Gaia EDR3 extinction law

 

In the broad Gaia passbands, the extinction coefficients depend both on the source spectral energy distribution and on the extinction itself (e.g., Jordi et al. 2010). Following the method presented in Danielski et al. (2018), we compute the extinction coefficient in the m band km = Am/A0, with A0 the extinction at 550 nm, as a function of the star's intrinsic colour or effective temperature (both denoted by X):

 

km = a1 + a2X + a3X2 + a4X3 + a5A0 + a6A02 + a7A03 + a8A0X + a9A0X2 + a10XA02

 

The above formula was fitted on a grid of extinctions convolving the Gaia EDR3 passbands presented in Riello et al. (2020) with Kurucz spectra (Castelli & Kurucz 2003) and the Fitzpatrick et al. (2019) extinction law for solar metallicity, 3500 K < Teff < 10000 K in steps of 250 K, and 0.01 < A0 < 20 mag with a step linearly increasing with 0.01 mag. We fitted the formula for main sequence stars assuming log g = 4.5.

We present another fit adapted for the top of the HR diagram (that is for giants and the top of the main sequence, up to MG ∼ 5 mag) where we adapt the surface gravity of the Kurucz spectra to the temperature with log g = 4 for Teff > 5250 and log g = −8.3 + 0.0023 Teff for cooler stars.

We performed both fits for various flavours of X = (GBP− GRP)0, (G − K)0, and TeffNorm=Teff / 5040 K and for the bands m = G, GBP, GRP, J, H, and K, with the near-infrared bands (J, H, K) corresponding to the 2MASS ones. The fit itself has a maximum uncertainty of 3.5%, 1.5%, and 1% in the G, GBP, and GRP bands, respectively. However, the main uncertainties are linked to the systematics (choice of the extinction law and of the reference spectra). The coefficients should not be extrapolated outside the extinction and temperature ranges used, the latter corresponding to -0.06 < (GBP − GRP )0 < 2.5 mag or −0.1 < (G − K)0 < 4.0 mag. The resulting coefficients are provided here in a zip file for download.

 

References:

 

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