4 Discussion

In this section we discuss the main physical and chemical aspects of the HV interstellar gas component observed in the far UV absorption line data, which is supplemented by both $\it IUE$ and visible line data.

4.1 The high velocity component

Our FUSE observations have revealed a well-resolved high velocity absorption feature at $V_{\rm LSR}$ $\sim$ +65 kms^-1in the O I, Ar I, N I, C I, Fe II and P II lines shown in the residual intensity profiles of Fig. 3. This HV feature is also well detected in the $\it IUE$profiles of the nine near ultraviolet lines shown in Fig. 6 (O I, Al II, Si II, S II, Fe II, C II*, Mg I and Mg II) and in both the Na I and Ca II visible line profiles (Sfeir 1999). Note that we have been unable to detect a similar interstellar HV feature in the $\it IUE$ spectra of the high ionization lines of Al III, C IV or Si IV. Upper limits for their equivalent width (derived from a conservative measurement of the local continuum noise) and their corresponding column density upper limit values can be found in Table 2. Inspection of the ionization potentials of the many lines exhibiting this HV absorption feature reveal that it is formed over a wide ionization range from 0 < I.P. < 23.3 eV (defined by the Na I and S II line detections). Clearly all these ions cannot physically co-exist within the same HV cloud and thus, based on the velocity structure revealed by the high resolution Na I and Ca II observations, we conjecture that this HV feature may be composed of several ionized and neutral gas shells expanding at slightly different velocities away from the center of the SNR.

We note that the doppler $\it b$ values derived from the high resolution observations of both Na I and Ca II for the two resolved HV components are <2.1 kms^-1, indicating a gas temperature <8000 K. Although we have derived $\it b$ values in fitting the UV line data, their usefulness in determining reliable gas temperatures is limited due to the dominant contribution of the instrumental resolution of both $\it IUE$ and $\it FUSE$ spectrographs in fitting the blended components in these spectral lines. Hence, for the HV component observed with $\it FUSE$, the average $\it b$ value is found to be $\sim$10 kms^-1, implying a typical gas temperature of <90000 K.

4.2 O VI line

In contrast to the low ionization interstellar lines, the high ionization absorption line profile of O VI shown in Fig. 7 is quite different in appearance.

Figure 7: FUSE spectra O VI absorption profile fitting for HD 47240; results are given in residual intensity. Solid bars on the continuum level indicate typical error size to the continuum level fit. The absorption line at $\sim$150 kms-1 is H2 1032.356 Å./TD>

It consists of a single main absorption component centered at $V_{\rm LSR} = +2.7$ kms^-1 and (given the present signal-to-noise of the $\it FUSE$ spectrum) is not accompanied by a high positive velocity component to a limit of N(O VI) $<7\times10^{12}$ cm^-2. We recall also that no HV components were detected for the high ionization lines of Si IV, C IV and Al III in our extracted $\it IUE$ spectra. Thus, it would appear that since the O VI absorption is formed over a restricted velocity range (-30 to +30 kms^-1), it is most likely that it can be associated with the high ionization gas found in the intervening 1400 pc of the general interstellar medium towards the Monoceros Loop and is not associated with the SNR itself. This lack of detectable high-ionization (high-temperature) HV absorbing SNR gas is somewhat surprising since SNR shock waves strongly affect the density and temperature structure of the ambient interstellar medium. Additionally, high-velocity features have been detected in high-ionization UV absorption lines towards other remnants such as the Vela SNR (Jenkins et al. 1976, 1984) and the Loops I and IV SNRs (Sembach $\&$ Savage 1997), and also high-velocity O VI, C IV and Si IV features are all routinely observed in $\it emission$ towards (bright filament) SNRs such as the Cygnus Loop (Long et al. 1992), the Vela SNR (Raymond et al. 1997) and Puppis A (Blair et al. 1995). However we note the absence of detections of such high-ionization features in the UV absorption spectra recorded towards the Shajn 147 SNR (Phillips & Godhalekar 1983). This SNR has a (Sedov) age of $\sim$ $9\times10^{4}$ years which is similar to that of the Monoceros Loop SNR (Graham 1982), but is far older than both the Vela and Cygnus Loop SNRs. Similarly both Shajn 147 and the Monoceros Loop have HV components with velocities <80 kms^-1, such that this magnitude of shock velocity may be insufficient to produce detectable O VI. This is supported by the the calculations of Shelton (1998) which show that after $\sim$ $2.5\times10^{5}$ years a typical SNR shock is too weak to heat the interstellar gas to more than 10⁵ K and the high-stage ions are no longer found near the shock front. Instead, they lie at the edge of the hot SNR bubble and such gas is rapidly cooling and recombining through the O VI, N V and C IV ions.

Clearly, as more $\it FUSE$ spectra of stars in the Monoceros Loop region become available to us, the spatial extent and range of ionization of the various absorbing components present throughout this evolved remnant will become clearer. For example, if the HV feature detected towards HD 47240 is ubiquitously detected in the FUSE absorption spectra of the other three target stars towards the Monoceros Loop, then this would argue strongly in favor of it being associated with a dense SNR shell arising from radiative cooling during the Sedov-Taylor phase. Such observations will be of particular importance in answering the outstanding question as to why there is (low-level) soft X-ray emission from the Monoceros Loop SNR but (as yet) no detectable O VI absorption/emission.