R. Oreiro

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 of a V361 Hya-type sdB variable (sdBV); seven show long-period pulsation characteristics 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.

An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry

The O9V star HD46202, which is a member of the young open cluster NGC 2244, was observed by the CoRoT satellite in October/November 2008 during a short run of 34 days. From the very high-precision light curve, we clearly detect β Cep-like pulsation frequencies with amplitudes of ∼0.1 mmag and below. A comparison with stellar models was performed using a χ 2 as a measure for the goodness-of-fit between the observed and theoretically computed frequencies. The physical parameters of our best-fitting models are compatible with the ones deduced spectroscopically. A core overshooting parameter αov = 0.10 ± 0.05 pressure scale height is required. None of the observed frequencies are theoretically excited with the input physics used in our study. More theoretical work is thus needed to overcome this shortcoming in how we understand the excitation mechanism of pulsation modes in such a massive star. A similar excitation problem has also been encountered for certain pulsation modes in β Cep stars recently modelled asteroseismically.

First Kepler results on compact pulsators – VI. Targets in the final half of the survey phase

We present results from the final 6 months of a survey to search for pulsations in white dwarfs (WDs) and hot subdwarf stars with the Kepler spacecraft. Spectroscopic observations are used to separate the objects into accurate classes, and we explore the physical parameters of the subdwarf B (sdB) stars and white dwarfs in the sample. From the Kepler photometry and our spectroscopic data, we find that the sample contains five new pulsators of the V1093 Her type, one AMCVn type cataclysmic variable and a number of other binary systems. This completes the survey for compact pulsators with Kepler. No V361 Hya type of short-period pulsating sdB stars were found in this half, leaving us with a total of one single multiperiodic V361 Hya and 13 V1093 Her pulsators for the full survey. Except for the sdB pulsators, no other clearly pulsating hot subdwarfs or white dwarfs were found, although a few low-amplitude candidates still remain. The most interesting targets discovered in this survey will be observed throughout the remainder of the Kepler mission, providing the most long-term photometric data sets ever made on such compact, evolved stars. Asteroseismic investigations of these data sets will be invaluable in revealing the interior structure of these stars and will boost our understanding of their evolutionary history.

A survey for pulsating subdwarf B stars with the Nordic Optical Telescope

Context. A search programme for pulsating subdwarf B stars was conducted with the Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009. Aims. The purpose of the programme was to significantly extend the number of rapidly pulsating sdB stars to better understand the properties of this new group of variable compact stars. Methods. Candidates were selected initially from the HS and HE surveys, but were supplemented with additional objects from other surveys. Short sequences of time-series photometry were made on the candidates to determine the presence of rapid pulsations. Results. In total twenty new pulsators were found in this survey, most of which have already been published and some extensively studied. We present four new short period pulsators, bringing the total of such pulsators up to 49. We also give limits on pulsation amplitudes for 285 objects with no obvious periodic variations, summarise the results of the survey, and provide improved physical parameters on the composite pulsators for which only preliminary estimates were published earlier.

Periodic Mass-loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD 50064

Aims. We aim to interpret the photometric and spectroscopic variability of the luminous blue variable supergiant HD 50064 ( V = 8.21). Methods. CoRoT space photometry and follow-up high-resolution spectroscopy with a time base of 137 d and 169 d, respectively, was gathered, analysed, and interpreted using standard time series analysis and light curve modelling methods, as well as spectral line diagnostics. Results. The space photometry reveals one period of 37 d, which undergoes a sudden amplitude change with a factor 1.6. The pulsation period is confirmed in the spectroscopy, which additionally reveals metal line radial velocity values differing by ∼30 km s ―1 depending on the spectral line and on the epoch. We estimate T eff ∼ 13 500 K, log g ∼ 1.5 from the equivalent width of Si lines. The Balmer lines reveal that the star undergoes episodes of changing mass loss on a time scale similar to the changes in the photometric and spectroscopic variability, with an average value of log M ≃ ―5 (in M ⊙ yr ―1 ). We tentatively interpret the 37 d period as the result of a strange mode oscillation.

AT LAST—A V777 HER PULSATOR IN THE KEPLER FIELD

We present the discovery of the first-and so far the only-pulsating white dwarf star located in the field of view of the Kepler spacecraft. During our ongoing effort to search for compact pulsator candidates that can benefit from the near-continuous coverage of Kepler, we recently identified a faint DB star from spectroscopy obtained with the William Herschel Telescope. After establishing its physical parameters to be T{sub eff} = 24,950 K and log g = 7.91 dex, placing it right in the middle of the V777 Her instability strip, we immediately submitted the target for follow-up space observations. The Kepler light curve reveals a pulsation spectrum consisting of five modes that follow a sequence roughly equally spaced in period with a mean spacing of 37 s. The three strongest modes show a triplet structure with a mean splitting of 3.3 {mu}Hz. We conclude that this object is a V777 Her pulsator with a mass of {approx}0.56 M{sub sun}, and very similar to the class prototype.

Massive unseen companions to hot faint underluminous stars from SDSS (MUCHFUSS) - Analysis of seven close subdwarf B binaries

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M > 1.0 M� ), neutron stars or stellar mass black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they exist in our Galaxy. First results are presented for seven close binary sdBs with short orbital periods ranging from � 0. 21 dt o 1.5 d. The atmospheric parameters of all objects are compatible with core helium-burning stars. The companions are most likely white dwarfs. In one case the companion could be shown to be a white dwarf by the absence of light-curve variations. However, in most cases late type main sequence stars cannot be firmly excluded. Comparing our small sample with the known population of close sdB binaries we show that our target selection method aiming at massive companions is efficient. The minimum companion masses of all binaries in our sample are high compared to the reference sample of known sdB binaries.

Multi-wavelength observations of the GRB 080319B afterglow and the modeling constraints

Context. We present observations of the GRB 080319B afterglow at optical, mm, and radio frequencies between a few hours and 67 days after the burst. Aims. We attempt to understand the nature of this extraordinarily bright explosion based on the observed properties and its comparison with afterglow models. Methods. Our observations and other published multiwavelength data were used to reconstruct the light curves and spectral energy distributions of the burst afterglow. Results. Our results indicate that the observed features of the afterglow agrees equally well with the inter stellar matter and the stellar wind density profiles of the circumburst medium. In the case of both density profiles, the maximum synchrotron frequency nu(m) is below optical value and the cooling break frequency nu(c) is below X-rays, similar to 10(4) s after the burst. The derived value of the Lorentz factor at the time of naked-eye brightness is also similar to 300 for a corresponding blast-wave size of similar to 10(18) cm. Conclusions. The numerical fit to the multiwavelength afterglow data constraints the values of physical parameters and the emission mechanism of the burst.

Time-resolved high-resolution spectroscopy of the bright pulsating subdwarf B star Balloon 090100001

We have obtained, for the first time, a time-resolved high-resolution spectroscopic data set of the bright sdBV Balloon 090100001. In the autumn of 2006 we gathered over 1600 time-resolved high-resolution spectra with the aim to identify the pulsation modes in Balloon 090100001 by studying the line-profile variations. In this paper we present the results obtained from 842 spectra secured in August 2006, aiming to identify the dominant pulsation mode in this star. We phase fold the spectra onto the known dominant period of 2.80746 mHz, to study the abundances and line-broadening as a function of pulsation phase. We find that the abundances do not stand out against those of other subdwarf B stars. Consequently, there is no way to demonstrate that the richness of the frequency spectrum and/or the high pulsation amplitudes are related to abundance effects. The abundances do not change with pulsational phase. The metal lines in Balloon 090100001 are much broader than for non-pulsating subdwarfs, and during the cycle of the main pulsation the line broadening varies with ∼2.5 km s −1 amplitude. Hence, we find clear evidence of pulsational broadening of the lines. We combine the information content of 56 narrow absorption lines through a cross-correlation technique, to create cross-correlation functions that for each individual pulsation phase approximate the average line-profile shape. The resulting profiles have a sufficient signal-to-noise ratio for a mode-identification analysis. From the cross-correlation analysis we find that the pulsation amplitude of the main mode decreased from 19 km s −1 in 2004 to 14.5 km s −1 in 2006. We present, for an sdB star, the first application of pulsation-mode identification based on line-profile variations of lines of heavy elements. To fit the line profiles we consider all modes with degree � ≤ 2 and associated azimuthal order m, and we use a model of the pulsational surface-velocity field. The models with � = 2 do not fit the profiles well, and consequently we can rule out quadrupole modes as the origin of the main pulsation mode in Balloon 090100001. Best fits are obtained from the model of a radial pulsation (� = 0) and the model of a dipole pulsation (� = 1).

Interpreting the line-profile variations of subdwarf B pulsators: the case of PG 1336-018 (NY Virginis)

Aims. We analyze the high-resolution time-resolved VLT/uves spectra of PG 1336−018, a rapidly pulsating subdwarf B (sdB) primary in a close orbit with an M5 companion, with the aim to detect the pulsational signal of the primary in line-profile variations. Methods. After removing the dominant radial-velocity component inherent to the orbital motion and taking only out-of-eclipse data we computed cross-correlation functions (CCFs) for each individual spectrum and assumed these to approximate the average line profile. We computed predictions of line-profile variations for pulsating sdB stars and present their diagnostic value for mode identification. Results. We detect the pulsation mode at 5435 μHz in the line-profile variations of the CCFs which allows us to characterize the detected pulsation mode of PG 1336−018. Conclusions. We model the pulsational perturbations of synthetic spectra for a star with physical parameters like PG 1336−018 and confront them with the observed spectra. We find that low order Balmer lines can be used as diagnostic tool to identify pulsation modes. A detailed line-profile analysis of the perturbed time-series spectra excludes the � = |m| = (3, 3), (1, 0), (2, 1) and (2, 0) modes, but does not allow a unique mode identification among the radial or sectoral dipole or quadrupole modes.