Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: We present an update of the grid of detailed atmosphere models and homogeneous synthetic spectra for hot, high-gravity subdwarf stars. High-resolution spectra and synthetic photometry were calculated in the wavelength range 1,000 \r{A} - 10,000 \r{A} using Non-LTE extensively line-blanketed atmosphere structures.

Abstract: We present an analysis of a flare on the Wolf 359 star based on simultaneous observations of TESS and XMM-Newton. A stellar flare with energy comparable to an X-class solar flare is analyzed on this star for the first time. The main goal of the study was to determine whether the same physical processes drive and occur in stellar flares as in the solar flares. We tried to estimate the flare class by various direct and indirect methods. Light curves and spectra in different energy ranges were used to determine the parameters and profiles of the flare. From the XMM-Newton EPIC-pn X-ray data, we estimated the temperature and emission measure during the flare. The thermodynamical timescale and the loop semi-length were also determined with two different methods. The RGS spectra enabled us to calculate the differential emission measure (DEM) distributions. The obtained DEM distributions have three components at temperature values of 3 MK, 7 MK, and 16-17 MK. The analysis of the line ratio in helium-like triplets allowed us to determine the plasma electron density. Our results for the flare loop on Wolf 359 were compared to typical parameters for solar flares observed with GOES and RHESSI. This supports our conclusion that the processes taking place in stellar flares are like those in solar flares. The determined geometrical parameters of the phenomenon do not differ from the values of analogs occurring on the Sun.

Abstract: Model orbits have been fitted to 27 physical double stars listed in the 1922 catalogue of Miller & Pitman (MP). A Markov Chain Monte Carlo technique was applied to estimate best fitting values and associated uncertainties for the orbital parameters. Dynamical masses were calculated using parallaxes from the Hipparcos and Gaia missions. These are not in strong agreement with the masses given by MP. This is surprising given the high correlation between the parallaxes from these missions and those listed by MP; unfortunately calculations are not given by MP nor are orbital parameters. The results of the current study are in good agreement with a recent study, as are comparisons with the orbital parameters listed by the Washington Double Star catalog, confirming the validity of the MCMC modelling.

Abstract: A large fraction of red-supergiant stars seem to be enshrouded by circumstellar material (CSM) at the time of explosion. Relative to explosions in a vacuum, this CSM causes both a luminosity boost at early times as well as the presence of symmetric emission lines with a narrow core and electron-scattering wings typical of type IIn supernovae (SNe). For this study, we performed radiation-hydrodynamics and radiative transfer calculations for a variety of CSM configurations (i.e., compact, extended, and detached) and documented the resulting ejecta and radiation properties. We find that models with a dense, compact, and massive CSM of ~0.5Msun can match the early luminosity boost of type II-P SNe but fail to produce type IIn-like spectral signatures (aka ``flash features''). These only arise if the photon mean free path in the CSM is large enough (i.e, if the density is low enough) to allow for a radiative precursor through a long-lived (i.e., a day to a week), radially extended unshocked optically thick CSM. The greater radiative losses and kinetic-energy extraction in this case boost the luminosity even for modest CSM masses -- this boost is delayed for a detached CSM. The inadequate assumption of high CSM density, in which the shock travels quasi adiabatically, overestimates the CSM mass and associated mass-loss rate. Our simulations also indicate that type IIn-like spectral signatures last as long as there is optically-thick unshocked CSM. Constraining the CSM structure therefore requires a combination of light curves and spectra, rather than photometry alone. We emphasize that for a given total energy, the radiation excess fostered by the presence of CSM comes at the expense of kinetic energy, as evidenced by the disappearance of the fastest ejecta material and the accumulation of mass in a dense shell. Both effects can be constrained from spectra well after the interaction phase.

Abstract: The double detonation is a widely discussed mechanism to explain Type Ia supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this scenario, a helium detonation is ignited in a surface helium shell on a carbon/oxygen white dwarf, which leads to a secondary carbon detonation. Explosion simulations predict high abundances of unburnt helium in the ejecta, however, radiative transfer simulations have not been able to fully address whether helium spectral features would form. This is because helium can not be sufficiently excited to form spectral features by thermal processes, but can be excited by collisions with non-thermal electrons, which most studies have neglected. We carry out a full non-local thermodynamic equilibrium (non-LTE) radiative transfer simulation for an instance of a double detonation explosion model, and include a non-thermal treatment of fast electrons. We find a clear He I {\lambda} 10830 feature which is strongest in the first few days after explosion and becomes weaker with time. Initially this feature is blended with the Mg II {\lambda} 10927 feature but over time separates to form a secondary feature to the blue wing of the Mg II {\lambda} 10927 feature. We compare our simulation to observations of iPTF13ebh, which showed a similar feature to the blue wing of the Mg II {\lambda} 10927 feature, previously identified as C I. Our simulation shows a good match to the evolution of this feature and we identify it as high velocity He I {\lambda} 10830. This suggests that He I {\lambda} 10830 could be a signature of the double detonation scenario.