2018A&A...609A..89L


Query : 2018A&A...609A..89L

2018A&A...609A..89L - Astronomy and Astrophysics, volume 609A, 89-89 (2018/1-1)

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

LATOUR M., CHAYER P., GREEN E.M., IRRGANG A. and FONTAINE G.

Abstract (from CDS):

Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized.
Aims. 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. Our goal is to determine their atmospheric parameters, chemical properties, and evolutionary status to better understand this class of stars that are en route to become white dwarfs.
Methods. 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.
Results. The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 63000K and logg (cm/s2) 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 25km/s. 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.
Conclusions. We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.

Abstract Copyright: © ESO, 2018

Journal keyword(s): stars: atmospheres - stars: abundances - subdwarfs - stars: fundamental parameters

Simbad objects: 11

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Number of rows : 11
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
#notes
1 CPD-71 172 ** 02 53 30.6948092520 -71 22 31.730091240   10.82 10.63     F2.5IV+sdOHe-wk 25 0
2 HD 269696 HS* 05 31 40.3594532592 -69 53 02.153902236 9.74 10.92 11.15     sdB+dM 193 0
3 BD+75 325 HS* 08 10 49.4903003712 +74 57 57.931973676 8.002 9.23 9.50 9.698 9.884 sdO5 253 0
4 AG+81 266 HS* 09 21 19.1766306264 +81 43 27.630508800 10.392 11.59 11.95 12.09 12.281 sdO 70 0
5 Feige 34 HS* 10 39 36.7358839104 +43 06 09.212615280 9.613 10.91 11.14 11.319 11.464 sdOp 679 0
6 CD-22 9142 HS* 11 50 38.8492465464 -23 20 34.600310232   11.57 11.79     sdOB 26 0
7 BD+18 2647 HS* 12 41 51.7899961800 +17 31 19.750566756 10.261 11.70 11.63 11.969 12.159 sdOpec 181 1
8 HD 350426 HS* 19 43 31.2053563008 +18 24 34.651848456 10.63 11.81 12.34 12.30 12.7 sdO 39 0
9 * P Cyg s*b 20 17 47.2020844 +38 01 58.552724 4.66 5.24 4.82 4.28 4.02 B1-2Ia-0ep 1176 1
10 BD+28 4211 HS* 21 51 11.0221915656 +28 51 50.367627468 8.922 10.25 10.58 10.656 10.831 sdO2VIIIHe5 798 0
11 Feige 110 HS* 23 19 58.3996844256 -05 09 56.171142864 10.360 11.45 11.50 11.970 12.145 sdO8VIIIHe5 636 1

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