Very-high-resolution spectroscopy of interstellar Na I
Open Access
- 1 December 1980
- journal article
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 193 (4) , 849-866
- https://doi.org/10.1093/mnras/193.4.849
Abstract
A Michelson interferometer, operating at a spectroscopic resolution of ∼6 × 105, has been used to study interstellar Na I in the lines-of-sight to α, δ Cyg and η Tau. Theoretical line profiles have been fitted to the observations to determine the column density and velocity width of Na I. The models show that at least seven individual clouds occur in the direction of α Cyg; single clouds dominate in the other two sight-lines. The major new result of the work is the discovery that Na I line widths are significantly smaller than previously supposed. In particular, hyperfine structure in the 2S1/2 ground state of Na I is seen in four clouds – three in α Cyg, the other in δ Cyg. In these clouds, $$b/\sqrt 2$$ is in the range 0.26–0.35 km s−1, $$N(\text{Na} \enspace\small\text {I}) = 0.4 - \gtrsim 19.0\times10^{11}\text{cm}^{-2}$$, and their average kinetic temperature is 260 K, or less if there is cloud turbulence. At a kinetic temperature of 80 K, the turbulent velocities of the interstellar clouds in α and δ Cyg are smaller than the expected value of the isothermal sound speed (∼ 0.7 km s−1), indicating that cloud turbulence is subsonic. In contrast, the cloud in η Tau is very turbulent, consistent with it occurring near to the Pleiades.
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