Early Optical and NIR Photometry and Optical Spectroscopy of SN 2012cg

ATel #4159; G. H. Marion, P. Challis, M. Hicken, K. Mandel, S. Meyer, R. P. Kirshner, R. J. Foley, A. Friedman, J. Irwin, W. Brown, M. L. Calkins (CfA), M. Wood-Vasey (U. Pittsburgh), J. C. Wheeler (U. Texas), & J. Vinko (U. Texas & U. Szeged)
on 8 Jun 2012; 12:51 UT
Credential Certification: Howie Marion (gmarion@cfa.harvard.edu)

The Harvard-Smithsonian Center for Astrophysics Supernova Group reports photometric and spectroscopic observations of SN 2012cg (ATEL #4115). Figures displaying these data are posted at http://www.cfa.harvard.edu/supernova/sn12cg.html

Photometry of SN 2012cg was obtained between May 18 - June 2 UT with the F. L. Whipple Observatory 1.2m telescope (+KeplerCam) in uBVri bands and between May 19 - June 5 with the Peters Automated Infrared Imaging Telescope (PAIRITEL) in JHKs bands. The optical photometry was converted to the u'BVr'i' standard-system following the description in Hicken et al. 2012. It is not s-corrected. No host-galaxy subtraction was performed so the final values may be slightly different.

The BayeSN method (Mandel, Narayan, & Kirshner 2011) was used to simultaneously fit the BVr'i'JH data. The H-band maximum is found to be 12.5 mag on May 29 UT and the J-band maximum to be 12.3 on May 30. A polynomial fit to the K-band data finds the maximum to be 12.4 on May 31. BayeSN estimates that the B-band maximum occurred on June 2 at 12.1 mag and that V-band maximum occurred on June 4 at 12.0 mag. The MASTER detection of SN 2012cg on May 15 (ATEL #4138) is therefore about 18 days before the date of B-band maximum.

Analysis of the optical and infrared colors near maximum using the BayeSN method suggests A_v ~0.6 +/- 0.15. The June 2 data show that the apparent B-V color near B-maximum is ~0.1 mag, indicating a color excess at maximum of E(B-V) = 0.2, consistent with this extinction estimate. These estimates will be refined as the light curve is extended. In addition Na I D is strongly detected in the early spectra from SN 2012cg, and we conclude that there is gas along the line of sight that is accompanied by some dust.

Spectra were obtained nightly with the FLWO 1.5m telescope (+ FAST) between May 18-25. The wavelength range of these data is 340-740 nm. The first spectrum, obtained May 18.8 (15 days before the estimated time of B-band maximum) shows a broad absorption feature from Si II 635.5 nm with blue wings that extend to nearly -30,000 km/s and a velocity at the absorption minimum of -21,500 km/s. A secondary absorption component is evident in this feature and if it is produced by Si II the velocity at minimum is about -10,000 km/s. If the feature is due to C II 658.0 nm, then the velocity is about -20,000 km/s. Si II velocity declines quickly with time to -13,100 km/s on May 22 and -11,500 km/s on May 25. Narrow absorption features from Na I D and Ca II H&K are strong in all spectra in this series. A feature from C II 658.0 nm is evident at all phases with velocities of -13,000 km/s on May 22 and -12,500 km/s on May 25.