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For the case of the
well-studied Vela SNR, the overall behavior of the velocities
of the absorption components
detected towards 45 stars was such that no ubiquitous
component could be identified that was consistent
with a radial expansion of the SNR (Jenkins et al 1984).
The HV interstellar lines detected towards Vela were thought to
be formed in shock-accelerated clouds, rather than behind a
shock expanding into a uniform medium. However, for the Shajn 147
SNR (in which only two lines-of-sight have been sampled
using the
satellite) a
common HV component at V = +80 kms-1 was detected towards
both stars observed by Phillips & Gondhalekar (1983).
Thus, since
we have (as yet) only sampled a single line-of-sight through
the Monoceros Loop SNR it is premature (although tempting) to
associate the HV feature at +65 kms-1 seen
towards HD 47240 with a global
expansion of the outer SNR shell.
Until absorption spectra along several more lines-of-sight
are obtained towards the Monoceros Loop we cannot, as yet,
distinguish whether this HV feature can be associated with shocks
within dense clouds that have been overtaken by the much faster
SNR shock wave, or whether the
high velocity motion is due to a gas cloud that has been
accelerated by the post-shock flow at some earlier epoch.
In addition,
we note that although HV high ionization interstellar lines of CIV and
SiIV were observed towards stars in the Vela SNR, such detections were
limited to about 40
of the lines-of-sight sampled. No HV components
were detected in the high ionization lines seen observed towards
Shajn 147. Thus, in the
case of SNRs the absence of presence does not always indicate
the presence of absence for highly ionized HV components!
For a SN explosion energy of
erg occuring
in a uniform medium of ambient density, 1 cm-3,
the relationship between
the SNR shell velocity (V)
and that the remnant age, t (in units of 105 years)
is given by Chevalier (1974) as:
V = 66.5
t-0.69 kms-1.
Under the simple (and as yet unsupported) assumption
that the
HV component we have detected towards
HD 47240 is representative of a SNR shell
with a resultant nebular expansion velocity of
50 kms-1 (Davies et al. 1978), then
we obtain an age for the Monoceros Loop
SNR of
years, in agreement with previous estimates for
this remnant
by Graham et al. (1982).
However, two important caveats must be taken into account in
this age determination.
The first is that the ambient interstellar density that the SNR
is expanding into is unknown, and is probably inhomogenous
by large magnitudes. Secondly,
we note that Leahy et al. (1986) have derived a far younger
age of
years for the Monoceros Loop SNR.
This younger age was derived by matching a model
with the shock radius, density (
= 0.001 atom cm-3) and
temperature of the X-ray emitting gas.
Thus, although at present our
observations cannot provide a definite
age for the Monoceros Loop remnant it would appear that
an age of 30000 to 150000 years seems appropriate.
This age range could be consistent
with the intepretation that the HV feature we have observed in both the
and
data represents that of an evolved
shocked SNR shell with no associated
high-ionization (high-temperature) absorption.
As stated previously, such high-ionization features have thus far
only been detected in
the ultraviolet towards SNRs with an age <15000 years (Cygnus Loop
and Vela SNR).
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Copyright ESO 2001