Examinando por Autor "De Burgos, A."

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Acceso Abierto
A detailed non-LTE analysis of LB-1: Revised parameters and surface abundances
(EDP Sciences, 2020-01-31) Simón Díaz, S.; Maíz Apellániz, J.; Lennon, Daniel J.; González Hernández, J. I.; Allende Prieto, C.; Castro, N.; De Burgos, A.; Dufton, P. L.; Herrero, A.; Toledo Padrón, B.; Smartt, S. J.; Agencia Estatal de Investigación (AEI); Ministerio de Economía y Competitividad (MINECO); Science and Technology Facilities Council (STFC); Simón Díaz, S. [0000-0003-1168-3524]; Maíz Apellániz, J. [0000-0003-0825-3443]; Lennon, D. J. [0000-0003-3063-4867]; González Hernández, J. I. [0000-0002-0264-7356]; Castro, N. [0000-0003-0521-473X]; De Burgos, A. [0000-0003-4729-0722]; Herrero, A. [0000-0001-8768-2179]; Toledo Padrón, B. [0000-0002-8194-215X]; Smartt, S. J. [0000-0002-8229-1731]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
Context. It has recently been proposed that LB-1 is a binary system at 4 kpc consisting of a B-type star of 8 M-circle dot and a massive stellar black hole (BH) of 70 M-circle dot. This finding challenges our current theories of massive star evolution and formation of BHs at solar metallicity. Aims. Our objective is to derive the effective temperature, surface gravity, and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH. Methods. We use the non-LTE stellar atmosphere code FASTWIND to analyze new and archival high-resolution data. Results. We determine (T-eff, log g) values of (14& x2006;000 +/- 500 K, 3.50 +/- 0.15 dex) that, combined with the Gaia parallax, imply a spectroscopic mass, from log g, of 3.2(-1.9)(+2.1) M(circle dot)3.2-1.9+2.1M circle dot$ 3.2<^>{+2.1}_{-1.9}\,M_\odot $ and an evolutionary mass, assuming single star evolution, of 5.2(-0.6)(+0.3) M(circle dot)5.2-0.6+0.3M circle dot$ 5.2<^>{+0.3}_{-0.6}\,M_\odot $. We determine an upper limit of 8 km s(-1) for the projected rotational velocity and derive the surface abundances; we find the star to have a silicon abundance below solar, and to be significantly enhanced in nitrogen and iron and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and Gaia yields similar results for T-eff and log g and a consistent distance around 2 kpc. Conclusions. We propose that the B-type star is a slightly evolved main sequence star of 3-5 M-circle dot with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the Gaia parallax. The goodness of fit of the Gaia astrometry also favors a high-inclination orbit. If the orbit is edge-on and the B-type star has a mass of 3-5 M-circle dot, the mass of the dark companion would be 4-5 M-circle dot, which would be easier to explain with our current stellar evolutionary models.
Acceso Abierto
Hubble spectroscopy of LB-1: Comparison with B+black-hole and Be+stripped-star models
(EDP Sciences, 2021-05-14) Lennon, Daniel J.; Maíz Apellániz, J.; Irrgang, A.; Bohlin, R. C.; Deustua, S.; Dufton, P. L.; Simón Díaz, S.; Herrero, A.; Casares, J.; Muñoz Darias, T.; Smartt, S. J.; González Hernández, J. I.; De Burgos, A.; Agencia Estatal de Investigación (AEI); Agencia Canaria de Investigación, Innovación y Sociedad de la Información (ACIISI); Ministerio de Economía y Competitividad (MINECO); Deutsche Forschungsgemeinschaft (DFG); National Aeronautics and Space Administration (NASA); Lennon, D. J. [0000-0003-3063-4867]
Context. LB-1 (alias ALS 8775) has been proposed as either an X-ray dim B-type star plus black hole (B+BH) binary or a Be star plus an inflated stripped star (Be+Bstr) binary. The latter hypothesis contingent upon the detection and characterization of the hidden broad-lined star in a composite optical spectrum. Aims. Our study is aimed at testing the published B+BH (single star) and Be+Bstr (binary star) models using a flux-calibrated UV-optical-IR spectrum. Methods. The Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) was used to obtain a flux-calibrated spectrum with an accuracy of ∼1%. We compared these data with non-local thermal equilibrium (non-LTE) spectral energy distributions (SED) and line profiles for the proposed models. The Hubble data, together with the Gaia EDR3 parallax and a well-determined extinction, were used to provide tight constraints on the properties and stellar luminosities of the LB-1 system. In the case of the Be+Bstr model we adopted the published flux ratio for the Be and Bstr stars, re-determined the Teff of the Bstr using the silicon ionization balance, and inferred Teff for the Be star from the fit to the SED. Results. The UV data strongly constrain the microturbulence velocity to ≲2 km s−1 for the stellar components of both models. We also find stellar parameters consistent with previous results, but with greater precision enabled by the Hubble SED. For the B+BH single-star model, we find the parameters (Teff, log(L/L⊙), Mspec/M⊙) of the B-type star to be (15 300 ± 300 K, 3.23−0.10+0.09, 5.2−1.4+1.8). For the Bstr star we obtain (12 500 ± 100 K, 2.70−0.09+0.09, 0.8−0.3+0.5), and for the Be star (18 900 ± 200 K, 3.04−0.09+0.09, 3.4−1.8+3.5). While the Be+Bstr model is a better fit to the He I lines and cores of the Balmer lines in the optical, the B+BH model provides a better fit to the Si IV resonance lines in the UV. The analysis also implies that the Bstr star has roughly twice the solar silicon abundance, which is difficult to reconcile with a stripped star origin. The Be star, on the other hand, has a rather low luminosity and a spectroscopic mass that is inconsistent with its possible dynamical mass. Conclusions. We provide tight constraints on the stellar luminosities of the Be+Bstr and B+BH models. For the former, the Bstr star appears to be silicon-rich, while the notional Be star appears to be sub-luminous for a classical Be star of its temperature and the predicted UV spectrum is inconsistent with the data. This latter issue can be significantly improved by reducing the Teff and radius of the Be star, at the cost, however, of a different mass ratio as a result. In the B+BH model, the single B-type spectrum is a good match to the UV spectrum. Adopting a mass ratio of 5.1 ± 0.1, from the literature, implies a BH mass of ∼21−8+9 M⊙.
Acceso Abierto
Variability of OB stars from TESS southern Sectors 1–13 and high-resolution IACOB and OWN spectroscopy
(EDP Sciences, 2020-07-13) Burssens, S.; Simón Díaz, S.; Bowman, D. M.; Holgado, G.; Michielsen, M.; De Burgos, A.; Castro, N.; Barbá, R. H.; Aerts, C.; Agencia Estatal de Investigación (AEI); European Research Council (ERC); National Aeronautics and Space Administration (NASA); Holgado, G. [0000-0002-9296-8259]; Bowman, D. [0000-0001-7402-3852]; Burssens, S. [0000-0002-1593-0863]; Aerts, C. [0000-0003-1822-7126]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
Context. The lack of high-precision long-term continuous photometric data for large samples of stars has impeded the large-scale exploration of pulsational variability in the OB star regime. As a result, the candidates for in-depth asteroseismic modelling have remained limited to a few dozen dwarfs. The TESS nominal space mission has surveyed the southern sky, including parts of the galactic plane, yielding continuous data across at least 27 d for hundreds of OB stars. Aims. We aim to couple TESS data in the southern sky with ground-based spectroscopy to study the variability in two dimensions, mass and evolution. We focus mainly on the presence of coherent pulsation modes that may or may not be present in the predicted theoretical instability domains and unravel all frequency behaviour in the amplitude spectra of the TESS data. Methods. We compose a sample of 98 OB-type stars observed by TESS in Sectors 1–13 and with available multi-epoch, high-resolution spectroscopy gathered by the IACOB and OWN surveys. We present the short-cadence 2 min light curves of dozens of OB-type stars, which have one or more spectra in the IACOB or OWN database. Based on these light curves and their Lomb–Scargle periodograms, we performed variability classification and frequency analysis. We placed the stars in the spectroscopic Hertzsprung–Russell diagram to interpret the variability in an evolutionary context. Results. We deduce the diverse origins of the mmag-level variability found in all of the 98 OB stars in the TESS data. We find among the sample several new variable stars, including three hybrid pulsators, three eclipsing binaries, high frequency modes in a Be star, and potential heat-driven pulsations in two Oe stars. Conclusions. We identify stars for which future asteroseismic modelling is possible, provided mode identification is achieved. By comparing the position of the variables to theoretical instability strips, we discuss the current shortcomings in non-adiabatic pulsation theory and the distribution of pulsators in the upper Hertzsprung–Russell diagram.