1 Introduction

The NASA Far Ultraviolet Spectroscopic Explorer ($\it FUSE$) was launched on June 24th 1999, and its instrumentation is ideally suited to probe interstellar plasma existing in a wide range of different physical conditions using the many astrophysically important absorption lines present throughout the 912 to 1187 Å wavelength region (Moos et al. 2000). A particularly interesting region for FUSE to study highly disturbed interstellar gas is the Monoceros Loop Supernova Remnant (SNR). This is a well evolved remnant of age $\sim$10⁵ years, is some 100 pc in diameter and lies at a distance of $\sim$1.6 kpc (Odegard 1986). The study of SNRs is important for understanding both the energy and ionization balance of the interstellar medium, since supernova explosions and strong stellar winds from OB stars are the dominant sources of high-temperature and high-velocity shocked gas in the interstellar medium (ISM). Much of the energy released in a supernova explosion is deposited in the kinetic energy of the ejecta which subsequently interacts with the ambient interstellar gas.

The emission characteristics of the gas associated with the Monoceros Loop SNR have been well studied at radio (Graham et al. 1982), X-ray (Leahy et al. 1986), $\gamma$-ray (Jaffe et al. 1997) and visible (Fesen et al. 1985) wavelengths. These and other studies indicate that the Monoceros Loop is probably interacting with the adjacent (and nearer) Rosette Nebula (NGC 2244), which contains a cluster of several ionizing early O-type stars (NGC 2264) known to be losing appreciable amounts of mass to the ambient ISM (Kuchar $\&$ Bania 1993). Remarkably for this well-known region, only one study of the Monoceros Loop/Rosette Nebula complex has been undertaken using interstellar absorption lines. Wallerstein $\&$ Jacobsen (1976) (hereafter WJ) have observed 25 stars in the direction of this SNR region using measurements of the interstellar Ca II K-line (3393 Å) and Na I D-lines (5890 Å) taken at a spectral resolution of 12 kms^-1. These observations revealed high-velocity absorption features at $V_{\rm LSR} = +69$ kms^-1 towards the star HD 47240 and another feature at $V_{\rm LSR} = -31$ kms^-1towards HD 47359, both of which have been associated with the expansion of the nebular gas.

Although the gross kinematical properties of the Monoceros Loop/Rosette Nebula region are well documented, the detailed interactions of the associated complex ionized, neutral and molecular gas structures interaction are still much debated. For example, although WJ detected gas cloud components with 5 different velocity values towards the 25 stars that they observed, only two of the velocities of these components were common to those found by H I 21 cm observations. Furthermore, both of the high-velocity cloud components observed by WJ in the Na I and Ca II lines towards HD 47240 and HD 47359 have anomalous Na I/Ca II ratios < 1.0, which is consistent with an enhancement of interstellar Ca due to the destruction of ambient interstellar dust grains by the passage of high-velocity shocks (Siluk $\&$ Silk 1976). Observations of stars associated with the Vela SNR by Danks $\&$ Sembach (1995) have shown similarly low values of the Na I/Ca II ratio for interstellar gas clouds with velocities up to $\sim$60 km s^-1. UV observations of the same remnant have revealed that the gas at high velocity exhibits higher than normal ionization and has unusually low abundances of N I and O I (Jenkins et al. 1998), whereas the depletion of Al, Si and Fe in high velocity interstellar gas clouds is substantially less (Jenkins et al. 1984). As part of a long-term program using the $\it FUSE$ satellite in which we hope to observe the absorption characteristics of the disturbed interstellar medium towards 4 early-type stars in the direction of Monoceros Loop SNR, we currently present preliminary observations of high velocity interstellar gas absorption seen towards the B1Ib star HD 47240 (M_v = 6.2, E(B-V) = 0.31) which lies just behind the south central region of the SNR at a distance of $\sim$1800 pc. These far ultraviolet data have been supplemented with archival near ultraviolet and newly presented high resolution visible absorption line data taken towards this star to further understand the physical and chemical nature of the high-velocity interstellar gas clouds associated with this expanding supernova remnant.