scholarly journals The Possible Role of Radiative Acceleration in Supporting Extended Atmospheres in Be Stars

1976 ◽  
Vol 70 ◽  
pp. 377-382 ◽  
Author(s):  
R. L. Kurucz ◽  
R. E. Schild
Keyword(s):  
Effective Temperature ◽  
Detailed Calculation ◽  
Be Stars ◽  
Be Star ◽  
Hα Emission ◽  
Two Peaks ◽  
Radiative Acceleration ◽  
Emission Strength ◽  
Peaked Function

A detailed calculation of the radiative acceleration in B-type stars shows it to be a double-peaked function of effective temperature at small optical depths. The two peaks are shown to coincide approximately with peaks in the distribution of mean Hα emission strength as a function of B - V color in Be stars. These facts suggest that radiation may play an important role in the support of the Be star extended atmosphere.

scholarly journals Rapid Variation of Shell Line Parameters in ζ Tau

1987 ◽  
Vol 92 ◽  
pp. 99-100
Author(s):  
M. Bossi ◽  
G. Guerrero ◽  
M. Scardia
Keyword(s):  
Binary System ◽  
Time Scales ◽  
The Other ◽  
Compact Set ◽  
Rapid Variation ◽  
Inverse Dispersion ◽  
Be Star ◽  
Hα Emission ◽  
Emission Strength

ζTau is a Be star which probably showed already in 1973 rapid variations in Hα emission strength with time scales of a few minutes (Bahng, 1976). It represents, moreover, the primary of a well-known binary system with a period of 132.91 days (e.g., Hynek and Struve, 1942), and its shell displays long term instabilities with time scales of some years (Delplace and Chambon, 1976). The basis of the present work is a compact set of 82 grating photographic spectrograms obtained at Merate by means of the 137 cm reflector with an inverse dispersion of about 35 Å/mm between Jan 17 and Jan 24, 1983. Forty four of these spectra cover the range between ˜ 4000 and ˜ 5000 Å, the other ones being centered on Hα.

scholarly journals Making a Be star: the role of rotation and pulsations

2013 ◽  
Vol 9 (S301) ◽  
pp. 465-466
Author(s):  
Coralie Neiner ◽  
Stéphane Mathis
Keyword(s):  
Angular Momentum ◽  
Critical Value ◽  
Circumstellar Disk ◽  
Be Stars ◽  
Stochastic Excitation ◽  
Stellar Rotation ◽  
Rapid Rotation ◽  
Be Star

AbstractThe Be phenomenon, i.e. the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT satellite brought clear evidence that Be outbursts are directly correlated to pulsations and rapid rotation. In particular the stochastic excitation of gravito-inertial modes, such as those detected by CoRoT in the hot Be star HD 51452, is enhanced thanks to rapid rotation. These waves increase the transport of angular momentum and help to bring the already rapid stellar rotation to its critical value at the surface, allowing the star to eject material. Below we summarize the recent observational and theoretical findings and describe the new picture of the Be phenomenon which arose from these results.

scholarly journals Binary Be Stars and Be Binaries

1992 ◽  
Vol 151 ◽  
pp. 147-156
Author(s):  
Dietrich Baade
Keyword(s):  
Angular Momentum ◽  
Momentum Transfer ◽  
Mass Exchange ◽  
White Dwarf ◽  
Be Stars ◽  
Star Model ◽  
Rapid Rotation ◽  
Sky Survey ◽  
Be Star

Two hypotheses have been put forward for the rôle of binarity in Be stars: (1) All Be stars are interacting binaries. (2) Roughly one-half of the observed Be stars are post-mass exchange binaries with compact companions. Contrary to (1), (2) does not attempt to explain also the existence of disks in Be stars. After the spin-up by mass and angular momentum transfer, the B star somehow has to succeed to form and maintain the disk. Since rapid rotation is only necessary but not sufficient for this transformation, the effect of duplicity would merely be to give more stars the opportunity to become a Be star. Model (1) is not nearly realistic as is also underlined by a new spectroscopic survey for cool companions. The verification of (2) on the basis of the ROSAT All-Sky Survey has just begun; but a serious deficiency of white dwarf companions is already apparent. Binarity currently provides no extra clue on the origin of the Be phenomenon.

scholarly journals A Survey of Mass Loss from Be and Shell Stars Using Ultraviolet Data from Copernicus

1976 ◽  
Vol 70 ◽  
pp. 179-189 ◽  
Author(s):  
J. M. Marlborough ◽  
Theodore P. Snow
Keyword(s):  
Mass Loss ◽  
Direct Evidence ◽  
Rotational Velocity ◽  
Stellar Winds ◽  
Be Stars ◽  
Dwarf Stars ◽  
Be Star ◽  
Effects Of Rotation ◽  
First Time

Ultraviolet spectra of intermediate resolution have been obtained with Copernicus of twelve objects classified as Be or shell stars, and an additional 19 dwarfs of spectral classes B0-B4. Some of these spectra show marked asymmetries in certain resonance lines, especially the Si iv doublet at λ 1400 Å, indicating the presence of outflowing material with maximum velocities of nearly 1000 km s−1. Direct evidence for mass loss at these velocities is seen for the first time in dwarf stars as late as B1.5. Later than B0.5, the only survey objects showing this phenomenon are Be stars. Among the stars considered there is a correlation between the presence of mass-loss effects and projected rotational velocity, suggesting that the UV flux from B1-B3 dwarfs is sufficient to drive high-velocity stellar winds only if rotation reduces the effective gravity near the equator. The role of mass-loss in producing the Be star phenomenon and the effects of rotation on mass loss are discussed.

scholarly journals Rotation and Mass Ejection: the Launching of Be-Star Disks

2004 ◽  
Vol 215 ◽  
pp. 515-524 ◽  
Author(s):  
Stanley P. Owocki
Keyword(s):  
Binary Systems ◽  
Line Emission ◽  
Circumstellar Disk ◽  
Balmer Line ◽  
Be Stars ◽  
Wide Binary ◽  
Orbital Mass ◽  
Be Star ◽  
Mass Ejection

The characteristic signature of Be Stars is the Balmer line emission understood to arise in a circumstellar disk. Unlike the accretion disks of protostars or mass-exchange binary systems, the evolved and generally single or wide-binary status of Be Stars seems to require that its disk must form from mass ejection (a.k.a. decretion) from the star itself. In this paper, I use analogies with launching orbital satellites to discuss two candidate processes (radiation, pulsation) for driving such orbital mass ejection, with particular emphasis on the role of the rapid, possibly near-critical, rotation of Be Stars in facilitating the formation of their signature disks.

scholarly journals NGC 6611 601: A hot pre-main sequence spectroscopic binary containing a centrifugal magnetosphere host star

2021 ◽  
Author(s):  
M E Shultz ◽  
E Alecian ◽  
V Petit ◽  
S Bagnulo ◽  
T Böhm ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Magnetic Star ◽  
Be Stars ◽  
Strong Emission ◽  
Surface Magnetic Field ◽  
Spectroscopic Binary ◽  
Be Star ◽  
Hα Emission ◽  
Strong Surface

Abstract W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large dataset of high-resolution spectropolarimetry. The star is a previously unreported spectroscopic binary, consisting of 2 B2 stars with a mass ratio of 1.8, masses of 12 M⊙ and 6.2 M⊙, in an eccentric 110-day orbit. The magnetic field belongs to the secondary, W 601 B. The Hα emission is consistent with an origin in W 601 B’s centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disk. However, the low value of log g = 3.8 determined via spectroscopic analysis, and the star’s membership in the young NGC 6611 cluster, are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the Hα line and the longitudinal magnetic field 〈Bz〉 is 1.13 d. Modelling of Stokes V and 〈Bz〉 indicates a surface dipolar magnetic field Bd between 6 and 11 kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to Hα-bright magnetic B-type stars such as σ Ori E. By contrast, the primary is an apparently non-magnetic (Bd < 300 G) pre-main sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.

scholarly journals The Surface Gravities of Be Stars

1976 ◽  
Vol 70 ◽  
pp. 69-78 ◽  
Author(s):  
Geraldine J. Peters
Keyword(s):  
Main Sequence ◽  
Be Stars ◽  
Line Broadening ◽  
Hydrogen Line ◽  
Wide Range ◽  
Hα Emission ◽  
The Mean ◽  
Emission Strength

In an attempt to shed some light on the origin of the material in the envelopes of Be stars, surface gravities were determined for 30 objects by comparing their observed profiles of Hγ and Hδ with those computed from the Princeton model atmospheres and the VCS theory of hydrogen line broadening. The program stars are predominately well-known Be stars and display a wide range of envelope spectra and v sin i. The mean and range in log g for the Be stars appear to be identical to that obtained from a similar analysis on non-Be stars. No correlation was found between log g and Hα emission strength or the strength and/or presence of emission of Fe II, O I λ 7774 Å, or the infrared Ca ii triplet. The suggestion made by Schild (1973) and Schild et al. (1974) that the extreme Be stars are in the post main sequence phase of rapid core contraction is weakened by the fact that there are several members of the class which have log g ≥ 3.8. All shell stars considered in the program appear to have low values of log g (≤3.5). Some possible explanations for this occurrence are discussed.

scholarly journals Spectroscopic Observations of Be Stars in the Photographic and Visual Regions

1982 ◽  
Vol 98 ◽  
pp. 109-124 ◽  
Author(s):  
Arne Slettebak
Keyword(s):  
Main Sequence ◽  
Review Paper ◽  
Planetary Nebulae ◽  
Be Stars ◽  
Early Type ◽  
Spectroscopic Observations ◽  
Be Star ◽  
Hα Emission

The term “Be star” has been used at times to describe classes of objects which are physically rather different from one another. While it could include early-type supergiant stars with Hα emission, early-type pre-main sequence nebular variables, or quasi-planetary nebulae like MWC 349, I will limit this review paper to a discussion of the “classical” Be stars. These are defined as stars of luminosity classes III to V, usually rapid rotators, which show normal B-type spectra with superposed Balmer (and sometimes Fe II) emission. Included also, however, will be the Oe stars and the A-type shell stars, which seem to represent extensions of the classical Be phenomenon to higher and lower temperatures, respectively.

scholarly journals A Survey of Hα in the Brighter Northern Be Stars

1976 ◽  
Vol 70 ◽  
pp. 29-29
Author(s):  
William P. Bidelman ◽  
Anthony J. Weitenbeck
Keyword(s):  
Graduate Student ◽  
High Latitude ◽  
The Other ◽  
University Of California ◽  
Be Stars ◽  
No Emission ◽  
Emission Component ◽  
Be Star ◽  
Hα Emission ◽  
The University

Hα observations have been made of the stars brighter than mv = 7.5 and north of δ = −30° contained in the three classical Be-star catalogues of Merrill and Burwell. At least one observation of each star was made at each of the two epochs 1958/9 and 1970/2 at the Lick Observatory and at the Warner and Swasey Observatory respectively. The dispersions used were 88 Å mm−1 at Lick and 66 Å mm−1 and 18 Å mm−1 (with image tube) at Warner and Swasey.Results were as follows: The total number of objects observed was 215. Of these, 32 were supergiants and will be discussed later. Another 16 stars were deemed abnormal in some manner (helium and/or forbidden emission, composite spectrum, markedly violet-displaced Hα emission component: an interesting group of five stars including the nebular variable AB Aurigae). Thirteen of the stars showed no emission at either epoch. Of the 154 presumably normal low-luminosity stars showing emission, 70 displayed shell structure in Hα at one or both epochs, though in over half of these cases the shell absorption was considered ‘weak’. Thirty of the 70 stars varied in their shell characteristics between the two epochs. The total number of stars whose Hα line varied in some respect was 43, or 26 percent of the 167 normal low-luminosity stars observed. Quite a number of the stars observed have been reported to show shell characteristics by others but were not so noted by us.Eight additional Be stars not contained in the Merrill-Burwell catalogues have also been observed. Seven of these are shell stars; the other is the unusually high latitude object HD 127617.It is a pleasure to acknowledge that many of the Lick observations were made by Dr Jack E. Forbes, a graduate student at the time in the Department of Astronomy at the University of California, Berkeley.

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