A Driving Mechanism for the Newly Discovered Class of Pulsating Subdwarf B Stars

Charpinet, S.; Fontaine, G.; Brassard, P.; Chayer, P.; Rogers, F. J.; Iglesias, Carlos A.; Dorman, Ben

doi:10.1086/310741

Cited by 300 publications

(315 citation statements)

References 17 publications

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“…marked "" He ] C ] Fe.ÏÏ An enhanced iron abundance at the surface might also increase the stellar radius slightly, thus decreasing the and this would also move the T eff , He ] C point somewhat redder. The iron enhancement seen in BHB stars does not appear to be present in the cooler EHB stars (see°6.1.3 ; also see Bedin et al 2000), but di †usion calculations (Charpinet et al 1997) show that the surface iron abundance can be enhanced to solar or even supersolar abundances at K, which is hotter T eff Z 30,000 than the canonical EHB but appropriate for blue-hook stars ; these iron enhancements o †er a possible driving mechanism for the pulsating sdB stars, which show solar iron abundances (Heber, Reid, & Werner 2000). Furthermore, it should be easier to enhance the iron abundance through radiative levitation if the atmosphere is mostly helium instead of hydrogen, because of the decrease in EUV opacity and the increase in the mean molecular weight in the atmosphere.…”

Section: Reddeningmentioning

confidence: 96%

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Brown

Sweigart

Lanz

et al. 2001

ApJ

197

394

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We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we Ðnd a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster u Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core Ñash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium Ñash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "" born again ÏÏ scenario for producing hydrogen-deÐcient stars following a late helium-shell Ñash. The Ñash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB e †ective temperatures at the transition between canonical and Ñash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to conÐrm that a late helium-core Ñash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

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Section: Reddeningmentioning

confidence: 96%

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Brown

Sweigart

Lanz

et al. 2001

ApJ

197

394

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“…One could simply enhance Fe uniformly through the entire stellar envelope (Charpinet et al 1996;Jeffery & Saio 2006a), but then the stellar evolution and pulsations would be unrealistically altered. A more sophisticated treatment assumes a diffusive equilibrium profile of Fe, see Charpinet et al (1997). These authors argued that the equilibrium state is reached on timescales much shorter than the sdB evolutionary timescale (∼10 5 compared to ∼10 8 years).…”

Section: Introductionmentioning

confidence: 99%

Gravitational settling in pulsating subdwarf B stars and their progenitors

Glebbeek

Thoul

et al. 2010

A&A

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Context. Diffusion of atoms can be important during quiescent phases of stellar evolution. Particularly in the very thin inert envelopes of subdwarf B stars, diffusive movements will considerably change the envelope structure and the surface abundances on a short timescale. Also, the subdwarfs will inherit the effects of diffusion in their direct progenitors, namely giants near the tip of the red giant branch. This will influence the global evolution and the pulsational properties of subdwarf B stars. Aims. We investigate the impact of gravitational settling, thermal diffusion and concentration diffusion on the evolution and pulsations of subdwarf B stars. Although radiative levitation is not explicitly calculated, we evaluate its effect by approximating the resulting iron accumulation in the driving region. This allows us to study the excitation of the pulsation modes, albeit in a parametric fashion. Our diffusive stellar models are compared with models evolved without diffusion. Methods. We use a detailed stellar evolution code to solve simultaneously the equations of stellar structure and evolution, including the composition changes due to diffusion. The diffusion calculations are performed for a multicomponent fluid using diffusion coefficients derived from a screened Coulomb potential. We constructed subdwarf B models with a mass of 0.465 M from a 1 M and 3 M zero-age main sequence progenitor. The low mass star ignited helium in an energetic flash, while the intermediate mass star started helium fusion gently. For each progenitor type we computed series with and without atomic diffusion. Results. Atomic diffusion in red giants causes the helium core mass at the onset of helium ignition to be larger. We find an increase of 0.0015 M for the 1 M model and 0.0036 M for the 3 M model. The effects on the red giant surface abundances are small after the first dredge up. The evolutionary tracks of the diffusive subdwarf B models are shifted to lower surface gravities and effective temperatures due to outward diffusion of hydrogen. This affects both the frequencies of the excited modes and the overall frequency spectrum. Especially the structure and pulsations of the post-non-degenerate sdB star are drastically altered, proving that atomic diffusion cannot be ignored in these stars. Sinking of metals could to some extent increase the gravities and temperatures due to the associated decrease in the stellar opacity. However, this effect should be limited as it is counteracted by radiative levitation.

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“…In particular, two types of pulsating sdBs have been identified. V361 Hya stars consist of sdBs with short period photometric modulations (2−10 min), explained as low order (n), low degree ( ) p-modes (Kilkenny et al 1997;Charpinet et al 1997), which coexist with sdBs not observed to vary at the high end of the temperature range for these stars. At the cooler end, a much larger fraction of sdBs seem to be pulsating, but with longer periods (∼1 h) and low amplitudes of a few millimagnitudes at best (Green et al 2003).…”

Section: Introductionmentioning

confidence: 99%

2M0415+0154: a new bright, short-period subdwarf B pulsator

Oreiro

Østensen

Green³

et al. 2009

A&A

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Context. The discovery of stellar oscillations in some subdwarf B stars presents the opportunity of studying the interior structure of these objects, thereby clarifying their possible formation channels. Aims. Our ongoing survey attempts to increase the number of pulsating hot subdwarfs to better understand the complexities of the various subgroups within the population of pulsators and to identify targets suitable for seismological analysis. Methods. Light curves for a list of potential pulsators were obtained at the Nordic Optical Telescope. The candidates were mainly selected based on similar T eff and log g as known pulsators. A frequency analysis of the light curves was performed to investigate the multiperiodic photometric variability.Results. From the sample we surveyed in 2007, 2M0415+0154 was the only star clearly showing photometric modulations. Followup photometry revealed three clear pulsation modes with periods between 143 and 150 s. Additional spectroscopic data was taken on 2M0415+0154 to refine its physical parameters. We find T eff = 34 000 ± 500 K, log g = 5.80 ± 0.05 dex, log (N He /N H ) = −1.60 ± 0.05, which are typical of sdB pulsators in this period range. No evidence of a companion was found. Conclusions. A new short-period sdB star has been discovered, 2M0415+0154. Its physical parameters and oscillation frequencies agree with those of other V361 Hya stars. The relative brightness and simple frequency spectrum of this star make it a suitable target for empirical mode identification, essential for a seismological analysis of simple pulsators.

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A Driving Mechanism for the Newly Discovered Class of Pulsating Subdwarf B Stars

Cited by 300 publications

References 17 publications

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Gravitational settling in pulsating subdwarf B stars and their progenitors

2M0415+0154: a new bright, short-period subdwarf B pulsator

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