Spectroscopic followup of three bright halo stars selected from SDSS and GALEX photometry

TL;DR

This study conducts high-resolution spectroscopic analysis of three bright halo stars from SDSS and GALEX data, revealing their chemical compositions and metallicities to better understand their nature and origins.

Contribution

It provides detailed chemical abundance measurements and physical parameters for three halo stars, including for the first time the elemental abundances for two of them.

Findings

Metallicities of the stars are approximately -1.3, -0.94, and -0.80.

Two stars show low-alpha element ratios, consistent with halo stars.

One star exhibits high rotational velocity (~40 km/s).

Abstract

We aim to reveal the nature of the Sloan Digital Sky Survey (SDSS) stars: SDSSJ100921.40+375233.9, SDSSJ015717.04+135535.9, and SDSSJ171422.43+283657.2, showing apparently high NUV excesses for their g-z colors, as expected for extremely low-metallicity stars. High resolution (R=60 000) spectra of the stars with a wide wavelength coverage were obtained to determine their chemical compositions with the Tull echelle spectrograph on the 2.7 m telescope at the McDonald Observatory. We derived the spectroscopic parameters Teff =5820+-125 K, log g =3.9+-0.2, and vt =1.1+-0.5 km/s for SDSSJ100921.40+375233.9, Teff=6250+-125 K, log g =3.7+-0.2, and vt =4.0+-0.5 km/s for SDSSJ015717.04+135535.9, and Teff=6320+-125 K, log g =4.1+-0.3, and vt =1.5+-0.5 km/s for SDSS J171422.43+283657.2, and elemental abundances were computed for 21 elements for J100921 and J171422 and for 19 elements for J015717 for the first time. We find metallicities of [Fe/H]= -1.30, -0.94, and -0.80 for SDSSJ100921.40+375233.9, J015717.04+135535.9, and J171422.43+283657.2, respectively. On the basis of calculated abundance ratios for J171422.43+283657.2 and J015717.04+135535.9, we also report that these two program stars have the expected composition of main-sequence halo turnoff stars, but with low-alpha abundances, i.e., the [alpha/Fe] ratio is ~0.0 for J171422.43+283657.2 and ~0.1 for J015717.04+135535.9. The latter one shows typical halo or thick-disk alpha-element abundances, but has a substantial rotational line broadening and vsini=40 +- 0.5 km/s.