2008CBET.1395....1Y

F. Yuan, University of Michigan; R. Quimby, California Institute of Technology; D. Chamarro, M. D. Sisson, and C. Akerlof, University of Michigan; and J. C. Wheeler, University of Texas, on behalf of the ROTSE collaboration, report the discovery of a supernova (mag about 18.5) in unfiltered CCD images taken on June 1.33 UT with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory. The new object is located at R.A. = 16h32m38s.27, Decl. = +41o27'33".2 (equinox 2000.0; uncertainty < 1"), which is 1".3 east and 2".4 north of the center of a galaxy (with z = 0.032 as measured by the Sloan Digital Sky Survey). A finding chart for 2008cw can be found at website URL http://www.rotse.net/rsvp/j163238.3+412733/j163238.3+412733.jpg. A spectrum (420-890 nm) of 2008cw, obtained on June 3.15 with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph) by M. Shetrone and E. Terrazas, shows it to be a supernova, the spectral features being roughly similar to those of the peculiar type-Ib supernova 2005bf at 32 days before maximum light (Folatelli et al. 2006, Ap.J. 641, 1039), except that the 560- to 600-nm region is more similar to that of the type-Ic supernova 1994I around 5 days before maximum light (Filippenko et al. 1995, Ap.J. 450, L11). Specifically, an absorption trough is seen at about 573 nm (z = 0.03 rest frame), which – if identified with He I/Na I – would correspond to a blueshift of about 8000 km/s. A weak P-Cyg profile, corresponding to the Ca II infrared triplet at this same velocity, is also detected. The O I 777-nm feature is not detected, a situation similar to that of SN 2005bf, but contamination from the host galaxy may help mask this feature and dilute the Ca II infrared triplet, as well.