S-type Stars Discovered in Medium-resolution Spectra of LAMOST DR9

• Published in: The Astrophysical journal 2022-06, Vol.931 (2), p.133
• Main Authors: Chen, Jing, Luo, A-Li, Li, Yin-Bi, Chen, Xiang-Lei, Wang, Rui, Li, Shuo, Du, Bing, Ma, Xiao-Xiao
• Format: Article
• Language: English
• Subjects: Astrophysics; Color; Iterative methods; S stars; Support vector machines
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac66de

In this paper, we report on 606 S-type stars identified from Data Release 9 of the LAMOST medium-resolution spectroscopic (MRS) survey; 539 of them are reported for the first time. The discovery of these stars is a three-step process, i.e., selecting with ZrO-band indices greater than 0.25, excluding non-S-type stars with the iterative Support Vector Machine method, and finally retaining stars with absolute bolometric magnitude larger than −7.1. The 606 stars are consistent with the distribution of known S-type stars in the color–magnitude diagram. We estimated the C/Os using the [C/Fe] and [O/Fe] provided by APOGEE and the MARCS model for S-type stars, respectively, and the results of the two methods show that the C/Os of all stars are larger than 0.5. Both the locations on the color–magnitude diagram and C/Os further verify the nature of our S-type sample. Investigating the effect of TiO and atmospheric parameters on ZrO with the sample, we found that log g has a more significant impact on ZrO than T eff and [Fe/H], and both TiO and log g may negatively correlate with ZrO. According to the criterion of Tian et al., a total of 238 binary candidates were found by the zero-point-calibrated radial velocities from the officially released catalog of LAMOST MRS and the catalog of Zhang et al. A catalog of these 606 S-type stars is available from the following link: doi.org/10.12149/101097 .