Discovery of a New Class of Pulsating Stars: Gravity-Mode Pulsators among Subdwarf B Stars

Green, E. M.; Fontaine, G.; Reed, M. D.; Callerame, K.; Seitenzahl, Ivo R.; White, Benjamin Thomas; Hyde, E. A.; Oestensen, R. H.; Cordes, O.; Brassard, P.; Falter, S.; Jeffery, Elizabeth; Schuh, S.; Giovanni, M. K.; Edelmann, H.; Rigby, J. Keith; Bronowska, A.

doi:10.1007/978-94-017-0799-2_10

Cited by 2 publications

(3 citation statements)

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“…Long-period variables constitute a newer, less extensively studied class of subdwarf B pulsator. Their variability was first announced by Betsy Green at the "Asteroseismology across the H-R diagram" conference held in Porto in July 2002 (Green et al 2003a; see also Green et al 2003b). Slowly pulsating sdB stars are distinctly cooler than their short period counterparts and show multi-periodic, low-amplitude (∼ 1 milli-magnitude) luminosity variations with typical periods between 0.8 and 1.6 hours.…”

Section: Introductionmentioning

confidence: 98%

“…These are about a factor of 30 longer than those of the EC 14026 stars, and automatically imply high radial order gravity modes (g-modes). Early ideas on a possible excitation mechanism for this new class of star included tidal excitation in a close binary system (Fontaine et al 2003a) and a mechanism involving a slow reviving of the hydrogen shell (Green et al 2003a), but proved unfruitful. A much more promising mechanism capable of qualitatively explaining the observed periods was brought forward more recently (Fontaine et al 2003b) and involves the same driving process that so successfully explains the instabilities in the EC 14026 stars.…”

Section: Introductionmentioning

confidence: 99%

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The Potential of Multicolor Photometry for Pulsating Subdwarf B Stars

Randall

Fontaine

Brassard

et al. 2005

ASTROPHYS J SUPPL S

121

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We investigate the potential of multicolor photometry for partial mode identification in both long-and shortperiod variable subdwarf B stars. The technique presented is based on the fact that the frequency dependence of an oscillation's amplitude and phase bears the signature of the mode's degree index l, among other things. Unknown contributing factors can be eliminated through the evaluation of the amplitude ratios and phase differences arising from the brightness variation in different wavebands, theoretically enabling the inference of the degree index from observations in two or more bandpasses. Employing a designated model atmosphere code, we calculate the brightness variation expected across the visible disk during a pulsation cycle in terms of temperature, radius, and surface gravity perturbations to the emergent flux for representative EC 14026 and PG 1716 star models. Nonadiabatic effects are considered in detail and found to be significant from nonadiabatic pulsation calculations applied to our state-of-the-art models of subdwarf B stars. Our results indicate that the brightness variations observed in subdwarf B stars are caused primarily by changes in temperature and radius, with surface gravity perturbations playing a small role. For PG 1716 stars, temperature effects dominate in the limit of long periods with the result that the oscillatory amplitudes and phases lose their period dependence and nonadiabatic effects become unimportant. Outside this regime, however, their values are strongly influenced by both factors. We find that the phase shifts between brightness variations in different wavebands are generally small but may lie above the experimental detection threshold in certain cases. The prospect of mode discrimination seems much more promising on the basis of the corresponding amplitude ratios. While in EC 14026 stars the amplitude ratios predicted are very similar for modes with l ¼ 0, 1, or 2, they are well separated from those of modes with l ¼ 3, l ¼ 5, and l ¼ 4 or 6, each of which form a distinct group. For the case of the PG 1716 stars it should be possible to discriminate between modes with l ¼ 1, 2, 4, or 6 and those of degree indices l ¼ 3 and l ¼ 5. Identifying modes within a given group is challenging for both types of pulsator and requires multicolor photometry of extremely high quality. Nevertheless, we demonstrate that it is feasible using the example of the largest amplitude peak detected for the fast pulsator KPD 2109+4401 by Jeffery et al.

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Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

The Potential of Multicolor Photometry for Pulsating Subdwarf B Stars

Randall

Fontaine

Brassard

et al. 2005

ASTROPHYS J SUPPL S

121

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“…The prototype p-mode pulsator EC 14026-2647 (or V361 Hya) was discovered by Kilkenny et al (1997). Later, Green et al (2003) discovered the g-mode prototype PG 1716+426 (or V1093 Her) and Schuh et al (2006) the hybrid prototype HS 0702+6043 (or DW Lyn). For both mode types, the driving mechanism is believed to be the κ-mechanism (Charpinet et al, 1997;Fontaine et al, 2003).…”

Section: Sdbv Starsmentioning

confidence: 99%

The search for substellar companions to subdwarf B stars in connection with evolutionary aspects

Lutz¹

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The formation and evolution of hot subluminous B stars (sdB stars in the following) is not entirely understood. Although the current evolutionary status, i. e. the position in the Hertzsprung-Russell diagram, can be characterized and interpreted quite well, the preceding evolution of these objects remains uncertain. Since sdB stars do not follow a standard evolution of low mass stars, there must have been some event or phase in their history which led to a departure of the standard description. It is well-established in the literature that this event is connected to an enhanced mass loss of the sdB progenitor star during its red giant phase. This enhanced mass loss is a prerequisite to explain the very existence of sdB stars, its cause is, however, matter of debate. In the case of subdwarf B stars in binary systems, formation scenarios invoking Roche-lobe overflow and common envelope evolution are most promising. Theory and observations agree quite well in this case. However, there are some issues concerning single sdB stars, i. e. such stars which are not part of a binary system and therefore do not have a stellar companion. Such single sdB stars are an integral part of this thesis. One particular hypothesis discussed in the literature predicts that even substellar companions like brown dwarfs or exoplanets may be able to have a decisive influence on the formation of single sdB stars. A major goal of this present thesis is to investigate this hypothesis by searching for substellar companions to subdwarf B stars. Another goal is to pin down the evolutionary status of the investigated sdB stars and to directly measure their evolutionary timescales. The precise purposes are hence the search for substellar companions and the determination of evolutionary timescales. To achieve the goals mentioned above, the so-called EXOTIME program has been set up. EXOTIME conducts a long-term monitoring of five pulsating sdB stars with means of ground based time-resolved relative photometry. xi I will motivate the basic questions and intentions which led to the idea of setting up this thesis and to investigate the subdwarf B stars with respect to substellar companions. Chapter three is intended to introduce the purpose and goals of the EXOTIME project and to briefly characterize the target stars central to this thesis. Subsequently, in chapter four I will present the data archive built up during the last years, with a special emphasis on the observation facilities and observational strategy. Chapter five provides a description of the methods being applied to investigate the subdwarf B stars for substellar companions. A focus is set on the O-C timing method and a comparison to various other exoplanet detection methods. The results of the analysis are extensively presented in chapter six. Chapter seven will discuss these results, put them into a broader context and address various other implications and applications. A summary and outlook, as provided in chapter eight, will conclude this thesis. xiv -Miles Kington Welcome to Kington...

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Discovery of a New Class of Pulsating Stars: Gravity-Mode Pulsators among Subdwarf B Stars

Cited by 2 publications

References 4 publications

The Potential of Multicolor Photometry for Pulsating Subdwarf B Stars

The Potential of Multicolor Photometry for Pulsating Subdwarf B Stars

The search for substellar companions to subdwarf B stars in connection with evolutionary aspects

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