Empirical photometric calibration of the Gaia Red Clump: colours,
  effective temperature and absolute magnitude
  Gaia Data Release 1 allows to recalibrate standard candles such as the Red
Clump stars. To use those, they first need to be accurately characterised. In
particular, colours are needed to derive the interstellar extinction. As no
filter is available for the first Gaia data release and to avoid the atmosphere
model mismatch, an empirical calibration is unavoidable. The purpose of this
work is to provide the first complete and robust photometric empirical
calibration of the Gaia Red Clump stars of the solar neighbourhood, through
colour-colour, effective temperature-colour and absolute magnitude-colour
relations, from the Gaia, Johnson, 2MASS, Hipparcos, Tycho-2, APASS-SLOAN and
WISE photometric systems, and the APOGEE DR13 spectroscopic temperatures. We
used a 3D extinction map to select low reddening red giants. To calibrate the
colour-colour and the effective temperature-colour relations, we developed a
MCMC method which accounts for all variable uncertainties and selects the best
model for each photometric relation. We estimate the Red Clump absolute
magnitude through the mode of a kernel-based distribution function. We provide
20 colour vs G-Ks relations and the first Teff vs G-Ks calibration. We obtained
the Red Clump absolute magnitudes for 15 photometric bands with, in particular,
M_Ks = -1.606 +/- 0.009 and M_G = (0.495 +/- 0.009) + (1.121 +/- 0.128)
(G-Ks-2.1). We present an unreddened Gaia-TGAS HR diagram and use the
calibrations to compare its Red Clump and its Red Giant Branch Bump with the
Padova isochrones.
