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Quantitative spectroscopy of extreme helium stars Model atmospheres and a non-LTE abundance analysis of BD+10°2179

Kupfer, T. and Przybilla, N. and Heber, U. and Jeffery, C. S. and Behara, N. T. and Butler, K. (2017) Quantitative spectroscopy of extreme helium stars Model atmospheres and a non-LTE abundance analysis of BD+10°2179. Monthly Notices of the Royal Astronomical Society, 471 (1). pp. 877-890. ISSN 0035-8711.

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Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems. In this paper, we present a detailed quantitative non-LTE spectral analysis for BD+10°2179, a prototype of this rare class of stars, using UV-Visual Echelle Spectrograph and Fiber-fed Extended Range Optical Spectrograph spectra covering the range from ∼3100 to 10 000 Å. Atmosphere model computations were improved in two ways. First, since the UV metal line blanketing has a strong impact on the temperature-density stratification, we used the ATLAS12 code. Additionally, We tested ATLAS12 against the benchmark code STERNE3, and found only small differences in the temperature and density stratifications, and good agreement with the spectral energy distributions. Secondly, 12 chemical species were treated in non-LTE. Pronounced non-LTE effects occur in individual spectral lines but, for the majority, the effects are moderate to small. The spectroscopic parameters give T_(eff) =17 300±300 K and log g = 2.80±0.10, and an evolutionary mass of 0.55±0.05 M_⊙. The star is thus slightly hotter, more compact and less massive than found in previous studies. The kinematic properties imply a thick-disc membership, which is consistent with the metallicity [Fe/H] ≈ −1 and α-enhancement. The refined light-element abundances are consistent with the white dwarf merger scenario. We further discuss the observed helium spectrum in an appendix, detecting dipole-allowed transitions from about 150 multiplets plus the most comprehensive set of known/predicted isolated forbidden components to date. Moreover, a so far unreported series of pronounced forbidden He I components is detected in the optical-UV.

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Kupfer, T.0000-0002-6540-1484
Additional Information:© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 June 16. Received 2017 June 15; in original form 2017 April 11. Based on observations obtained at the European Southern Observatory, proposals 077.D-0458(A) and 077.D-0458(B). TK acknowledges support by the Erasmus Internship Program which supported his stay at Armagh observatory where parts of this paper were written. The authors thank the staff at the ESO Paranal and La Silla observatories for performing the spectroscopic observations in service mode. We would also like to thank the anonymous referee for constructive criticism that helped to improve the manuscript. This research has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. Research at the Armagh Observatory and Planetarium is supported by a grant-in-aid from the Northern Ireland Department for Communities. CSJ acknowledges support from the UK Science and Technology Facilities Council (STFC) Grant No. ST/M000834/1.
Funding AgencyGrant Number
Northern Ireland Department for CommunitiesUNSPECIFIED
Science and Technology Facilities Council (STFC) ST/M000834/1
Subject Keywords:line: formation, stars:abundances, stars: atmospheres, stars: fundamental parameters, stars: individual: BD+10°2179
Record Number:CaltechAUTHORS:20170911-134809880
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Official Citation:T. Kupfer, N. Przybilla, U. Heber, C. S. Jeffery, N. T. Behara, K. Butler; Quantitative spectroscopy of extreme helium stars Model atmospheres and a non-LTE abundance analysis of BD+10°2179, Monthly Notices of the Royal Astronomical Society, Volume 471, Issue 1, 11 October 2017, Pages 877–890,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81300
Deposited By: Ruth Sustaita
Deposited On:11 Sep 2017 22:17
Last Modified:11 Sep 2017 22:17

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