A MODEL OF THE GALAXY FOR PREDICTING STAR COUNTS IN THE INFRARED

We present a model of the Galaxy to reproduce star counts in the R, I, J, H, K, L, [12mum] and [25mum] bands, for any given direction and se nsitivity. The Galaxy is represented as the sum of a spheroidal compon ent given by a Hernquist density law, two disk components with scale h eights 100 pc and 390 pc, and a spiral arm structure. Absolute magnitu des in each spectral band and for each luminosity class are obtained f rom V-lambda color indices, presented in form of polynomial fits as a function of spectral type. The interstellar extinction is taken into a ccount, based on the known distribution of galactic gas in atomic and molecular forms. The contribution of circumstellar dust shells to the luminosity of late type giants and supergiants is studied and taken in to account; all giant stars later than M6 are found to present a dust shell. The effect of the large amplitude of variability of these stars is also considered. The model predictions have not been compared with observations for all the bands included in our code due to the lack o f observational data. Extensive comparisons are presented for the K ba nd, over the 1 - 11 magnitude range for many directions in the Galaxy, and comparisons with the IRAS PSC are presented at 12mum and 25mum. T he model reproduces correctly the overall star counts in the K band, a t 12 mum and at 25 mum, although an increase of the parameter represen ting the intensity of the spiral structure is required for the IRAS ba nds, and our description of the local arm (l = 80-degrees) is not yet satisfactory. The scale-length of the disk is found to be about 2.6 kp c for the 390 pc disk, and seems to be larger (about 4 kpc) for the 10 0 pc disk, which contains a younger population.