Abstract
As part of a multifaceted effort to exploit better the asteroseismological potential of the pulsating sdB star Feige 48, we present an improved spectroscopic analysis of that star based on new grids of NLTE, fully line-blanketed model atmospheres. To that end, we gathered four high S/N time-averaged optical spectra of varying spectral resolution from 1.0 \AA\ to 8.7 \AA, and we made use of the results of four independent studies to fix the abundances of the most important metals in the atmosphere of Feige 48. The mean atmospheric parameters we obtained from our four spectra of Feige 48 are : Teff= 29,850 $\pm$ 60 K, log $g$ = 5.46 $\pm$ 0.01, and log N(He)/N(H) = $-$2.88 $\pm$ 0.02. We also modeled for the first time the He II line at 1640 \AA\ from the STIS archive spectrum of the star and we found with this line an effective temperature and a surface gravity that match well the values obtained with the optical data. With some fine tuning of the abundances of the metals visible in the optical domain we were able to achieve a very good agreement between our best available spectrum and our best-fitting synthetic one. Our derived atmospheric parameters for Feige 48 are in rather good agreement with previous estimates based on less sophisticated models. This underlines the relatively small effects of the NLTE approach combined with line blanketing in the atmosphere of this particular star, implying that the current estimates of the atmospheric parameters of Feige 48 are reliable and secure.
Highlights
Feige 48 is a well known rapidly pulsating subdwarf B star
Our fitting procedure found the following parameters for Feige 48: Teff = 29 504 K, log g = 5.41 and log N (He)/N (H) = −2.90. These results are in very good agreement with previous spectroscopic estimates, indicating that metal line-blanketing in NLTE – modeled for the first time here – is not a dominant factor in the atmosphere of Feige 48
Our spectroscopic analysis of Feige 48 leads to parameters similar to those already found in the literature on the basis of less sophisticated models
Summary
Feige 48 is a well known rapidly pulsating subdwarf B (sdB) star. Asteroseismic analyses carried out on this star have found it, among other things, to have a mass of 0.46 M [1]. The synchronous rotation between Feige 48 and its invisible companion (most likely a white dwarf) forming a binary system with an orbital period of 9.02 h has been established using asteroseismology [2]. Both of these important results rely on independent estimates of the effective temperature and surface gravity provided by spectroscopic analyses. These analyses are usually made by fitting simultaneously the Balmer and helium lines of an observed spectrum with a grid of model atmospheres. We want to redetermine these parameters by using state-of-the-art NLTE model atmospheres that include the line-blanketing effect of metallic elements
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