The remarkable high pressure of the local Leo Cold Cloud

David M. Meyer*, J. T. Lauroesch, J. E.G. Peek, Carl Heiles

*Corresponding author for this work

Research output: Contribution to journalArticle

18 Scopus citations

Abstract

Using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope, we have obtained high-resolution ultraviolet spectra of the C I absorption toward two stars behind the Local Leo Cold Cloud (LLCC). At a distance (20pc) that places it well inside the Local Bubble, the LLCC is the nearest example of the coldest known (T 20 K) diffuse interstellar clouds. The STIS measurements of the C I fine-structure excitation toward HD 85259 and HD 83023 indicate that the thermal gas pressure of the LLCC is much greater than that of the warm clouds in the Local Bubble. The mean LLCC pressure measured toward these two stars (60,000cm-3 K) implies an H I density of 3000cm-3 and a cloud thickness of 200AU at the 20K cloud temperature. Such a thin, cold, dense structure could arise at the collision interface between converging flows of warm gas. However, the measured LLCC pressure is appreciably higher than that expected in the colliding-cloud interpretation given the velocity and column density constraints on warm clouds in the HD 85259 and HD 83023 sightlines. Additional STIS measurements of the Zn II, Ni II, and Cr II column densities toward HD 85259 indicate that the LLCC has a modest "warm cloud" dust depletion pattern consistent with its low dust-to-gas ratio determined from H I 21cm and 100 μm observations. In support of the inferred sheet-like geometry for the LLCC, a multi-epoch comparison of the Na I absorption toward a high-proper-motion background star reveals a 40% column density variation indicative of LLCC Na I structure on a scale of 50AU.

Original languageEnglish (US)
Article number119
JournalAstrophysical Journal
Volume752
Issue number2
DOIs
StatePublished - Jun 20 2012

Keywords

  • ISM: atoms
  • ISM: clouds
  • ISM: structure
  • solar neighborhood

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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