scholarly journals The Pulsational Behaviour of Variable Stars in the Lower Part of the Instability Strip

2000 ◽  
pp. 227-242 ◽  
Author(s):  
Ennio Poretti
Keyword(s):  
Variable Stars ◽  
Instability Strip

scholarly journals The Theoretical Situation

1974 ◽  
Vol 59 ◽  
pp. 3-34
Author(s):  
Icko Iben
Keyword(s):  
Cataclysmic Variables ◽  
Theoretical Work ◽  
Variable Stars ◽  
Important Variable ◽  
Type I ◽  
Instability Strip ◽  
Rr Lyrae ◽  
Current Thinking ◽  
The Status ◽  
Advanced Stages

The domain of this symposium is so wide – essentially anything at all having to do with stars – that it is impossible to do more than cursory justice to even a small portion of the matters to be discussed. My contribution will therefore be limited primarily to a discussion of the status of theoretical work bearing on the behavior of stars that evolve through the classical instability strip that extends from the region of Cepheids to the domain of RR Lyrae stars. Discussion of other extremely important variable stars such as cataclysmic variables, Mira and irregular variables, flare stars, β-Canis Majoris stars, and δ-Scuti and small amplitude variables will here be mentioned only in passing; presumably, most of these stars will be discussed at length by other speakers at this symposium. Further little attention will be paid to the thermal instability that is initiated in the helium-burning region of a double-shell-source star and to current thinking about the progenitors of type I and type II supernovae; presumably, these topics will be discussed in Warsaw in the symposium on advanced stages of evolution.

Non-variable stars inside the ZZ Ceti instability strip

1995 ◽  
Vol 4 (2) ◽  
Author(s):  
S. O. Kepler ◽  
O. Giovannini ◽  
A. Kanaan ◽  
M. A. Wood ◽  
C. F. Claver
Keyword(s):  
Variable Stars ◽  
Instability Strip

scholarly journals Discovery of a second pulsating intermediate helium-enriched sdOB star

2019 ◽  
Vol 623 ◽  
pp. L12 ◽  
Author(s):  
M. Latour ◽  
E. M. Green ◽  
G. Fontaine
Keyword(s):  
Galactic Halo ◽  
Local Thermodynamic Equilibrium ◽  
Variable Stars ◽  
Light Curves ◽  
Atmospheric Parameters ◽  
Instability Strip ◽  
Long Period ◽  
Non Local ◽  
Halo Population ◽  
Low Amplitude

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. So far, only one other He-enriched sdOB star (LS IV−14 ° 116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fit our optical spectrum of the star with our grid of non-local thermodynamic equilibrium (NLTE) model atmospheres and derived the following atmospheric parameters: Teff = 36120 ± 230 K, log g = 5.93 ± 0.04, and log N(He)/N(H) = −0.32 ± 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV−14 ° 116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV−14 ° 116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear, but a possible scenario involves the ϵ-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.

scholarly journals Multicolour studies of β Cephei stars in the LMC

2010 ◽  
Vol 6 (S272) ◽  
pp. 286-287
Author(s):  
Chris A. Engelbrecht ◽  
Fabio A. M. Frescura ◽  
Sashin L. Moonsamy
Keyword(s):  
Variable Stars ◽  
Instability Strip ◽  
Model Calculations ◽  
Preliminary Results

AbstractPreliminary results of a four-week multi-colour photometric campaign on previously identified β Cephei stars as well as newly-discovered variable stars in two respective LMC fields are presented. Besides the two targeted β Cephei stars, at least six further presumed B variables are detected. The strongest identified periods appear to lie on the longward end of the galactic β Cep instability strip, as predicted by model calculations.

scholarly journals Rate of change of the pulsation periods in the PG 1159 star PG 0122+200

2008 ◽  
Vol 4 (S252) ◽  
pp. 157-162
Author(s):  
G. Vauclair ◽  
J.-N. Fu ◽  
J.-E. Solheim ◽  
S.-L. Kim ◽  
M. Chevreton ◽  
...  
Keyword(s):  
White Dwarf ◽  
Rapid Evolution ◽  
Theoretical Models ◽  
Variable Stars ◽  
Rate Of Change ◽  
Point Of View ◽  
Short Time Scale ◽  
Theoretical Point ◽  
Instability Strip ◽  
Core Cooling

AbstractThe pre-white dwarf pulsators of PG 1159 type, or GW Virginis variable stars, are in a phase of rapid evolution towards the white dwarf cooling sequence. The rate of change of their nonradial g-mode frequencies can be measured on a reasonably short time scale. From a theoretical point of view, it was expected that one could derive the rate of cooling of the stellar core from such measurements. At the cool end of the GW Virginis instability strip, it is predicted that the neutrinos flux dominates the cooling. PG 0122+200 which defines the red edge of the instability strip is in principle a good candidate to check this prediction. It has been followed-up through multisite photometric campaigns for about fifteen years. We report here the first determination of the rate of change of its 7 largest amplitude frequencies. We find that the amplitudes of the frequency variations are one to two orders of magnitude larger than predicted by theoretical models based on the assumption that these variations are uniquely caused by cooling. The time scales of the variations are much shorter than the ones expected from a neutrino dominated core cooling. These results point to the existence of other mechanisms responsible for the frequency variability. We discuss the role of nonlinearities as one possible mechanism.

scholarly journals Maximizing the Educational Value of Student Research Participation

1990 ◽  
Vol 105 ◽  
pp. 119-124
Author(s):  
Emilia Pisani Belserene
Keyword(s):  
Research Participation ◽  
Variable Stars ◽  
Instability Strip ◽  
Educational Value ◽  
Scientific Goal ◽  
Educational Potential ◽  
To Receive ◽  
Student Assistants ◽  
Constant Period ◽  
Single Constant

The Maria Mitchell Observatory monitors variable stars photographically and has lately begun to receive photometric data remotely. The staff consists solely of the director and undergraduate student assistants during summer and January vacations. Research topics are chosen for both their scientific interest and their educational potential.The scientific goal is to improve variable-star statistics by answering any unresolved observational questions. Lately the emphasis is on pulsators in the Cepheid instability strip. Can we watch the stars grow older? We look for deviations from a single, constant period.

scholarly journals Oscillations of HB Red Variable Stars

1998 ◽  
Vol 185 ◽  
pp. 401-402
Author(s):  
D.R. Xiong ◽  
Q. L. Cheng ◽  
L. Deng
Keyword(s):  
Globular Clusters ◽  
Turbulent Viscosity ◽  
Variable Stars ◽  
Instability Strip ◽  
Rr Lyrae ◽  
Short Period ◽  
Spatial Oscillations ◽  
The Right ◽  
Thermodynamic Coupling ◽  
Low Amplitude

Using a nonlocal time-dependent theory of convection, we have calculated the linear non-adiabatic oscillations of the Horizontal Branch (HB) stars, with both the dynamic and thermodynamic coupling between convection and oscillations been carefully treated. Turbulent pressure and turbulent viscosity have been included consistently in our equations of non-adiabatic pulsation. When the coupling between convection and oscillations is ignored, for all models with Te ≤ 7350K, the fundamental through the second overtone are pulsationally unstable; while for Te ≤ 6200K all the models are unstable up to (at least) the 9th overtone. When the coupling between convection and oscillations is included, the RR Lyrae instability strip is very well predicted. Within the strip most models are pulsationally unstable only for the fundamental and the first few overtones. Turbulent viscosity is an important damping mechanism. Being exclusively distinct from the luminous red variables (long period variables), the HB stars to the right of the RR Lyrae strip are pulsationally stable for the fundamental and low-order overtones, but become unstable for some of the high-order overtones. This may provide a valuable clue for the short period, low amplitude red variables found outside the red edge of the RR Lyrae strip on the H-R diagram of globular clusters. Moreover, we present a new radiation modulated excitation mechanism functioning in radiation flux gradient regions. The effects of nonlocal convection and the dynamic coupling between convection and oscillations are discussed. The spatial oscillations of the thermal variables in the pulsational calculations have been effectively suppressed.

scholarly journals A Proposed Pulsation Driving Mechanism for γ Doradus Variable Stars

2002 ◽  
Vol 185 ◽  
pp. 502-505 ◽  
Author(s):  
J.A. Guzik ◽  
A.B. Kaye ◽  
P.A. Bradley ◽  
A.N. Cox ◽  
C. Neuforge-Verheecke ◽  
...  
Keyword(s):  
Convection Zone ◽  
Variable Stars ◽  
Driving Mechanism ◽  
Pulsation Period ◽  
Instability Strip

AbstractWe propose a pulsation driving mechanism for γ Doradus variable stars. This mechanism requires that the local convective timescale at the base of the envelope convection zone be comparable to or longer than the pulsation period. This mechanism explains both the red and blue edges of the γ Dor instability strip. We predict that some candidate γ Dor stars summarized by Handler (1999) may actually be too hot or too cool to be γ Dor variables.

scholarly journals Line Profile Variations in the Spectra of the γ Dor Star HR 2740

1998 ◽  
Vol 185 ◽  
pp. 387-388
Author(s):  
E. Poretti ◽  
L. Mantegazza ◽  
C. Koen ◽  
P. Martinez ◽  
F. Breuer ◽  
...  
Keyword(s):  
Light Curve ◽  
Line Profile ◽  
Frequency Analysis ◽  
Variable Stars ◽  
Instability Strip ◽  
Satisfactory Solution ◽  
Long Time ◽  
The Relationship

HR 2740 was a target for a photometric campaign carried out at La Silla (ESO) and Sutherland (SAAO) from 1997 January 14 to 1997 February 11 (Poretti et al., 1997). The campaign revealed that HR 2740 is one of the brightest γ Dor stars, a class of variable stars located near the cool border of the instability strip, and in which gravity pulsation modes are excited. Four frequencies were identified (f1=1.0434, f2=0.9951, f3=1.1088, f4=0.9019 c d−1), which together yield a satisfactory solution to the observed light curve. The frequency analysis was not simple, but thanks to the large coverage in longitude we could separate the effect of aliasing on the two terms f3=1.1088 c d−1 and f4=0.9019 c d−1, linked by the relationship f4 = 2 - f3. Moreover, only the long time baseline allowed us to resolve the two close terms f1=1.0434 c d−1 and f2=0.9951 c d−1. See Poretti et al. (1997) for a detailed discussion.

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