A NLTE analysis of the ζ Aurigae B-type secondary I. Determination of the fundamental stellar parameters

Published

October 12, 2021

Author(s)

P D. Bennett, A Brown, J L. Linsky

Abstract

We present a model atmosphere analysis of the B star secondary of ζ Aurigae (B5 V+K4Ib), and determine its stellar parameters. A grid of model atmospheres and synthetic spectra were computed for stellar parameters typical of mid-B starts, using the TLUSTY and SYNSPEC codes of Hubeny & Lanz (1992) with the lines and continua of H and He calculates in NLTE. We observed ζ Aur with the GHRS at two epochs far from eclipse, and by carefully removing the wind features we obtain the intrinsic photospheric spectra of the B star. The photospheric spectra of ζ Aur B are compared to a series of synthetic spectra calculated with varying stellar parameters and the best fit is determined using a lest-squares technique. We find T_eff=15450+or-}140K, log g=3.86+or-}0.05, and v sin i=200+or-}15km s^-1. The corresponding spectral type, using the effective temperature scale of Underhill et al. (1979), is B5 V.The C I UV 5,76 & 7 resonance multiplets (1277-1281 Angstrom}) and the Si II UV 4 (1260-65 Angstrom}) and UV 5 (1190-97 Angstrom}) resonance multiplets are observed to be much weaker than our models predict. We examine possible causes of the Si II line weakness, and conclude that an underabundance due to NLTE effects is the probable explanation. We empirically determine effective departure coefficients, b_i, which should be interpreted as mean values over the line-forming regions. We find log b(C I)=-1.84 and log b(Si II)=-1.00. This NLTE effect for C I is almost certainly real, and is due to photoionization from low-lying levels. Studies of Si II and III atoms computed in NLTE show that the Si II resonance lines are formed essentially in LTE. We suggest that autoionization of Si II(neglected in previous modeling) may shift the silicon ionization balance enough to account for the weakness of the Si II lines. Our present models do not include line blanketing; line-blanketed models should give lower photoionization rates for Si II which would tend to counter NLTE effects.We emphasize that despite the problems in modeling the Si II lines, the resultant synthetic spectra are in excellent agreement with the other observed spectral line. The objective of this study is to determine the stellar parameters of the Aur B star, and to accurately model the emergent flux spectrum of this star. We do not believe the uncertainties in modeling the Si II profiles comprise the results of this analysis.

Citation

Astrophysical Journal

Pub Type

Journals

Keywords

early-type - stars, effective temperatures - stars, individual-ultraviolet, model atmospheres - stars, NLTE analysis - stars, spectra

Citation

Bennett, P. , Brown, A. and Linsky, J. (2021), A NLTE analysis of the ζ Aurigae B-type secondary I. Determination of the fundamental stellar parameters, Astrophysical Journal (Accessed November 8, 2024)