Topic
Shell star
About: Shell star is a research topic. Over the lifetime, 151 publications have been published within this topic receiving 1440 citations.
Papers published on a yearly basis
Papers
TL;DR: In this paper, the velocity law in most, if not all, disks of Be stars is roughly Keplerian and central quasi emission bumps (CQEs) in photospheric lines were found in 75% of all normal shell stars.
Abstract: Echelle observations are presented and discussed for 23 of the 27 known "normal" shell stars brighter than about 6.5 mag. In addition to those typical cases, three stars with known transitions between emission & shell and pure emission line appearance, and three rapidly rotating B stars without records of line emission (Bn stars) are added to the sample. Long-term V/R emission-line variability and central quasi emission bumps (CQEs) in photospheric lines were found in 75% of all normal shell stars. This strongly suggests that the velocity law in most, if not all, disks of Be stars is roughly Keplerian. Both phenomena may occur in the same star but not at the same time. This is in agreement with the previous conclusion that CQEs only form in the presence of negligible line-of-sight velocities while long-term V/R variations are due to non-circular gas particle orbits caused by global disk oscillations. V/R variations associated with binary orbits are much less pronounced. Similarly, phase lags between different lines were detected in long-term V/R variable stars only. A binary fraction of only one-third is too low to support binary hypotheses as an explanation of the Be phenomenon. CQEs detected in 3 out of 19 Bn stars reveal the presence of disk-like equatorial concentrations of matter in B stars without emission lines. Accordingly, there seem to be intermediate cases between disk-free B stars and Be stars. Previous claims of the existence of shell stars with low v sin i could not be confirmed. Shell stars are Be stars viewed equator-on, and their observed rotational velocities are indistinguishable from the equatorial ones which are the same as in Be stars. The mean fraction of the critical rotation velocity is 81 ± 12%. The standard deviation is comparable to, or even less than, the observational uncertainties. Since this would require star-to-star differences to be negligible, which is unrealistic, the correlation between the widths of strong spectral lines and the stellar rotation velocities may be truncated or severely distorted at its extreme end. A number of not previously known facts about individual stars is also reported.
131 citations
TL;DR: In this article, rotational velocities for 66 metallic-line and 123 normal AS-Ag IV or V stars are given and used to determine the frequency distributions of equatorial rotational velocity.
Abstract: Rotational velocities for 66 metallic-line and 123 normal AS-Ag IV or V stars are given and used to determine the frequency distributions of equatorial rotational velocities. Those distributions have only a 1.3 percent overlap, which is probably due to a small contamination of the two samples. If so, then the rotational velocity is a necessary and sufficient parameter to detennine whether, within certain spectral type and age ranges, a star will have a well- developed metallic-line or normal spectrum. Among the 35 most rapidly rotating normal stars, eight were found to have shell spectra. Most of the shell stars occur during or just after the overall stellar contraction when the stars have depleted most of their core hydrogen; one shell star has apparently recently contracted to the main sequence. These shell stars seem to be the A-type counterpants of the Be stars. (auth)
69 citations
TL;DR: In this paper, the authors make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion, and find the tidal timescale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years.
Abstract: Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice-versa and also the Type II X-ray outbursts. We find the tidal timescale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this timescale depending on the orbital parameters of the binary. The timescales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.
54 citations
Journal Article•
TL;DR: In this paper, the authors observe des variations of Ca II-K in β Pictoris, and discuss the origin and origin of these variations, as well as l'origine of ces variations.
Abstract: On a observe des variations sur des echelles de temps mensuelles, journalieres et horaires dans le profil de raie d'absorption circumstellaire Ca II-K dans β Pictoris. Discussion sur l'origine de ces variations
31 citations
TL;DR: In this article, the authors present an intensive monitoring of high-resolution spectra of the Ca−sc ii} K line in the A7IV shell star $\Phi$ Leo at very short (minutes, hours), short (night to night), and medium (weeks, months) timescales.
Abstract: We present an intensive monitoring of high-resolution spectra of the Ca {\sc ii} K line in the A7IV shell star $\Phi$ Leo at very short (minutes, hours), short (night to night), and medium (weeks, months) timescales. The spectra show remarkable variable absorptions on timescales of hours, days, and months. The characteristics of these sporadic events are very similar to most that are observed toward the debris disk host star $\beta$ Pic, which are commonly interpreted as signs of the evaporation of solid, comet-like bodies grazing or falling onto the star. Therefore, our results suggest the presence of solid bodies around $\Phi$ Leo. To our knowledge, with the exception of $\beta$ Pic, our monitoring has the best time resolution at the mentioned timescales for a star with events attributed to exocomets. Assuming the cometary scenario and considering the timescales of our monitoring, our results indicate that $\Phi$ Leo presents the richest environment with comet-like events known to date, second only to $\beta$ Pic.
28 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2021 | 2 |
| 2019 | 1 |
| 2018 | 2 |
| 2016 | 6 |
| 2015 | 5 |
| 2014 | 2 |