Observations of the 21-CM Hydrogen Line Toward High-Latitude Stars

Abstract

We have obtained 21-cm spectra in 213 positions toward early-type stars with galactic latitude numerically greater than 200 and declination greater than _400. A velocity range of ±50 km sec~1 with respect to the local standard of rest was covered with a resolution of 1.3 km sec~', and in most cases a resolution of 0 53 km sec' was also used. We used the 140-foot telescope and the 101-channel autocor- relation function receiver of the National Radio Astronomy Observatory with a system-noise tempera- ture of 200° K and 10-minute integrations. We give the velocities of 268 well-resolved features from these spectra. About half of the stars have published observations of interstellar calcium or sodium lines. We find that most of Adams's velocity measurements for calcium lines agree to within 2.5 km sec' with our measurements in the same direction. However, there is little agreement between our hydrogen-line com- ponents and calcium-line components from the Adams or Munch and Zirin catalogs when the velocity with respect to the local standard of rest is 23 km sec1 or more. The integrated brightness temperatures found from the 21-cm spectra are the basis for a study of the galactic gas layer near the Sun. On the assumption that the Sun is at the center of an optically thin layer, stratified parallel to the galactic equator, our observations give 2.5 X 1020 atoms per cm2 for the projected density of hydrogen atoms. If the layer is allowed to have an arbitrary pole, our observations fit best when the galactic coordinates of the pole are 1 = 1800, b = 75°, a value that differs by 4° from the pole of the Gould's belt system obtained by Shapley. The projected density in this new coordinate system is 9.0 X 1019 atoms per cm2. However, the concept of a stratified optically thin layer is at best a crude description of the local gas, for the rms deviation of an individual observation is 3.3 times the value which the best-fitting model predicts for the integrated brightness temperature at the pole