Interstellar and Circumstellar Optical and Ultraviolet Lines toward SN 1998S

Abstract

We have observed SN 1998S which exploded in NGC 3877, with the Utrecht Echelle Spectrograph (6-7 km s-1 FWHM) at the William Herschel Telescope and with the E230M echelle of the Space Telescope Imaging Spectrograph (8 km s-1 FWHM) aboard the Hubble Space Telescope. Both data sets were obtained at two epochs, separated by 19 (optical) and 7 days (UV data). From our own Galaxy we detect interstellar absorption lines of Ca II K, Fe II λλ2600, 2586, 2374, 2344, Mg I λ2852, and probably Mn II λ2576, at vLSR = -95 km s-1 arising from the outer edge of the High Velocity Cloud Complex M. We derive gas-phase abundances of [Fe/H] = -1.4 and [Mn/H] = -1.0, values which are very similar to warm disk clouds found in the local ISM. This is the first detection of manganese from a Galactic HVC, and we believe that the derived gas-phase abundances argue against the HVC material having an extragalactic origin. At the velocity of NGC 3877 we detect interstellar Mg I λ2852, Mn II λλ2576, 2594, 2606, Ca II K and Na I D2, D1 absorption lines, spanning a velocity range of -102 to +9 km s-1 from the systemic velocity of the galaxy (910 km s-1). Surprisingly, the component at -102 km s-1 is seen to increase by a factor of 1 dex in N(Na I) between 1998 March 20 and April 8, and in N(Mg I) between 1998 April 4 and April 11. Unusually, our data also show narrow Balmer, He I, and metastable UV Fe II P Cygni profiles, with a narrow absorption component superimposed on the bottom of the profile's absorption trough. Both the broad and narrow components of the optical lines are seen to increase substantially in strength between March 20 and April 8. The broad absorption covers ~350 km s-1 and is seen in Mg II λλ2796, 2803 absorption as well, although there is no evidence of narrow Mg II emission forming a P Cygni profile. There is some suggestion that this shelf has decreased in strength over 7 days between April 4 and April 11. Most of the low-ionization absorption can be understood in terms of gas corotating with the disk of NGC 3877, provided the supernova is at the back of the disk as we observe it, and the H I disk is of a similar thickness to our own Galaxy. However, the variable component seen in all the other lines, and the accompanying emission which forms the classic P Cygni profiles, most likely arise in slow-moving circumstellar outflows originating from the red supergiant progenitor of SN 1998S.