Binary star population of the Sculptor dwarf galaxy^⋆

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Santa Cruz de Tenerife, Spain
e-mail: jmarroyo@iac.es
² Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, 38205 La Laguna, Santa Cruz de Tenerife, Spain
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands

Received: 8 May 2023
Accepted: 8 July 2023

Abstract

Aims. We aim to compute the binary fraction of “classical” dwarf spheroidal galaxies (dSphs) that are satellites of the Milky Way (MW). This value can offer insights into the binary fraction in environments that are less dense and more metal-poor than our own galaxy. Additionally, knowledge of the binary fraction in dwarf galaxies is important with respect to avoiding overestimations of their dark matter content, inferred from stellar kinematics.

Methods. We refined an existing method from the literature, placing an emphasis on providing robust uncertainties on the value of the binary fraction. We applied this modified method to a VLT/FLAMES dataset for Sculptor, specifically acquired for the purpose of velocity monitoring of individual stars, as well as to literature datasets for other six MW “classical” dSphs. In all cases, the targeted stars were mainly red giant branch stars, with expected masses of around 0.8 M_⊙. The VLT/FLAMES dataset offers the most precise binary fractions compared to literature datasets, due to its time baseline of 12 yr, along with at least nine repeated observations for each star.

Results. We found that the binary fraction of Sculptor is 0.55_−0.19^+0.17. We find that it is important to take into account the Roche lobe overflow for constraining the period distribution of binary stars. In contrast to what has recently been proposed in the literature, our analysis indicates that there is no evidence to support varying the properties of the binary stellar population or their deviations from those established for the solar neighborhood, based on the sample of MW dSphs analyzed here.