Valérie Van Grootel

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.

Kepler observations of the beaming binary KPD 1946+4340

The Kepler Mission has acquired 33.5 d of continuous 1-min photometry of KPD 1946+4340, a short-period binary system that consists of a subdwarf B star (sdB) and a white dwarf. In the light curve, eclipses are clearly seen, with the deepest occurring when the compact white dwarf crosses the disc of the sdB (0.4 per cent) and the more shallow ones (0.1 per cent) when the sdB eclipses the white dwarf. As expected, the sdB is deformed by the gravitational field of the white dwarf, which produces an ellipsoidal modulation of the light curve. Spectacularly, a very strong Doppler beaming (also known as Doppler boosting) effect is also clearly evident at the 0.1 per cent level. This originates from the sdB’s orbital velocity, which we measure to be 164.0 ± 1. 9k m s −1 from supporting spectroscopy. We present light-curve models that account for all these effects, as well as gravitational lensing, which decreases the apparent radius of the white dwarf by about 6 per cent, when it eclipses the sdB. We derive system parameters and uncertainties from the light curve using Markov chain Monte Carlo simulations. Adopting a theoretical white dwarf mass–radius relation, the mass of the subdwarf is found ,

EARLY ASTEROSEISMIC RESULTS FROM KEPLER: STRUCTURAL AND CORE PARAMETERS OF THE HOT B SUBDWARF KPD 1943+4058 AS INFERRED FROM g-MODE OSCILLATIONS

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time that g-mode seismology can be exploited quantitatively for stars on the extreme horizontal branch, all previous successful seismic analyses having been confined so far to short-period, p-mode pulsators. We demonstrate that current models of hot B subdwarfs can quite well explain the observed g-mode periods, while being consistent with independent constraints provided by spectroscopy. We identify the 18 pulsations retained in our analysis as low-degree (l = 1 and 2), intermediate-order (k = –9 through –58) g-modes. The periods (frequencies) are recovered, on average, at the 0.22% level, which is comparable to the best results obtained for p-mode pulsators. We infer the following structural and core parameters for KPD 1943+4058 (formal fitting uncertainties only): T eff = 28,050 ± 470 K, log g = 5.52 ± 0.03, M * = 0.496 ± 0.002 M ☉, log (M env/M *) = –2.55 ± 0.07, log (1 – M core/M *) = –0.37 ± 0.01, and X core(C+O) = 0.261 ± 0.008. We additionally derive the age of the star since the zero-age extended horizontal branch 18.4 ± 1.0 Myr, the radius R = 0.203 ± 0.007 R ☉, the luminosity L = 22.9 ± 3.13 L ☉, the absolute magnitude MV = 4.21 ± 0.11, the reddening index E(B – V) = 0.094 ± 0.017, and the distance d = 1180 ± 95 pc.

The radius and mass of the close solar twin 18 Scorpii derived from asteroseismology and interferometry

The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interferometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation 134.4 ± 0.3 μHz and angular and linear radiuses

Testing the forward modeling approach in asteroseismology II. Structure and internal dynamics of the hot B subdwarf component in the close eclipsing binary system PG 1336-018

Aims. We present a stringent test on the forward modeling technique in asteroseismology by confronting the predictions of a detailed seismic analysis of the pulsating subdwarf component in the unique close eclipsing binary system PG 1336−018 with those derived independently from modeling the binary light curve of the system. We also take advantage of the observed rotationally-split rich period spectrum to investigate the internal dynamics of the pulsating component in this system expected to be tidally locked. Methods. We carry out numerical exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use a recently updated version that now incorporates the effects of stellar rotation on the pulsation properties. We thus search in parameter space for the optimal model that objectively leads to the best simultaneous match of the 25 periods (including rotationally-split components) observed in PG 1336−018. For the first time, we also attempt to precisely reconstruct the internal rotation profile of the pulsator from its oscillations. Results. Our principal result is that our seismic model, which closely reproduces the observed periods, is remarkably consistent with one of the best-fitting possible solutions uncovered independently from the binary light curve analysis, in effect pointing to the correct

The rapid rotation and complex magnetic field geometry of Vega

Context. The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields. Aims. We aim to confirm the field detection reported by Lignieres et al. (2009, A&A, 500, L41) and provide additional observational constraints about the field characteristics, by modelling the large-scale magnetic geometry of the star and by investigating a possible seasonal variability of the reconstructed field topology. Methods. We analyse a total of 799 high-resolution circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. Using about 1100 spectral lines, we employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20 000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. Results. We confirm the detection of circularly polarized signatures in the mean line profiles. The signal shows up in four independent data sets acquired with both NARVAL and ESPaDOnS. The amplitude of the polarized signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 ± 0.008 d. The reconstruction of the magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. Conclusions. The repeated observational evidence that Vega possesses a weak photospheric magnetic field strongly suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars now investigatable with the current generation of stellar spectropolarimeters.

A polarity reversal in the large-scale magnetic field of the rapidly rotating sun HD 190771

Aims. We investigate the long-term evolution of the large-scale photospheric magnetic field geometry of the solar-type star HD 190771. With fundamental parameters very close to those of the Sun except for a shorter rotation period of 8.8 d, HD 190771 provides us with a first insight into the specific impact of the rotation rate in the dynamo generation of magnetic fields in 1 Mstars. Methods. We use circularly polarized, high-resolution spectra obtained with the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute cross-correlation line profiles with high signal-to-noise ratio to detect polarized Zeeman signatures. From three phase-resolved data sets collected during the summers of 2007, 2008, and 2009, we model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler imaging and follow its temporal evolution. Results. The comparison of the magnetic maps shows that a polarity reversal of the axisymmetric component of the large-scale mag- netic field occurred between 2007 and 2008, this evolution being observed in both the poloidal and toroidal magnetic components. Between 2008 and 2009, another type of global evolution occured, characterized by a sharp decrease of the fraction of magnetic energy stored in the toroidal component. These changes were not accompanied by significant evolution in the total photospheric magnetic energy. Using our spectra to perform radial velocity measurements, we also detect a very low-mass stellar companion to HD 190771.

Asteroseismology in action: a test of spin-orbit synchronism in the close binary system Feige 48

Aims. In this paper, we provide a test of spin-orbit synchronism in the close binary system Feige 48. This system is made of a rapidly pulsating subdwarf B (sdB) star with an unseen companion, most likely a white dwarf. The presence of nonradial oscillations offers the opportunity to infer the inner profile and period of rotation of the primary star through asteroseismology. This constitutes the key element for testing spin-orbit synchronism in depth, since stellar internal layers are inaccessible to traditional techniques. Methods. We carried out a new asteroseismic analysis of Feige 48 following the so-called “forward modeling” approach. This is done with our latest optimization algorithms, which have been updated to incorporate the effect of stellar rotation on pulsations, assuming various internal rotation laws. In this analysis, the simultaneous match of all the frequencies observed in Feige 48 leads objectively to the full identification of the pulsation modes through the determination of the indices k, � , m. It also leads to determining the structural and rotational parameters of Feige 48. Results. Our optimal model, obtained with a solid-body rotation law, is characterized by a spin period of 9.028 ± 0.480 h. This value is remarkably similar to the system’s orbital period of 9.024 ± 0.072 h, measured independently from radial velocity variations. We further demonstrate that the hypothesis of differential rotation of the core – including a fast rotating core – must be eliminated for Feige 48. Conclusions. These results strongly imply that Feige 48 rotates as a solid body in a tidally locked system. This constitutes the first explicit demonstration of spin-orbit synchronism in a binary star by asteroseismic means.

Structural and core parameters of the hot B subdwarf KPD 0629-0016 from CoRoT g-mode asteroseismology

Context. The asteroseismic exploitation of long period, g-mode hot B subdwarf pulsators (sdBV s ), undermined so far by limitations associated with ground-based observations, has now become possible, thanks to high quality data obtained from space such as those recently gathered with the CoRoT (COnvection, ROtation, and planetary Transits) satellite. Aims. We propose a detailed seismic analysis of the sdBV s star KPD 0629-0016, the first compact pulsator monitored with CoRoT, using the g-mode pulsations recently uncovered by that space-borne observatory during short run SRa03. Methods. We use a forward modeling approach on the basis of our latest sdB models, which are now suitable for the accurate computation of the g-mode pulsation properties. The simultaneous match of the independent periods observed in KPD 0629-0016 with those of the models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of the structural and core parameters of the star. Results. The optimal model we found closely reproduces the 18 observed periods retained in our analysis at a 0.23% level on average. These are identified as low-degree (l = 1 and 2), intermediate-order (k = -9 through -74) g-modes. The structural and core parameters for KPD 0629-0016 are the following (formal fitting errors only): T eff = 26290 ± 530 K, log g = 5.450 ± 0.034, M * = 0.471 ± 0.002 M ⊙ , log (M env /M * ) = ―2.42 ± 0.07, log (1 ― M core /M * ) = ―0.27 ± 0.01, and X core (C+O) = 0.41 ± 0.01. We additionally derive an age of 42.6 ± 1.0 Myr after the zero-age extreme horizontal branch, the radius R = 0.214 ± 0.009 R ⊙ , the luminosity L = 19.7 ± 3.2 L ⊙ , the absolute magnitude M v = 4.23 ± 0.13, the reddening index E(B - V) = 0.128 ± 0.023, and the distance d = 1190 ± 115 pc. Conclusions. The advent of high-precision time-series photometry from space with instruments like CoRoT now allows as demonstrated with KPD 0629-0016 the full exploitation of g-modes as deep probes of the internal structure of these stars, in particular for determining the mass of the convective core and its chemical composition.

Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456

Aims.The principal aim of this project is to determine the structural parameters of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our work forms part of an ongoing programme to constrain the internal characteristics of hot B subdwarfs with the long-term goal of differentiating between the various formation scenarios proposed for these objects. So far, a detailed asteroseismic interpretation has been carried out for 6 such pulsators, with apparent success. First comparisons with evolutionary theory look promising, however it is clear that more targets are needed for meaningful statistics to be derived. Methods: The observational pulsation periods of PG 0911+456 were extracted from rapid time-series photometry using standard Fourier analysis techniques. Supplemented by spectroscopic estimates of the stars mean atmospheric parameters, they were used as a basis for the ?forward modelling? approach in asteroseismology. The latter culminates in the identification of one or more ?optimal? models that can accurately reproduce the observed period spectrum. This naturally leads to an identification of the oscillations detected in terms of degree ? and radial order k, and infers the structural parameters of the target. Results: The high S/N low- and medium resolution spectroscopy obtained led to a refinement of the atmospheric parameters for PG 0911+456, the derived values being T_eff = 31 940 ± 220 K, log g = 5.767 ± 0.029, and log He/H = -2.548 ± 0.058. From the photometry it was possible to extract 7 independent pulsation periods in the 150-200 s range with amplitudes between 0.05 and 0.8% of the stars mean brightness. There was no indication of fine frequency splitting over the 68-day time baseline, suggesting a very slow rotation rate. An asteroseismic search of parameter space identified several models that matched the observed properties of PG 0911+456 well, one of which was isolated as the ?optimal? model on the basis of spectroscopic and mode identification considerations. All the observed pulsations are identified with low-order acoustic modes with degree indices ? = 0,1,2 and 4, and match the computed periods with a dispersion of only 0.26%, typical of the asteroseismological studies carried out to date for this type of star. The inferred structural parameters of PG 0911+456 are T_eff = 31 940 ± 220 K (from spectroscopy), log {g} = 5.777 ± 0.002, Mast/M? = 0.39 ± 0.01, log{M_env/Mast} = -4.69 ± 0.07, R/R? = 0.133 ± 0.001 and L/L? = 16.4 ± 0.8. We also derive the absolute magnitude MV = 4.82 ± 0.04 and a distance d = 930.3 ± 27.4 pc. This study made extensive use of the computing facilities offered by the Calcul en Midi-Pyrenees (CALMIP) project and the Centre Informatique National de lEnseignement Superieur (CINES), France. Some of the spectroscopic observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution.