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A Comparison of the Components in Interstellar Sodium and Calcium.
Routly, Paul McRae; Spitzer, Lyman, Jr.
Astrophysical Journal, vol. 115, p.227 (ApJ Homepage)
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Radial velocities and equivalent widths of the interstellar D lines, including a number of faint components, were obtained for some twenty stars from high-dispersion spectrograms taken at the Mount Wilson Observatory. Corresponding measures in Ca II were also obtained for four stars. This material, together with additional measures obtained previously by other investigators, was employed in a comparison between Na and Ca. For each component N(Na I) and N(Ca II), the numbers of absorbing Na I atoms and Ca II ions in the line of sight, were found whenever possible, as were also the values of the Doppler widths b(Na I) and b(Ca II). For strong components, the data indicate that b(Ca II) in kilometers per second is about 1.5 times b(Na I), in qualitative agreement with Wilson's analysis; but for weaker lines these Doppler widths are more nearly equal. On the average, a decrease occurs in N(Na I)/N(Ca II) as the residual radial velocity of the component increases in absolute value; in low-velocity clouds Na I tends to be more abundant than Ca II, while in high-velocity clouds the reverse is true. This result apparently explains why the strong saturated lines of Na I and Ca II do not lie on the same curve of growth [b(Ca II) > b(Na I)]. Consideration is given to the possibility that differences of ionization between high-velocity and low-velocity clouds may this variation of abundance ratio with cloud velocity. In a rapidly moving cloud the kinetic temperature should be increased, since the intercloud gas in front of the cloud will be compressed and heated. At a temperature somewhere between 5000°and 10,000°, collisional ionization of Na I by electrons would reduce the ratio N(Na I)/N(Ca II) by the observed factor. However it is uncertain whether the temperature can be so high, and other mechanisms may be important.

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