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

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 &lt;20 au. Therefore, an LBI survey allows us to explore separations previously unexplored. Our sample consists of 54 A-type primaries with estimated masses between 1.44 and 2.93 M ⊙ and ages 10–790 Myr, which we observed with the Michigan Infra-Red Combiner-eXeter and Michigan Young Star Imager at Center for High Angular Resolution Astronomy instruments at the Center for High Angular Resolution Astronomy 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.95</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.92</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and a companion frequency of 0.25 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.17</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> over mass ratios 0.1–1.0 and projected separations 0.01–27.54 au. 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-type stars and that close companions to B-type stars are potentially more frequent, which may be indicative of increased disk fragmentation for stars ≳3 M ⊙ .