Research project P4S/251/Gaia-BRASS (Research action P4S)
Gaia-BRASS : unlocking the full potential of Gaia’s spectra for distance measurement and stellar physics
Gaia-BRASS addresses a critical limitation in ESA-Gaia's fourth Data Release (DR4): the Radial Velocity Spectrometer (RVS) pipeline produces unreliable radial velocity measurements for cool, variable M-type stars, particularly Miras and other Long Period Variables, because their spectra are dominated by broad, phase-dependent molecular bands (e.g., from TiO, VO) that current theoretical spectral templates cannot reproduce accurately. As a result, many RV curves exhibit large, unphysical offsets and spurious structure, preventing their use in precision astrophysics, especially in the heavily dust-obscured regions toward the Galactic center where Miras shine brightest in the infrared and serve as vital distance indicators.
To overcome this, Gaia-BRASS will develop and implement a new, robust radial velocity pipeline that combines various key innovations; refined synthetic spectra with updated molecular line lists and atmosphere models calibrated on high-quality Gaia observations and a curated library of empirical template spectra drawn from real M-star observations to capture phase-dependent changes. Applied to about 300,000 M-type stars from DR4, these methods will deliver the first large-scale, homogeneous catalog of validated RV curves, with precision better than 1 km/s, enabling clean separation of true pulsation from binary-induced variability.
These improved RV measurements will power the construction of significantly tighter Period–Luminosity relations for Miras, leveraging RV-derived pulsation periods combined with Gaia photometry and extinction-corrected distances to achieve much smaller uncertainties towards the inner Galaxy. The refined calibrations will not only enhance Galactic structure studies but also enable more accurate distance estimates to nearby galaxies, extending the reach of standard candles beyond the domain of Cepheids and tightening constraints on the Hubble constant, thereby directly supporting cosmological missions such as NASA/ESA/CSA-JWST.
Crucially, all outcomes, including validated epoch spectra, RV time series, quality flags, and the full processing pipeline software, will be made publicly accessible through Gaia-BRASS, an open, interactive database hosted at the Royal Observatory of Belgium. This platform will extend the successful BRASS infrastructure with advanced tools for online spectral analysis, phase-resolved viewing, and customisable data downloads. The project will also train a postdoctoral researcher in cutting-edge spectroscopic analysis and scientific data infrastructure, strengthening expertise for future large-scale surveys. Importantly, the refined RV methods, empirical and synthetic template libraries, and validated wavelength regions developed for Gaia-BRASS will be directly integrated into the ongoing DR5 pipeline upgrades led by Gaia-DPAC-CU6, turning lessons learned from DR4’s limitations into robust, community-tested improvements for Gaia’s final and most comprehensive data release in 2030.
