Multiwavelength Absolute Magnitudes and Colours of Red Clump Stars in
  {\it Gaia} Era
  This study presents the multi-wavelength investigation of the absolute
magnitudes and colours of the red clump (RC) stars selected from APOGEE and
GALAH DR2 combined catalogue which is complemented with {\it Gaia} DR2
astrometric data and multi-wavelength photometric data of {\it GALEX} GR6/7,
SDSS DR7, {\it Gaia} DR2, 2MASS and {\it WISE} sky surveys. The analyses are
centred on the different distance estimation methods using {\it Gaia}
trigonometric parallaxes, (1/$\varpi$) and Bayes statistics, and chemically
defined Galactic disc populations on [$\alpha$/Fe]$\times$[Fe/H] plane. Such
investigation questions the long studied problem of the population effects on
RC luminosity. Using two different distance estimation approach, (i) chemical
thin and chemical thick disc RC stars are shown to have different absolute
magnitudes, while colours remain the same in all photometric bands. Absolute
magnitudes vary between -0.12 and +0.13 mag for the 1/$\varpi$ with the change
of the Galactic population. This variation in absolute magnitudes is found to
be larger for the other method. (ii) The Besan\c con population synthesis model
of Galaxy for 2MASS photometry, in which the absolute magnitude difference
between chemical populations were found between -0.35 and -0.40 mag from thin
disc to thick disc. When results compared with each other, differences of
absolute magnitudes are about three times larger in the model than
observations. We confirm that the RC absolute magnitudes depend on
$\alpha$-element abundances of Galactic populations.
