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6 Energetics and remnant age

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 $\it IUE$ 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 $3 \times 10^{50}$ 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 $\times$ 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 $t = 1.5 \times 10^{5}$ 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 $3\times10^{4}$ years for the Monoceros Loop SNR. This younger age was derived by matching a model with the shock radius, density ($\rho$ = 0.001 atom cm-3) and temperature of the X-ray emitting gas. Thus, although at present our $\it FUSE$ 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 $\it FUSE$ and $\it IUE$ 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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