Helium emission lines in the type Ic supernova 1999cq

We present the first unambiguous detection of helium emission lines in spectra of Type Ic supernovae (SNe Ic). The presence of He I lines with FWHM ≈ 2000 km s^-1 and the distinct absence of any other intermediate-width emission (e.g., Hα) implies that the ejecta of SN Ic 1999cq are interacting with dense circumstellar material composed of almost pure helium. This strengthens the argument that the progenitors of SNe Ic are core-collapse events in stars that have lost both their hydrogen and helium envelopes, either through a dense wind or mass transfer to a companion. In this way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that helped firmly establish a physical connection between Type Ib and Type II supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid rise followed by a quick decline. SN 1999cq is also found to exhibit a high level of emission at blue wavelengths (≲ 5500 Å), likely resulting from either an unusually large amount of iron and iron-group element emission or uncharacteristically low reddening compared with other SNe Ic.