THE INTERSTELLAR D1 LINE AT HIGH-RESOLUTION

Observations at a resolving power R = lambda/DELTA-lambda = 5 x 10(5) or a velocity resolution DELTA-upsilon = 0.6 km s-1 are reported of the interstellar D1 line of Na I in the spectra of gamma-Cas, delta-Ori, epsilon-Ori, pi-Sco, delta-Cyg, and alpha-Cyg. An echelle grating was used in a double-pass configuration with a CCD detector in the coude spectrograph of the 2.7 m reflector at McDonald Observatory. At least 42 kinematically distinct clouds are detected along the light paths to the five more distant stars, in addition to a single cloud seen toward delta-Cyg. The absorption lines arising in 13 of the clouds are sufficiently narrow and unblended to reveal clearly resolved hyperfine structure components split by 1.05 km s-1. An additional 13 clouds apparently show comparably narrow, but more strongly blended, lines. For each individual cloud, upper limits T(max) and (upsilon-t(max)) on the temperature and the turbulent velocity, respectively, are derived by fitting the observed lines with theoretical absorption profiles. Twenty-three of the 43 clouds show T(max) < 450 K and (3)1/2 x (upsilon-t(max)) < 0.70 km s-1, the sound speed which corresponds to T = 80 K. The most stringent upper limits obtained for any of the clouds, from unblended lines, are T(max) < 160 K and (upsilon-t(max)) < 0.24 km s-1.