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

in Astronomy and Astrophysics (2009), 507

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 ... [more ▼]

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. This analysis forms part of a long-term programme geared towards distinguishing between different proposed formation scenarios for hot B subdwarfs on the basis of their internal characteristics. So far, secure asteroseismic solutions have been computed for 9 of these pulsators, and first comparisons with results from evolutionary calculations look promising. The new data comprise 30 h of fast time-series photometry obtained with SUSI2 at the NTT on La Silla, Chile, as well as 1 h 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 4σ detection threshold. The spectroscopic observations led to the following estimates of the atmospheric parameters of EC 09582-1137: T_eff = 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 observed oscillations 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 0.57%. Non-adiabatic calculations reveal all theoretical modes in the observed period range to be unstable, an important a posteriori consistency check for the validity of the optimal model. The inferred structural parameters of EC 09582-1137 are T_eff = 34 806 K (from spectroscopy), log{g} = 5.788±0.004, Mast = 0.485±0.011 Msun, log{(M_env/Mast)} = -4.39±0.10, R = 0.147±0.002 Rsun, and L = 28.6±1.7 Lsun. We additionally derive the absolute magnitude MV = 4.44±0.05 and the distance d = 1460±66 pc. [less ▲]

Progress in the asteroseismic analysis of the pulsating sdB star PG 1605+072

Conference (2009, July)

The hot pulsating sdB star PG 1605+072 exhibits uncommon spectroscopic and pulsation prop- erties, and is one of the biggest challenge in the field of sdB star modeling. Two hypotheses have been proposed ... [more ▼]

The hot pulsating sdB star PG 1605+072 exhibits uncommon spectroscopic and pulsation prop- erties, and is one of the biggest challenge in the field of sdB star modeling. Two hypotheses have been proposed to explain its unusually rich pulsation spectrum. The first is the natural explanation of a fast-rotating pulsator, which lifts the (2l+1)-fold degeneracy of the frequency components. Another approach, where PG 1605+072 can be seen as a slow rotator, considers that the numerous low amplitude frequency components are second- and third-order harmonics and nonlinear com- binations of the highest amplitude frequencies. We investigated the two hypotheses in the light of asteroseismology, using our latest tools - including pulsation codes that incorporate star rotation and new generation complete sdB models. The results of both approaches are presented, showing interesting results and raising new questions for our understanding of this mysterious sdB star. [less ▲]

Internal dynamics from asteroseismology for two sdB pulsators in close binary systems

in Communications in Asteroseismology (2008, December), 157

Since their discovery eleven years ago, short-period pulsating sdB stars have proved their potential for quantitative asteroseismological studies. We have recently updated our astero- seismic diagnostic ... [more ▼]

Since their discovery eleven years ago, short-period pulsating sdB stars have proved their potential for quantitative asteroseismological studies. We have recently updated our astero- seismic diagnostic tools in order to incorporate the effects of stellar rotation on pulsations, assuming various internal rotation laws. It is possible, with these new tools, to determine the internal rotation profile of two short-period pulsating sdB stars residing in close binary systems, namely Feige 48 and PG 1336 -018. They exhibit orbital periods of 9.024 h and 2.424 h respectively, as measured from spectroscopy. For the two stars, we show that spin-orbit synchronism is reached from the surface down to ˜ 0.22 R∗ and 0.55 RxE2x88x97 , respectively. The rotation of deeper layers cannot be inferred with the type of modes p-modes observed in short-period pulsating sdB stars. These results can potentially provide new elements to test tidal friction theories, particularly the angular momentum transport, in close binary systems. [less ▲]

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

in Astronomy and Astrophysics (2008), 489

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 ... [more ▼]

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 one. The latter indicates a mass of Mast = 0.466±0.006 Msun and a radius of Rast = 0.15±0.01 Rsun for the sdB star. In comparison, our seismic analysis, combined to high-quality time-averaged spectroscopy, leads to the following estimates of the basic structural parameters of the sdB component: Mast = 0.459±0.005 Msun, Rast = 0.151±0.001 Rsun, log g = 5.739±0.002, Teff = 32 740 ± 400 K, and log(Menv/Mast) = -4.54±0.07. We also find strong evidence that the sdB star has reached spin-orbit synchronism and rotates as a solid body down to at least r ~ 0.55 Rast. We further estimate that higher-order perturbation effects due to rotation and tidal deformation of the star are insufficient to alter in a significant way the proposed asteroseismic solution itself (i.e., the derived structural parameters and rotation properties). Future efforts to improve further the accuracy of the seismic models will clearly have to incorporate such effects, however. Conclusions: We conclude that our approach to the asteroseismology of sdB stars has passed a fundamental test with this analysis of PG 1336-018. The structural parameters and inferences about the internal dynamics of this star derived in the present paper through this approach should rest on very solid grounds. More generally, our results underline the power and usefulness of the forward modeling method in asteroseismology, despite historical misgivings about it. [less ▲]

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

in Astronomy and Astrophysics (2008), 488

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 ... [more ▼]

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. [less ▲]

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

in Astronomy and Astrophysics (2008), 483

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 ... [more ▼]

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, l, 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. [less ▲]

An Asteroseismological Study of the Rapidly Pulsating Subdwarf B Star PG 0911+456

in Leaflet - Astronomical Society of the Pacific (2008, January), 392

We present the analysis and interpretation of 57 hours of time-series photometry obtained with the new Mont4kccd on Mt. Bigelow, Arizona, for the rapidly pulsating sdB star PG 0911+456. Thanks to the ... [more ▼]

We present the analysis and interpretation of 57 hours of time-series photometry obtained with the new Mont4kccd on Mt. Bigelow, Arizona, for the rapidly pulsating sdB star PG 0911+456. Thanks to the impressively high S/N of the data we were able to extract 7 independent periodicities in the 140-200 s range with amplitudes down to 0.05 % of the mean stellar brightness. On the basis of these we performed an asteroseismological search for the optimal model and derived the star's fundamental parameters. Given that there are now 12 sdB stars for which asteroseismology has been performed, the results are starting to become significant from a statistical point of view and evolutionary trends are slowly emerging. [less ▲]

Constraints on Seismic Models of Balloon 090100001 through High-Sensitivity Multicolour Photometry

in Leaflet - Astronomical Society of the Pacific (2008, January), 392

We report on an analysis of high S/N ratio UBV multicolour photometry of the bright pulsating sdB star Balloon 090100001 gathered at the Canada-France-Hawaii Telescope with the help of the Montreal 3 ... [more ▼]

We report on an analysis of high S/N ratio UBV multicolour photometry of the bright pulsating sdB star Balloon 090100001 gathered at the Canada-France-Hawaii Telescope with the help of the Montreal 3-channel photometer LAPOUNE. Through an application of the color-amplitude method described in Randall et al. (2005), we were able to identify the degree index ℓ of several individual pulsation modes. These identifications provide extremely useful constraints on possible seismic models for Balloon 090100001. [less ▲]

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

in Astronomy and Astrophysics (2007), 476

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 ... [more ▼]

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 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. 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 star's 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 ℓ = 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/Msun = 0.39 ± 0.01, log{M_env/Mast} = -4.69 ± 0.07, R/Rsun = 0.133 ± 0.001 and L/Lsun = 16.4 ± 0.8. We also derive the absolute magnitude MV = 4.82 ± 0.04 and a distance d = 930.3 ± 27.4 pc. [less ▲]

Elaboration du modèle thermique du télescope SWAP à bord du satellite PROBA-2

Master's dissertation (2005)