Multiplicity and period distribution of Population II field stars in solar vicinity

TL;DR

This study analyzes the multiplicity and orbital period distribution of metal-poor Population II stars in the solar vicinity, revealing unique binary characteristics and implications for galactic formation history.

Contribution

It provides the first detailed distribution of binary periods among Population II subdwarfs and compares their multiplicity with globular cluster stars, challenging formation theories.

Findings

Binary fraction among subdwarfs is high with a ratio of 147:64:9:1.

Orbital period distribution peaks at 100-1000 days, differing from thin disc stars.

Field halo stars are unlikely to originate solely from globular cluster destruction.

Abstract

We examine a sample of 223 F, G and early K metal-poor subdwarfs ([m/H]<-1) with high proper motions ($\mu > 0.2"/$ year) at the distances of up to 250 pc from the Sun. By means of our own speckle interferometric observations conducted on the 6 m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, and the spectroscopic and visual data taken from the literature, we determine the frequency of binary and multiple systems in this sample. The ratio of single, binary, triple and quadruple systems among 221 primary components of the sample is 147:64:9:1. We show that the distribution of orbital periods of binary and multiple subdwarfs is asymmetric in the range of up to $P=10^{10}$ days, and has a maximum at $P=10^{2}-10^{3}$ days, what differs from the distribution, obtained for the thin disc G dwarfs (Duquennoy & Mayor 1991). We estimated the number of undetected companions in our sample. Comparing the frequency of binary subdwarfs in the field and in the globular clusters, we show that the process of halo field star formation by the means of destruction of globular clusters is very unlikely in our Galaxy. We discuss the multiplicity of old metal-poor stars in nearby stellar streams.