scholarly journals Why do hot subdwarf stars pulsate?

2015 ◽  
Vol 11 (A29B) ◽  
pp. 521-524
Author(s):  
Stephan Geier
Keyword(s):  
Red Giants ◽  
Horizontal Branch ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Long Period ◽  
Sdb Stars ◽  
Short Period ◽  
Instability Regions

AbstractHot subdwarf B stars (sdBs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. Several different kinds of pulsators are found among those stars. The mechanism that drives those pulsations is well known and the theoretically predicted instability regions for both the short-period p-mode and the long-period g-mode pulsators match the observed distributions fairly well. However, it remains unclear why only a fraction of the sdB stars pulsate, while stars with otherwise very similar parameters do not show pulsations. From an observers perspective I review possible candidates for the missing parameter that makes sdB stars pulsate or not.

scholarly journals Progress in the study of pulsating subdwarf B stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 589-595
Author(s):  
M. D Reed
Keyword(s):  
Differential Rotation ◽  
Observational Constraints ◽  
Interior Structure ◽  
Horizontal Branch ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Rotation Periods ◽  
Horizontal Branch Stars ◽  
Short Period

AbstractDuring Kepler's main mission, nearly 20 pulsating subdwarf B (sdB: extreme horizontal branch stars) were discovered. Many of these stars were observed for three years, accumulating over 1.5 million observations. Only through these extended observations have we been able to identify pulsation modes, applying constraints for structure models. Discoveries include nearly-evenly-spaced asymptotic period overtones which represent the interior structure and rotationally-induced frequency multiplets from which we have learned that rotation periods are long, even when in short-period binaries. This paper reviews progress on observational constraints and highlights some of our discoveries including radially differential rotation, conflicting stratification indicators and mode lifetimes.

Asteroseismology of tidally distorted sdB stars

2019 ◽  
Vol 489 (3) ◽  
pp. 3066-3072 ◽  
Author(s):  
Holly P Preece ◽  
C Simon Jeffery ◽  
Christopher A Tout
Keyword(s):  
Normal Modes ◽  
Frequency Shifts ◽  
B Stars ◽  
Tidal Distortion ◽  
Common Envelope ◽  
Subdwarf B Stars ◽  
Sdb Stars ◽  
Short Period ◽  
Split Mode ◽  
Orbital Periods

ABSTRACT Most subdwarf B stars are located in post-common envelope binaries. Many are in short-period systems subject to tidal influence, and many show pulsations useful for asteroseismic inference. In combination, one must quantify when and how tidal distortion affects the normal modes. We present a method for computing tidal distortion and associated frequency shifts. Validation is by application to polytropes and comparison with previous work. For typical sdB stars, a tidal distortion to the radius of between $0.2\,$ and $2\,$ per cent is generated for orbital periods of 0.1 d. Application to numerical helium core-burning stars identifies the period and mass-ratio domain where tidal frequency shifts become significant and quantifies those shifts in terms of binary properties and pulsation modes. Tidal shifts disrupt the symmetric form of rotationally split multiplets by introducing an asymmetric offset to modes. Tides do not affect the total spread of a rotationally split mode unless the stars are rotating sufficiently slowly that the rotational splitting is smaller than the tidal splitting.

scholarly journals The Scale Height of Blue Halo subdwarf B Stars

1995 ◽  
Vol 164 ◽  
pp. 403-403
Author(s):  
K.S. De Boer ◽  
A. Theissen ◽  
U. Heber ◽  
S. Moehler
Keyword(s):  
Spatial Distribution ◽  
Scale Height ◽  
Horizontal Branch ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Sdb Stars ◽  
The Past ◽  
Halo Stars ◽  
Red Giant ◽  
Branch Type

Numerous blue halo stars have been discovered during the past decade (e.g. the Palomar-Green Survey). Many are horizontal branch type, being HBA, HBB, sdB, or sdO star. The sdB stars (Teff between ≈ 18 000 and ≈ 30 000 K) are the endproducts of evolution in the red-giant phase. This means that the spatial distribution of the sdB stars can be used to get insight in the population nature and in their spatial origin.

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 TESS observations of pulsating subdwarf B stars: extraordinarily short-period gravity modes in CD−28° 1974

2020 ◽  
Vol 493 (4) ◽  
pp. 5162-5169 ◽  
Author(s):  
M D Reed ◽  
K A Shoaf ◽  
P Németh ◽  
J Vos ◽  
M Uzundag ◽  
...  
Keyword(s):  
Main Sequence ◽  
Orbital Motion ◽  
Proper Motions ◽  
Reflection Effect ◽  
Echelle Spectrograph ◽  
Single Star ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Short Period ◽  
Typical Period

ABSTRACT Transiting Exoplanet Survey Satellite (TESS) observations show CD−28° 1974 to be a gravity(g)-mode-dominated hybrid pulsating subdwarf B (sdBV) star. It shows 13 secure periods that form an ℓ = 1 asymptotic sequence near the typical period spacing. Extraordinarily, these periods lie between 1500 and 3300 s, whereas typical $\ell = 1\, g$ modes in sdBV stars occur between 3300 and 10 000 s. This indicates a structure somewhat different from typical sdBV stars. CD−28° 1974 has a visually close F/G main-sequence companion 1.33 arcsec away, which may be a physical companion. Gaia proper motions indicate a comoving pair with the same distance. A reanalysis of Ultraviolet and Visual Echelle Spectrograph (UVES) spectra failed to detect any orbital motion and the light curve shows no reflection effect or ellipsoidal variability, making an unseen close companion unlikely. The implication is that CD−28° 1974 has become a hot subdwarf via single star or post-merger evolution.

Asteroseismological Studies of Long‐Period Variable Subdwarf B Stars. I. A Multisite Campaign on PG 1627+017

2006 ◽  
Vol 643 (2) ◽  
pp. 1198-1218 ◽  
Author(s):  
S. K. Randall ◽  
G. Fontaine ◽  
E. M. Green ◽  
P. Brassard ◽  
D. Kilkenny ◽  
...  
Keyword(s):  
B Stars ◽  
Subdwarf B Stars ◽  
Long Period ◽  
Period Variable

A Driving Mechanism for the Newly Discovered Long‐Period Pulsating Subdwarf B Stars

2003 ◽  
Vol 597 (1) ◽  
pp. 518-534 ◽  
Author(s):  
G. Fontaine ◽  
P. Brassard ◽  
S. Charpinet ◽  
E. M. Green ◽  
P. Chayer ◽  
...  
Keyword(s):  
Driving Mechanism ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Long Period

scholarly journals First Kepler results on compact pulsators - V. Slowly pulsating subdwarf B stars in short-period binaries

2010 ◽  
Vol 409 (4) ◽  
pp. 1509-1517 ◽  
Author(s):  
S. D. Kawaler ◽  
M. D. Reed ◽  
R. H. Østensen ◽  
S. Bloemen ◽  
D. W. Kurtz ◽  
...  
Keyword(s):  
B Stars ◽  
Subdwarf B Stars ◽  
Short Period

scholarly journals Some Blue Stars of Peculiar Type in the Region of the South Galactic Pole

1973 ◽  
Vol 50 ◽  
pp. 245-250
Author(s):  
J. A. Graham ◽  
A. Slettebak
Keyword(s):  
Classification Criteria ◽  
Type Star ◽  
Horizontal Branch ◽  
The South ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Horizontal Branch Stars ◽  
Photometric Observations ◽  
Late Type Star ◽  
Objective Prism

uvby photometric observations have been used in conjunction with slit spectra to classify 90 stars which were noted as peculiar by Slettebak and Brundage in a recent objective prism survey of the South Galactic Pole region. In this paper, we review the photometric classification criteria and identify in the Slettebak-Brundage list, 8 subdwarf O stars, 10 subdwarf B stars, 10 horizontal branch stars, 1 white dwarf star and 26 late subdwarf stars. Three stars with outstanding peculiarities are SB (Slettebak-Brundage) 58 which is a helium subdwarf O star, SB 319 (CD–38°245), a late type star with extremely weak metal lines and SB 845 (BD–13°6465), an A type star with a very small Balmer discontinuity.

scholarly journals K2 observations of the pulsating subdwarf B stars UY Sex and V1405 Ori

2020 ◽  
Vol 492 (4) ◽  
pp. 5202-5217
Author(s):  
M D Reed ◽  
M Yeager ◽  
J Vos ◽  
J H Telting ◽  
R H Østensen ◽  
...  
Keyword(s):  
Rotation Period ◽  
B Stars ◽  
Subdwarf B Stars ◽  
Fundamental Properties ◽  
The Core ◽  
Rotation Periods ◽  
Short Period ◽  
Marginal Detection

ABSTRACT We processed and analysed K2 observations of the pulsating subdwarf-B (sdBV) stars UY Sex and V1405 Ori. We detect 97 p-mode pulsations in UY Sex while we discover V1405 Ori to be a rare rich hybrid pulsator with over 100 p-mode pulsations and 19 g-mode pulsations. We detect frequency multiplets, which we use to identify pulsation modes as well as determine rotation periods. For UY Sex, we find a rotation period of the envelope of 24.6 ± 3.5 d and for V1405 Ori, we find a rotation period of 0.555 ± 0.029 d for the p modes and a marginal detection of 4.2 ± 0.4 d for the g modes. We discover that V1405 Ori is unique among sdBV stars observed to date. It is a rich hybrid pulsator, allowing us to simultaneously probe the envelope and interior; its frequency multiplets indicate V1405 Ori to be rotating differentially with the core rotating more slowly than the envelope, and it is also in a short-period binary (0.398 d) with an envelope that is nearly but not quite tidally locked. For both stars, we have obtained spectroscopic follow-up observations and examine combining them with Gaia parallaxes and archival photometry to determine fundamental properties. Our derived masses are inconsistent with spectroscopy and previous determinations and indicate problems with the methodology.

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