scholarly journals Evolutionary study of the Be star 28 Tau

2010 ◽  
Vol 6 (S272) ◽  
pp. 274-275
Author(s):  
Nazhatulshima Ahmad ◽  
Mohd. Zambri Zainuddin ◽  
Mohd. Sahar Yahya ◽  
Peter P. Eggleton ◽  
Hakim L. Malasan
Keyword(s):  
Critical Velocity ◽  
Effective Temperature ◽  
Main Sequence ◽  
Evolutionary Model ◽  
Rotation Velocity ◽  
Photometric Data ◽  
Rapid Rotation ◽  
Evolutionary Study ◽  
Be Star ◽  
Hr Diagram

AbstractWe present an evolutionary study of 28 Tau, a Be star, in connection with its rapid rotation. The photometric data during the absence of its envelope in 1921 have been used to determine the effective temperature and luminosity of the star at the main sequence of the HR diagram. From an evolutionary model, we found that the mass and radius of the star are about 3.2 M⊙ and 3.2 R⊙ respectively. The equatorial rotation velocity of the star, νe found to be close to its critical velocity, νcr where νe/νcr ≃ 0.87.

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 Stellar models of ε Eridani constrained by stellar seismology

1988 ◽  
Vol 123 ◽  
pp. 285-286
Author(s):  
D. B. Guenther
Keyword(s):  
Binary System ◽  
Effective Temperature ◽  
Characteristic Frequency ◽  
Metal Content ◽  
Main Sequence ◽  
Rapid Rotation ◽  
Chromospheric Activity ◽  
Frequency Spacing ◽  
Stellar Models ◽  
Stellar Seismology

Because Epsilon Eridani (Z = 0.013) appears about 0.2 mag above the Z = 0.02 zero-age main sequence, zero-age models of Epsilon Eridani must be evolved off the ZAMS to ages much older than its rapid rotation and high chromospheric activity suggests. To resolve this conflict we propose that either Epsilon Eridani is part of a binary system or that its metal content has been underestimated. To test these hypotheses we use not only the stellar constraints of luminosity and effective temperature but also impose that the characteristic frequency spacing of the oscillation spectra of the models match the value observed by Noyes et al. (1984).

scholarly journals Non — LTE analysis of massive O-stars

1984 ◽  
Vol 105 ◽  
pp. 265-268
Author(s):  
K. P. Simon ◽  
R. P. Kudritzki
Keyword(s):  
Mass Loss ◽  
Effective Temperature ◽  
Main Sequence ◽  
Stellar Mass ◽  
Photometric Data ◽  
Helium Abundance ◽  
Loss Rates

Non-LTE analyses of five very massive O-stars yield effective temperature, gravity and helium abundance of their photospheres. From these results together with photometric data an estimation of the stellar mass is possible. The comparison of the position in the log g — log Teff diagram relative to theoretical evolutionary tracks allows three compatibility-checks: (1) masses, (2) helium-abundance and (3) age of the stars. From these checks it is inferred, that (a) the stars are in an advanced main-sequence-phase, and (b) they have suffered from considerable mass-loss. Furthermore it seems to be likely that (c) mass-loss-rates do not scale with N > 200, (d) the age of the stars in the η Carinae region is less than 2 mio yrs., and (e) HD 93128 is in the background of Tr14.

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 Photometric and spectroscopic variability of the Be star 48 Lib: The relation between photometric variations and rotation

2018 ◽  
Vol 35 ◽  
Author(s):  
D. Ozuyar ◽  
S. Caliskan ◽  
I. R. Stevens ◽  
A. Elmasli
Keyword(s):  
High Precision ◽  
Spectroscopic Data ◽  
Photometric Data ◽  
Time Interval ◽  
Line Intensities ◽  
Disk Structure ◽  
Be Star

AbstractThe purpose of this paper is to determine the origin of the photometric variations of 48 Lib using the data from theSTEREOand to investigate their relations with the disk structure. The photometric data comprise a period of five years from 2007 to 2011. The spectroscopic data covering the same time interval are provided from theBeSSdatabase. The Hαlines are examined by measuring their equivalent widths and line intensities. Hαvariations are then compared with those displayed by the photometric data. From the photometry, high-precision results (10−5c d−1in frequency and 10−4mag in amplitude) are obtained. It is detected that the star has shown 24 frequencies, mainly clustered around the peaks at 2.48896(1) and 5.08150(2) c d−1. The analysis reveals that the photometric frequencies are not due to pulsation, but caused by the rotation, and that the remaining frequencies arise from transient activities on or just above the photosphere. Also, it is shown that the spectroscopic data exhibit a significant Hαvariability, and that the Hαline variation depends on the variation of frequency and amplitude, something which has been often proposed in the literature but has never before been demonstrated observationally. This proves that the disk structure and photometric variations are related.

scholarly journals PTPS Candidate Exoplanet Host Star Radii Determination with CHARA Array

2011 ◽  
Vol 7 (S282) ◽  
pp. 203-204
Author(s):  
Paweł Zieliński ◽  
Martin Vaňko ◽  
Ellyn Baines ◽  
Andrzej Niedzielski ◽  
Aleksander Wolszczan
Keyword(s):  
Effective Temperature ◽  
Critical Parameter ◽  
Spectroscopic Analysis ◽  
Evolutionary Model ◽  
Stellar Mass ◽  
Surface Temperatures ◽  
Penn State ◽  
Chara Array ◽  
Model Dependency

AbstractWe propose to measure the radii of the Penn State - Toruń Planet Search (PTPS) exoplanet host star candidates using the CHARA Array. Stellar radii estimated from spectroscopic analysis are usually inaccurate due to indirect nature of the method and strong evolutionary model dependency. Also, the so-called degeneracy of stellar evolutionary tracks due to convergence of many tracks in the giant branch decreases the precision of such estimates. However, the radius of a star is a critical parameter for the calculation of stellar luminosity and mass, which are often not well known especially for giants. With well determined effective temperature (from spectroscopy) and radius, the luminosity may be calculated precisely. In turn also stellar mass may be estimated much more precisely. Therefore, direct radii measurements increase precision in the determination of planetary candidates masses and the surface temperatures of the planets.

scholarly journals The Position of the Central Stars of PN on the HR Diagram

1989 ◽  
Vol 131 ◽  
pp. 481-492
Author(s):  
S. R. Pottasch
Keyword(s):  
Effective Temperature ◽  
Critical Role ◽  
Central Stars ◽  
Hr Diagram

Central stars can be placed on the HR diagram if their effective temperature (Teff) and radii are known. Knowledge of the radius can sometimes be replaced by another indication of the luminosity. The distance, which always plays an important, really critical role, is not well known. This is the essential reason that there is so much uncertainty about the position on the HR diagram.

scholarly journals Determining the evolutionary stage of HD163899 on the basis of its oscillation spectrum

2015 ◽  
Vol 11 (A29B) ◽  
pp. 567-572
Author(s):  
Jakub Ostrowski ◽  
Jadwiga Daszyńska-Daszkiewicz ◽  
Henryk Cugier
Keyword(s):  
Effective Temperature ◽  
Main Sequence ◽  
Rotational Velocity ◽  
Evolutionary Stage ◽  
Oscillation Spectrum ◽  
New Interpretation ◽  
Massive Models

AbstractWe present the new interpretation of the oscillation spectrum of HD 163899 based on the new determinations of the effective temperature, mass-luminosity ratio and rotational velocity. These new parameters strongly prefer the more massive models than previously considered. Now it is also possible that the star could be in the main sequence stage. Using the oscillation spectrum as a gauge, we intend to establish which stage of evolution corresponds better to HD 163899.

scholarly journals Search for Magnetic Stars at Early Stages of Evolution

1993 ◽  
Vol 137 ◽  
pp. 669-671
Author(s):  
Yu. V. Glagolevskij
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Rotation Velocity ◽  
Electric Vector ◽  
Magnetic Star ◽  
Stellar Rotation ◽  
Magnetic Stars ◽  
Hydrogen Lines ◽  
Direction Of Polarization ◽  
Gaseous Envelope

Young stars, as a rule, are too faint for measurements of magnetic field either by photographic method with the use of Zeeman analizer, or photoelectrically from hydrogen lines. That is why it is necessary to look for indirect ways of magnetic field detection, for example, by measurement of polarization. Ae/Be Herbig stars without a magnetic field are surrounded by a gaseous envelope in the form of a globe or a spheroid, flattened along the rotational axes (as dependent on stellar rotation velocity), and also by a gaseous-dust accretion disc in the plane of equator. There are powerful flows in gaseous envelopes of stars, connected with mass loss and accretion. If a star is a magnetic oblique rotator (as a magnetic star of the Main Sequence), then the gaseous envelope may acquire the shape of alon-gated ellipsoid with the major axes coincident with that of dipole (Dolginov et al., 1979). From the poles there arises a jet flow controlled by a magnetic field, as in He-r and He-w stars, having already reached the Main Sequence (Barker et al., 1982). Calculations show (Dolginov et al., 1979), that maximum polarization in the extended envelope p ≈ 4% arises when the ratio of ellipsoid axes is ≈ 2.5b. The electric vector of the dominating oscillation of the light wave is perpendicular to the plane through the axis of symmetry of the ellipsoid and the line of sight. Naturally, the magnetosphere rotates together with the star, involving the gaseous envelope, resulting in the variation of the degree and direction of polarization. Additional polarization is created by the polar jets, where the direction of the dominating oscillations of the electric vector is perpendicular to the axis of the polar stream, and value of maximal polarization may reach 5% along the beam.

scholarly journals Spectro-interferometric observations of a sample of Be stars

2019 ◽  
Vol 621 ◽  
pp. A123 ◽  
Author(s):  
Y. R. Cochetti ◽  
C. Arcos ◽  
S. Kanaan ◽  
A. Meilland ◽  
L. S. Cidale ◽  
...  
Keyword(s):  
Critical Velocity ◽  
Velocity Ratio ◽  
Density Wave ◽  
Wave Structure ◽  
Maximum Size ◽  
Rotation Velocity ◽  
Variable Stars ◽  
High Mass ◽  
Be Stars ◽  
Infrared Excess

Context. Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. Aims. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. Methods. We performed spectro-interferometric observations of 26 Be stars in the region of the Brγ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral (R ≃ 12 000) and high spatial (θmin = 4 mas) resolutions. Results. We modeled 18 Be stars with emission in the Brγ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD 28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V̅/V̅c = 0.75 (where Vc is the critical velocity), and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2–13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. Conclusions. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.

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