173 research outputs found
Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV–14°116 revisited
No full textInternational audienceHot subdwarf stars of spectral types O and B represent a poorly understood phase in the evolution of low-mass stars, in particular of close compact binaries. A variety of phenomena are observed, which make them important tools for several astronomical disciplines. For instance, the richness of oscillations of many subdwarfs are important for asteroseismology. Furthermore, hot subdwarfs are among the most chemically peculiar stars known. Two intermediate He-rich hot subdwarf stars, LS IV-14°116 and Feige 46, are particularly interesting, because they show extreme enrichments of heavy elements such as Ge, Sr, Y, and Zr, which are strikingly similar in both stars. In addition, both stars show light oscillations at periods incompatible with standard pulsation theory and form the class of V366 Aqr variables. We investigated whether the similar chemical compositions extend to more complete abundance patterns in both stars and validate the pulsations in Feige 46 using its recent TESS light curve. High-resolution optical and near-ultraviolet spectroscopy are combined with non-local thermodynamical-equilibrium model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC to consistently determine detailed metal abundance patterns in both stars. Many previously unidentified lines were identified for the first time with transitions originating from Ga III, Ge III-IV, Se III, Kr III, Sr II-III, Y III, Zr III-IV, and Sn IV, most of which have not yet been observed in any star. The abundance patterns of 19 metals in both stars are almost identical, light metals being only slightly more abundant in Feige 46, while Zr, Sn, and Pb are slightly less enhanced compared to LS IV-14°116. Both abundance patterns are distinctively different from those of normal He-poor hot subdwarfs of a similar temperature. The extreme enrichment in heavy metals of more than 4 dex compared to the Sun is likely the result of strong atmospheric diffusion processes that operate similarly in both stars while their similar patterns of C, N, O, and Ne abundances might provide clues to their as yet unclear evolutionary history. Finally, we find that the periods of the pulsation modes in Feige 46 are stable to better than Ṗ ≲ 10-8 s s-1. This is not compatible with Ṗ predicted for pulsations driven by the ɛ-mechanism and excited by helium-shell flashes in a star that is evolving, for example, onto the extended horizontal branch
The Arizona-Montréal Spectroscopic Survey of hot subluminous stars
No full textContext . Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45-0.5 M ⊙ range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population. Aims . Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios. Methods . We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature ( T eff ), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the Gaia parallax was used to compute the stellar parameters radius, luminosity, and mass. Results . Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with T eff > 14 kK. We detected the presence of helium stratification in six sdB stars with T eff around 30 kK, making them good candidates for also showing 3 He enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (log g-T eff ) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 M ⊙ , consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 M ⊙ . In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 M ⊙ . This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass (<0.45 M ⊙ ) sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p - and g -mode instability regions
The asteroseismic analysis of the pulsating sdB Feige 48 revisited
No full textpeer reviewedWe present a new detailed analysis of the short period pulsating sdB star Feige 48, based on the same observations than in our previous work (Charpinet et al., 2005, A&A 443, 251) but with our new codes including the rotation of the star. Feige 48 is supposed to be a relatively fast rotator, showing a fine structure in its pulsation spectrum about 28 µHz on three groups in the 9 frequencies. On the other side, Feige 48 belongs to a close binary system (likely with a white dwarf) where the orbital period is 9.02 +/- 0.03h (S. O'Toole 2004). With our new codes including the rotation (for a given rotation law, we calculate associated multiplets for each frequency with the perturbative approach on the first order), avoiding us to make assumptions about the m=0 modes, we were able to fit all the 9 periods together, leading to sligthly different estimations for the structural parameters of Feige 48 than in our previous work. Moreover, the rotation of the star is found to be solid with a period of 32,540s 9.038h, very close to
the orbital period of the system determined by spectroscopy, confirming the reasonable assumption that the system is tidally locked. In this context, it was possible to exclude that Feige 48 has a fast rotating core in term of much poorer merit functions
Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC 10001893
No full textpeer reviewe
Finding Compact Hot Subdwarf Binaries in the Galactic Disc
No full textContains fulltext :
127861.pdf (Author’s version preprint ) (Closed access
A Target or the Next XCov23 Campaign: KPD 1930+2752
No full textAbstractKPD 1930+2752 is a pulsating subdwarf B star with a particularly rich period spectrum, likely due to significant rotational splitting. It is also a short period (~ 2h17m), close binary system (as revealed by the presence of an ellipsoidal variation in the light curve) containing an unseen companion - almost certainly a white dwarf. Radial velocity measurements indicate that the total binary mass exceeds the Chandrasekhar limit. KPD 1930+2752 is therefore the first known (non-X-ray source) candidate for a Type Ia supernova (SNe Ia) progenitor that should explode within an astrophysically interesting timescale (~ 200 Myr) after the two stars merge due to gravitational wave radiation. Additionally, the tidal distortions are indicative of the strong tidal force incurred on the pulsating star by its companion. This will allow us to test the effects such strong tidal forces have on pulsations. Hence, this object presents an outstanding astrophysical interest, and deserves to be observed with the WET network to fully resolve the complex pulsation pattern of the pulsating sdB star, as well as the details of the ellipsoidal modulation. The asteroseismological analysis of the pulsation data obtained with the WET, along with a detailed analysis of the folded, high S/N ellipsoidal variation will lead to strong physical constraints on this particularly interesting stellar object.</jats:p
A Target or the Next XCov23 Campaign: KPD 1930+2752
No full textKPD 1930+2752 is a pulsating subdwarf B star with a particularly rich period spectrum, likely due to significant rotational splitting. It is also a short period (~ 2h17m), close binary system (as revealed by the presence of an ellipsoidal variation in the light curve) containing an unseen companion - almost certainly a white dwarf. Radial velocity measurements indicate that the total binary mass exceeds the Chandrasekhar limit. KPD 1930+2752 is therefore the first known (non-X-ray source) candidate for a Type Ia supernova (SNe Ia) progenitor that should explode within an astrophysically interesting timescale (~ 200 Myr) after the two stars merge due to gravitational wave radiation. Additionally, the tidal distortions are indicative of the strong tidal force incurred on the pulsating star by its companion. This will allow us to test the effects such strong tidal forces have on pulsations. Hence, this object presents an outstanding astrophysical interest, and deserves to be observed with the WET network to fully resolve the complex pulsation pattern of the pulsating sdB star, as well as the details of the ellipsoidal modulation. The asteroseismological analysis of the pulsation data obtained with the WET, along with a detailed analysis of the folded, high S/N ellipsoidal variation will lead to strong physical constraints on this particularly interesting stellar object
On the Origin of the Small-Frequency Spacings Found in the Pulsation Spectra of Hot B Subdwarf Stars
No full textWe examine in this paper the nature of the intriguing small-frequency spacings that have been reported in the pulsation spectra of rapidly pulsating hot B subdwarf stars. It has been said that these mysterious and puzzling spacings should not be present if current models of these stars and their pulsations are any guide to reality. They lead to highly accurate fits to observed frequency data and have been suggested to contain valuable (but still encoded) information about the inner structure of sdB stars. The empirical relation that leads to such high-accuracy fits involves a zero-point frequency, two small-frequency spacings (a factor of 15-30 smaller than the so-called large-frequency spacing for acoustic modes in the asymptotic regime), and two sets of integers, all optimized to best match the observed frequencies. After investigating its true nature, we have to report that this empirical relation contains, in fact, no physical information and is a purely numerical artifact. In particular, we show that the two best-fit frequency spacings used in the proposed relation correspond basically to the largest common denominators between the real frequency spacings in the observed sequence: one acting on a coarse scale and the other on a finer scale. As common denominators, the numerical values of these spacings depend sensitively on the signal-to-noise ratio (S/N) of the observations and cannot therefore represent intrinsic properties of a pulsating star. We suggest that, for the time being, progress on the front of the asteroseismology of sdB stars might still best be done with the help of physical models such as those pioneered by Brassard et al
Ten years of asteroseismic modelling of pulsating B subdwarf stars: achievements, challenges, and prospects. Veröffentlichungen der Kommission für Astronomie|Communications in Asteroseismology|Communications in Asteroseismology 150 150|
No full textInter-comparison of the g-, f- and p-modes calculated using different oscillation codes for a given stellar model
No full textIn order to make asteroseismology a powerful tool to explore stellar interiors, different numerical codes should give the same oscillation frequencies for the same input physics. Any differences found when comparing the numerical values of the eigenfrequencies will be an important piece of information regarding the numerical structure of the code. The ESTA group was created to analyze the non-physical sources of these differences. The work presented in this report is a part of Task 2 of the ESTA group. Basically the work is devoted to test, compare and, if needed, optimize the seismic codes used to calculate the eigenfrequencies to be finally compared with observations. The first step in this comparison is presented here. The oscillation codes of nine research groups in the field have been used in this study. The same physics has been imposed for all the codes in order to isolate the non-physical dependence of any possible difference. Two equilibrium models with different grids, 2172 and 4042 mesh points, have been used, and the latter model includes an explicit modelling of semiconvection just outside the convective core. Comparing the results for these two models illustrates the effect of the number of mesh points and their distribution in particularly critical parts of the model, such as the steep composition gradient outside the convective core. A comprehensive study of the frequency differences found for the different codes is given as well. These differences are mainly due to the use of different numerical integration schemes. The number of mesh points and their distribution are crucial for interpreting the results. The use of a second-order integration scheme plus a Richardson extrapolation provides similar results to a fourth-order integration scheme. The proper numerical description of the Brunt-Väisälä frequency in the equilibrium model is also critical for some modes. This influence depends on the set of the eigenfunctions used for the solution of the differential equations. An unexpected result of this study is the high sensitivity of the frequency differences to the inconsistent use of values of the gravitational constant (G) in the oscillation codes, within the range of the experimentally determined ones, which differ from the value used to compute the equilibrium model. This effect can provide differences for a given equilibrium model substantially larger than those resulting from the use of different codes or numerical techniques; the actual differences between the values of G used by the different codes account for much of the frequency differences found here. © 2007 Springer Science+Business Media B.V
- …
