The Small Separation A-Star Companion Population: Tentative Signatures of Enhanced Multiplicity with Primary Mass

TL;DR

This study uses long-baseline interferometry to explore the multiplicity of intermediate-mass A-type stars at close separations, revealing a flat separation distribution and suggesting increased companion frequency with primary mass.

Contribution

It provides new observational data on close companions of A-type stars and introduces a Bayesian analysis of their multiplicity properties, filling gaps left by previous surveys.

Findings

Separation distribution is consistent with being flat.

Estimated companion frequency is approximately 0.25.

Close companion population may be more frequent around more massive stars.

Abstract

We present updated results from our near-infrared long-baseline interferometry (LBI) survey to constrain the multiplicity properties of intermediate-mass A-type stars within 80 pc. Previous adaptive optics surveys of A-type stars are incomplete at separations $<$ 20au. Therefore, a LBI survey allows us to explore separations previously unexplored. Our sample consists of 54 A-type primaries with estimated masses between 1.44-2.93 M$_{\odot}$ and ages 10-790 Myr, which we observed with the MIRC-X and MYSTIC instruments at the CHARA Array. We use the open source software CANDID to detect two new companions, seven in total, and we performed a Bayesian demographic analysis to characterize the companion population. We find the separation distribution consistent with being flat, and we estimate a power-law fit to the mass ratio distribution with index -0.13$^{+0.92}_{-0.95}$ and a companion frequency of 0.25$^{+0.17}_{-0.11}$ over mass ratios 0.1-1.0 and projected separations 0.01-27.54au. We find a posterior probability of 0.53 and 0.04 that our results are consistent with extrapolations based on previous models of the solar-type and B-type companion population, respectively. Our results suggest that the close companion population to A-type stars is comparable to that of solar-types and that close companions to B-type stars are potentially more frequent which may be indicative of increased disk fragmentation for stars $\gtrsim$ 3M$_{\odot}$.