Galactic S Stars: Investigations of Color, Motion, and Spectral Features

TL;DR

This paper presents a spectral atlas and analysis of bright S stars, exploring their spectral features, colors, and motions to distinguish intrinsic from extrinsic types and confirm their giant status.

Contribution

It provides a new digital spectral atlas of 57 S stars, defines spectral indices, and investigates their colors and motions to improve classification and understanding.

Findings

S stars have colors similar to carbon and M stars, making them hard to distinguish by color alone.

Infrared colors help identify intrinsic versus extrinsic S stars.

Most S stars in the sample are confirmed to be giants.

Abstract

Known bright S stars, recognized as such by their enhanced s-process abundances and C/O ratio, are typically members of the asymptotic giant branch (AGB) or the red giant branch (RGB). Few modern digital spectra for these objects have been published, from which intermediate resolution spectral indices and classifications could be derived. For published S stars we find accurate positions using the Two-Micron All Sky Survey (2MASS), and use the FAST spectrograph of the Tillinghast reflector on Mt. Hopkins to obtain the spectra of 57 objects. We make available a digital S star spectral atlas consisting of 14 spectra of S stars with diverse spectral features. We define and derive basic spectral indices that can help distinguish S stars from late-type (M) giants and carbon stars. We convolve all our spectra with the SDSS bandpasses, and employ the resulting gri magnitudes together with 2MASS JHK mags to investigate S star colors. S stars have colors similar to carbon and M stars, and are therefore difficult to distinguish by color alone. Using near and mid-infrared colors from IRAS and AKARI, we identify some of the stars as intrinsic (AGB) or extrinsic (with abundances enhanced by past mass-transfer). We also use V band and 2MASS magnitudes to calculate a temperature index for stars in the sample. We analyze the proper motions and parallaxes of our sample stars to determine upper and lower limit absolute magnitudes and distances, and confirm that most are probably giants.