Hot Subdwarf Stars Observed in LAMOST DR1 - Atmospheric parameters from single-lined spectra

Abstract

We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 non-composite spectra subdwarf stars were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. Most of the sdB stars scatter near the Extreme Horizontal Branch in the $T_{\mathrm{eff}}-\log g$ diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near $T_{\mathrm{eff}}=45000$ K and $\log (g)=5.8$. The sdB population separates into several nearly parallel sequences in the $T_{\mathrm{eff}}-\mathrm{He}$ abundance diagram with clumps corresponding to those in the $T_{\mathrm{eff}}-\log g$ diagram. Over $38000$ K (sdO) stars show abundance extremes, they are either He-rich or He-deficient and we observe only a few stars in the $-1<\log (y)<0$ abundance range. With increasing temperature these extremes become less prominent and the He abundance approaches to $\log (y)\sim -0.5$. A unique property of our sample is that it covers a large range in apparent magnitudes and galactic latitudes, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch (2009) and we conclude that He-rich and He-deficient sdB stars ($\log (y)<1$) probably origin from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the luminosity and helium abundance plane, which suggests that these atmospheric parameters are correlated.

Accepted To The Astrophysical Journal

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