A HIGH-RESOLUTION SURVEY OF INTERSTELLAR NA-I-D-1-LINES

We present high-resolution (0.5 km s(-1)) spectra, obtained with the M cDonald Observatory 2.7 m coude echelle spectrograph, of interstellar Na I D-1 absorption toward 38 bright stars. Numerous narrow, closely b lended absorption components, showing resolved Na I hyperfine structur e, are evident in these spectra; such narrow components appear in both low halo and quite local gas, as well as in gas toward more distant d isk stars. We have used the method of profile fitting in an attempt to determine column densities, line widths, and velocities for the indiv idual interstellar clouds contributing to the observed absorption line s. The resulting sample of 276 clouds is significantly larger, and lik ely more complete, than several previous samples of ''individual'' int erstellar clouds, and allows more precise investigation of various sta tistical properties. We find that the cloud column density (N) and lin e width parameter (b) are not correlated, for 0.3 km s(-1) less than o r similar to b less than or similar to 1.5 km s(-1) and 10.0 cm(-2) le ss than or similar to log [N(Na I)] less than or similar to 11.6 cm(-2 ). The median b is about 0.73 km s(-1), the median log N is about 11.0 9 cm(-2), and the median separation between adjacent components is abo ut 2.0 km s(-1). All these are overestimates of the true median values , however, due to our inability to completely resolve all the componen t structure present in some cases; even at a resolution of 0.5 km s(-1 ), we may have discerned only 60% of the full number of individual com ponents actually present. The one-dimensional dispersion of component velocities, in the local standard of rest, is approximately 8.6 km s(- 1); the distribution of velocities is broader and displaced to more ne gative velocities for the weaker components. If 80 K is a representati ve temperature for the interstellar clouds seen in Na I absorption, th en at least 38% (and probably the majority) of the clouds have subsoni c internal turbulent motions. The range in N(H I) observed at a given N(Na I) increases as N(Na I) decreases below about 10(11) cm(-2), so t hat N(Na I) becomes a less reliable predictor of N(H I) at low column densities. These spectra will be quite useful in future detailed studi es of these lines of sight with the HST GHRS echelle-to determine accu rate velocities and to reveal the detailed interstellar component stru ctures that cannot be discerned at the 3.5 km s(-1) resolution availab le with the GHRS.