Binary Contamination in the SEGUE sample: Effects on SSPP Determinations of Stellar Atmospheric Parameters

TL;DR

This study investigates how unresolved binary stars affect the accuracy of stellar atmospheric parameters derived from SEGUE data, revealing significant biases at certain signal-to-noise ratios and proposing adjusted uncertainty estimates.

Contribution

It models binary contamination effects on SSPP parameter determinations using synthetic spectra and quantifies the impact on temperature and metallicity measurements.

Findings

Approximately 10% of blended G-K dwarf pairs are affected by secondary stars.

Binary contamination introduces uncertainties of about 140 K in temperature and 0.17 dex in metallicity at S/N~10.

Binary effects must be considered for accurate stellar parameter estimation in large surveys.

Abstract

Using numerical modeling and a grid of synthetic spectra, we examine the effects that unresolved binaries have on the determination of various stellar atmospheric parameters for SEGUE targets measured using the SEGUE Stellar Parameter Pipeline (SSPP). To model undetected binaries that may be in the SEGUE sample, we use a variety of mass distributions for the primary and secondary stars in conjunction with empirically determined relationships for orbital parameters to determine the fraction of G-K dwarf stars, as defined by SDSS color cuts, that will be blended with a secondary companion. We focus on the G-K dwarf sample in SEGUE as it records the history of chemical enrichment in our galaxy. To determine the effect of the secondary on the spectroscopic parameters, we synthesize a grid of model spectra from 3275 to 7850 K (~0.1 to 1.0 \msun) and [Fe/H]=-0.5 to -2.5 from MARCS model atmospheres using TurboSpectrum. We analyze both "infinite" signal-to-noise ratio (S/N) models and degraded versions, at median S/N of 50, 25 and 10. By running individual and combined spectra (representing the binaries) through the SSPP, we determine that ~10% of the blended G-K dwarf pairs with S/N>=25 will have their atmospheric parameter determinations, in particular temperature and metallicity, noticeably affected by the presence of an undetected secondary. To account for the additional uncertainty from binary contamination at a S/N~10, uncertainties of ~140 K and ~0.17 dex in [Fe/H] must be added in quadrature to the published uncertainties of the SSPP. (Abridged)