Helium Emission Lines in the Type I[CLC]c[/CLC] Supernova 1999[CLC]cq[/CLC]

We present the first unambiguous detection of helium emission lines inspectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with fullwidth at half maximum ~ 2000 km/s, and the distinct absence of any otherintermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic1999cq are interacting with dense circumstellar material composed of almostpure helium. This strengthens the argument that the progenitors of SNe Ic arecore-collapse events in stars that have lost both their hydrogen and heliumenvelopes, either through a dense wind or mass-transfer to a companion. In thisway, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J thathelped firmly establish a physical connection between Type Ib and Type IIsupernovae. The light curve of SN 1999cq is very fast, with an extremely rapidrise followed by a quick decline. SN 1999cq is also found to exhibit a highlevel of emission at blue wavelengths (< 5500 A), likely resulting from eitheran unusually large amount of iron and iron-group element emission oruncharacteristically low reddening compared with other SNe Ic.Comment: 17 pages (AASTeX V5.0), 4 figures, accepted for publication in the Astronomical Journa