Dense Gas Contours Surrounding the Local Cavity: Comparison with the Soft X-Ray Background Emission Map

Abstract

The Local Cavity (also called the Local Bubble) is a region devoid of cold neutral hydrogen surrounding the Sun to 100 pc, and is known to contain a few groups of warm (5,000-10,000K) and very diffuse (0.1 cm−3) clouds, like the Fluff containing the Local Cloud. The local cavity (LC) is thought to be filled with hot gas at 106K, the presumed source of the soft X-ray background emission (e.g. Snowden et al. 1998). The gas pressure within the LC, derived from the soft X-ray intensity, is about nT 10,000 cm−3 K. By comparison, the gas pressure in the local cloud surrounding the Sun is about nT 2,500 cm−3K (Lallement 1999), and in a few other diffuse clouds it is of the same order (Jenkins 2002). The magnetic field within the LC and in the diffuse clouds embedded in the hot gas is unknown. The multiphase structure of the local interstellar medium is not yet fully understood: what is the origin of the LC?, a blown stellar or SNR bubble? or simply a typical interarm-type region (Breitschwerdt 2001)?, what is responsible for the pressure difference between the hot gas and the clouds embedded in the hot gas? what is the origin of the high ionization of helium and argon in the local diffuse clouds? (Wolff et al. 1999; Jenkins et al. 2000; see Cox 1997 for a review). An extensive effort to map the interstellar gas and the LC boundaries has been undertaken a few years ago, taking advantage of the Hipparcos precise parallaxes. Mapping the dense boundary is the easiest part, and can be done using neutral sodium absorption measurements towards nearby stars. As a matter of fact, NaI is a good surrogate for cold (T≤2,000K) and dense (≥1cm−3) HI gas and is detected in the visible thanks to the well known Na-D doublet at