scholarly journals Non-Radial Pulsation In Be Stars

1986 ◽  
Vol 7 ◽  
pp. 265-272
Author(s):  
John R. Percy
Keyword(s):  
Mass Transfer ◽  
Critical Velocity ◽  
Main Sequence ◽  
Radial Pulsation ◽  
Be Stars ◽  
Hydrogen Line ◽  
Main Sequence Stars ◽  
Rapid Rotation ◽  
B Stars ◽  
Effective Gravity

Be stars are B stars in which emission has been observed in at least one hydrogen line on at least one occasion. Some Be stars are pre-main-sequence stars, mass-transfer binaries, or supergiant stars with extended atmospheres. The majority, however, are classical Be stars: single stars on or near the main sequence. An important characteristic of these stars seems to be their rapid rotation – close to but not at the “critical” velocity at which the effective gravity vanishes at the equator.

scholarly journals The Evolution of Rapidly Rotating B Stars (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 486-499
Author(s):  
Robert Connon Smith
Keyword(s):  
Main Sequence ◽  
Review Paper ◽  
Rotation Axis ◽  
Evolutionary Stage ◽  
Radial Pulsation ◽  
Be Stars ◽  
Rotating Stars ◽  
B Stars ◽  
Sequence Band ◽  
Main Effects

AbstractBe stars are located in or near the main-sequence band for non-rotating stars. Although this stage of evolution is relatively well understood, there are two main effects that make it impossible to say whether all Be stars are in the same stage of evolution and, if so, what that stage is. One effect is the spread in observed magnitude and colour as a result of rotation. Correction for rotation is not possible because of the unknown inclination of any particular star's rotation axis to the line of sight and because it is not clear what the internal angular momentum distribution is or how it changes as a result of evolution. The other effect is that there are uncertainties in the theoretical evolutionary tracks because the amount of convective overshooting is unclear. Other mixing mechanisms that might in principle also confuse the tracks seem to be small near the main sequence. If Be stars are related to the β Cephei and 53 Persei stars in the same part of the HR diagram, then the non-radial pulsation properties of Be stars may give a clue to their evolutionary state. The existence of a circumstellar disc or ring, however, tells us very little about the evolutionary stage of the underlying star. A useful way forward may be to try to understand individual stars in as much detail as possible.

scholarly journals A Spectroscopic Search for Non-Radial Pulsations in Early B-Type Stars

2002 ◽  
Vol 185 ◽  
pp. 204-205 ◽  
Author(s):  
C. Schrijvers ◽  
J.H. Telting ◽  
J. De Ridder
Keyword(s):  
Line Profile ◽  
Spectral Resolution ◽  
Main Sequence ◽  
High Spectral Resolution ◽  
Radial Pulsation ◽  
Main Sequence Stars ◽  
B Stars ◽  
Low Degree ◽  
Intermediate Degree ◽  
Radial Pulsations

It is known that early-B type main-sequence stars can be unstable against pulsations induced by the κ mechanism. Traditionally, radial and low-degree non-radial β Cephei-like pulsations have been detected photometrically; nowadays radial, low-degree and intermediate-degree pulsations can be detected spectroscopically through line-profile variations (LPVs). Whereas most of the brightest early-B type stars have been photometrically monitored for variability (72 of the 541 O9.5-B3 III-V stars in the BSC are classified as β Cephei stars), a systematic LPV survey to look for pulsational behaviour was lacking. Here we present some preliminary results of the southern part of our Spectroscopic survey of early-B type stars. We have made a high spectral resolution survey of the Si III λ 4552, 4567, 4574 Å triplet in early-B type stars in order to look for LPVs due to non-radial pulsation (NRP). Our sample consists of 82 southern O9.5-B3 stars with spectral classes III-V, taken from the BSC; 27% of all O9.5-B2.5 III-V stars in the BSC were sampled. This sample is not fully representative for all nearby early-B stars, because we concentrated on stars with high rotational velocities, because many of the stars are members of the Sco-Cen associations, and because we focused on the β Cephei stars in the BSC. As much as 40% of the β Cephei stars in the BSC with v sin г ≥ 25 km s-1 were sampled. The observations were carried out with the ESO CAT telescope at resolution R∼65000. Our targets were observed during 6 runs in September 1995 – June 1998. Some stars were observed many times, but most targets were observed only once or twice.

scholarly journals Chromospheres, Coronae, and Mass Loss in Stars Hotter than the Sun

1980 ◽  
Vol 5 ◽  
pp. 525-531 ◽  
Author(s):  
Theodore P. Snow
Keyword(s):  
Mass Loss ◽  
Main Sequence ◽  
Emission Lines ◽  
Be Stars ◽  
Main Sequence Stars ◽  
B Stars ◽  
X Ray ◽  
Ob Stars ◽  
F Stars ◽  
Simple Dependence

AbstractReviews of the mass-loss characteristics of OB stars have been published recently, and the present review therefore emphasizes the A and F stars and very recent results on O and B stars. For the F stars, chromospheric indicators are present in the form of emission lines, seen in visible and ultraviolet wavelengths. Winds are present in A supergiants, but not in main sequence stars, although at least a few of the latter are X-ray sources, indicating the possible existence of coronae. Most OB supergiants are X-ray sources as well, indicating, along with the presence of super-ionization, that these stars have coronae. On the main sequence, the O stars and some B stars (including Be stars in many cases) have mass loss with highly-ionized species in the wind. The winds in the O and B stars are commonly variable. The mass-loss rates do not show a simple dependence on luminosity, contrary to the predictions for radiatively-driven winds.

scholarly journals NonRadial Pulsations and the Be Phenomenon

1987 ◽  
Vol 92 ◽  
pp. 463-463
Author(s):  
G. D. Penrod
Keyword(s):  
Relaxation Oscillation ◽  
Radial Pulsation ◽  
Be Stars ◽  
B Stars ◽  
Long Period ◽  
Period 2 ◽  
Short Period ◽  
Essential Components ◽  
Emission Stars ◽  
High Degree

AbstractOver the last three years I have obtained about 2000 spectra of a sample of 25 rapidly rotating Bn and Be stars. All but two of the program stars show obvious line-profile variations due to non-radial oscillations. The non-emission stars are each pulsating in one or two short-period high-degree (l = 4 to 10) modes, while the Be stars are in all cases pulsating in a long-period % = 2 mode, and often in a short-period high-Z mode as well. The amplitude of the pulsations in several stars (λ Eri, o And, ζ 0ph, and 2 Vul) is correlated with the occurrence of Be outbursts. The amplitude of the pulsations is largest before the outbursts, declines slowly during the emission phases to a fraction of its previous amplitude, and then slowly recovers to its previous amplitude, a few months before the onset of the next outburst. The correspondence between the presence of a long-period % = 2 mode and Ha emission in rapidly rotating B stars strongly suggests that non-radial pulsation and rapid rotation are the essential components which enable single early B stars to become Be stars. The time scale between Be outbursts probably reflects the relaxation oscillation cycle of the I = 2 mode excitation and damping.

scholarly journals Main-sequence broadening, Be stars, and stellar rotation inhand χ Persei

1994 ◽  
Vol 162 ◽  
pp. 151-152
Author(s):  
J. Denoyelle ◽  
C. Aerts ◽  
C. Waelkens
Keyword(s):  
Main Sequence ◽  
Large Fraction ◽  
Variable Stars ◽  
Theoretical Studies ◽  
Be Stars ◽  
Stellar Rotation ◽  
B Stars ◽  
Fundamental Parameters ◽  
Magnitude Diagram ◽  
Double Cluster

The double cluster h andxPersei is one of the richest clusters containing early-B stars, and therefore is important for observational and theoretical studies on the fundamental parameters of massive stars. The colour-magnitude diagram of the double cluster shows an important scatter (see Figure 1). It has long been known thathandxPersei are extremely rich in Be stars (Slettebak 1968). Our previous contention (Waelkens et al. 1990) that the large-amplitude variable stars we discovered are also Be stars, could be confirmed for a few objects. Rotation velocities for stars inhandxPersei are usually high, which is not surprising in view of the large fraction of Be stars.

MAIN SEQUENCE-LIKE STARS WITH SOLITONIC CORES IN EFFECTIVE GRAVITY MODELS

1999 ◽  
Vol 08 (06) ◽  
pp. 695-703
Author(s):  
NIMMI ROOPRAI ◽  
DAKSH LOHIYA
Keyword(s):  
Main Sequence ◽  
Gravity Models ◽  
Main Sequence Stars ◽  
Effective Gravity ◽  
Normal Star ◽  
Astrophysical Objects ◽  
Generic Class ◽  
Radiative Zone ◽  
Characteristic Features

Lee–Wick type nontopological solitons (NTS's) are known to arise in a generic class of nonminimally coupled theories. Size of these solutions depend upon the nature and the amount of their associated conserved charge. Such NTS's would have characteristic features and would represent novel astrophysical objects. We explore the possibility of having main sequence stars condensing around small NTS's. In particular, if such an NTS is embedded in the radiative zone of a star, the exterior attributes could be indistinguishable from those of a normal star. Possible application to stellar modeling is explored.

scholarly journals Where do Be stars stand in the picture of rotational mixing?

2010 ◽  
Vol 6 (S272) ◽  
pp. 85-86
Author(s):  
Paul R. Dunstall ◽  
Ines Brott ◽  
Philip L. Dufton ◽  
Chris J. Evans
Keyword(s):  
Critical Velocity ◽  
Rotational Velocity ◽  
Magellanic Clouds ◽  
Evolutionary Status ◽  
Evolutionary Models ◽  
Be Stars ◽  
Atmospheric Parameters ◽  
B Stars ◽  
The Absolute ◽  
Fast Rotating

AbstractAtmospheric parameters and photospheric abundances have been estimated for 60 Be-type stars located in 4 fields over the Magellanic Clouds. Particular attention has been given to the absolute nitrogen abundances to test theories of rotational mixing, an important factor in the evolutionary status of B-type stars, Hunter et al. (2008). The analysis used the non-LTE atmospheric code TLUSTY and required the implementation of a procedure to compensate for possible contamination due to the presence of a circumstellar disc. Through comparison with evolutionary models of fast rotating B-type stars and projected rotational velocity distributions our results support the theory that Be-type stars are typically faster rotators than B stars, but the measured nitrogen enhancements appear to be significantly less than expected for Be stars rotating with velocities greater than 70% of their critical velocity

scholarly journals Study of a sample of faint Be stars in the exofield of CoRoT

2018 ◽  
Vol 613 ◽  
pp. A70 ◽  
Author(s):  
T. Semaan ◽  
A. M. Hubert ◽  
J. Zorec ◽  
J. Gutiérrez-Soto ◽  
Y. Frémat ◽  
...  
Keyword(s):  
Power Spectrum ◽  
Main Sequence ◽  
Power Spectra ◽  
Circumstellar Disks ◽  
Light Curves ◽  
Radial Pulsation ◽  
Be Stars ◽  
Time Frequency ◽  
Mass Ejection ◽  
Low Amplitude

Context. The class of Be stars are the epitome of rapid rotators in the main sequence. These stars are privileged candidates for studying the incidence of rotation on the stellar internal structure and on non-radial pulsations. Pulsations are considered possible mechanisms to trigger mass-ejection phenomena required to build up the circumstellar disks of Be stars. Aims. Time series analyses of the light curves of 15 faint Be stars observed with the CoRoT satellite were performed to obtain the distribution of non-radial pulsation (NRP) frequencies in their power spectra at epochs with and without light outbursts and to discriminate pulsations from rotation-related photometric variations. Methods. Standard Fourier techniques were employed to analyze the CoRoT light curves. Fundamental parameters corrected for rapid-rotation effects were used to study the power spectrum as a function of the stellar location in the instability domains of the Hertzsprung–Russell (H-R) diagram. Results. Frequencies are concentrated in separate groups as predicted for g-modes in rapid B-type rotators, except for the two stars that are outside the H-R instability domain. In five objects the variations in the power spectrum are correlated with the time-dependent outbursts characteristics. Time-frequency analysis showed that during the outbursts the amplitudes of stable main frequencies within 0.03 c d−1 intervals strongly change, while transients and/or frequencies of low amplitude appear separated or not separated from the stellar frequencies. The frequency patterns and activities depend on evolution phases: (i) the average separations between groups of frequencies are larger in the zero-age main sequence (ZAMS) than in the terminal age main sequence (TAMS) and are the largest in the middle of the MS phase; (ii) a poor frequency spectrum with f ≲ 1 cd−1 of low amplitude characterizes the stars beyond the TAMS; and (iii) outbursts are seen in stars hotter than B4 spectral type and in the second half of the MS. Conclusions. The two main frequency groups are separated by δf = (1.24 ± 0.28) × frot in agreement with models of prograde sectoral g-modes (m = −1, −2) of intermediate-mass rapid rotators. The changes of amplitudes of individual frequencies and the presence of transients correlated with the outburst events deserve further studies of physical conditions in the subatmospheric layers to establish the relationship between pulsations and sporadic mass-ejection events.

scholarly journals The Rotation of Pre-Main Sequence Stars

1980 ◽  
Vol 5 ◽  
pp. 835-837
Author(s):  
Leonard V. Kuhi ◽  
Stuart Vogel
Keyword(s):  
Angular Momentum ◽  
Spectral Type ◽  
Main Sequence ◽  
Rotational Velocity ◽  
The Sun ◽  
Main Sequence Stars ◽  
Lower Mass ◽  
B Stars ◽  
Large Numbers ◽  
Rapid Deceleration

Kraft (1970) obtained the rotational velocities for large numbers of stars located in the field and in clusters of different ages. He noted that (a) among the field stars those stars with strong Call K emission had larger rotational velocities than those without; (b) stars in the Hyades and Pleiades (which are much younger than the field) had both larger rotational velocities and stronger Call K emission than field stars; (c) there was a pronounced break at spectral type early F in v sini as a function of spectral type and (d) the distribution of angular momentum per unit, mass J(M⊚) was proportional to M0.57 for main sequence stars with mass M > 1.5 Mʘ. This distribution predicted a v sini of ˜75 km/sec for stars of lower mass (e.g. G type) but such high velocities were not seen in the Pleiades nor in the sun. This implied a more rapid deceleration of v sini for lower mass stars and led to estimates of the e-folding time of ˜4×l08 years for stars of 1.2 M⊚ to reduce their v sini from that of the Pleiades to that of the Hyades and ˜4×l09 years to go from the Hyades to the sun’s v sini. We note also that the age of the Pleiades is approximately equal to the pre-main sequence lifetime of a 1.0 M0 star so that the zero-age main sequence cannot have J(M) α M0.57 for ˜1 M0 stars. Skumanich (1972) showed that both the Call k emission and the rotational velocity decayed as the (age)-½ for main-sequence stars.

scholarly journals Nonlinear hydrodynamic simulations of radial pulsations in massive stars

2013 ◽  
Vol 9 (S301) ◽  
pp. 449-450 ◽  
Author(s):  
Catherine Lovekin ◽  
Joyce A. Guzik
Keyword(s):  
Main Sequence ◽  
Radial Pulsation ◽  
Helium Abundance ◽  
Initial Model ◽  
Main Sequence Stars ◽  
Solar Mass ◽  
Dramatic Decrease ◽  
Hydrodynamic Simulations ◽  
Density Relation ◽  
Radial Pulsations

AbstractWe investigate the radial pulsation properties of massive main-sequence stars using both linear and non-linear calculations. Using 20, 40, 60 and 85 solar-mass models evolved by Meynet et al. (1994), we calculate nonlinear hydrodynamic envelope models including the effects of time-dependent convection. Many of these models are massive enough to lose a significant amount of mass as they evolve, which also reveals more helium-rich layers. This allows us to investigate the dependence of pulsation on mass, metallicity and surface helium abundance. We find that as a model loses mass, the periods become longer relative to the period predicted by the period-mean density relation (period × $\sqrt{\overline{\rho}}$ is proportional to a constant, Q) for the initial model. Increased surface helium abundance causes a dramatic decrease in the period relative to that expected from Q, while changing the metallicity had little impact on the expected periods.

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