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

Borja Anguiano

TL;DR

Gaia has transformed binary-star studies with global astrometric, photometric, and spectroscopic data, but its spectroscopic reach is incomplete due to the RVS limit and scanning cadence. The paper advocates a dedicated wide-field, multi-epoch spectroscopic survey deeper than Gaia's RVS, with flexible cadence to recover ultra-short, long-period, low-amplitude, and compact-object binaries. This program would provide a bias-correctable census of Galactic multiplicity and supply essential spectroscopic context for time-domain and multi-messenger facilities like Rubin/LSST, Roman, and LISA. By mapping the full parameter space of binaries across the Milky Way, it underpins future studies of Galactic structure, stellar evolution, and gravitational-wave progenitors in the era of precision astrometry and multi-messenger astronomy.

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}.

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

TL;DR

Gaia has transformed binary-star studies with global astrometric, photometric, and spectroscopic data, but its spectroscopic reach is incomplete due to the RVS limit and scanning cadence. The paper advocates a dedicated wide-field, multi-epoch spectroscopic survey deeper than Gaia's RVS, with flexible cadence to recover ultra-short, long-period, low-amplitude, and compact-object binaries. This program would provide a bias-correctable census of Galactic multiplicity and supply essential spectroscopic context for time-domain and multi-messenger facilities like Rubin/LSST, Roman, and LISA. By mapping the full parameter space of binaries across the Milky Way, it underpins future studies of Galactic structure, stellar evolution, and gravitational-wave progenitors in the era of precision astrometry and multi-messenger astronomy.

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}.

Paper Structure

This paper contains 3 sections, 1 figure.

Table of Contents

  1. Motivation
  2. Gaia's binary revolution and its limits
  3. A dedicated spectroscopic survey of binary stars

Figures (1)

  • Figure 1: Gaia-DR5--like orbital completeness in a toy Milky Way binary population. We draw $\sim 8\times10^{4}$ binaries from a smooth, Raghavan-like period--eccentricity distribution with random masses and inclinations, and assign toy heliocentric distances (100--4000 pc) to model Gaia's astrometric signal. We then propagate them through a simplified Gaia model including RVS and astrometric errors, number of visits and mission baseline. The left and middle panels show, for bright ($G\le 14$) and faint ($G>14$) stars, the mean reliability weight of the recovered orbital solution in the $(P,e)$ plane, combining RV and astrometry; shaded regions highlight ultra-short periods (aliasing and phase smearing) and very long periods with $N_{\rm cycles}\ll1$ in DR5. The right panel shows the corresponding RV-only reliability in the $(q,K)$ plane, emphasising that low-$q$, low-$K$ systems lie below Gaia's sensitivity for robust spectroscopic orbits.