An Ultraviolet Spectral Atlas of Interstellar Lines toward SN 1987A

Abstract

We present a spectral atlas of the interstellar spectrum toward SN 1987A in the Large Magellanic Cloud. The data used in constructing the atlas were obtained with the IUE satellite as part of the ESA Target of Opportunity Program for observing bright supernovae and were collected within the first few days following discovery of SN 1987A. At that time the supernova was sufficiently bright at ultraviolet wavelengths to allow short exposures to be made even in the high-resolution mode of IUE. Twelve high-resolution spectra were taken, evenly divided between the short- and long-wavelength cameras. The resulting spectra have good resolution and very low background levels and are of a higher quality than previously obtained for Magellanic Cloud stars. The atlas was constructed from two or more common echelle orders having adequate exposure levels; it extends from 1250 to 3200 A. The atlas includes almost 200 lines from 12 interstellar species, covering a wide range of ionization, namely: C I, II, IV; O I; Mg I, II; Al II, III; Si II, IV; Cl I; Cr II; Mn II; Fe II; Ni II; and Zn II. Fine structure lines of C I, II, and Si II are also seen, and there are a number of unidentified features. Optical spectroscopy at very high resolution has shown the extraordinarily large number of interstellar components in this direction. In Ca II there are at least 27 velocity components spread over 300 km s^-1^. At the lower resolution of IUE, the ultraviolet profiles are difficult to interpret. Nevertheless, we identify 10 different velocity groups at - 17, 15, 26, 70, 129, 171, 206, 226, 264, and 281 km s^-1^; they have optical counterparts except for the features at - l7 and possibly 206 km s^-1^. The dominant LMC component occurs at 281 km s^-1^ and may be part of the extensive sheet of material in front of the H II region, 30 Doradus. Fine-structure lines occur in a separate cloud at 264 km s^-1^. Only high ions occur in the cloud at 206 km s^-1^. We interpret our results in the context of existing optical and ultraviolet studies of interstellar gas toward the LMC.