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 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 Be and Bn stars: Balmer discontinuity and stellar-class relationship

2020 ◽  
Vol 634 ◽  
pp. A18
Author(s):  
Y. R. Cochetti ◽  
J. Zorec ◽  
L. S. Cidale ◽  
M. L. Arias ◽  
Y. Aidelman ◽  
...  
Keyword(s):  
High Resolution ◽  
Main Sequence ◽  
Rotational Velocity ◽  
Evolutionary Stage ◽  
Circumstellar Envelope ◽  
Be Stars ◽  
Rotating Stars ◽  
Circumstellar Material ◽  
Low Mass ◽  
Rapidly Rotating Stars

Context. A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material. Aims. We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods. We obtained low- and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hα line. Results. Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the V sin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with V sin i ≲ 250 km s−1, and in absorption in both Be and Bn stars with V sin i ≳ 50 km s−1. Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions. We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

The Evolution of Rapidly Rotating B/Be Stars

1982 ◽  
Vol 98 ◽  
pp. 299-302 ◽  
Author(s):  
A. S. Endal
Keyword(s):  
Rotation Rate ◽  
Main Sequence ◽  
Critical Value ◽  
Moment Of Inertia ◽  
Maxwellian Distribution ◽  
Be Stars ◽  
Rotating Stars ◽  
Hydrogen Burning ◽  
Critical Rotation ◽  
The Moment

Rotation can significantly change the moment-of-inertia of a main sequence star. As a result, the ZAMS rotation rate need only be within ~30% of the critical value in order to reach critical rotation during the hydrogen burning stage. Calculations of the evolution of rotating stars show that the Be stars result from a normal (Maxwellian) distribution of B-star rotation velocities.

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.

scholarly journals A Study of Be Stars in Clusters

1976 ◽  
Vol 70 ◽  
pp. 31-32
Author(s):  
R. Schild ◽  
W. Romanishin
Keyword(s):  
Main Sequence ◽  
Sequence Evolution ◽  
Be Stars ◽  
Rotating Stars ◽  
Time Duration ◽  
Hydrogen Burning ◽  
Fourfold Increase ◽  
Cluster Membership ◽  
Galactic Clusters ◽  
Short Time

Calibrated spectrograms at Hα of 566 stars in 29 young galactic clusters led to the detection of 41 Be stars in clusters. Using cluster membership we have inferred ages and intrinsic (B – V) colors of Be stars to permit a discussion of their evolutionary states.Rotating stars can become Be stars in their early hydrogen burning evolution away from the main sequence. Both the fraction of stars showing hydrogen emission and the strength of emission appear to vary little during the first 80% of post main sequence evolution. However, at the onset of gravitational core contraction, both the fraction of stars showing emission and their mean emission strength undergo a fourfold increase. Many stars in the core contraction phase develop an intrinsic (B – V) excess of 0.15 mag. due to the H− free-bound continuum radiation. Because of the great strength of Hα emission and the short time duration of the effect, the extreme Be stars would be excellent probes for studies of spiral structure and would also serve as probes for studies of ages and distances of extragalactic systems.Analysis of the corrected colors of the Be stars in clusters suggests that the Lucy and Solomon (1970) mechanism for reduction of effective surface gravity by ultraviolet resonance line scattering is probably important for the hotter Be stars. However, a discrepancy exists for the cooler stars between the predicted and observed colors.

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 Evolution of the rotational properties and nitrogen surface abundances of B-Type stellar populations

2014 ◽  
Vol 9 (S307) ◽  
pp. 102-103
Author(s):  
A. Granada ◽  
G. Meynet ◽  
S. Ekström ◽  
C. Georgy ◽  
L. Haemmerlé
Keyword(s):  
Main Sequence ◽  
Chemical Species ◽  
Stellar Populations ◽  
Be Stars ◽  
Population Synthesis ◽  
Rotating Stars ◽  
Surface Enrichment ◽  
Stellar Rotation ◽  
Abundance Anomalies ◽  
Synthetic Intermediate

AbstractStellar evolution models predict that rotation induces the mixing of chemical species, with the subsequent surface abundance anomalies relative to single non-rotating models, even during the main sequence (MS) evolution. The lack of measurable nitrogen surface enrichment in MS rotating stars, such as Be stars, has been interpreted as being in conflict with evolutionary models (e.g. Lennon et al. 2005; Hunter et al. 2008). In order to have an insight on the kind of ambient we do or we do not expect to find enriched rotating stars, we use our new population synthesis code, to produce synthetic intermediate-mass stellar populations fully accounting for stellar rotation effects, and study their evolution in time.

scholarly journals New models of massive and Wolf-Rayet stars with mass loss and rotation

1999 ◽  
Vol 193 ◽  
pp. 177-186 ◽  
Author(s):  
André Maeder
Keyword(s):  
Mass Loss ◽  
Massive Stars ◽  
Life Time ◽  
Structural Equations ◽  
Be Stars ◽  
Rotating Stars ◽  
Physical Treatment ◽  
B Stars ◽  
History Of ◽  
New Treatments

We give results of models of massive stars which have a detailed physical treatment of rotation, including structural equations for shellular rotation, new treatments of shears in differentially rotating stars and of meridional circulation, together with mass loss rates depending on rotation. For a 20 M⊙ star, He- and N-enrichments at the stellar surface already occur during the MS phase for moderately low rotational velocities, thus most supergiants are enriched in helium and nitrogen. A long B- and A-supergiant phase results from rotational mixing, with some primary nitrogen formed at this stage. For the most massive stars, rotation makes the star to enter the WR stage during the MS phase thus avoiding the LBV and red supergiant stage.The WR life-times are considerably increased by rotation and the minimum mass for forming WR stars is lowered. Interestingly enough, the increase of the WN life-time is larger than for WC stars, so that rotation leads to a decrease of the WC/WN number ratio. Also, the fraction of transition WN/WC stars is much larger at higher rotation.Finally, on the basis of clusters in the SMC, in the LMC and towards the galactic interior and exterior, we show that for clusters with ages between about 1 and 3 x 107 yr the fraction of Be stars with respect to normal B stars is larger at lower metallicities. This may suggest a higher rotation at lower metallicities for massive stars, due to a different history of star formation.

scholarly journals g-Mode instability in the main sequence B-type stars

1994 ◽  
Vol 162 ◽  
pp. 69-69
Author(s):  
W.A. Dziembowski ◽  
P. Moskalik ◽  
A.A. Pamyatnykh
Keyword(s):  
Main Sequence ◽  
High Order ◽  
Large Region ◽  
Excitation Mechanism ◽  
Instability Domain ◽  
Mode Instability ◽  
Period Range ◽  
Photometric Variability ◽  
B Stars ◽  
Sequence Band

We show that the OPAL opacities, in addition to explaining the origin of β Cep stars pulsations, also predict existence of a large region in the Main Sequence band at lower luminosities, where high-order g-modes of low harmonic degrees, l, are unstable. The excitation mechanism remains the same and is due to the usual κ-effect acting in the metal opacity bump (T ≈ 2 × 105K). The new instability domain nearly bridges the gap in spectral types between δ Set and β Cep stars. Periods of unstable modes are in the range 0.4–3.5 days for l = 1 and l = 2. We propose that this excitation mechanism causes photometric variability in the Slowly Pulsating B-type stars (SPB stars, Waelkens 1991) and perhaps in other B stars whose variability in the same period range has been reported.

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