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7 Summary

We have obtained a far ultraviolet spectrum of the star HD 47240 covering the wavelength range 990-1187 Å using the NASA $\it FUSE$ satellite. Using these new FUV data supplemented by visible and $\it IUE$ archival absorption spectra taken towards this star, we have detected a high-velocity absorption component at $V_{\rm LSR}$ = +65 kms-1 whose origin can be associated with the highly disturbed SNR gas. This high-velocity feature is formed only in lines with an ionization potential range up to 23.3 eV. High spectral resolution visible observations of this feature reveal a more complex velocity structure (with at least two velocity components at $V_{\rm LSR}$ = +60 and +71 kms-1), suggesting that it is composed of several ionized and neutral gas shells expanding at slightly different velocities.

This high-velocity feature has not been detected in any of the high-ionization absorption lines of O VI, C IV or Si IV. Similar non-detections of these high ionization lines have been reported for HV gas associated with the Shajn 147 SNR (which is of a similar age to the Monoceros Loop), whereas both detections and non-detections of HV high ions have been reported towards the (younger) Vela SNR which is expanding at velocities >100 kms-1. Thus, until more lines-of-sight are sampled towards the Monoceros Loop SNR this present lack of HV high ion line detectability cannot be attributed solely to the low (V< 80 kms-1) shock velocity and evolved nature of the expanding remnant gas.

We have derived elemental abundances (relative to that of S) for gas associated with the HV component. A pattern in which the refractory elements of Fe, Si and Al are all less depleted (i.e. more abundant) than in the cold, interstellar disk gas has been found. This pattern of overabundance of refractory elements found for both the Monoceros Loop and Vela SNRs is consistent with the levels of depletion found in warm disk gas, and is probably due to grain sputtering processes. In contrast N, O, and Ar are all more depleted in the Vela and Monoceros Loop SNR HV gas than in both the cold and warm interstellar disk gas. This apparent element deficiency can be attributed to ionization effects of the SNR gas. The element phosphorus is essentially undepleted in the high-velocity gas, in agreement with observations of interstellar gas in the galactic disk.

Finally, we believe that an age of 30000 to 150000 years for the Monoceros Loop SNR is appropriate. Such an evolved remnant appears to possess absorption properties more like those of the Shajn 147 SNR as opposed to those of the far younger (age <15000 years) Cygnus and Vela SNRs.

Acknowledgements

We wish to thank the NASA $\it FUSE$ Operations and Science Center at the Johns Hopkins University which is operated for NASA under contract NAS5-32985. Particular thanks go to Dr. Bill Blair, Dr. Robin Shelton and Dr. Ed Jenkins for very useful discussions and important suggestions that have greatly improved this paper. DS, SS and BYW all acknowledge funding through the NASA $\it FUSE$science support contract with the Experimental Astrophysics Group at the Space Sciences Laboratory, UC Berkeley.


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Copyright ESO 2001