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 Gaia DR2 astrometric data and multi-wavelength photometric data of GALEX GR6/7, SDSS DR7, Gaia DR2, 2MASS and WISE sky surveys. The analyses are centred on the different distance estimation methods using Gaia trigonometric parallaxes, (1/ ) and Bayes statistics, and chemically defined Galactic disc populations on [α/Fe]×[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/ with the change of the Galactic population. This variation in absolute magnitudes is found to be larger for the other method. (ii) The Besançon 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 α-element abundances of Galactic populations.

We used the updated [Fe/H] abundances of 168 F-G type dwarfs and calibrated them to a third order polynomial in terms of reduced ultraviolet excess, δ 0.41 defined with ugr data in the SDSS. We estimated the M g absolute magnitudes for the same stars via the re-reduced Hipparcos parallaxes and calibrated the absolute magnitude offsets, ∆M g , relative to the intrinsic sequence of Hyades to a third order polynomial in terms of δ 0.41 . mean of the residuals and the corresponding standard deviation for the metallicity calibration are 0 and 0.137 mag; while, for the absolute magnitude calibration they are 0 and 0.179 mag, respectively. We applied our procedures to 23,414 dwarf stars in the Galactic field with the Galactic coordinates 85• and size 78 deg 2 . We estimated absolute magnitude M g dependent vertical metallicity gradients as a function of vertical distance Z. The gradients are deep in the range of 0 < Z ≤ 5 kpc, while they are very small positive numbers beyond Z = 5 kpc. All dwarfs with 5 < M g ≤ 6 mag are thin-disc stars and their distribution shows a mode at (g − r) 0 ≈ 0.38 mag, while the absolute magnitudes 4 < M g ≤ 5 are dominated by thick disc and halo stars, i.e. the apparently bright ones (g 0 ≤ 18 mag) are thick-disc stars with a mode at (g − r) 0 ∼ 0.38 mag, while the halo population is significant in the faint stars (g 0 > 18 mag).

We estimated iron and metallicity gradients in the radial and vertical directions with the F and G type dwarfs taken from the Radial Velocity Experiment Data Release 4 database. The sample defined by the constraints Z max ≤ 825 pc and e p ≤ 0.10 consists of stars with metal abundances and space velocity components agreeable with the thin-disc stars. The radial iron and metallicity gradients estimated for the vertical distance intervals 0 < Z max ≤ 500 and 500 < Z max ≤ 800 pc are d[Fe/H]/dR m = −0.083 ± 0.030 and d[Fe/H]/dR m = −0.048 ± 0.037 dex kpc −1 , and d[M/H]/dR m = −0.063 ± 0.011 and d[M/H]/dR m = −0.028 ± 0.057 dex kpc −1 , respectively, where R m is the mean Galactocentric distance. The iron and metallicity gradients for less number of stars at further vertical distances, 800 < Z max ≤ 1500 pc, are mostly positive. Compatible iron and metallicity gradients could be estimated with guiding radius (R g ) for the same vertical distance intervals 0 < Z max ≤ 500 and 500 < Z max ≤ 800 pc, i.e. d[Fe/H]/dR g = −0.083 ± 0.030 and d[Fe/H]/dR g = −0.065 ± 0.039 dex kpc −1 ; d[M/H]/dR g = −0.062 ± 0.018 and d[M/H]/dR g = −0.055 ± 0.045 dex kpc −1 . F and G type dwarfs on elongated orbits show a complicated radial iron and metallicity gradient distribution in different vertical distance intervals. Significant radial iron and metallicity gradients could be derived neither for the sub-sample stars with R m ≤ 8 kpc, nor for the ones at larger distances, R m > 8 kpc. The range of the iron and metallicity abundance for the F and G type dwarfs on elongated orbits, [−0.13, −0.01), is similar to the thin-disc stars, while at least half of their space velocity components agree better with those of the thick-disc stars. The vertical iron gradients estimated for the F and G type dwarfs on circular orbits are d[Fe/H]/dZ max = −0.176 ± 0.039 dex kpc −1 and d[Fe/H]/dZ max = −0.119 ± 0.036 dex kpc −1 for the intervals Z max ≤ 825 and Z max ≤ 1500 pc, respectively.

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We used the spectroscopic and astrometric data provided from the GALAH DR2 and Gaia DR2, respectively, for a large sample of stars to investigate the behaviour of the [α/Fe] abundances via two procedures, i.e. kinematically and spectroscopically. With the kinematical procedure, we investigated the distribution of the [α/Fe] abundances into the high/low probability thin disc, and high/low probability thick-disc populations in terms of total space velocity, [Fe/H] abundance, and age. The high probability thin-disc stars dominate in all sub-intervals of [α/Fe], including the rich ones: [α/Fe]> 0.3 dex, where the high probability thickdisc stars are expected to dominate. This result can be explained by the limiting apparent magnitude of the GALAH DR2 (V < 14 mag) and intermediate Galactic latitude of the star sample. Stars in the four populations share equivalent [α/Fe] and [Fe/H] abundances, total space velocities and ages. Hence, none of these parameters can be used alone for separation of a sample of stars into different populations. High probability thin-disc stars with abundance −1.3 < [Fe/H] ≤ −0.5 dex and age 9 < τ ≤ 13 Gyr are assumed to have different birth places relative to the metal rich and younger ones. With the spectroscopic procedure, we separated the sample stars into α-rich and α-poor categories by means of their ages as well as their [α/Fe] and [Fe/H] abundances. Stars older than 8 Gyr are richer in [α/Fe] than the younger ones. We could estimate the abundance [α/Fe]=0.14 dex as the boundery separating the α-rich and α-poor sub-samples in the [α/Fe]×[Fe/H] plane.2 Karaali et al. (2003, 2005):where it is assumed that the Galactic space velocity components, U LSR , V LSR , and W LSR of the stellar populations in the thin disc, thick disc, and halo have Gaussian distributions, k = [(2π) 3/2 σ U σ V σ W ] −1 normalizes the expression, σ U , σ V , and σ W are the characteristic velocity dispersions, and V asym is the asymmetric drift. Thus, one gets two relative probabilities for a star as in the following: