Highly ionized gas in the local ISM: Some like it hot?

¹ Experimental Astrophysics Group, Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu
² Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France

Received: 23 December 2004
Accepted: 18 February 2005

Abstract

We present -STIS medium-resolution spectra ( km s^-1) of the ultraviolet interstellar absorption lines observed towards 4 early-type stars located within the local interstellar medium (ISM), with sight-line distances <186 pc in the general direction of the Loop I superbubble (, ). These data have been supplemented with high resolution ( km s^-1) visible absorption observations of the NaI D-lines towards these 4 stars. Our main discovery is the detection of highly ionized absorption components of CIV, SiIV and NV towards the two most distant targets, HD 127381 and HD 142256. These lines-of-sight are known to cross both the near and far neutral interface boundaries to the Loop I cavity, in addition to intersecting the fragmented shell of neutral and partially ionized gas that defines the boundary to the Local Bubble. However, the presently measured narrow line profile-widths and their measured absorption intensities are found to be incompatible with theoretical models that predict high ion absorption due to the presence of evaporating cloud conduction interfaces. We conclude that the formation of high ions in the local ISM is highly dependent on the location of the absorbing gas clouds with respect to nearby sources of both hot X-ray emitting gas and/or photo-ionization.
Our observations have also revealed at least 6 gas clouds with distances ranging from 5 pc to 150 pc along these sight-lines. We have detected a cloud of neutral and partially ionized gas with a velocity of -15 km s^-1 and a hydrogen column density of log N cm^-2 that is thought to define the boundary to the Local Bubble cavity at a distance of ~90 pc in this galactic direction. The far neutral boundary to the Loop I superbubble cavity is also detected at a distance of 150-180 pc and is composed of two cold clouds moving at velocities close to  km s^-1 possessing a combined hydrogen column density of log N cm^-2. In contrast, we have also detected three low density, warm and partially ionized diffuse clouds with average velocities of ~-10, -23 and -32 km s^-1, that are all located within a distance of ~150 pc. The cloud component at  km s^-1 may be associated with the very local “G-cloud” at a distance of <5 pc, but we also provide evidence for its placement at a greater distance. The measured velocities of the majority of the gas clouds we have detected along all 4 sight-lines are consistent with an inflow of gas into the LB cavity from the direction of the Loop I superbubble. This gas is flowing through a region of fragmentation at a distance of ~90 pc that represents the interaction region between the Loop I and Local Bubble cavities.