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D. V. Bowen (ATC, Edinburgh/Johns Hopkins Univ.), K. Roth (Johns Hopkins Univ/IfA(UH)), D. M. Meyer (Northwestern Univ.), J. C. Blades (STScI)
We present multi-epoch, echelle-resolution optical and HST UV spectra of SN1998S which exploded in NGC3877 in March 1998. We detect narrow absorption lines from the host galaxy due to Ca~II, Na~I, Mg~I, Mg~II, Mn~II and Fe~II, although individual components show a wide variety of column density ratios. Of these narrow components, however, the column density from one line can clearly be seen to vary substantially on the time-scale of two weeks. Further, this absorbing cloud also shows variable absorption in both hydrogen Balmer and He~I lines on the same timescales, along with rare Mg~II excited lines. Narrow Fe~II fine-structure lines are also seen throughout the HST spectra at the same velocity.
In addition, broad, highly complex, asymmetric absorption is seen up to 600~km~s-1 blueward of the strong narrow complex in NGC3877, in Mg~II and Fe~II, as well as in hydrogen Balmer and He~I absorption. These structures also vary in strength with time. We associate this absorption from circumstellar absorption around the supernova. Balmer, He~I, [OIII], and [N~II] emission lines are also evident in the spectra, arising from the environment of the supernovae. The combination of narrow absorption, combined with these emission features, gives rise to classic P-Cygni type profiles, although whether the physical mechanisms which produce the features are the same as the conventional interpretation remains unclear. We will discuss possible origins for all the velocity components observed from NCG 3877, including which are most likely to be associated with circumstellar material and which lie outside of the supernova's sphere of influence.
We conclude by examining the detection of Fe~II and Mn~II from a High-Velocity Cloud in Complex M of the Milky Way, as well as several intermediate-velocity clouds.
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