Brief Description of Starcount Model

The infrared Galactic stellar distribution model was developed by Jarrett and collaborators (1992; 1994), whose underlying design is inspired by the classic starcount model created by Bahcall & Soneira (1980) for optical wavelengths. The simplicity (and ultimate limitation) of this model is to incorporate two distinct populations of stars: disk giants and dwarfs, and spheroid giants and dwarfs. Jarrett adopted the discrete formalism of Elias (1978), Jones et al. (1981) and Garwood & Jones (1987), extending the model to the near-infrared (1-10 microns). This updated model includes the class III evolved giants, class IV subdwarfs, class V main sequence, brown dwarfs (types L and T), and AGB populations. As with the Bahcall & Soneira model, the stars are distributed in two primary large-scale components: disk (exponential profile) and spheroid (R1/4 profile). Colors, luminosity, and number density per spectral type are based on the empirical data from Wielen (1974),Wielen et al. (1983), Koornneef (1983), Reid & Gilmore (1982), Hawkins & Bessell (1988), Bessell (1990), Bessell & Brett (1988), and L/T-dwarf luminosity function courtesy of Davy Kirkpatrick in private correspondence (2000). The interstellar extinction is applied as a smooth exponential function of galactic position, characterized by its own scale height and disk length. In total there are several adjustable parameters, including disk/spheroid scale lengths, scale heights, luminosity function and dispersion, and extinction/reddening laws.

The model parameters were tuned using deep optical and infrared star counts. The optical portion of the model was tuned and validated using deep CCD observations, V < 24 mag, R < 22 mag, of a diverse set of fields, encompassing both low and high stellar number density (Jarrett 1992; Jarrett et al. 1994). The near-infrared portion was tuned and validated using 2MASS star counts, for J < 15.5 and Ks < 14 mag (and 1 mag deeper observations from the 2MASS Extended Mission), for fields ranging from 0 < |b| < 90 deg (cf. Cambresy et al. 2002). The model performs well (+- 10%) for |b| > 20 deg. An illustrative example of the performance for l = 90, b = 30 deg is given in Cambresy et al. 2002, Fig. 13. At low galactic latitudes the model performs well overall, but in spots has small but significant differences compared with 2MASS, probably due to differential extinction from clouds along the line of sight, and from overdensities of stars from spiral arms.


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