PROBING THE INTERSTELLAR MEDIUM ALONG THE LINES OF SIGHT TO SUPERNOVAE SN 1994D AND SN 1994I

摘要

We present high-resolution (～ 8 km s~(-1)) echelle observations of SN 1994D and 1994I covering the optical Na D interstellar absorption lines and the region around Hα. The high-quality spectra reveal a complex pattern of Na D absorption systems along the path to both supernovae. The velocity systems can be identified with contributions from the Galactic disk, the host galaxy of each supernova, and absorbing material having "anomalous" velocities inconsistent with simple rotation within our Galaxy or the respective host galaxies. The anomalous velocity gas along the line of sight to SN 1994D may be intergalactic in origin and is infal-ling toward NGC 4526, the Virgo cluster host galaxy of SN 1994D. We consider several hypotheses for the anomalous velocity systems seen toward SN 1994I. The most likely explanation is that they are optical counterparts of "high-velocity clouds," normally studied in 21 cm emission, associated with either the Galactic halo or the halo of the host galaxy NGC 5194 (M51). These metal-bearing, low-column density clouds may be much more common than previously thought. At least some metal-line QSO absorption systems may be associated with high-velocity clouds in the halos of intervening galaxies. We use the derived Na column densities to estimate the extinction of the two supernovae. For SN 1994D, we obtain A_V = 0.08_(-0.04)~(+0.08) mag, consistent with independent estimates based on considerations of the supernova colors near maximum brightness. Several complications conspire to make the extinction estimate for SN 1994I very uncertain. Our best estimate yields A_V = 3.1_(-1.5)~(+3.1) mag, although there are reasons to believe that the true extinction is approx < 1.4 mag. Finally, we note that no narrow Hα emission or absorption is detected to 2 σ equivalent width limits of 3 mA in the spectrum of SN 1994D, a Type Ia supernova. Unfortunately, this observation does not provide very significant constraints on models of the progenitor; SN 1994D was observed ～ 3 weeks after maximum brightness, by which time little if any Hα would be expected even if circumstellar hydrogen were present.