11.1.1 Astrophysical parameters in Gaia DR3

Author(s): Orlagh L. Creevey, Ulrike Heiter, Frédéric Thévenin

Source characterisation in DR3, based on mean BP/RP and RVS spectra, astrometry and photometry, was performed by the coordination unit 8 (CU8), entitled Astrophysical Parameters. Characterisation implies stellar and extra-galactic class probabilities, object-type labels, and parametrisations with associated uncertainties. We refer to these as 1D data products. It also includes multi-dimensional (MD) data products such as a 2D extinction map and a self-organising map of outliers. All of these products, appearing in 538 unique fields, are found in one of 10 tables in the Gaia DR3 archive, with a subset of these also copied to gaia_source. A synthetic description of the contents of these tables is given in Table 11.1.

These products were produced by the CU8 Astrophysical Parameters Inference System (Apsis) chain which consists of 14 modules, see Section 11.1.2. In comparison, in Gaia DR2, only two modules produced five stellar parameters and uncertainties based on the integrated $G$, $G_{\mathrm{BP}}$, $G_{\mathrm{RP}}$ photometry and astrometry ($T_{\mathrm{eff}}$, $A_{\mathrm{G}}$, $E(G_{\mathrm{BP}}-G_{\mathrm{RP}})$, $ℛ$, $ℒ$), see Andrae et al. (2018). Creevey et al. (2023) presents the overall content of the archive produced by Apsis, while Fouesneau et al. (2023) and Delchambre et al. (2023) focus on the validation of the stellar and non-stellar content, respectively.

The 43 main 1D independent data products in Gaia DR3 comprise stellar atmospheric parameters ($T_{\mathrm{eff}}$, $\log g$, $[\mathrm{M}/\mathrm{H}]$, [$\alpha$/Fe], activity index, $v\sin i$), binary component parameters ($T_{\mathrm{eff}}$ 1, $T_{\mathrm{eff}}$ 2, $\log g$ 1, $\log g$ 2), chemical abundances ([Fe/M], [Fe ii/M], [N/Fe], [Mg/Fe], [Si/Fe], [S/Fe], [Ca/Fe], [Ti/Fe], [Cr/Fe], [Ni/Fe], [Zr/Fe], [Ce/Fe], [Nd/Fe]), equivalent widths of H${}_{\alpha }$ and of a CN line, stellar evolutionary parameters ($ℛ$, $ℒ$, $ℳ$, age, evolutionary stage, absolute $G$ magnitude, gravitational redshift), interstellar medium characterisation parameters (extinction – monochromatic and in $G$, $G_{\mathrm{BP}}$, $G_{\mathrm{RP}}$ bands, colour-excess, diffuse interstellar band characteristics) and distance, classification parameters (class labels from three different modules, class probabilities for 12 different classes from two modules, and a spectral type), quasar and galaxy redshifts, and information on each neuron in the self-organising map from the outlier analysis. Other 1D data products include auxiliary information such as quality flags, bolometric correction and goodness-of-fit indicators. We refer to all of these products collectively as astrophysical parameters (APs).

The MD data products comprise a 2D total galactic extinction map at four HEALPix levels and at the optimal HEALPix level, a table of outlier properties classified as neurons in a self-organising map and the corresponding BP/RP spectra, as well as Markov-Chain Monte Carlo (MCMC) samples for stellar parameters from two of the Apsis modules. An overview of the data products is given in Section 11.1.3.