Wide companions to M and L subdwarfs with Gaia and the Virtual Observatory^⋆

¹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid (UCM), Plaza de Ciencias 3, 28040 Madrid, Spain
e-mail: fcojgonz@ucm.es
² Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
³ Spanish Virtual Observatory, Spain
⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
⁶ School of Astronomy and Space Science, Key Laboratory of Ministry of Education, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
⁷ Departamento de Economía Financiera y Contabilidad e Idioma moderno. Facultad de Ciencias Jurídicas y Sociales. Universidad Rey Juan Carlos (URJC), Campus de Vicálvaro, 28032 Madrid, Spain

Received: 3 February 2021
Accepted: 19 April 2021

Abstract

Aims. The aim of the project is to identify wide common proper motion companions to a sample of spectroscopically confirmed M and L metal-poor dwarfs (also known as subdwarfs) to investigate the impact of metallicity on the binary fraction of low-mass metal-poor binaries and to improve the determination of their metallicity from the higher-mass binary.

Methods. We made use of Virtual Observatory tools and large-scale public surveys to look in Gaia for common proper motion companions to a well-defined sample of ultracool subdwarfs with spectral types later than M5 and metallicities below or equal to −0.5 dex. We collected low-resolution optical spectroscopy for our best system, which is a binary composed of one sdM1.5 subdwarf and one sdM5.5 subdwarf located at ∼1360 au, and for another two likely systems separated by more than 115 000 au.

Results. We confirm one wide companion to an M subdwarf, and infer a multiplicity for M subdwarfs (sdMs) of 1.0_−1.0^+2.0% for projected physical separations of up to 743 000 au. We also find four M–L systems, three of which are new detections. No colder companion was identified in any of the 219 M and L subdwarfs of the sample, mainly because of limitations on the detection of faint sources with Gaia. We infer a frequency of wide systems for sdM5–9.5 of 0.60_−0.60^+1.17% for projected physical separations larger than 1 360 au (up to 142 400 au). This study shows a multiplicity rate of 1.0_−1.0^+2.0% in sdMs, and 1.9_−1.9^+3.7% in extreme M subdwarfs. We did not find any companion for the ultra M subdwarfs of our sample, establishing an upper limit of 5.3% on binarity for these objects.