Seven-Period Asteroseismic Fit of the Kepler DBV

We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the $\ell$ and $\rm{m}$ identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relatively hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling. At least a few of the modes appear to be stable on evolutionary timescales and could allow us to obtain a measurement of the rate of cooling with monitoring of the star over the course of the next few years with ground-based follow-up.Comment: 24 pages, 4 figures. 2 tables. Published fall 2014 in the Astrophysical Journa

Three new pulsating sdB stars discovered with SuperWASP

We present an analysis of three new pulsating subdwarf B stars discovered in the Super Wide Angle Search for Planets archive. Two of the stars, J1938+5609 and J0902−0720, are p- mode pulsators; J1938+5609 shows a pulsation at 231.62 d−1 (P = 373 s; 2681 µHz) with an amplitude of 4 mmag, whereas J0902−0720 pulsates at frequencies 636.74 (P = 136 s; 7370 µHz) and 615.34 d−1 (P = 140 s; 7122 µHz), with amplitudes 7.27 and 1.53 mmag, respectively. The third star, J2344−3427, is a hybrid pulsator with a p-mode frequency at 223.16 d−1 (P = 387 s; 2583 µHz) and a corresponding amplitude of 1.5 mmag, and g modes in the frequency range 8.68–28.56 d−1 (P = 3025–9954 s; 100–331 µHz) and amplitudes between 0.76 and 1.17 mmag. Spectroscopic results place J1938+5609 and J2344−3427 among the long-period or hybrid pulsators, suggesting there may be further modes in these stars below our detection limits, with J0902−0720 placed firmly amongst the p-mode pulsators

Deep asteroseismic sounding of the compact hot B subdwarf pulsator KIC02697388 from Kepler time series photometry

peer reviewedContext. Contemporary high precision photometry from space provided by the Kepler and CoRoT satellites generates significant breakthroughs in terms of exploiting the long-period, g-mode pulsating hot B subdwarf (sdBVs) stars with asteroseismology. Aims: We present a detailed asteroseismic study of the sdBVs star KIC02697388 monitored with Kepler, using the rich pulsation spectrum uncovered during the ~27-day-long exploratory run Q2.3. Methods: We analyse new high-S/N spectroscopy of KIC02697388 using appropriate NLTE model atmospheres to provide accurate atmospheric parameters for this star. We also reanalyse the Kepler light curve using standard prewhitening techniques. On this basis, we apply a forward modelling technique using our latest generation of sdB models. The simultaneous match of the independent periods observed in KIC02697388 with those of models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of some of the parameters of the star. Results: The light curve analysis reveals 43 independent frequencies that can be associated with oscillation modes. All the modulations observed in this star correspond to g-mode pulsations except one high-frequency signal, which is typical of a p-mode oscillation. Although the presence of this p-mode is surprising considering the atmospheric parameters that we derive for this cool sdB star (Teff = 25 395 ± 227 K, log g = 5.500 ± 0.031 (cgs), and log N(He) /N(H) = -2.767 ± 0.122), we show that this mode can be accounted for particularly well by our optimal seismic models, both in terms of frequency match and nonadiabatic properties. The seismic analysis leads us to identify two model solutions that can both account for the observed pulsation properties of KIC02697388. Despite this remaining ambiguity, several key parameters of the star can be derived with stringent constraints, such as its mass, its H-rich envelope mass, its radius, and its luminosity. We derive the properties of the core proposing that it is a relatively young sdB star that has burnt less than ~34% (in mass) of its central helium and has a relatively large mixed He/C/O core. This latter measurement is in line with the trend already uncovered for two other g-mode sdB pulsators analysed with asteroseismology and suggests that extra mixing is occurring quite early in the evolution of He cores on the horizontal branch. Conclusions: Additional monitoring with Kepler of this particularly interesting sdB star should reveal the inner properties of KIC02697388 and provide important information about the mode driving mechanism and the helium core properties

A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi

After the initial discoveries fifteen years ago, over 200 extrasolar planets have now been detected. Most of them orbit main-sequence stars similar to our Sun, although a few planets orbiting red giant stars have been recently found. When the hydrogen in their cores runs out, main-sequence stars undergo an expansion into red-giant stars. This expansion can modify the orbits of planets and can easily reach and engulf the inner planets. The same will happen to the planets of our Solar System in about five billion years and the fate of the Earth is matter of debate. Here we report the discovery of a planetary-mass body (Msini = 3.2M_(Jupiter)) orbiting the star V 391 Pegasi at a distance of about 1.7 astronomical units (au), with a period of 3.2 years. This star is on the extreme horizontal branch of the Hertzsprung–Russell diagram, burning helium in its core and pulsating. The maximum radius of the red-giant precursor of V 391 Pegasi may have reached 0.7 au, while the orbital distance of the planet during the stellar main-sequence phase is estimated to be about 1 au. This detection of a planet orbiting a post-red-giant star demonstrates that planets with orbital distances of less than 2 au can survive the red-giant expansion of their parent stars

A small survey of UV-bright stars around the northern ecliptic pole: seeking new p-mode sdB variables for the TESS mission

status: publishe

The blue-edge problem of the V1093 Herculis instability strip revisited using evolutionary models with atomic diffusion

We have computed a new grid of evolutionary subdwarf B star (sdB) models from the start of central He burning, taking into account atomic diffusion due to radiative levitation, gravitational settling, concentration diffusion, and thermal diffusion. We have computed the non-adiabatic pulsation properties of the models and present the predicted p-mode and g-mode instability strips. In previous studies of the sdB instability strips, artificial abundance enhancements of Fe and Ni were introduced in the pulsation driving layers. In our models, the abundance enhancements of Fe and Ni occur naturally, eradicating the need to use artificial enhancements. We find that the abundance increases of Fe and Ni were previously underestimated and show that the instability strip predicted by our simulations solves the so-called blue edge problem of the subdwarf B star g-mode instability strip. The hottest known g-mode pulsator, KIC 10139564, now resides well within the instability strip even when only modes with low spherical degrees (l ≤ 2) are considered.7 pages, 7 figures. Accepted for publication in Astronomy & Astrophysicsstatus: publishe

KIC 7668647: a 14 day beaming sdB plus WD binary with a pulsating subdwarf

The recently discovered subdwarf B (sdB) pulsator KIC 7668647 is one of the 18 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars in general. We use new ground-based low-resolution spectroscopy, and the near-continuous 2.88 year Kepler light curve, to reveal that KIC 7668647 consists of a subdwarf B star with an unseen white-dwarf companion with an orbital period of 14.2 d. An orbit with a radial-velocity amplitude of 39 km s-1 is consistently determined from the spectra, from the orbital Doppler beaming seen by Kepler at 163 ppm, and from measuring the orbital light-travel delay of 27 s by timing of the many pulsations seen in the Kepler light curve. The white dwarf has a minimum mass of 0.40 M⊙. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, and find that nitrogen and iron have abundances close to solar values, while helium, carbon, oxygen and silicon are underabundant relative to the solar mixture. We use the full Kepler Q06-Q17 light curve to extract 132 significant pulsation frequencies. Period-spacing relations and multiplet splittings allow us to identify the modal degree ℓ for the majority of the modes. Using theg-mode multiplet splittings we constrain the internal rotation period at the base of the envelope to 46-48 d as a first seismic result for this star. The few p-mode splittings may point at a slightly longer rotation period further out in the envelope of the star. From mode-visibility considerations we derive that the inclination of the rotation axis of the sdB in KIC 7668647 must be around ~60°. Furthermore, we find strong evidence for a few multiplets indicative of degree 3 ≤ ℓ ≤ 8, which is another novelty in sdB-star observations made possible by Kepler. Based on observations obtained by the Kepler spacecraft, the Nordic Optical Telescope and the William Herschel TelescopeAppendix A is available in electronic form at http://www.aanda.orgarXiv admin note: text overlap with arXiv:1206.3872status: publishe

ASTEROSEISMIC FINGERPRINTS OF ROTATION AND MIXING IN THE SLOWLY PULSATING B8 V STAR KIC 7760680

We present the first detection of a rotationally affected series consisting of 36 consecutive high-order sectoral dipole gravity modes in a slowly pulsating B (SPB) star. The results are based on the analysis of four years of virtually uninterrupted photometric data assembled with the Kepler Mission, and high-resolution spectra acquired using the HERMES spectrograph at the 1.2 m Mercator Telescope. The specroscopic measurements place KIC 7760680 inside the SPB instability strip, near the cool edge, given its fundamental parameters of {{T}eff}=11650+/- 210 K, log g=3.97+/- 0.08 dex, microturbulent velocity {{xi }t}=0.0-0.0+0.6 km {{s}-1}, vsin i=61.5+/- 5.0 km {{s}-1}, and [M/H]=0.14+/- 0.09 dex. The photometric analysis reveals the longest unambiguous series of gravity modes of the same degree \ell with consecutive radial order n, which carries clear signatures of chemical mixing and rotation. With such exceptional observational constraints, this star should be considered as the Rosetta stone of SPBs for future modeling, and brings us a step closer to the much-needed seismic calibration of stellar structure models of massive stars.6 pages, 4 figures, 1 table; accepted for publication in ApJ Letters, one reference updated with arxiv identifierstatus: publishe

Influence of restricted FOV and CCD binning in SH-WFS on the performance of NAOMI

Using a unique combination of empirical data collected simultaneously by the science camera (INGRID) and the wave front sensor in NAOMI plus the same night profiles of the turbulent layers measured by SLODAR, we discuss the accuracy of the analytic approach to modelling of AO performance. The WFS frames recorded for different atmospheric conditions allow us to make a detailed investigation of the influence of a restricted field of view and sampling of the WFS on the accuracy of the centre of gravity and its propagation to the residual variance. The predictions of Strehl, FWHM and FWHE derived for NAOMI-INGRID using our analytic approach are compared with on-sky performance demonstrated during the commissioning and science observations with NAOMI