A high-resolution survey of interstellar Na I D1 lines
D. E. Welty, L. M. Hobbs, V. P. Kulkarni;
ApJ, 1994, 436, 152
ABSTRACT:We present high-resolution (0.5 km/s) spectra, obtained with the McDonald Observatory 2.7 m coude echelle spectrograph, of interstellar Na I D1
absorption toward 38 bright stars.
Numerous narrow, closely blended absorption
components, showing resolved Na I hyperfine structure, 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 disk stars.
We have used the method of
profile fitting in an attempt to determine column densities, line widths, and
velocities for the individual interstellar clouds contributing to the observed
absorption lines.
The resulting sample of 276 clouds is significantly larger,
and likely more complete, than several previous samples of 'individual'
interstellar clouds, and allows more precise investigation of various statistical
properties.
We find that the cloud column density (N) and line width parameter (b) are
not correlated, for 0.3 km/s approximately less than b approximately less
than 1.5 km/s and 10.0/sq cm approximately less than log (N(Na I))
approximately less than 11.6/sq cm.
The median b is about 0.73 km/s, the median log N is
about 11.09/sq cm, and the median separation between adjacent components is
about 2.0 km/s.
All these are overestimates of the true median values,
however, due to our inability to completely resolve all the component structure
present in some cases; even at a resolution of 0.5 km/s, we may have discerned
only 60% of the full number of individual components actually present.
The
one-dimensional dispersion of component velocities, in the local standard of rest, is
approximately 8.6 km/s; the distribution of velocities is broader and displaced to
more negative velocities for the weaker components.
If 80 K is a
representative temperature for the interstellar clouds seen in Na I absorption, then
at least 38% (and probably the majority) of the clouds have subsonic
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(exp 11)/sq cm, so that 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 studies of these lines of sight
with the Hubble Space Telescope (HST) Goddard High Resolution
Spectrograph (GHRS) echelle -- to determine accurate velocities and to reveal the
detailed interstellar component structures that cannot be discerned at the 3.5
km/s resolution available with the GHRS.
KEYWORDS: data reduction, interstellar matter, sodium, stellar luminosity, stellar mass, stellar spectra, absorption spectra, gauss equation, hubble space telescope, hydrogen, space observations (from earth), statistical distributions
PERSOKEY:absorption, h_i, turbulence, optical, ,
CODE: welty94