The luminosities and the spatial distribution of stars detected on a two micron sky survey

Citation

Hughes, Evan Eugene (1969) The luminosities and the spatial distribution of stars detected on a two micron sky survey. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FFMK-TS05. https://resolver.caltech.edu/CaltechETD:etd-10282008-144348

Abstract

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An infrared sky survey, carried out at the California Institute of Technology, has resulted in a catalog of the 5612 sources whose flux at 2.2μ was measured to be greater than 2.5 x 10[superscript -15] watts cm[superscript -2]μ[superscript -1]. (Neugebauer, G., and Leighton, R. B., Two Micron Sky Survey, A Preliminary Catalog, National Aeronautics and Space Administration, Washington, D.C., 1969.) The positions and the fluxes at 2.2μ and 0.8μ of the stars in the catalog are taken here as the input data for a determination of the luminosities and the distribution in space of these stars. The 0.8μ measurements are used only for the purpose of dividing the catalog stars into four groups on the basis of the ratio of 2.2μ flux to 0.8μ flux.

For each of the four groups data are presented which show the distribution of the stars in spectral type and in galactic longitude and latitude. Also presented is the distribution n(t) of an observed quantity t. This quantity is defined as the logarithm of the square of the ratio r[subscript max]/z, where r[subscript max] is the maximum distance at which a star can be detected and z is its distance from the galactic plane. An integral equation, which relates n(t) to the space density D(z) and the luminosity distribution, is used to derive luminosity distributions for the stars. The method makes use of the decrease in D(z) at some characteristic distance z = σ to determine the distribution of r[subscript max]/σ.

The results indicate that the least red (i.e., smallest ratio of fluxes) 35 percent of the stars are generally giant stars of type MO or earlier and are not luminous enough to be detected beyond the characteristic half-width σ of their z distribution, where 200 < σ < 300 parsecs (pc). The next 20 percent in order of redness are giant stars of spectral type M0 - M3 and can be detected to an average distance about twice their characteristic half-width σ, where 150 < σ < 300 parsecs.

The reddest 45 percent of the stars are generally giants of spectral type later than M3 and can be detected to distances about five times σ, on the average. The luminosity function derived from these reddest stars may be written in terms of absolute K (i.e., 2.2μ) magnitudes M[subscript K] as ([...]) exp [...] per unit volume per unit magnitude, where D[subscript o] = 29/σ[superscript 3], M[subscript o] = -4.3 - 5 1og[subscript 10](σ/100 pc), and [...] = 1.2. The σ for these stars is probably in the range 200 to 400 parsecs. This luminosity function does not account for about 200 of the stars in the reddest group which form an excess concentration of stars within 1 or 2 degrees of the galactic equator. These excess stars are interpreted as being supergiants with σ [...] 50 parsecs and r[subscript max] [...] 5 kiloparsecs.

An estimate of the gradient of the space density in the galactic plane is made for the reddest 45 percent of the stars. The estimate is that the space density decreases by a factor of 10[superscript -0.15] in a distance of 5σ.

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