HOT SUBDWARF STARS OBSERVED IN LAMOST DR1-ATMOSPHERIC PARAMETERS FROM SINGLE-LINED SPECTRA

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 subdwarf stars with non-composite spectra 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-eff-log g diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near T-eff = 45,000 K and log g = 5.8. The sdB population separates into several nearly parallel sequences in the T-eff-He abundance diagram with clumps corresponding to those in the T-eff-log g diagram. Over 38,000 K (sdO) stars show abundance extremes; they are either He-rich or He-deficient and we observe only a few stars in the abundance range -1< log y < 0. With increasing temperature these extremes become less prominent and the He abundance approaches log y similar to-0.5. A unique property of our sample is that it covers a large range in apparent magnitude and galactic latitude, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch and we conclude that He-rich and He-deficient sdB stars (log y <. 1) probably originate from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the plane of luminosity and helium abundance, which suggests that these atmospheric parameters are correlated.