C. Rodríguez-López

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.

Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf

Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this paper, we investigate the radial velocity variations of Kapteyns star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 days using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indicies and archival ASAS-3 photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyns star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seems a remarkable feat given the peculiar origin and kinematic history of Kapteyns star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.

The γ Doradus CoRoT target HD 49434: I. Results from the ground-based campaign

Context. We present an extensive ground-based photometric and spectroscopic campaign of the γ Dor CoRoT target HD 49434. This campaign was a preparatory step of the CoRoT satellite observations, which occurred between October 2007 and March 2008. Aims. With satellite data, detection of low-degree pulsation modes only is achievable, and, as no filters are available, with poor identification. Ground-based data promise eventually to identify additional modes and provide extra input for the identification: spectroscopic data allows the detection of high-degree modes and an estimate of the azimuthal number m. We attempt to detect and identify as many pulsation modes as possible from the ground-based dataset of the γ Dor star HD 49434, and anticipate the CoRoT results. Methods. We searched for frequencies in the multi-colour variations, the pixel-to-pixel variations across the line profiles, and the moments variations in a large dataset, consisting of both multi-colour photometric and spectroscopic data from different observatories, using different frequency analysis methods. We performed a tentative mode identification of the spectroscopic frequencies using the Moment Method and the Intensity Period Search Method. We also completed an abundance analysis. Results. The frequency analysis clearly indicates the presence of four frequencies in the 0.2−1.7 d −1 interval, as well as six frequencies in the 5−12 d −1 domain. The low frequencies are typical of γ Dor variables, while the high frequencies are common to δ Sct pulsators. We propose that the frequency 2.666 d −1 is the rotational frequency. All modes, for which an identification was possible, appear to be high-degree modes (3 ≤ � ≤ 8). We did not find evidence for a possible binary nature of the star HD 49434. The element abundances that we derived are consistent with values obtained in previous analyses. Conclusions. We classify the γ Dor star HD 49434 as a hybrid pulsator, which pulsates simultaneously in p -a ndg-modes. This implies that HD 49434 is an extremely interesting target for asteroseismic modelling.

SEISMOLOGY OF β CEPHEI STARS: DIFFERENTIALLY ROTATING MODELS FOR INTERPRETING THE OSCILLATION SPECTRUM OF ν ERIDANI

A method for the asteroseismic analysis of β Cephei stars is presented and applied to the star ν Eridani. The method is based on the analysis of rotational splittings and their asymmetries using differentially rotating asteroseismic models. Models with masses around 7.13 M ☉, and ages around 14.9 Myr, were found to fit better in 10 of the 14 observed frequencies, which were identified as the fundamental radial mode and the three l = 1 triplets g1, p1, and p2. The splittings and aymmetries found for these modes recover those provided in the literature, except for p2. For this last mode, all its non-axysimmetric components are predicted by the models. Moreover, opposite signs of the observed and predicted splitting asymmetries are found. If identification is confirmed, this can be a very interesting source of information about the internal rotation profile, in particular in the outer regions of the star. In general, the seismic models that include a description for shellular rotation yield slightly better results as compared with those given by uniformly rotating models. Furthermore, we show that asymmetries are quite dependent on the overshooting of the convective core, which makes the present technique suitable for testing the theories describing the angular momentum redistribution and chemical mixing due to rotationally induced turbulence.

The pulsating hot subdwarf Balloon 090100001: results of the 2005 multisite campaign

We present the results of a multisite photometric campaign on the pulsating B-type hot subdwarf star Balloon090100001 (Bal09). The star is one of the two known hybrid hot subdwarfs with both long- and short-period oscillations, theoretically attributed to g and p modes. The campaign involved eight telescopes with three obtaining UBVR data, four B-band data and one Stromgren uvby photometry. The campaign covered 48 nights, providing a temporal resolution of 0.36μHz with a detection threshold of about 0.2mmag in B-filter data. Bal09 has the richest pulsation spectrum of any known pulsating subdwarf B star, and our analysis detected 114 frequencies including 97 independent and 17 combination ones. Most of the 24 g-mode frequencies are between 0.1 and 0.4mHz. Of the remaining 73, presumably p modes, 72 group into four distinct regions near 2.8, 3.8, 4.7 and 5.5mHz. The density of frequencies requires that some modes must have degrees l larger than 2. The modes in the 2.8 mHz region have the largest amplitudes. The strongest mode (f1) is most likely radial, while the remaining ones in this region form two nearly symmetric multiplets: a triplet and quintuplet, attributed to rotationally split l = 1 and 2 modes, respectively. We find clear increases of splitting in both multiplets between the 2004 and 2005 observing campaigns, amounting to ~15 per cent on average. The observed splittings imply that the rotational rate in Bal09 depends on stellar latitude and is the fastest on the equator. We also speculate on the possible reasons for the changes of splitting. The only plausible explanation we find is torsional oscillation. This hypothesis, however, needs to be verified in the future by detailed modelling. In this context, it is very important to monitor the splittings on a longer time-scale as their behaviour may help to explain this interesting phenomenon. The amplitudes of almost all terms detected in both 2004 and 2005 were found to vary. This is evident even during one season; for example, amplitudes of modes f8 and fC were found to change by a factor of 2-3 within about 50 d during 2005. We use a small grid of models to constrain the main mode (f1), which most likely represents the radial fundamental pulsation. The groups of p-mode frequencies appear to lie in the vicinity of the consecutive radial overtones, up to the third one. Despite the large number of g-mode frequencies observed, we failed to identify them, most likely because of the disruption of asymptotic behaviour by mode trapping. The observed frequencies were not, however, fully exploited in terms of seismic analysis which should be done in the future with a larger grid of reliable evolutionary models of hot subdwarfs.

Pulsations in M dwarf stars

ABSTRACT We present the results of the first theoretical non-radial non-adiabatic pulsationalstudy of M dwarf stellar models with masses in the range 0.1 to 0.5 M⊙. We find thefundamental radial mode to be unstable due to an ǫ mechanism, caused by deuterium(D-) burning for the young 0.1 and 0.2 M⊙models, by non-equilibrium He 3 burningfor the 0.2 and 0.25 M⊙models of 10 4 Myr, and by a flux blocking mechanism for thepartially convective 0.4 and 0.5 M⊙models once they reach the age of 500 Myr. Theperiods of the overstable modes excited by the D-burning are in the range 4.2 to 5.2 hfor the 0.1 M⊙models and is of order 8.4 h for the 0.2 M⊙models. The periods of themodes excited by He 3 burning and flux blocking are in the range 23 to 40 min. Themore massive and oldest models are more promising for the observational detection ofpulsations, as their ratio of instability e-folding time to age is more favourable.Key words: stars: oscillations. 1 INTRODUCTIONM dwarf stars are cold, low-mass stars on the main sequencethat have recently gained attention as being the best can-didates to harbour Earth-like planets (Delfosse et al. 2006).Although they make up about 75% of the stars in our Galaxyand the solar neighbourhood, their main physical parame-ters, such as their radii, are poorly understood, and hard todetermine observationally due to their low luminosities.Evidence has been accumulating for discrepancies be-tween theoretically predicted radii and effective tempera-tures of low mass stars (LMS), and those derived from obser-vational data. Observational studies of LMS in eclipsing bi-naries and single LMS have shown that their radii are about4 to 20% larger, and their temperatures as much as 5% lowerthan predicted by theoretical models (see e.g. Morales et al.2009 and references therein). This discrepancy is usually at-tributed to effects associated with chromospheric activity ormetallicity (see e.g. L´opez-Morales 2007).Another reported problem is age discrepancy: althoughmean ages derived from lithium isochrones for LMS in younggroups agree with those derived from Hertzsprung-Russelldiagram (HRD) isochrones, the two methods often give dis-cordant ages when applied to the individual stars in theyoung groups (see e.g. Yee & Jensen 2010 -YJ10).Magnetic inhibition of convection provides a possible so-lution to both problems. This leads to models having larger

NO EVIDENCE for ACTIVITY CORRELATIONS in the RADIAL VELOCITIES of KAPTEYN'S STAR

This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. IOP Publishing is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The version of record is available online at: https://doi.org/10.3847/0004-637X/830/2/74

HD 172189: another step in furnishing one of the best laboratories known for asteroseismic studies

HD 172189 is a spectroscopic eclipsing binary system with a rapidly-rotating pulsating δ Scuti component. It is also a member of the open cluster IC 4756. These combined characteristics make it an excellent laboratory for asteroseismic studies. To date, HD 172189 has been analysed in detail photometrically but not spectroscopically. For this reason we have compiled a set of spectroscopic data to determine the absolute and atmospheric parameters of the components. We determined the radial velocities (RV) of both components using four different techniques. We disentangled the binary spectra using KOREL, and performed the first abundance analysis on both disentangled spectra. By combining the spectroscopic results and the photometric data, we obtained the component masses, 1.8 and 1.7 Mȯ, and radii, 4.0 and 2.4 Rȯ, for inclination i = 73.2°, eccentricity e = 0.28, and orbital period Π = 5.70198 days. Effective temperatures of 7600 K and 8100 K were also determined. The measured v sin i are 78 and 74 km s-1, respectively, giving rotational periods of 2.50 and 1.55 days for the components. The abundance analysis shows [Fe/H] = -0.28 for the primary (pulsating) star, consistent with observations of IC 4756. We also present an assessment of the different analysis techniques used to obtain the RVs and the global parameters.

Detection of a multishell planetary nebula around the hot subdwarf O-type star 2MASS J19310888+4324577

Context. The origin of hot subdwarf O-type stars (sdOs) remains unclear since their discovery in 1947. Among others, a post- Asymptotic Giant Branch (post-AGB) origin is possible for a fraction of sdOs. Aims. We are involved in a comprehensive ongoing study to search for and to analyze planetary nebulae (PNe) around sdOs with the aim of establishing the fraction and properties of sdOs with a post-AGB origin. Methods. We use deep Hand (Oiii) images of sdOs to detect nebular emission and intermediate resolution, long-slit optical spec- troscopy of the detected nebulae and their sdO central stars. These data are complemented with other observations (archive images, high-resolution, long-slit spectroscopy) for further ana lysis of the detected nebulae. Results. We report the detection of an extremely faint, complex PN around 2MASS J19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system. The PN shows a bipolar and an elliptical shell, whose major axes are oriented perpendi cular to each other, and high-excitation structures outside the two shells. WISE archive images show faint, extended emission at 12µm and 22µm in the inner nebular regions. The internal nebular kinematics, derived from high resolution, long-slit spectra, is co nsistent with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main axis mainly perpendicular to the line of sight. The nebular spectrum only exhibits H�, Hand (Oiii)��4959,5007 emission lines, but suggests a very low-excitation ((Oiii)/H�≃1.5), in strong contrast with the absence of low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow, ionized helium absorp- tions that confirm the previous sdO classification and sugges t an effective temperature≥60000 K. The binary nature of 2M1931+4324, its association with a complex PN, and several properties of the system provide strong support for the idea that binary central stars are a crucial ingredient in the formation of complex PNe.

Has a Star Enough Energy to Excite the Thousand of Modes Observed with CoRoT

The recent analyses of the light curves provided by CoRoT have revealed pulsation spectra of unprecedented richness and precision-in particular, thousands of pulsating modes and a clear distribution of amplitudes with frequency. In the community, some scientists have started doubting the validity of the classical tools to analyze these very accurate light curves. This work provides the asteroseismic community with answers to this question showing that (1) it is physically possible for a star to excite at a time and with the observed amplitudes such a large number of modes; and (2) that the kinetic energy accumulated in all those modes does not destroy the equilibrium of the star. Consequently, mathematical tools presently applied to the analyses of light curves can a priori be trusted. This conclusion is even more important now, when a large amount of space data coming from Kepler is currently being analyzed. The power spectrum of different stellar cases and the non-adiabatic code GraCo have been used to estimate the upper limit of the energy per second required to excite all the observed modes and their total kinetic energy. A necessary previous step for this study is to infer the relative radial pulsational amplitude from the observed photometric amplitude, scaling our linear pulsational solutions to absolute values. The derived upper limits for the required pulsational energy were compared with (1) the luminosity of the star; and (2) the gravitational energy. We found that both upper energy limits are orders of magnitude smaller.