Companions to nearby stars with astrometric acceleration. II

TL;DR

This study used adaptive optics to resolve and analyze companions to nearby stars with astrometric acceleration, enhancing understanding of stellar multiplicity and the effects of companions on astrometric measurements.

Contribution

It presents new resolved companions and re-measures known pairs, clarifies the impact of companions on Hipparcos astrometry, and improves multiplicity statistics for nearby dwarfs.

Findings

Resolved 18 new sub-arcsecond companions.

Re-measured 7 known astrometric pairs.

Bias in Hipparcos data due to companions is generally below formal errors.

Abstract

Hipparcos astrometric binaries were observed with the NICI adaptive optics system at Gemini-S, completing the work of Paper I (Tokovinin et al. 2012). Among the 65 F, G, K dwarfs within 67pc of the Sun studied here, we resolve 18 new sub-arcsecond companions, re-measure 7 known astrometric pairs, and establish the physical nature of yet another three wider companions. The 107 astrometric binaries targeted at Gemini so far have 38 resolved companions with separations under 3". Modeling shows that bright enough companions with separations on the order of an arcsecond can perturb the Hipparcos astrometry when they are not accounted for in the data reduction. However, the resulting bias of parallax and proper motion is generally below formal errors and such companions cannot produce fake acceleration. This work contributes to the multiplicity statistics of nearby dwarfs by bridging the gap between spectroscopic and visual binaries and by providing estimates of periods and mass ratios for many astrometric binaries.