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 seven days ( UV data ) .

From our own Galaxy we detect interstellar absorption lines of \ion Ca2 K , \ion Fe2 \lambda \lambda 2600 , 2586 , 2374 , 2344 , \ion Mg1 \lambda 2852 , and probably \ion Mn2 \lambda 2576 , at v _ { LSR } \ > = \ > -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 \ion Mg1 \lambda 2852 , \ion Mn2 \lambda \lambda 2576 , 2594 , 2606 , \ion Ca2 K and \ion Na1 D _ { 2 } , D _ { 1 } 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 ^ { > } _ { \sim } \ > 1 dex in N ( \ion Na1 ) between 20-March-1998 and 8-April-1998 , and in N ( \ion Mg1 ) between 4-April-1998 and 11-April-1998 .

Unusually , our data also show narrow Balmer , \ion He1 , and metastable UV \ion Fe2 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 \sim 350 km s ^ { -1 } and is seen in \ion Mg2 \lambda 2796 , 2803 absorption as well , although there is no evidence of narrow \ion Mg2 emission forming a P-Cygni profile .
There is some suggestion that this shelf has decreased in strength over seven days between April-4 and April-11 .

Most of the low-ionization absorption can be understood in terms of gas co-rotating with the disk of NGC 3877 , providing 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 .