A Survey of Local Interstellar Hydrogen from OAO-2 Observations of Lyman Alpha Absorption.

Abstract

The Wisconsin far-ultraviolet spectrometer aboard OAO-2 observed the wavelength region near 1216 A for 69 stars of spectral type B2 or earlier. From the strength of the observed interstellar L a absorption, atomic hydrogen column densities were derived over distances averaging 300 pc away from the sun. Measurements of the equivalent widths of this absorption were found to be inappropriate for deducing column densities because of the blending of nearby strong stellar lines by the 12 A wide instrumental profile of the OAO. Instead/ the OAO data were compared to synthetic ultraviolet spectra, originally derived from earlier higher resolution rocket observations , which were computer processed to simulate the effects of absorption by different amounts of hydrogen followed by the instrumental blending. An average volume density njj = 0.6 atoms cm" 3 was derived using all the stars. For stars nearer the sun (r<140 pc) we obtained nn = 0.25 atoms cm" 3. The fact that no stars were observed much beyond 1 kpc precludes our obtaining any general information on large-scale galactic structure. However there is evidence for a pronounced enrichment of gas toward a number of stars in Scorpius, and a general deficiency seems to exist in the 180° < I 11 < 270° sector. A comparison of L a column densities https://ntrs.nasa.gov/search.jsp?R=19720024179 2020-05-18T15:54:27+00:00Z 238 SCIENTIFIC RESULTS OF OAO-2 to those derived from observations of 21-cm emission shows a very poor correlation, which demonstrates the importance of the unavoidable differences in sampling geometry. The L a column densities correlated reasonably well with D line, H and K line and color excess measurements for the respective stars, and the average ratios = 3.5 x 10~ 9 , = 2-5 x IQ-9 and = 5 x 10 21 atoms cm~ 2 mag-1 were obtained. For assumed ionization equilibrium constants and a model for the interstellar medium, we find the observed ratios imply the total abundances = 2.3 x 10~ 7 and = 6.8 x 10~ 9. For a very recent grain model we derive ~ 10 ° from the relation.