Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey

TL;DR

This study uses VLTI/PIONIER interferometry to detect faint stellar companions around main sequence stars, revealing new binaries and suggesting many A-type stars may have undetected companions, impacting exozodiacal disc studies.

Contribution

First-time detection of five stellar companions around main sequence stars using infrared interferometry, improving binary star statistics among A-type stars.

Findings

Five new stellar companions resolved around A-type stars.

Approximately 13% of A-type stars are likely undetected visual binaries.

About half of nearby A-type stars could be resolved as binaries with current techniques.

Abstract

The Exozodi survey aims to determine the occurrence rate of bright exozodiacal discs around nearby main sequence stars using infrared interferometry. Although the Exozodi survey targets have been carefully selected to avoid the presence of binary stars, the results of this survey can still be biased by the presence of unidentified stellar companions. Using the PIONIER data set collected within the Exozodi survey, we aim to search for the signature of point-like companions around the Exozodi target stars. We use both the closure phases and squared visibilities collected by PIONIER to search for companions within the ~100 mas interferometric field of view. The presence of a companion is assessed by computing the goodness of fit to the data for a series of binary models with various separations and contrasts. Five stellar companions are resolved for the first time around five A-type stars: HD 4150, HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD 16555 was independently resolved by speckle interferometry while we were carrying out the survey). In the most likely case of main sequence companions, their spectral types range from A5V to K4V. Three of these stars were already suspected to be binaries from Hipparcos astrometric measurements, although no information was available on the companions themselves so far. In addition to debiasing the statistics of the Exozodi survey, these results can also be used to revise the fraction of visual binaries among A-type stars, suggesting that an extra ~13% A-type stars are visual binaries in addition to the ones detected in previous direct imaging surveys. We estimate that about half the population of nearby A-type stars could be resolved as visual binaries using a combination of state-of-the-art interferometry and single-aperture imaging, and we suggest that a significant fraction of these binaries remains undetected to date.