The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the Wide Angle Search for Planets (WASP) data base, i.e. an eclipsing binary star in ...which an A-type dwarf star (WASP 1628+10 A) eclipses the remnant of a disrupted red giant star (WASP 1628+10 B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u
′, g
′, r
′) with the ULTRACAM instrument and medium-resolution spectroscopy. The properties of the remnant are well matched by models for stars in a rarely observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low mass white dwarf composed almost entirely of helium, i.e. we confirm that WASP 1628+10 B is a precursor of a helium white dwarf (pre-He-WD). WASP 1628+10 A appears to be a normal A2 V star with a mass of 1.36 ± 0.05 M⊙. By fitting models to the spectrum of this star around the Hγ line we find that it has an effective temperature T
eff, A = 7500 ± 200 K and a metallicity Fe/H = −0.3 ± 0.3. The mass of WASP 1628+10 B is only 0.135 ± 0.02 M⊙. The effective temperature of this pre-He-WD is approximately 9200 K. The ULTRACAM photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles d−1, which is typical for δ Sct-type pulsations often observed in early A-type stars like WASP 1628+10 A. We also observe frequencies near 114 and 129 cycles d−1, much higher than the frequencies normally seen in δ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10 B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.
According to our current cosmological model, galaxies like the Milky Way are expected to experience many mergers over their lifetimes. The most massive of the merging galaxies will be dragged towards ...the disc plane, depositing stars and dark matter into an accreted disc structure. In this work, we utilize the chemodynamical template developed in Ruchti et al. to hunt for accreted stars. We apply the template to a sample of 4675 stars in the third internal data release from the Gaia-ESO Spectroscopic Survey. We find a significant component of accreted halo stars, but find no evidence of an accreted disc component. This suggests that the Milky Way has had a rather quiescent merger history since its disc formed some 8–10 billion years ago and therefore possesses no significant dark matter disc.
Many low-mass and extremely low-mass white-dwarf stars are currently being found in the field of the Milky Way. Motivated by this finding, we present a detailed nonadiabatic pulsation study of such ...stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M, which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Discoveries of additional members of this new class of pulsating stars and their analysis in the context of the theoretical background presented in this paper will shed new light on the evolutionary history of their progenitor stars.
ABSTRACT
We introduce the public version of the BAyesian STellar Algorithm (BASTA), an open-source code written in Python to determine stellar properties based on a set of astrophysical observables. ...BASTA has been specifically designed to robustly combine large data sets that include asteroseismology, spectroscopy, photometry, and astrometry. We describe the large number of asteroseismic observations that can be fit by the code and how these can be combined with atmospheric properties (as well as parallaxes and apparent magnitudes), making it the most complete analysis pipeline available for oscillating main-sequence, subgiant, and red giant stars. BASTA relies on a set of pre-built stellar isochrones or a custom-designed library of stellar tracks, which can be further refined using our interpolation method (both along and across stellar tracks or isochrones). We perform recovery tests with simulated data that reveal levels of accuracy at the few percent level for radii, masses, and ages when individual oscillation frequencies are considered, and show that asteroseismic ages with statistical uncertainties below 10 per cent are within reach if our stellar models are reliable representations of stars. BASTAis extensively documented and includes a suite of examples to support easy adoption and further development by new users.
Context. The overall evolution of low-mass stars is heavily influenced by the processes occurring in the stellar interior. In particular, mixing processes in convectively unstable zones and ...overshooting regions affect the resulting observables and main sequence lifetime. Aims. We aim to study the effects of different convective boundary definitions and mixing prescriptions in convective cores of low-mass stars and to distinguish the existence, size, and evolutionary stage of the central mixed zone by means of asteroseismology. Methods. We implemented the Ledoux criterion for convection in our stellar evolution code, together with a time-dependent diffusive approach for mixing of elements when semiconvective zones are present. We compared models with masses ranging from 1.1 M⊙ to 2 M⊙ computed with two different criteria for convective boundary definition and included different mixing prescriptions within and beyond the formal limits of the convective regions. Using calculations of adiabatic oscillations frequencies for a large set of models, we developed an asteroseismic diagnosis using only l = 0 and l = 1 modes based on the ratios of small to large separations r01 and r10 defined by Roxburgh & Vorontsov (2003, A&A, 411, 215). We analyzed the sensitivity of this seismic tool to the central conditions of the star during the main sequence evolution. Results. The seismic variables r01 and r10 are almost linear in the expected observable frequency range, and we show that their slope depends simultaneously on the central hydrogen content, the extent of the convective core, and the amplitude of the sound-speed discontinuity at the core boundary. By considering about 25 modes and an accuracy in the frequency determinations as expected from the CoRoT and Kepler missions, the technique we propose allows us to detect the presence of a convective core and to discriminate the different sizes of the homogeneously mixed central region without the need for a strong prior knowledge on the stellar mass.
We report the discovery of the likely white dwarf companions to radio millisecond pulsars 47 Tuc Q and 47 Tuc S in the globular cluster 47 Tucanae. These blue stars were found in near-ultraviolet ...images from the Hubble Space Telescope for which we derived accurate absolute astrometry, and are located at positions consistent with the radio coordinates to within 0.016 arcsec (0.2σ). We present near-ultraviolet and optical colours for the previously identified companion to millisecond pulsar 47 Tuc U, and we unambiguously confirm the tentative prior identifications of the optical counterparts to 47 Tuc T and 47 Tuc Y. For the latter, we present its radio-timing solution for the first time. We find that all five near-ultraviolet counterparts have U
300 − B
390 colours that are consistent with He white dwarf cooling models for masses ∼0.16–0.3 M⊙ and cooling ages within ∼0.1–6 Gyr. The Hα − R
625 colours of 47 Tuc U and 47 Tuc T indicate the presence of a strong Hα absorption line, as expected for white dwarfs with an H envelope.
Asteroseismology has the capability of precisely determining stellar properties that would otherwise be inaccessible, such as radii, masses, and thus ages of stars. When coupling this information ...with classical determinations of stellar parameters, such as metallicities, effective temperatures, and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Stromgren survey for Asteroseismology and Galactic Archaeology has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similar to the solar neighborhood. Here we present the first results from a stripe centered at a Galactic longitude of 74degrees and covering latitude from about 8degrees to 20degrees, which includes almost 1000 K giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clumps in a population of field stars with a ratio of 2 to 1 and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalog is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Stromgren survey.
What Is the Neon Abundance of the Sun? Bahcall, John N; Basu, Sarbani; Serenelli, Aldo M
The Astrophysical journal,
10/2005, Volume:
631, Issue:
2
Journal Article
Peer reviewed
Open access
We have evolved a series of 13 complete solar models that utilize different assumed heavy-element compositions. Models that are based upon the heavy-element abundances recently determined by Asplund ...and coworkers are inconsistent with helioseismological measurements. However, models in which the neon abundance is increased by 0.4 -0.5 dex to log N(Ne) = 8.29 c 0.05 on the scale in which log N(H) = 12 are consistent with the helioseismological measurements even though the other heavy-element abundances are in agreement with the determinations of Asplund et al. These results sharpen and strengthen an earlier study by Antia & Basu. The predicted solar neutrino fluxes are affected by the uncertainties in the composition by less than their 1 s theoretical uncertainties.
This paper explores the right-hand side of the forming limit diagram (FLD) for a BCC material in order to test the crystallographic slip assumptions. BCC crystals are considered with either 24 or 48 ...slip systems (BCC24 or BCC48). Identical uniaxial stress responses are assumed in order to compare the predicted FLDs. FLDs are performed using a rate-dependent polycrystal viscoplastic model together with the Marciniak–Kuczynski (M–K) approach. It is verified that the predictions of the limit strains carried out with the full-constraints (MK-FC) model are strongly affected by the selected deformation modes, showing unrealistically high limit strains in balanced-biaxial tension. Much more reliable values are found with the viscoplastic self-consistent (MK-VPSC) approach using either a BCC24 or BCC48 assumptions, enhancing the relevance of the selected transition scale model. Discrepancies between the numerical results, obtained using MK-FC and MK-VPSC, are interpreted in terms of the differences in the active slip systems selected by each model, and consequently, in the predicted lattice rotations and local curvature of the yield locus. Finally, it is found that the calculation of the FLD with MK-VPSC, using 48 slip systems, successfully predicts the right-hand side experimental tendency observed in a low carbon steel sheet metal obtained by bulge test.
PDF
