BINARY CONTAMINATION IN THE SEGUE SAMPLE: EFFECTS ON SSPP DETERMINATIONS OF STELLAR ATMOSPHERIC PARAMETERS

We examine the effects that unresolved binaries have on the determination of various stellar atmospheric parameters for targets from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) using numerical modeling, a grid of synthetic spectra, and the SEGUE Stellar Parameter Pipeline (SSPP). The SEGUE survey, a component of the Sloan Digital Sky Survey-II (SDSS-II) project focusing on Galactic structure, provides medium resolution spectroscopy for over 200,000 stars of various spectral types over a large area on the sky. To model undetected binaries that may be in this 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, defined by SDSS color cuts as having 0.48 <= (g - r)(0) <= 0.75, 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, specifically effective temperature, surface gravity, metallicity, and [alpha/Fe], we synthesize a grid of model spectra from 3275 to 7850 K and [Fe/H] = -0.5 to -2.5 from MARCS model atmospheres using TurboSpectrum. These temperature and metallicity ranges roughly correspond to a stellar mass range of 0.1-1.0M(circle dot). We assume that both stars in the pair have the same metallicity. We analyze both infinite signal-to-noise ratio (S/N) models and degraded versions of the spectra, at median S/N of 50, 25, and 10. By running individual and combined spectra (representing the binaries) through the SSPP, we determine that similar to 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; namely, they will be shifted beyond the expected SSPP uncertainties. Shifts in [Fe/H] largely result from the shifts in temperature caused by a secondary. The additional uncertainty from binarity in targets with S/N >= 25 is similar to 80 K in temperature and similar to 0.1 dex in [Fe/H]. The effect on surface gravity and [alpha/Fe] is even smaller. As the S/N of targets decreases, the uncertainties from undetected secondaries increase. For S/N = 10, 40% of the G-K dwarf sample is shifted beyond expected uncertainties for this S/N in effective temperature and/or metallicity. To account for the additional uncertainty from binary contamination at an S/N similar to 10, the most extreme scenario, uncertainties of similar to 140 K and similar to 0.17 dex in [Fe/H] must be added in quadrature to the published uncertainties of the SSPP.