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Optical studies of interstellar material in low density regions of the GalaxyWe analyze high-resolution Na I and Ca II interstellar absorption line data obtained in an earlier spectroscopic survey of 57 stars along extended sight lines through the Galactic disk and halo. We find that the Na I lines trace a diffuse cloudy medium and the CA II lines trace both the cloudy medium and a more extended (intercloud) medium. High latitude and interarm sight lines that do not cross spiral arms have clouds that are more diffuse on average than those along sight lines that cross spiral arms. Spiral structure may play an important role in determinating the average absorption properties along extended sight lines and/or interesting physical differences may exist between sight lines that cross spiral arms and those that do not. These might include a harder radiation field and/or higher electron tempertures along the high latitude and 'clean' interarm sight lines. On average, 10% of the Ca II column density occurs at velocities forbidden by the Galactic rotation law by more than 10 km/s. In contrast, only a small precentage of the Na I column density occurs at these velocites. The Ca II to Na I ratio increases by a factor of 15 over forbidden velocities from 0 to 50 km/s and rises rapidly thereafter. A two component model of the Ca II column density per unit velocity over the range l = 325 deg to 360 deg indicates that two distinct distributions exists, one with sigma = 8 km/s and one with sigma = 21 km/s. As much as 60% of the Ca II column density at forbidden velocities may be associated with the faster distribution, which we attribute to warm intercloud material. We estimate expontential scale heights of 0.4-0.5 kpc for the neutral gas traced by the E(B-V), Na I, and H I distributions along the low density sight lines, and we find that Ca II has a larger scale height of 0.8 kpc.
Document ID
19950056025
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Sembach, K. R.
(Massachusetts Institute of Technology, Cambridge, MA United States)
Danks, A. C.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 16, 2013
Publication Date
September 1, 1994
Publication Information
Publication: Astronomy and Astrophysics
Volume: 289
Issue: 2
ISSN: 0004-6361
Subject Category
Astronomy
Accession Number
95A87624
Funding Number(s)
CONTRACT_GRANT: NAG5-26555
Distribution Limits
Public
Copyright
Other

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