Mapping the Milky Way with Gaia Bp/Rp spectra I: Systematic flux corrections and atmospheric parameters for 68 million stars

Abstract

Gaia Bp/Rp spectra for over two hundred million stars have great potential for mapping metallicity across the Milky Way. We aim to construct an alternative catalog of atmospheric parameters from Gaia Bp/Rp spectra by fitting them with synthetic spectra based on model atmospheres, and provide corrections to the Bp/Rp fluxes according to stellar colors, magnitudes, and extinction. We use GaiaXPy to obtain calibrated spectra and apply FERRE to match the corrected Bp/Rp spectra with models and infer atmospheric parameters. We train a neural network using stars in APOGEE to predict flux corrections as a function of wavelength for each target. Based on the comparison with APOGEE parameters, we conclude that our estimated parameters have systematic errors and uncertainties in $T_{\mathrm{eff}}$, $\log g$, and [M/H] about $-38\pm 167$ K, $0.05\pm 0.40$ dex, and $-0.12\pm 0.19$ dex, respectively, for stars in the range $4000\le T_{\mathrm{eff}}\le 7000$ K. The corrected Bp/Rp spectra show better agreement with both models and Hubble Space Telescope CALSPEC data. Our correction increases the precision of the relative spectrophotometry of the Bp/Rp data from $3.2\%-3.7\%$ to $1.2\%-2.4\%$. Finally, we have built a catalog of atmospheric parameters for stars within $4000\le T_{\mathrm{eff}}\le 7000$ K, comprising $68,394,431$ sources, along with a subset of $124,188$ stars with $[M/H]\le -2.5$. Our results confirm that the Gaia Bp/Rp flux calibrated spectra show systematic patterns as a function of wavelength that are tightly related to colors, magnitudes, and extinction. Our optimization algorithm can give us accurate atmospheric parameters of stars with a clear and direct link to models of stellar atmospheres, and can be used to efficiently search for extremely metal-poor stars.

17 pages, 12 figures, revised version: incorporated referee comments, minor revisions and corrected typos, published in A&A, catalogs and code in https://doi.org/10.5281/zenodo.14028588