A Search for Binary Stars at Low Metallicity

TL;DR

This study investigates the binary star fraction among extremely metal-poor stars using high-precision Doppler measurements to understand star formation in the early Galaxy and whether binary formation is suppressed at very low metallicities.

Contribution

It provides initial measurements of binary fractions at [Fe/H] < -2.0 using Doppler observations, exploring the existence of a metallicity threshold for binary formation.

Findings

Binary fraction measurements at low metallicity.

Errors of 50 to 300 m/s in Doppler measurements.

Implications for star formation in the early Galaxy.

Abstract

We present initial results measuring the companion fraction of metal-poor stars ([Fe/H]$<-$2.0). We are employing the Lick Observatory planet-finding system to make high-precision Doppler observations of these objects. The binary fraction of metal-poor stars provides important constraints on star formation in the early Galaxy (Carney et al. 2003). Although it has been shown that a majority of solar metallicity stars are in binaries, it is not clear if this is the case for metal-poor stars. Is there a metallicity floor below which binary systems do not form or become rare? To test this we are determining binary fractions at metallicities below [Fe/H]$=-2.0$. Our measurments are not as precise as the planet finders', but we are still finding errors of only 50 to 300 m/s, depending on the signal-to-noise of a spectrum and stellar atmosphere of the star. At this precision we can be much more complete than previous studies in our search for stellar companions.