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Updated Physical Parameters of SN 2012cg

ATel #4215; G. H. Marion, P. Challis, M. Hicken, K. Mandel, S. Meyer, R. P. Kirshner, R. J. Foley, A. Friedman, J. Irwin, (CfA) W. M. Wood-Vasey (U. Pittsburgh), J. C. Wheeler (U. Texas), J. Vinko (U. Texas & U. Szeged), K. Rines & S. Wilhelmy (Western Washington University), and L. Macri (Texas A&M)
on 26 Jun 2012; 16:25 UT
Credential Certification: Howie Marion (gmarion@cfa.harvard.edu)

Subjects: Infra-Red, Optical, Supernovae

Referred to by ATel #: 4226

The Harvard-Smithsonian Center for Astrophysics Supernova Group reports photometric and spectroscopic observations of SN 2012cg (ATEL #4115, #4159). We find that SN 2012cg has a slow decline rate and low expansion velocities. BayeSN fits to the data show that SN 2012cg has significant dust extinction (A_v ~ 0.67 mag). We find R_v = 2.7 +/- 0.5, which is consistent with the Milky Way value of 3.1 and mildly inconsistent with the extremely low values reported for some highly reddened SN (e.g., R_v = 1.59 +/- 0.07 for SN 2002cv; Elias-Rosa et al. 2008, MNRAS, 384, 107). Fits to the data with MLCS2k2.v007 agree with BayeSN that this is a slow decliner with Delta = -0.37 and t_Bmax = 56081.6. For an assumed R_v = 3.1, MCLS2k2 finds A_v = 0.54 +/- 0.13.

Figures displaying these data are posted at http://www.cfa.harvard.edu/supernova/sn12cg.html.

OIR photometry of SN 2012cg was obtained from May 18 to June 20 UT with the F. L. Whipple Observatory 1.2m telescope (+KeplerCam) and the Peters Automated Infrared Imaging Telescope (PAIRITEL). The BayeSN method (Mandel, Narayan, & Kirshner 2011, ApJ 730, 120) was used to simultaneously fit the BVr'i'JH data. JHKs fits to the NIR templates found in Wood-Vasey et al. 2008, ApJ, 689, 377.

We note that the first B-V data point (about 3 days after the explosion) is very blue. Both FLWO data and KAIT data exhibit this characteristic (Silverman et al. 2012, ApJL (submitted) arXiv1206.1328). This result is consistent with predictions for early B-V behavior in the case of SN interaction with a Red Giant companion (Kasen 2010, ApJ, 708, 1025).

BayeSN results:
A_V = 0.67 +/- 0.09 for R_V = 2.7 +/- 0.5

dm15(B) = 0.86 +/- 0.03
dm15(V) = 0.48 +/- 0.03

B-V at Bmax = 0.12 mag
Bmax - Vmax = 0.13 mag

B_at_Bmax = 12.11 at MJD 56081.0 (June 2.5)
V_at_Vmax = 11.98 at MJD 56083.4
r'_at_r'max = 12.02 at MJD 56082.0
i'_at_i'max = 12.39 at MJD 56079.5
J_at_Jmax = 12.28 at MJD 56078.3
H_at_Hmax = 12.52 at MJD 56076.6
Ks_at_Kmax = 12.47 at MJD 56079.7

(Peak magnitudes and times of maximum for r', i', & Ks bands were found by a polynomial fit to the data.)

Optical spectra were obtained with the FLWO 1.5m telescope (+ FAST) and the Hobby-Eberly Telescope (Ramsey et al. 1998) at McDonald Observatory using the Marcario Low-Resolution Spectrograph (Hill et al. 1998) from May 18 through June 20. The first spectrum, obtained May 18.8 (15 days before the time of B-band maximum) shows the Si II 6355 A velocity at the absorption minimum of -21,600 km/s. Si II velocities decline quickly with time to -11,300 km/s on May 24 (-8d). The velocity reaches about -10,000 km/s near the time of B_max and stays nearly constant thereafter. We note that the low expansion velocity and relatively high R_v is consistent with the trend between expansion velocity and R_v for highly reddened SN (Wang et al., 2009, ApJL, 699, 139). Narrow absorption features from Na I D and Ca II H&K are strong in all spectra.

We are grateful for support from the National Science Foundation through AST-0907903 to Harvard University.