Probing the Interstellar Medium Along the Lines of Sight to Supernovae 1994D and 1994I

Previous abstract Next abstract

Session 12 -- Interstellar medium
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[12.06] Probing the Interstellar Medium Along the Lines of Sight to Supernovae 1994D and 1994I

L.C.Ho and A.V.Filippenko (U.C. Berkeley)

We present high-resolution ($\sim$ 8 km s$^{-1}$) echelle observations of SN~1994D and 1994I covering the optical Na~D interstellar absorption lines and the region around H$\alpha$. 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 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 infalling 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 $A_V\,=\,0.08^{+0.08}_{-0.04}$ mag, consistent with independent estimates based on considerations of the light curve. Several complications conspire to make the extinction estimate for SN~1994I very uncertain. Our best estimate yields $A_V\,=\,3.1^{+3.1}_{-1.5}$ mag, although there are reasons to believe that the true extinction is ${_<\atop^{\sim}}$ 1.4 mag. Finally, we note that no narrow H$\alpha$ emission or absorption is detected to 2$\sigma$ equivalent width limits of 3 m\AA\ 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 $\sim 3$ weeks after maximum brightness, by which time little if any H$\alpha$ would be expected even if circumstellar hydrogen were present.

Monday program listing