Issue |
A&A
Volume 436, Number 2, June III 2005
|
|
---|---|---|
Page(s) | 615 - 632 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20042611 | |
Published online | 30 May 2005 |
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
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.
Key words: ISM: bubbles / ISM: kinematics and dynamics
© ESO, 2005
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