WE have listed1 three requirements that have to be fulfilled for determinations of the mean volume density of the interstellar neutral hydrogen, nH, from Lyman α absorption measurements, (a) Stars that are likely to be surrounded by circumstellar envelopes, such as early-type giants and supergiants, have to be excluded, because circumstellar envelopes can give rise to a Lyman α emission sufficient to affect the absorption measurements by a significant amount. The best candidates for a meaningful measurement are therefore early-type main-sequence stars or horizontal branch stars. (b) Good spectral resolution (Δλ ≤ 2 Å) is needed to resolve Lyman α from other strong nearby absorption features. (c) Observations of a large number of stars are needed in order to reduce the error in the determination of nH due to the rather large uncertainties in the measured equivalent widths and in the adopted distance to each star.
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MACCHETTO, F., PANAGIA, N. Neutral Interstellar Hydrogen in the Solar Neighbourhood. Nature Physical Science 235, 56–58 (1972) doi:10.1038/physci235056a0
Applied Physics (1974)