Abstract
Aims: We offer a new, simpler picture of the local interstellar medium, made of a single continuous cloud enveloping the Sun. This new outlook enables the description of a diffuse cloud from within and brings to light some unexpected properties. Methods: We re-examine the kinematics and abundances of the local interstellar gas, as revealed by the published results for the ultraviolet absorption lines of MgII, FeII, and HI. Results: In contrast to previous representations, our new picture of the local interstellar medium consists of a single, monolithic cloud that surrounds the Sun in all directions and accounts for most of the matter present in the first 50 parsecs around the Sun. The cloud fills the space around us out to about 9 pc in most directions, although its boundary is very irregular with possibly a few extensions up to 20 pc. The cloud does not behave like a rigid body: gas within the cloud is being differentially decelerated in the direction of motion, and the cloud is expanding in directions perpendicular to this flow, much like a squashed balloon. Average HI volume densities inside the cloud vary between 0.03 and 0.1 cm-3 over different directions. Metals appear to be significantly depleted onto grains, and there is a steady increase in depletion from the rear of the cloud to the apex of motion. There is no evidence that changes in the ionizing radiation influence the apparent abundances. Secondary absorption components are detected in 60% of the sight lines. Almost all of them appear to be interior to the volume occupied by the main cloud. Half of the sight lines exhibit a secondary component moving at about -7.2 km/s with respect to the main component, which may be the signature of a shock propagating toward the cloud's interior.
Highlights
The study of the local interstellar medium (LISM) is interesting for two reasons: (1) an obvious aspect is the knowledge of the structure, the physical conditions, and the evolution of the gaseous environment surrounding our solar system, which allows us to understand better the interactions between the two and (2) the LISM is an excellent laboratory for studying the basic physics at work in diffuse gas
Frisch & Schwadron (2013) claimed that they had evidence for a correlation between the 975 Å radiation field at the Sun and the Mg II and Fe II abundances, based on nine lines of sight in the LIC. We looked for such a correlation in our sample of 57 Components 1 by plotting the Mg II and Fe II abundances against Galactic longitude, taken as a very approximate indicator for the strength of their 975 Å radiation field, according to Fig. 12 of Frisch et al (2012) that shows that the field increases smoothly from l = 100◦, where it is minimum, to l = 280◦, where it is maximum
We have re-examined the sample of LISM sight lines (d ≤ 100 pc) observed at high spectral resolution in the UV lines of Fe II and Mg II
Summary
The study of the local interstellar medium (LISM) is interesting for two reasons: (1) an obvious aspect is the knowledge of the structure, the physical conditions, and the evolution of the gaseous environment surrounding our solar system, which allows us to understand better the interactions between the two and (2) the LISM is an excellent laboratory for studying the basic physics at work in diffuse gas. Following the pioneering work of Crutcher (1982), Lallement & Bertin (1992) have identified a coherent velocity vector creating a CaII absorption component in the spectra of six nearby stars located in the anti-Galactic center direction. Both investigators identified it with the gas of interstellar origin that flows into the heliosphere, producing the Lα and He λ584 backscattering inside the solar system. Identification of a single circum-heliospheric cloud from Mg II and Fe II kinematics
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