Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Abstract

We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK$+$81 266, and LS II$+$18 9, among which the first three are used as standard stars for flux calibration. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 63 000 K and log g in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S, are depleted. The stars have very similar abundances, although AGK$+$81 266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s${}^{-1}$. The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and Hipparcos distances are in good agreement for our three brightest stars.

arXiv commentAccepted for publication in Astronomy & Astrophysics

References in corpus (38)

• The chemical composition of the Sun
• Validation of the new Hipparcos reduction
• Correcting for the Effects of Interstellar Extinction
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
• A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars
• The origin of sdB stars (II)
• The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K
• Ultraviolet Radiation from Evolved Stellar Populations -- I. Models
• Hot subluminous stars
• The binary fraction of extreme horizontal branch stars
• The Origin of Extreme Horizontal Branch Stars
• A new catalogue of Stroemgren-Crawford uvbybeta photometry
• A selection of hot subluminous stars in the GALEX survey II. Subdwarf atmospheric parameters
• Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars
• Modeling He-rich subdwarfs through the hot-flasher Scenario
• Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy
• A Spitzer White Dwarf Infrared Survey
• Radial velocity measurements of white dwarfs
• High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216
• The deuterium-to-oxygen ratio in the interstellar medium
• The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project
• Hot subdwarf stars in close-up view - III. Metal abundances of subdwarf B stars
• The Gaia spectrophotometric standard stars survey. I. Preliminary results
• Hot subdwarf stars in close-up view - II. Rotational properties of single and wide binary subdwarf B stars
• Discovery of a variable lead-rich hot subdwarf: UVO 0825+15
• A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars
• Stellar wind models of subluminous hot stars
• The rapid evolution of the exciting star of the Stingray Nebula
• The Gaia spectrophotometric standard stars survey - III. Short-term variability monitoring
• On the sdOB primary of the post common-envelope binary AA Doradus (LB 3459)
• The magnetic fields of hot subdwarf stars
• A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum
• A NLTE analysis of the hot subdwarf O star Bd+28 4211. I. The UV spectrum
• The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. II. NLTE spectral analysis of the OB-type subdwarf Feige 110
• FUSE spectroscopy of sdOB primary of the post common-envelope binary LB 3459 (AA Dor)
• Three new X-ray emitting sdO stars discovered with Chandra
• EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited

Cited by in corpus (14)

• The population of hot subdwarf stars studied with Gaia -- IV. Catalogues of hot subluminous stars based on Gaia EDR3
• Hot subdwarf Atmospheric parameters, Kinematics, and Origins -- Based on 1587 hot subdwarf stars observed in Gaia DR2 and LAMOST DR7
• Hot UV-bright stars of galactic globular clusters
• Blue Large-Amplitude Pulsators: the Possible Surviving Companions of Type Ia Supernovae
• The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
• A quantitative NLTE analysis of visual and ultraviolet spectra of four helium-rich subdwarf O stars
• Hot subdwarf wind models with accurate abundances I. Hydrogen dominated stars HD 49798 and BD+18${}^{\circ }$2647
• A 500 pc volume-limited sample of hot subluminous stars II. Atmospheric parameters, mass distribution, and kinematics
• Spectroscopic survey of faint planetary-nebula nuclei. II. The subdwarf O central star of Fr 2-30
• TIC 5724661: A Long-Period Binary with a Pulsating sdB Star and $\delta$ Scuti Variable
• Kinematics of Hot Subdwarfs from the Gaia DR2 Catalogue
• Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34
• Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields
• Rotational broadening and conservation of angular momentum in post-extreme horizontal branch stars