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 Ultraviolet Observations, Stellar Winds, and Mass Loss for Be Stars

1982 ◽  
Vol 98 ◽  
pp. 361-376 ◽  
Author(s):  
J. M. Marlborough
Keyword(s):  
Mass Loss ◽  
Stellar Winds ◽  
Be Stars ◽  
Be Star ◽  
Ultraviolet Observations

Although the first ultraviolet (UV) observation of an astronomical source was obtained in 1946, the first UV observations of Be stars were not obtained until 1964. In this review of UV data covering the period since 1964, the term Be star will be assumed to include Oe stars as well (Frost and Conti, 1976). An earlier review of this subject is by Heap (1976).

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 Apparent Variation of v sin i and Rotational Modulation in Be Stars

2004 ◽  
Vol 215 ◽  
pp. 93-94
Author(s):  
C. Neiner ◽  
S. Jankov ◽  
M. Floquet ◽  
A. M. Hubert
Keyword(s):  
Fourier Transform ◽  
Magnetic Dipole ◽  
Be Stars ◽  
Line Profiles ◽  
Transform Method ◽  
Rotational Modulation ◽  
Be Star ◽  
Apparent Variation ◽  
The Fourier Transform ◽  
First Time

v sin i was determined by applying the Fourier transform method to the line profiles of two classical Be Stars. A variation is observed in the apparent v sin i which corresponds to the main frequencies associated to nrp modes. Rotational modulation is observed in wind sensitive UV lines of the Be star ω Ori and is associated with an oblique magnetic dipole which is discovered for the first time in a classical Be star.

scholarly journals Non-radial Pulsations of BA Supergiants and Be Stars

1999 ◽  
Vol 169 ◽  
pp. 312-319
Author(s):  
Dietrich Baade
Keyword(s):  
Mass Loss ◽  
Background Noise ◽  
The Other ◽  
Be Stars ◽  
Physical Processes ◽  
Other Hand ◽  
Be Star ◽  
The Subject ◽  
Radial Pulsations

If observing time and number of photons are not the limit, it will probably be very difficult to find any Be star or BA supergiant that is not variable. Moreover, there is hardly any major set of observations that is not tempting to explain at least partly in terms of nonradial (g-mode) pulsations. Since a few years ago, such conjectures are also theoretically permissible because improved opacity calculations have established the classical к-mechanism as a viable source of pulsation driving (cf. Pamyatnykh, these proceedings).Contrary to Be stars, it can for any given BA supergiant nevertheless be arbitrarily difficult to diagnose nonradial pulsations (NRP’s) with certainty because they need to be detected against considerable background ‘noise’ of other physical processes, most of which are related to mass loss and/or rotation. To make things worse, there is some evidence that NRP’s can have some effect on the dynamics of the mass loss. On the other hand, variable and non-spherical winds is the subject of this Colloquium, and this paper is accordingly biased towards the interplay between pulsation and mass loss.

scholarly journals An Unusually Stable and Short Spectroscopic Period of the Be Star 28 CMa

1982 ◽  
Vol 98 ◽  
pp. 167-170
Author(s):  
D. Baade
Keyword(s):  
Time Scale ◽  
Emission Lines ◽  
Be Stars ◽  
Stable Period ◽  
Be Star ◽  
First Time ◽  
Calar Alto

Spectroscopic (1970: ESO, 12 Å/mm, 6 spectra kindly put at my disposal by Prof. A. Van Hoof; 1976: ESO, 12 Å/mm; 1977: Calar Alto Observatory, 42 Å/mm; 1979: ESO, 12 Å/mm) and photometric (1976: ESO and Cerro Tololo, Hβ, uvby) observations of 28 CMa (B2-3 IV-Ve; 3.52 < mV < 4.18, irregular variations on the time scale of months or years reported; vrot = 80 km/s) revealed a very complex variability. All observed individual types of variations are known from at least a few other Be stars. In 28 CMa, however, for the first time a highly significant correlation between the various variations is established by a stable common period. The period is 1.365 days which seems to be the shortest stable period presently known of any Be star. There is no indication that the star's behaviour changed between 1970 and 1979. Only the equivalent widths of the emission lines increased noticeably.

scholarly journals Stellar Winds and Mass-Loss Rates from Be Stars

1982 ◽  
Vol 98 ◽  
pp. 377-385 ◽  
Author(s):  
Theodore P. Snow
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Stellar Winds ◽  
Be Stars ◽  
Line Profiles ◽  
Sample Group ◽  
Show Evidence ◽  
Acceleration Zone ◽  
Low Levels ◽  
Loss Rates

Resonance-line profiles of SiIII and SiIV lines in 22 B and Be stars have been analyzed in the derivation of mass-loss rates. Of the 19 known Be or shell stars in the sample group, all but one show evidence of winds. It is argued that for stars of spectral type B1.5 and later, SiIII and SiIV are the dominant stages of ionization, and this conclusion, together with theoretical fits to the line profiles, leads to mass-loss rates between 10-11 and 3 × 10-9 for the stars. The rate of mass loss does not correlate simply with stellar parameters, and probably is variable with time. The narrow FeIII shell lines often seen in the ultraviolet spectra of Be stars may arise at low levels in the wind, below the strong acceleration zone. The mass-loss rates from Be stars are apparently insufficient to affect stellar evolution.

scholarly journals Magnetic Fields in Be Stars? (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 38-48
Author(s):  
Paul K. Barker
Keyword(s):  
Magnetic Fields ◽  
Field Strength ◽  
Review Paper ◽  
Stellar Winds ◽  
Be Stars ◽  
Circumstellar Envelopes ◽  
Star Models ◽  
Upper Limits ◽  
Be Star

AbstractNo mean longitudinal or toroidal magnetic fields have yet been detected on any classical Be star. Models of stellar winds and circumstellar envelopes around magnetic Be stars are not appreciably constrained by present observed upper limits on field strength. A few magnetic Be stars do exist among the helium strong stars, but these objects show spectral phenomenology which is unmistakably distinct from that shown by every other object known as a Be star.

scholarly journals Mixing Due to Angular Momentum Transfer in Evolving Sunlike Stars

1991 ◽  
Vol 145 ◽  
pp. 71-83
Author(s):  
Pierre Demarque
Keyword(s):  
Angular Momentum ◽  
Open Clusters ◽  
Striking Difference ◽  
Red Giants ◽  
Dwarf Stars ◽  
Physical Mechanisms ◽  
History Of ◽  
Effects Of Rotation ◽  
Halo Population

The physical mechanisms that control the rotational history of stars like the Sun are reviewed, and the likely role of the associated rotational mixing is described. Results of stellar evolution calculations that include the effects of rotation are also described. The same theory, based on a single calibration for stars of different masses and metallicities, can successfully explain the observed Li and Be depletion in young open clusters and Li in field dwarf stars of the halo population, and the striking difference between metal-rich and metal- poor abundance patterns. There is evidence, both observational and theoretical, that angular momentum remains hidden in stellar interiors into advanced phases of evolution. Internal rotation also offers a natural explanation for the CNO isotopic anomalies observed among red giants. Finally, some of the questions raised by recent progress are considered.

scholarly journals Problems of Gaseous Motion Around Stars

1973 ◽  
Vol 51 ◽  
pp. 22-47
Author(s):  
Su-Shu Huang
Keyword(s):  
Gravitational Field ◽  
Binary Systems ◽  
Rotational Velocity ◽  
Point Of View ◽  
Temperature Sensitive ◽  
Be Stars ◽  
Jet Streams ◽  
Circumstellar Material ◽  
Be Star ◽  
Gaseous Envelopes

A distinction is drawn between radial and tangential modes of ejection from stars, and the possible flow patterns are described. They are: expanding streams, falling streams, jet streams, circulatory streams, and gaseous envelopes. Motion around Be stars is discussed at some length, as a preliminary to studying more complicated flow in binary systems. The rotational velocity of the Be star is insufficient to form the ring. It appears likely that radial instability is temperature sensitive. Rings and disks in binary systems are discussed from the point of view of periodic orbits for particles within the gravitational field of such a system. The formation of these rings is discussed. The expected relation between rotational velocity of the ring and the orbital period is discussed. The relation of circumstellar streams to period changes is mentioned. Finally, the influence of magnetic fields on the circumstellar material and the system is discussed.

scholarly journals Non-radially Pulsating Hot Stars: Non-radial Pulsations and Be Phenomenon

1999 ◽  
Vol 169 ◽  
pp. 329-336
Author(s):  
Yoji Osaki
Keyword(s):  
Mass Loss ◽  
Relaxation Oscillation ◽  
Stellar Atmosphere ◽  
Circumstellar Disk ◽  
Circumstellar Envelope ◽  
Be Stars ◽  
Hot Stars ◽  
Disk Model ◽  
Rotating Star

AbstractWe discuss a possible role of non-radial oscillations as a cause of mass-loss in hot stars. In particular, we propose a working model for the episodic mass-loss in Be stars. In this model, equatorial mass loss is thought to be driven by wave-breaking phenomenon of large-amplitude non-radial waves and a circumstellar disk could thus be formed around the equatorial plane of a rapidly rotating star. A kind of relaxation-oscillation cycle could be established between the Be phase and non-Be phase, in which an interplay between non-radial oscillations in stellar atmosphere and the circumstellar disk is essential. We also discuss a viscous decretion-disk model for the circumstellar envelope around Be stars.

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