Ultraviolet Observations of Interstellar Absorption Lines toward SN 1987A

Abstract

An average of all high-resolution UV spectra of SN 1987A recorded by IUE a few days after the explosion reveals numerous interstellar absorption lines between 1200 and 3100 A. Features produced in the neutral medium of our Galaxy and the LMC (e.g., those arising from O I, C I, C II, Mg I, Mg II, Fe II, Mn II, Si II, Ni II, etc.), together with lines from much higher stages of ionization (Al III, Si IV, and C IV), were recorded with far greater fidelity than for previous observations toward stars in the LMC, owing to the supernova's extraordinary brightness and very smooth continuum. Although profiles are shown for a large number of species, our study emphasizes the absorption produced by the more highly ionized species including Al III, Si IV, and C IV. The observed absorption line velocity structure which covers the heliocentric velocity range from -30 to 330 km s^-1^ is very complex. In our discussions, we assume that the absorption with v < 120 km s^-1^ occurs in the Milky Way and its halo, while the absorption with v > 190 km s^-1^ occurs in the LMC. The line profiles for Al III, Si IV, and C IV are converted into plots of optical depth (and column density) versus velocity. In the velocity range which we believe pertains to the Milky Way, we find that the profiles for Si IV and C IV are quite similar and have much less structure than the Al III profile. On relating column densities, we find that while the C IV to Si IV ratio is relatively constant (ranging from 3 to 5) over the velocity range from 0 to 100 km s^-1^, the C IV to Al III and Si IV to Al III ratios vary by nearly a factor of 10. This suggests that the C IV and Si IV along this sight line in the Milky Way and its halo may have a common origin which differs from that for Al III. Al III is almost certainly produced by ultraviolet photoionization in the gas of the Milky Way disk and low halo. A different origin is therefore suggested for Si IV and C IV. Absorption lines created by gas in the immediate surroundings of the supernova are hard to differentiate from the very strong features created by foreground, more generally distributed material in the LMC. The best line for highlighting gas very near the supernova, the 1264 A line from Si II in an excited fine-structure level, indicated that out to ~ 10 pc from the supernova the density of a mass-loss wind from the progenitor in its red giant phase is consistent with a total flux divided by the wind velocity M/v_w_ < 1 x 10^-6^ M_sun_ yr^-1^ (km s^-1^)^-1^. With the small accumulation of gas from such a wind, it does not appear possible that the pulse of energetic photons created during the supernova's shock breakout could explain the strong Si IV and C IV features at LMC velocities.