arXiv subjects

R. H. Østensen

Publications and source records attributed to R. H. Østensen.

40 records · Page 3Linked to original sources

arXiv2010

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

The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for nonradial pulsations. We present our analysis of two sdB stars that are found to be g-mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M-dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer-duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal-to-noise, perhaps revealing lower amplitude periodicities that could confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high-overtone g-mode pulsations.

astro-ph.SR↗

arXiv2010

First Kepler results on compact pulsators I. Survey target selection and the first pulsators

We present results from the first two quarters of a survey to search for pulsations in compact stellar objects with the Kepler spacecraft. The survey sample and the various methods applied in its compilation are described, and spectroscopic observations are presented to separate the objects into accurate classes. From the Kepler photometry we clearly identify nine compact pulsators, and a number of interesting binary stars. Of the pulsators, one shows the strong, rapid pulsations typical for a V361 Hya type sdB variable (sdBV), seven show long-period pulsations characteristic of V1093 Her type sdBVs, and one shows low-amplitude pulsations with both short and long periods. We derive effective temperatures and surface gravities for all the subdwarf B stars in the sample and demonstrate that below the boundary region where hybrid sdB pulsators are found, all our targets are pulsating. For the stars hotter than this boundary temperature a low fraction of strong pulsators (<10 per cent) is confirmed. Interestingly, the short-period pulsator also shows a low-amplitude mode in the long-period region, and several of the V1093 Her pulsators show low amplitude modes in the short-period region, indicating that hybrid behaviour may be common in these stars, also outside the boundary temperature region where hybrid pulsators have hitherto been found.

astro-ph.SR↗

arXiv2010

2M1938+4603: A rich, multimode pulsating sdB star with an eclipsing dM companion observed with Kepler

2M1938+4603 (KIC 9472174) displays a spectacular light curve dominated by a strong reflection effect and rather shallow, grazing eclipses. The orbital period is 0.126 days, the second longest period yet found for an eclipsing sdB+dM, but still close to the minimum 0.1-d period among such systems. The phase-folded light curve was used to detrend the orbital effects from the dataset, and the resulting amplitude spectrum shows a rich collection of pulsation peaks spanning frequencies from ~50 to 4500 uHz. The presence of a complex pulsation spectrum in both the p-mode and the g-mode regions has never been seen before in a compact pulsator. Eclipsing sdB+dM stars are very rare, with only seven systems known and only one with a pulsating primary. Pulsating stars in eclipsing binaries are especially important since they permit masses derived from seismological model fits to be cross checked with orbital mass constraints. We present a first analysis of this star based on the Kepler 9.7-day commissioning light curve and extensive ground-based photometry and spectroscopy that allow us to set useful bounds on the system parameters. We derive a radial-velocity amplitude K_1 = 65.7 +/- 0.6 km/s, inclination angle i = 69.45 +/- 0.20 degrees, and find that the masses of the components are M_1 = 0.48 +/- 0.03 and M_2 = 0.12 +/- 0.01 solar masses.

astro-ph.SR↗

arXiv2010

Spin-resolved spectroscopy of the intermediate polar DQ Her

We present high-speed spectroscopic observations of the intermediate polar DQ Herculis. Doppler tomography of two He I lines reveals a spiral density structure in the accretion disc around the white dwarf primary. The spirals look very similar to the spirals seen in dwarf novae during outburst. DQ Her is the first well established intermediate polar in which spirals are seen, that are in addition likely persistent because of the system's high mass transfer rate. Spiral structures give an alternative explanation for sidebands of the WD spin frequency that are found in IP light curves. The Doppler tomogram of He II 4686 indicates that a large part of the emission is not disc-like. Spin trails of spectra reveal a pulsation in the He II 4686 emission that is believed to result from reprocessing of X-rays from the white dwarf's magnetic poles in the accretion flow close to the WD. We confirm the previous finding that the pulsation is only visible in the red-shifted part of the line when the beam points to the back side of the disc. The absence of reprocessed light from the front side of the disc can be explained by obscuration by the front rim of the disc, but the absence of extra emission from the blue-shifted back side of the disc is puzzling. Reprocessing in accretion curtains can be an answer to the problem and can also explain the highly non-Keplerian velocity components that are found in the He II 4686 line. Our spin trails can form a strong test for future accretion curtain models, with the possibility of distinguishing between a spin period of 71s or 142s. Spin trails of data taken at selected orbital phases show little evidence for a significant contribution of the bright spot to the pulsations and allow us to exclude a recent suggestion that 71s is the beat period and 70.8s the spin period.

astro-ph.SR↗