Search results for "STARS: ATMOSPHERE"
showing 5 items of 5 documents
H-He collision-induced satellite in the Lyman alpha profile of DBA white dwarf stars
2020
The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centered around 1160~\AA\/ in the blue wing of the Lyman-$\alpha$ line. It is extremely apparent in WD 1425+540 recently observed with HST COS. With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman-$\alpha$ seen in this and other white dwarf spectra. Our previous work predicting this absorption feature was limited by molecular…
VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), σ Oph, BM Sco, and HD 206859
2017
Aims. We add four warmer late-type supergiants to our previous spectro-interferometric studies of red giants and supergiants. Methods. We measure the near-continuum angular diameter, derive fundamental parameters, discuss the evolutionary stage, and study extended atmospheric atomic and molecular layers. Results. V766 Cen (=HR 5171 A) is found to be a high-luminosity (log L/L = 5.8 ± 0.4) source of effective temperature 4290 ± 760 K and radius 1490 ± 540 R, located in the Hertzsprung-Russell (HR) diagram close to both the Hayashi limit and Eddington limit; this source is consistent with a 40 M evolutionary track without rotation and current mass 27-36 M. V766 Cen exhibits Na i in emission a…
Theoretical profiles of the Mg + resonance lines perturbed by collisions with He
2016
International audience; The effects of collision broadening by He are central to understanding the opacity of cool stellar atmospheres. Aims. DZ white dwarfs show metal lines which are, in many cases, believed to come from some rocky material, a remnant of a former exoplanetary system. The analysis of the Mg + resonance lines is a valuable method to determine the chemical abundances in these systems. Methods. Unified profiles of the strongest of the UV lines of Mg + have been calculated in the semi-classical approach using very recent ab initio potential energies. Results. We present the first theoretical line profile calculations of the resonance lines of Mg + that have been perturbed by h…
X-ray optical depth diagnostics of T Tauri accretion shocks
2009
In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this is plasma heated in the accretion shock or whether it is coronal plasma. High-resolution X-ray spectroscopy allows us to measure individual line f…
Recent X-ray studies of stellar cycles and long-term variability
2006
AbstractWe discuss recent X-ray studies of stellar cycles and long-term variability.