination with an occultation of the H$\alpha$ emission by the photosphere., Comment: accepted by Astronomy Letters

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1–13.1 MPa) and over a range of oxygen fugacity (log fO2 − (log fO2)IW = −2.5 to −1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5–25 wt.%) of chondritic planetesimals. Most experiments (IW −2.5 to −1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun’s photosphere (e.g. CI, H and LL compositions) produced low-degree melts (

We investigate the rising flux tube and the formation of sunspots in an unprecedentedly deep computational domain that covers the whole convection zone with a radiative magnetohydrodynamics simulation. Previous calculations had shallow computational boxes (< 30 Mm) and convection zones at a depth of 200 Mm. By using our new numerical code R2D2, we succeed in covering the whole convection zone and reproduce the formation of the sunspot from a simple horizontal flux tube because of the turbulent thermal convection. The main findings are (1) The rising speed of the flux tube is larger than the upward convection velocity because of the low density caused by the magnetic pressure and the suppression of the mixing. (2) The rising speed of the flux tube exceeds 250 m/s at a depth of 18 Mm, while we do not see any clear evidence of the divergent flow 3 hr before the emergence at the solar surface. (3) Initially, the root of the flux tube is filled with the downflows and then the upflow fills the center of the flux tube during the formation of the sunspot. (4) The essential mechanisms for the formation of the sunspot are the coherent inflow and the turbulent transport. (5) The low-temperature region is extended to a depth of at least 40 Mm in the matured sunspot, with the high-temperature region in the center of the flux tube. Some of the findings indicate the importance of the deep computational domain for the flux emergence simulations., 17 pages, 24 figures. Accepted for publication by MNRAS

Observations of scattering polarization and the Hanle effect in various spectral lines are increasingly used to complement traditional solar magnetic field determination techniques. One of the strongest scattering polarization signals in the photosphere is measured in the Sr I line at 4607.3 Å when observed close to the solar limb. Here, we present the first observational evidence of Hanle rotation in the linearly polarized spectrum of this at several limb distances. We observed with the Zurich IMaging POLarimeter, ZIMPOL, at the IRSOL observatory, with exceptionally good seeing conditions, allowing for long integration times. We combined the fast modulating polarimeter with a slow modulator installed in front of the telescope. This combination allows the measurement of spectropolarimetric data being highly precise and unprecedentedly accurate. Fixing the reference direction for positive Stokes $Q$ parallel to the limb, we detect singly-peaked $U/I$ signals well above the noise level. We can exclude instrumental origin for such $U/I$ signals. These signatures are exclusively found in the Sr I line, but not in the adjoining Fe I line, therefore eliminating the Zeeman effect as the mechanism responsible for their appearance. However, we find a clear spatial correlation between the circular polarization produced by the Zeeman effect and the $U/I$ amplitudes. This suggests that the detected $U/I$ signals are the signatures of Hanle rotation caused by a spatially resolved magnetic field. A novel measurement technique allows for determining the absolute level of polarization with unprecedented precision. Using this technique, high-precision spectropolarimetric observations reveal for the first time unambiguous $U/I$ signals due to Hanle rotation in the Sr I line., 7 pages, 4 figures, accepted for publication in A&A

Full list of authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang, Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.; Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša, Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez, Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.; Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu, Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer, James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen, Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz, Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili, Teimuraz; Zuccarello, Francesca.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a future UK-led international network of ground-based solar telescope stations. SAMNet, at its full capacity, will continuously monitor the Sun’s intensity, magnetic, and Doppler velocity fields at multiple heights in the solar atmosphere (from photosphere to upper chromosphere). Each SAMM sentinel will be equipped with a cluster of identical telescopes each with a different magneto-optical filter (MOFs) to take observations in K I, Na D, and Ca I spectral bands. A subset of SAMM stations will have white-light coronagraphs and emission line coronal spectropolarimeters. The objectives of SAMNet are to provide observational data for space weather research and forecast. The goal is to achieve an operationally sufficient lead time of e.g., flare warning of 2–8 h and provide many sought-after continuous synoptic maps (e.g., LoS magnetic and velocity fields, intensity) of the lower solar atmosphere with a spatial resolution limited only by seeing or diffraction limit, and with a cadence of 10 min. The individual SAMM sentinels will be connected to their master HQ hub where data received from all the slave stations will be automatically processed and flare warning issued up to 26 h in advance. © R. Erdélyi et al., Published by EDP Sciences 2022., RE is grateful to the Science and Technology Facilities Council (STFC, grant numbers ST/M000826/1 and ST/V005979/1) the UK and the Royal Society for enabling this research. RE also acknowledges the CAS Presidents International Fellowship Initiative, Grant No. 2019VMA052 and the TOP CLLD 7.1.1-16-H-ERFA-2019-00207-Magyar Napfizikai Alaptvány ID Nr 1-8-2019/P support.MBK acknowledges STFC grant ST/S000518/1 to Aberystwyth University. MBK also acknowledges the open research program of CAS Key Laboratory of Solar Activity, National Astronomical Observatories, No. KLSA201610. RE and MBK both acknowledge the CAS Key Laboratory of Solar Activity, National Astronomical Observatories Commission for Collaborating Research Program for the support received to carry out part of this work. IGH and DP acknowledge support from EPSRC (grant EP/R002061/1). RB acknowledges the support by the Croatian Science Foundation under project 7549 “Millimeter and submillimeter observations of the solar chromosphere with ALMA”. RB, AH, and AMV acknowledge the support from the Austrian-Croatian Bilateral Scientific Project “Comparison of ALMA observations with MHD-simulations of coronal waves interacting with coronal holes”. AK and PG acknowledge the support from grant VEGA 2/0048/20. AAP acknowledges NASA 80NSSC18K1242 grant and grant No. 19-02-00088 by RFBR. Sunspot Solar Observatory is a partnership between NSF (1945705), the National Solar Observatory, the state of New Mexico, and consortium partners. TZ acknowledges funding from the Austrian Science Fund (FWF, project P30695-N27). FZ acknowledges support by the Università degli Studi di Catania (Piano per la Ricerca Università di Catania 2016–2018 – Linea di intervento 1 “Chance”; Linea di intervento 2 “Dotazione ordinaria”; Fondi di Ateneo 2020–2022, Università di Catania, Linea Open Access), by the Italian MIUR-PRIN grant 2017APKP7T on “Circumterrestrial Environment: Impact of Sun-Earth Interaction”. YH acknowledges funding from NSFC (11790300,11790301). DK has received funding from the Sêr Cymru II scheme, part-funded by the European Regional Development Fund through the Welsh Government. © R. Erdélyi et al., Published by EDP Sciences 2022., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

Photoexcited forbidden lines at visible and infrared wavelengths provide important diagnostics for the coronal magnetic field via scattering induced polarization and the Zeeman effect. In forward models, the polarized formation of these lines is often treated assuming a simplified exciting radiation field consisting only of the photospheric quiet-Sun continuum, which is both cylindrically-symmetric relative to the solar vertical and unpolarized. In particular, this assumption breaks down near active regions, especially due to the presence of sunspots and other surface features that modify the strength and anisotropy of the continuum radiation field. Here we investigate the role of symmetry-breaking on the emergent polarized emission in high resolution models of the active corona simulated with the MURaM code. We treat the full 3D unpolarized continuum radiation field of the photosphere that excites the coronal ions and compare the cases where the symmetry-breaking effects of the photospheric features are included or ignored. Our discussion focuses on the key observables soon to be available by the National Science Foundation’s Daniel K Inouye Solar Telescope. The results indicate that while symmetry breaking can in principle have a large effect, its role is relatively minor for the simulated active region, largely due to the low inherent polarization fraction emitted by forbidden lines in denser active region plasmas.

Long-term periodicities in the solar irradiance are often observed with periods proportional to the solar rotational period of 27 days. These periods are linked either to some internal mechanism in the Sun or said to be higher harmonics of the rotation without further discussion of their origin. In this article, the origin of the peaks in periodicities seen in the solar extreme ultraviolet (EUV) and ultraviolet (UV) irradiance around the 7, 9, and 14 days periods is discussed. Maps of the active regions and coronal holes are produced from six images per day using the Spatial Possibilistic Clustering Algorithm (SPoCA), a segmentation algorithm. Spectral irradiance at coronal, transition-region/chromospheric, and photospheric levels are extracted for each feature as well as for the full disk by applying the maps to full-disk images (at 19.3, 30.4, and 170 nm sampling in the corona/hot flare plasma, the chromosphere/transition region, and the photosphere, respectively) from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) from January 2011 to December 2018. The peaks in periodicities at 7, 9, and 14 days as well as the solar rotation around 27 days can be seen in almost all of the solar irradiance time series. The segmentation also provided time series of the active regions and coronal holes visible area (i.e. in the area observed in the AIA images, not corrected for the line-of-sight effect with respect to the solar surface), which also show similar peaks in periodicities, indicating that the periodicities are due to the change in area of the features on the solar disk rather than to their absolute irradiance. A simple model was created to reproduce the power spectral density of the area covered by active regions also showing the same peaks in periodicities. Segmentation of solar images allows us to determine that the peaks in periodicities seen in solar EUV/UV irradiance from a few days to a month are due to the change in area of the solar features, in particular, active regions, as they are the main contributors to the total full-disk irradiance variability. The higher harmonics of the solar rotation are caused by the clipping of the area signal as the regions rotate behind the solar limb.

White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from astronomical unit (au)-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and accrete onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.

Sunspots host a large variety of oscillatory phenomena, whose properties depend on the nature of the wave modes and the magnetic and thermodynamic structure of the spot. Umbral chromospheric oscillations exhibit significant differences compared to their photospheric counterparts. They show an enhanced power and a shorter dominant period, from waves with an amplitude of a few hundred meters per second in the five-minute band at the photosphere, to amplitudes of several kilometers per second in the three-minute band at the chromosphere. Various models have been proposed to explain this behaviour, including the presence of a chromospheric resonance cavity between the photosphere and the transition region. Jess et al. (2020, Nature Astronomy, 4, 220) claimed the detection of observational evidence supporting this model, obtained from the comparison of spectropolarimetric observations and numerical simulations. Here, it is shown that the observational insight reported by Jess et al. is not a common property of sunspots. More importantly, numerical modelling also shows that it is not an unequivocal signature of an acoustic resonator., Comment: Matters Arising published in Nature Astronomy. The publisher's final edited version of this article is available at https://rdcu.be/b5JCS

One of the main science motivations for the ESA PLAnetary Transit and Oscillations (PLATO) mission is to measure exoplanet transit radii with 3% precision. In addition to flares and starspots, stellar oscillations and granulation will enforce fundamental noise floors for transiting exoplanet radius measurements. We simulate light curves of Earth-sized exoplanets transiting continuum intensity images of the Sun taken by the HMI instrument aboard SDO to investigate the uncertainties introduced on the exoplanet radius measurements by stellar granulation and oscillations. After modeling the solar variability with a Gaussian process, we find that the amplitude of solar oscillations and granulation is of order 100 ppm -- similar to the depth of an Earth transit -- and introduces a fractional uncertainty on the depth of transit of 0.73% assuming four transits are observed over the mission duration. However, when we translate the depth measurement into a radius measurement of the planet, we find a much larger radius uncertainty of 3.6%. This is due to a degeneracy between the transit radius ratio, the limb-darkening, and the impact parameter caused by the inability to constrain the transit impact parameter in the presence of stellar variability. We find that surface brightness inhomogeneity due to photospheric granulation contributes a lower limit of only 2 ppm to the photometry in-transit. The radius uncertainty due to granulation and oscillations, combined with the degeneracy with the transit impact parameter, accounts for a significant fraction of the error budget of the PLATO mission, before detector or observational noise is introduced to the light curve. If it is possible to constrain the impact parameter or to obtain follow-up observations at longer wavelengths where limb-darkening is less significant, this may enable higher precision radius measurements., 11 pages, 8 figures, accepted in MNRAS

Little is known of the activity and differential rotation of low luminosity, early M dwarfs from direct observation. We present the first stellar activity analysis of star-spots and faculae for the hot Jupiter hosting M1V dwarf Kepler-45 from $\it Kepler$ transit light curves. We find star-spot and facula temperatures contrasting a few hundred degrees with the quiet photosphere, hence similar to other early M dwarfs having a convective envelope surrounding a radiative core. Star-spots are prominent close to the centre of the stellar disc, with faculae prominent towards the limbs, similar to what is observed for the Sun. Star-spot and facula mean sizes are about 40 and 45 × 103 km, respectively, and thus faculae occupy a 10 per cent larger surface area than the star-spots. A short-term activity cycle of about 295 d is observed that is reminiscent of those seen for other cool dwarfs. Adopting a solar-type differential rotation profile (faster equatorial rotation than polar rotation), our star-spot and facula temporal mapping indicates a rotation period of 15.520 ± 0.025 d at the transit latitude of −33.2°. From the mean stellar rotation of 15.762 d, we estimate a rotational shear of 0.031 ± 0.004 rad d−1, or a relative differential rotation of 7.8 ± 0.9 per cent. Kepler-45’s surface rotational shear is thus consistent with observations and theoretical modelling of other early M dwarfs that indicate a shear of less than 0.045 rad d−1 and no less than 0.03 rad d−1 for stars with similar stellar rotation periods.

The validity of the lambda-meter method for determining the quiet Sun velocity field using the Si I 1082.7 nm line is investigated. To this end, the intensity profiles of this line were calculated for the solar disk center by means of NLTE simulations in a three-dimensional model atmosphere describing the small-scale magnetic activity in the quiet solar photosphere. The velocity field recovered using the lambda-meter method from theoretical NLTE profiles of the Si I 1082.7 nm line was compared with the velocity field from the model atmosphere. The influence of atmospheric and instrumental effects on the results is considered. These effects are atmospheric turbulence and light diffraction by telescope aperture, such as VTT, GREGOR, and EST/DKIST. It is shown that, in the case of observations of the Si I 1082.7 nm line on large-diameter telescopes like GREGOR and EST/DKIST with a spatial resolution substantially better than 0.27″, the lambda-meter method provides reliable values of the velocity field for the lower and upper solar photosphere. For the middle photosphere, the correlation between the inferred and the real velocities is worse, particularly when using the smaller diameter telescopes like VTT. Under a poor spatial resolution exceeding 2″, information about the velocity field can be obtained only for the uppermost photospheric layers. For this case, the lambda-meter velocities turn out to be noticeably smaller than the real values.

The distributions of temperature and density in the sunspot atmosphere are crucial for an understanding of the process of energy transfer from the photosphere upward to the corona in strong magnetic fields. The joint analysis of millimeter observations at ALMA (Atacama Large Millimeter/Submillimeter Array) and observations performed in the microwave range (centimeter wavelengths) with the RATAN-600 radio telescope of the Russian Academy of Sciences provides new data on the temperature distribution and physical processes at different altitudes above a spot. The NOAA 12470 active region was observed and mapped at 1.3 and 3 mm (ALMA) and 2–10 cm (RATAN-600) in December 2015. These observations are analyzed, and the results are compared to models of sunspot atmospheres. The fundamental problems arising in the study of atmospheres of active regions in the millimeter and centimeter ranges are determined, and the importance of the 3–18 mm range in the physics of the generation and transfer of energy for corona heating is demonstrated.

A distinctive feature of chromospheric magnetic fields in the facular regions on the Sun is their very fine spatial structure. In accordance with the observations performed in Ca II K and H line (the level of the lower chromosphere, about 0.5–1 Mm over the photosphere), numerous “slender fibrils mapping the magnetic field” were recorded. It is noted that “…the loops are organized in canopy-like arches” (Jafarzadeh et al., 2017). In this paper we perform some modification of the facular model presented by Solov’ev and Kirichek (2019) as a steady magnetic “fountain” with many thin trickles of plasma flows. The radial-azimuthal temperature profiles of the facula are calculated. It is shown that the difference in the vertical and horizontal scales of the system does not improve the correspondence between the model parameters of the faculae and their observed characteristics.

The characteristics of electron acceleration to relativistic energies by sub-Dreicer quasi-stationary electric fields under the conditions of the lower atmosphere of the Sun are considered. The proposed model makes it possible to explain the spatial location of hard X-ray, optical, and gyrosynchrotron sources in the chromosphere/photosphere observed during limb solar flares. Possible mechanisms of electric field generation in the lower atmosphere are discussed.