Binarity Beyond Gaia: The case for a dedicated spectroscopic survey of binary stars

TL;DR

This paper advocates for a dedicated spectroscopic survey to complement Gaia's binary star data, aiming to better sample the full range of binary parameters and improve understanding of stellar multiplicity in the Milky Way.

Contribution

It proposes a new, optimized spectroscopic survey to address Gaia's limitations, enabling comprehensive binary star statistics and supporting future astrophysical missions.

Findings

Gaia's RVS limit leaves gaps in binary parameter space

A dedicated survey can detect ultra short and long period binaries

The survey will enable bias-corrected stellar multiplicity census

Abstract

Stellar multiplicity is a fundamental ingredient of stellar astrophysics, yet binary statistics across the Galaxy remain poorly constrained. The \emph{Gaia} mission has revolutionised binary star astrophysics by delivering high-precision astrometry, photometry and global radial velocities, and by providing hundreds of thousands of non-single-star solutions in DR3. However, the RVS magnitude limit, mission time span and scanning law impose strong selection effects in period, mass ratio, inclination and semi-amplitude, leaving large regions of the binary parameter space either sparsely sampled or effectively inaccessible. In this white paper we outline the case for a dedicated, wide-field, multi-epoch spectroscopic survey explicitly optimised for binary science: deeper than the \emph{Gaia} RVS limit, with flexible cadence from hours to years, and with moderate to high spectral resolution. Using a simplified forward model of \emph{Gaia} DR5-like performance, we highlight the populations for which robust orbital solutions will be rare (ultra short period, very long period, low-amplitude and compact-object binaries), and show how a ``Binarity Beyond \emph{Gaia}'' survey would fill these gaps. Such a programme would deliver a bias correctable census of stellar multiplicity across the Milky Way and provide the spectroscopic backbone needed to exploit binary samples from \emph{Rubin}/LSST, \emph{Roman} and \emph{LISA}.