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In article <C1JLxC.5H9@dartvax.dartmouth.edu> Frederick.A.Ringwald@dartmouth.edu (Frederick A. Ringwald) writes:
>If one detects interstellar Na D lines in a hot star's spectrum, how
>exactly does one find the column density of Na D? No, it's not in
>Osterbrock's book - at least, I couldn't find it, and it's only very
>briefly mentioned in Spitzer's book. Please e-mail me literature
>references, and I'll post a summary in a week or so,
>
>Fred Ringwald
>Department of Physics & Astronomy
>Dartmouth College
>Hanover, NH 03755-3528

The description in Spitzer is complete, if terse.  Also remember that
everything in that book is in cgs units unless otherwise stated.  Briefly,
you measure the equivalent widths of the lines (you *do* have both D lines,
don't you?), and fit a curve of growth to them.  The standard curve of
growth gives W(lambda)/lambda as a function of N*f*lambda, where W(lambda)
is the equivalent width, lambda is the wavelength, N is the column
density, and f is the oscillator strength of the line (all the atomic
physics rolled into one number).

If you can partially resolve the line, there are newer methods that show
some promise.  See, for example, recent papers in the Ap. J. by Sembach
and Savage.

If you have more questions, e-mail me (edgar@wisp4.physics.wisc.edu).  I
have some software for fitting curves of growth to doublet lines.  It's
partly described in Edgar, Diplas & Savage, in the Ap J about 1988 or 1989.


---------
Richard J. Edgar  (edgar@wisp4.physics.wisc.edu)
University of Wisconsin--Madison, Department of Physics
"It all depends, of course, upon whether or not it
depends or not, of course, if you take my meaning"