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Register a new userTowards asteroseismology of the non-radial pulsating sdB star PG 1605+072
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S. Falter
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The recently discovered new class of sdB pulsators (sdBV) offers a powerful possibility for the investigation of their interior and thus their evolutionary history. The first step towards applying asteroseismologic tools is the identification of pulsation modes. We reoport on simultaneous spectroscopic and multi-band photometric time series observations of PG 1605+072 and analyse its radial velocity and light curve.
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Three members of the new class of pulsating sdB stars (sdBV or EC 14026 stars) are analysed from Keck HIRES spectra using line blanketed NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, He/H), metal abundances and rotational velocities are determined. As is typical for sdB stars, all programme stars are found to be helium deficient, with a He abundance ranging from 1/80 solar for Feige 48 to 1/3 solar for PG 1219+534, probably due to diffusion. Most metals are also depleted. The abundances of C, O, Ne, Mg, Al and Si in the high gravity programme stars KPD 2109+4401 and PG 1219+534 are considerably lower than in the lower gravity stars Feige 48 and PG 1605+072 which could be explained by an equilibrium between gravitational settling and radiative levitation. Surprisingly iron is solar to within error limits in all programme stars irrespective of their gravity, confirming predictions from diffusion calculations. The metal lines are very sharp and allow the microturbulent velocity to be constrained to be lower than 5km/s (KPD2109+4401, PG 1219+534). Also the projected rotational velocities have to be very low (vrot sini<10km/s). For Feige 48 the limits are even tighter (vmicro<=3km/s, vrot sini<=5km/s).
PG 0014+067 is one of the most promising pulsating subdwarf B stars for seismic analysis, as it has a rich pulsation spectrum. The richness of its pulsations, however, poses a fundamental challenge to understanding the pulsations of these stars, as the mode density is too complex to be explained only with radial and nonradial low degree (l < 3) p-modes without rotational splittings. One proposed solution, for the case of PG 0014+067 in particular, assigns some modes with high degree (l=3). On the other hand, theoretical models of sdB stars suggest that they may retain rapidly rotating cores, and so the high mode density may result from the presence of a few rotationally-split triplet (l=1), quintuplet (l=2) modes, along with radial (l=0) p-modes. To examine alternative theoretical models for these stars, we need better frequency resolution and denser longitude coverage. Therefore, we observed this star with the Whole Earth Telescope for two weeks in October 2004. In this paper we report the results of Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067. We find that the frequencies seen in PG 0014+067 do not appear to fit any theoretical model currently available; however, we find a simple empirical relation that is able to match all of the well-determined frequencies in this star.
Four members of the new class of pulsating sdB stars are analysed from Keck HIRES spectra using NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, log(He/H)), metal abundances and rotational velocities are determined. Balmer line fitting is found to be consistent with the helium ionization equilibrium for PG1605+072 but not so for PG1219+534 indicating that systematic errors in the model atmosphere analysis of the latter have been underestimated previously. All stars are found to be helium deficient probably due to diffusion. The metals are also depleted with the notable exception of iron which is solar to within error limits in all four stars, confirming predictions from diffusion calculations of Charpinet et al. (1997). While three of them are slow rotators (vsini < 10km/s), PG1605+072 displays considerable rotation (vsini = 39km/s, P<8.7h) and is predicted to evolve into an unusually fast rotating white dwarf. This nicely confirms a prediction by Kawaler (1999) who deduced a rotation velocity of 130km/s from the power spectrum of the pulsations which implies a low inclination angle of the rotation axis.
We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC 6791. We confirm that B4 is a reflection effect binary with an sdB component and a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s to 7643 s. Several of the pulsation modes show symmetric splitting by 0.62 microHz. Attributing this to rotational splitting, we conclude that the sdB component has a rotation period of approximately 9.63 d, indicating that tidal synchronization has not been achieved in this system. Comparison with theoretical synchronization time provides a discriminant between various theoretical models.
At present, a large number of pulsating white dwarf (WD) stars is being discovered either from Earth-based surveys such as the Sloan Digital Sky Survey, or through observations from space (e.g., the Kepler mission). The asteroseismological techniques allow us to infer details of internal chemical stratification, the total mass, and even the stellar rotation profile. In this paper, we first describe the basic properties of WD stars and their pulsations, as well as the different sub-types of these variables known so far. Subsequently, we describe some recent findings about pulsating low-mass WDs.
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