Be stars in Gaia

Note: The following is information gathered while performing a series of quick checks. It is intended to be used as a starting off point, but specific use cases should be verified.

Gaia typically lists G-band apparent magnitude (m), as well as BP and RP colours. Using these, one can verify the position of the Be star on the HR diagram.

Typically, the listed magnitudes need to be corrected for reddening. The amount of reddening can be approximated through an absorption coefficient Av (Ag for the Gaia band), that can be estimated from the fitted Nh obtained through X-ray spectral fitting (or the nh query on HEASARC). See here for example. The value obtained here is not equivalent to Ag, but is typically a bit off (see this Gaia DR2 paper).

For distance:

- Take 1/parallax if star is "outside" the disk of the galaxy. This can be estimated using the distance and the galactic latitude.

- Otherwise, use Bailer Jones corrected geometric distances.

Gaia magnitudes:

Gaia G absolute magnitude is given by: M = m - 5*log(dist)+5;

Dereddenned: M_dered = m - 5*log(dist)+5-Ag;

Color excess: BP-RP = Bmag-Rmag;

Dereddenned: BP-RP_dered = Bmag - Rmag - Ag/2;

The factor 2 only applies to sources with parallax less than 1mas, and was taken from the Gaia DR2 paper above.

To reproduce the star's position on the Gaia HR diagram, plot M_dered over BP-RP_dered. OBe stars should show up on the top left. See the standard version here.