2 Observations and data processing

The spectral integrations accumulated with the $\it FUSE$instrument were obtained in the histogram integration mode on March 15th, 2000. The observations were performed with the star centered in the $30 \times 30$ arcsec aperture of the LiF1 channel of the $\it FUSE$ spectrograph, but due to observational difficulties no data were obtained in the $\it FUSE$ SiC spectral channels (which cover the short wavelength end of the bandpass). Thus, the far UV data presented in this paper consists of eleven separate, 500 s long integrations that cover the 990-1187 Å wavelength region, and all data have been individually processed using version 1.8.3 of the $\it FUSE$ science data reduction (CALFUSE) pipeline. This processing of the detector raw histogram data nominally accounts for geometric image distortions, background level subtraction, image drift due to instrument thermal problems, detector deadtime, wavelength and flux calibration (Sahnow et al. 2000).

Each of the 11 processed spectra were co-added and averaged using standard IDL data reduction routines. The data contained in the LiF1a and LiF1b detector segments were deemed to be of the best quality and these were subsequently used in the following spectral analysis. (However, we note that the LiF2a and LiF2b spectra were used to check for consistency in all the line detections that we shall now report.) For the purposes of this preliminary paper we have restricted our analysis to the two wavelength regions shown in Figs. 1 and 2 that cover the 1030-1050 Å and 1130-1155 Å ranges respectively.

Figure 1: HD 47240 - Average FUSE LiF1a wavelength corrected spectrum.

Figure 2: HD 47240 - Average FUSE LiF1b wavelength corrected spectrum.

The zero point of the wavelength scale was determined with reference to the many H₂molecular lines that were detected in both spectral regions. We have assumed that these lines occur at the same velocity as that of the visible CH molecular line at 4232.5 Å (i.e. V = + 3 kms^-1), as observed at a spectral resolution of 5 kms^-1 by Sfeir (1999). For the convenience of comparison with radio data, all velocities in this paper are in the local standard of rest (LSR) frame. The resultant $\it FUSE$ spectra typically have a S/N ratio >15:1 and a velocity resolution of $\sim$13 kms^-1 as determined from absorption line-profile fitting of the weak interstellar lines of H₂ at 1041.2 Å and 1047.6 Å .

In addition, we have supplemented the $\it FUSE$ far UV data with high spectral resolution observations of the Na I (5890 Å) and Ca II (3933 Å) interstellar lines taken at the Lick Observatory and with ultraviolet observations of HD 47240 extracted from the on-line $\it IUE$ data archive (Rodriguez-Pascual et al. 1999) that have a spectral resolution of $\sim$15 kms^-1, which is comparable to that of the $\it FUSE$ data. Further details of the visible and $\it IUE$ data gained towards HD 47240 and other stars in the Monoceros Loop SNR have been presented in the Ph.D. Thesis of Sfeir (1999).