Observations and asteroseismological analysis of the rapid subdwarf B pulsator EC 09582-1137

We made photometric and spectroscopic observations of the rapidly pulsating subdwarf B star EC 09582-1137 with the aim of determining the target's fundamental structural parameters from asteroseismology. The new data comprise ~ 30 hours of fast time-series photometry obtained with SUSI2 at the NTT on La Silla, Chile, as well as 1 hour of low-resolution spectroscopy gathered with EMMI, also mounted on the NTT. From the photometry we detected 5 independent harmonic oscillations in the 135-170 s period range with amplitudes up to 0.5% of the mean brightness of the star. In addition, we extracted two periodicities interpreted as components of a rotationally split multiplet that indicate a rotation period of the order of 2-5 days. We also recovered the first harmonic of the dominant pulsation, albeit at an amplitude below the imposed 4-sigma detection threshold. The spectroscopic observations led to the following estimates of the atmospheric parameters of EC 09582-1137: Teff = 34,806+-233 K, log g = 5.80+-0.04, and log[N(He)/N(H)] = - 1.68+-0.06. Using the observed oscillations as input, we searched in model parameter space for unique solutions that present a good fit to the data. Under the assumption that the two dominant observed periodicities correspond to radial or dipole modes, we were able to isolate a well-constrained optimal model that agrees with the atmospheric parameters derived from spectroscopy. The inferred structural parameters of EC 09582-1137 are Teff = 34,806 K (from spectroscopy), log g = 5.788+-0.004, M = 0.485+-0.011 M_solar, log(M_env/M_star) = - 4.39+-0.10, R = 0.147+-0.002 R_solar, and L = 28.6+-1.7 L_solar. We additionally derive the absolute magnitude M_V = 4.44+-0.05 and the distance d = 1460+-66 pc.Comment: accepted for publication in Astronomy and Astrophysic

PG 1613+426: a new sdB pulsator

We report the detection of short period oscillations in the hot subdwarf B (sdB) star PG 1613+426 from time-series photometry carried out with the 91-cm Cassegrain telescope of the Catania Astrophysical Observatory. This star, which is brighter than the average of the presently known sdB pulsators, with B = 14.14 mag, has $T_{\rm eff}=34 400 {\rm K}$ and $\log g = 5.97$, its position is near the hot end of the sdB instability strip, and it is a pulsator with a well observed peak in the power spectrum at $144.18\pm 0.06 \rm s$. This star seems to be well suited for high precision measurements, which could detect a possible multi-mode pulsation behaviourComment: 3 pages, 4 figures. to appear on A&

Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338-1925

We obtain time-series spectrophotometry observations at the VLT with the aim of partially identifying the dominant oscillation modes in the rapidly pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude and phase variations. From the data gathered, we detect four previously known pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2 and 2.3 % of the star's mean brightness. We also determine the atmospheric parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K, log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty estimates quoted refer to the formal fitting errors. Finally, we calculate the observed monochromatic amplitudes and phases for the periodicities extracted using least-squares fitting to the light curves obtained for each wavelength bin. These observed quantities are then compared to the corresponding theoretical values computed on the basis of dedicated model atmosphere codes and also taking into account non-adiabatic effects. We find that the quality of the data is sufficient to identify the dominant pulsation at 146.9 s as a radial mode, while two of the lower amplitude periodicities must be low-degree modes with l=0-2. This is the first time that monochromatic amplitudes and phases have been used for mode identification in a subdwarf B star, and the results are highly encouraging.Comment: 11 pages. Accepted for publication in Astronomy & Astrophysic

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

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. First comparisons of asteroseismic values with evolutionary theory look promising, however it is clear that more targets are needed for meaningful statistics to be derived. 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 star's 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 l and radial order k, and infers the structural parameters of the target. 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 star's mean brightness. 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 l=0,1,2 and 4, and match the computed periods with a dispersion of only ~0.26 %.Comment: accepted for publication in A&A, 14 pages, 13 figure

First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode

We present the discovery of nonradial pulsations in a hot subdwarf B star based on 30.5 days of nearly continuous time-series photometry using the \emph{Kepler} spacecraft. KIC 010139564 is found to be a short-period pulsator of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes whose periods range from 130 to 190 seconds. It also shows one periodicity at a period of 3165 seconds. If this periodicity is a high order g-mode, then this star may be the hottest member of the hybrid DW Lyn stars. In addition to the resolved pulsation frequencies, additional periodic variations in the light curve suggest that a significant number of additional pulsation frequencies may be present. The long duration of the run, the extremely high duty cycle, and the well-behaved noise properties allow us to explore the stability of the periodic variations, and to place strong constraints on how many of them are independent stellar oscillation modes. We find that most of the identified periodicities are indeed stable in phase and amplitude, suggesting a rotation period of 2-3 weeks for this star, but further observations are needed to confirm this suspicion.Comment: 10 pages, accepted for publication in MNRA

RAT J0455+1305: A rare hybrid pulsating subdwarf B star

We present results on the second-faintest pulsating subdwarf B (sdB) star known, RAT J0455+1305, derived from photometric data obtained in 2009. It shows both short and long periods oscillations, theoretically assigned as pressure and gravity modes. We identify six short-period frequencies (with one being a combination) and six long-period frequencies. This star is the fourth hybrid sdB star discovered so far which makes it of special interest as each type of mode probes a different part of the star. This star is similar to the sdB hybrid pulsator Balloon 090100001 in that it exhibits short-period mode groupings, which can be used to identify pulsation parameters and constrain theoretical models.Comment: published in MNRA

Testing the forward modeling approach in asteroseismology. I. Seismic solutions for the hot B subdwarf Balloon 090100001 with and without a priori mode identification

Context: Balloon 090100001, the brightest of the known pulsating hot B subdwarfs, exhibits simultaneoulsy both short- and long-period pulsation modes, and shows relatively large amplitudes for its dominant modes. For these reasons, it has been studied extensively over the past few years, including a successful experiment carried out at the Canada-France-Hawaii Telescope to pin down or constrain the value of the degree index ℓ of several pulsation modes through multicolor photometry. Aims: The primary goal of this paper is to take advantage of such partial mode identification to test the robustness of our standard approach to the asteroseismology of pulsating subdwarf B stars. The latter is based on the forward approach whereby a model that best matches the observed periods is searched for in parameter space with no a priori assumption about mode identification. When successful, this method leads to the determination of the global structural parameters of the pulsator. As a bonus, it also leads, after the fact, to complete mode identification. For the first time, with the availability of partial mode identification for Balloon 090100001, we are able to evaluate the sensitivity of the inferred seismic model to possible uncertainty in mode identification. Methods: We carry out a number of exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use the set of ten periods corresponding to the independent pulsation modes for which values of ℓ have been either formally identified or constrained through multicolor photometry in Balloon 090100001. These exercises differ in that they assume different a priori mode identification. Results: Our primary result is that the asteroseismic solution stands very robust, whether or not external constraints on the values of the degree ℓ are used. Although this may come as a small surprise, the test proves to be conclusive, and small differences in mode identification among the ten modes do not affect in any significant way, at the typical accuracy presently achieved, the final emergent seismic model. This is due to the structure of the p-mode pulsation spectra in sdB stars. In all cases, the inferred structural parameters of Balloon 090100001 remain practically unchanged. They correspond, and this constitutes our second important result, to a star beyond the TAEHB with T_eff = 28 000 ± 1 200 K, log g = 5.383 ± 0.004, M⋆/Msun = 0.432 ± 0.015, and log{M_env/M⋆} = -4.89 ± 0.14. Other structural parameters are also derived.Peer reviewe

High-resolution spectropolarimetric observations of hot subdwarfs

We report on high-resolution spectropolarimetric observations of the hot subdwarf stars HD 76431 and Feige 66, using the ESPaDOnS echelle spectropolarimeter at CFHT. We compute cross-correlation Stokes I and V line profiles to enhance the signal-to-noise ratio. We then average all available cross-correlation profiles of each star to further decrease the noise level. Although both targets were previously reported to host kilo-gauss magnetic fields, we do not derive any evidence of large-scale photospheric fields from our sets of observations, in spite of tight error bars on the longitudinal field of the order of 60 gauss for HD 76431 and 200 gauss for Feige 66. A new analysis of FORS1 observations of HD 76431, which provided the basis for the original claim of field detection, confirms the absence of any detectable Zeeman signature, with an error bar of about 100 gauss on the longitudinal magnetic field.Comment: Proceedings of the fifth meeting on hot subdwarf stars and related object