High-resolution observations of interstellar Na I and Ca II towards the southern opening of the 'Local Interstellar Chimney': probing the disc—halo connection

Crawford, Ian and Lallement, R. and Price, R.J. and Sfeir, D.M. and Wakker, B.P. and Welsh, B.Y. (2002) High-resolution observations of interstellar Na I and Ca II towards the southern opening of the 'Local Interstellar Chimney': probing the disc—halo connection. Monthly Notices of the Royal Astronomical Society 337 (2), pp. 720-730. ISSN 0035-8711.

Abstract

We present high-resolution (R = 400 000) observations of interstellar Ca II and Na I absorption lines towards seven stars in the direction of the southern opening of the recently identified Local Interstellar Chimney. These lines of sight probe the lower Galactic halo (0.3 ≲∣z∣≲ 2.5 kpc), without the complication of sampling dense foreground interstellar material. In addition to components with velocities expected from Galactic rotation, these stars also exhibit components with negative local standard of rest velocities, which are contrary to the sense of Galactic rotation for the sightlines observed. After a discussion of possible origins for these peculiar velocities, we conclude that at least some of them result from gas falling towards the Galactic plane from distances of ∣z∣≳ 300 pc. The narrow linewidths are generally inconsistent with temperatures as high as the ∼6000 K generally assumed for the so-called Lockman layer. Rather, the picture that emerges is one of a scattered, generally infalling, population of high-∣z∣ diffuse clouds, seemingly not very different from those encountered in the local interstellar medium. Overall, we argue that our results are most consistent with a ‘Galactic fountain’ model.

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