Interstellar Absorption Lines in Distant Stars. I. Northern Milky way.

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Abstract
Measures of radial velocities and intensities of the Ca ii and/or Na I interstellar lines are given for 112 stars near the galactic plane and with longitudes between 550 and 1600. In stars at distances not smaller than about 2 kpc two strong components are observed, with relative separations depending on galactic longitude and reaching a maximum of about 30 km/sec around 1 = 1000. It is shown that this separation is an effect of galactic rotation, and it is inferred that the interstellar gas clouds producing the observed lines are arranged in space along the spiral arms outlined by the space distribution of H ii regions. The connection between data derived from the optical absorption lines and other indicators of spiral structure, such as the space distributions of 0 associations and interstellar hydrogen, is discussed. It is found that the interstellar gas around distant 0 associations has systematic expanding motions, revealed by line components shifted to the violet with respect to the velocities of the associations by amounts of the order of 20 km/sec. The mean intensity-ratio-curves of the Na I and Ca ii doublets in nearby stars are rediscussed. It is shown that they may be accounted for by a velocity distribution of the interstellar clouds exponential in form and with mean speed constant within a spiral arm rather than by a Gaussian distribution with dispersion increasing with distance from the observer. An indication has been found that the 1?(Na i) for the lines formed in the Perseus arm is larger by factors between 1.2 and 1.5 than for the lines formed in the Orion arm. The existence of variations in shape of the velocity distribution of the interstellar clouds, in relation to the geometry of the spiral arms, is suggested.

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