A different approach for the estimation of Galactic model parameters

Abstract

We estimated the Galactic model parameters by means of a new approach based on the comparison of the observed space density functions per absolute magnitude interval with a unique density law for each population individually, and via the procedure in situ for the field SA 114 (alpha = 22(h)40(m)00(s), delta = 00degrees00'00"; l = 68.degrees15 b = 48degrees38; 4.239 deg(2); epoch 2000). The separation of stars into different populations has been carried out by their spatial distribution. The new approach reveals that model parameters are absolute-magnitude-dependent. The scaleheight for a thin disc decreases monotonically from absolutely bright [M(g') = 5] to absolutely faint [M(g') = 13] stars in the range 265-495 pc, but there is a discontunity at the absolute magnitude M(g') = 10 where the sech(2) density law replaces the exponential one. The range of the scaleheight for a thick disc, dominant in the absolute magnitude interval 5 < M(g') <= 9, is less: 805-970 pc. The local space density for a thick disc relative to a thin disc decreases from 9.5 to 5.2 per cent when one goes from absolutely bright to faint magnitudes. The halo is dominant in three absolute magnitude intervals, namely 5 < M(g') less than or equal to 6, 6 < M(g') <= 7, and 7 < M(g') less than or equal to 8, and the axial ratio for this component is almost the same for these intervals where c a similar to 0.7. The same holds for the local space density relative to the space density of the thin disc with range (0.02-0.15) per cent. The model parameters estimated by comparison of the observed space density functions combined for three populations per absolute magnitude interval with the combined density laws agree with the cited values in the literature. Also, each parameter is equal to at least one of the corresponding parameters estimated for different absolute magnitude intervals by the new approach. We argue that the most appropriate Galactic model parameters are those that are magnitude dependent.