Galactic Density Gradients.

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galactic Density Gradients. Bok, Bart J. Abstract An attempt is made to investigate the gradients of the star density in the galactic plane for two sections of the Milky Way. First a study is made for several subregions of the change of color with distance for stars of known spectral types. Since it has been shown by several investigators that the ratio of total absorption for photographic light to the observed color excess in a given color system is sensibly constant for regions outside conspicuous dark nebulae, it is possible to estimate roughly the increase of the total absorption for photographic light in each subregion. Star counts to faint limits of magnitude and statistics of the distribution of spectral types provide the data from which the change of star density with increasing distance from the sun can be deter- mined. The basic material for the section in Monoceros (X = 1800, $ = ~0) consists of blue- red color indices of the early-type stars of known spectral type. The fields fall in a part of the Milky Way for which the surface distribution of the stars is extremely regu- lar and unaffected by obvious local obscuration. It is therefore interesting to fmd that space reddening clearly affects the colors of the fainter stars of early spectral types. If the assumption is made that the total photographic absorption for any given star equals three times the color excess on the blue-red system (X = 4500 A, X2 6200 A), the photographic absorption is found to increase regularly with distance for distances up to 1500 parsecs. For the fields within three degrees of the galactic equator the rate of increase is measured by the value Apg = Im/kpc. The corresponding value for the fields at average galactic latitude +5° is Apg = o'~4/kpc. The density analyses for the B8 to Ao and A2 to A5 stars show the presence of considerable negative density gradients. It is important to notice that the rapid drop in the star density apparently is not shared by the stars at large; star counts for Selected Area 98-which falls within the region covered by the analysis-indicate a fairly constant star density for distances up to 1000 parsecs from the sun. The second section of the Milky Way for which approximate density gradients are determined lies in Cepheus and Cassiopeia. The basic material consists of star counts, spectral types, and color indices for Selected Areas 8, 9, i8, and 19. The Cepheus- Cassiopeia section of the Milky Way is by no means free from local obscuration. S.A. 8 is in a region where the surface distribution is decidedly irregular, S.A. 9 is in a fairly uniform field, S.A. i8 is bordered by obscuration, and S.A. iç is adjacent to a large dark cloud. The counted areas and the fields for which the colors were measured were adjusted so as to include only uniform fields of smallest obscuration. A factor four is used for the conversion of excesses on the international scale to total photo- graphic absorptions. The resulting minimum values for the corresponding coefficients of absorption are: S.A. 8: Apg = I'~2/kpc for o < r < 2000 parsecs; S.A. ~: Apg im6/kpc for o < r < 1500 parsecs; S.A. i8: Apg = o~5/kpc for o < r < 2000 parsecs; S.A. 19: Apg = o~o/kpc for r < 800 parsecs, Apg = I~'2/kpc for 800 <r < 2000 parsecs. S.A. i8, at ~ = +6°, is the oniy area at some distance from the galactic plane. Star densities are computed for intervals Bo to B5, B6 to B9, Ao to A4, A5 to A9, and-from star counts-for all stars combined. It is found that the star density drops generally with increasing distance from the sun for distances up to 2000 parsecs. In contrast to the results for the fields in Monoceros, it is generally * This paper was presented at the symposium on galactic and extragalactic struc- ture, held in connection with the dedication of the McDonald Observatory May 5-8, 1939 Publication: The Astrophysical Journal Pub Date: September 1939 DOI: 10.1086/144103 Bibcode: 1939ApJ....90..249B full text sources ADS |