2011CBET.2860....1V

J. Vinko, University of Szeged; J. C. Wheeler and E. Chatzopoulos, University of Texas; G. H. Marion, Harvard-Smithsonian Center for Astrophysics; K. Sarneczky, Konkoly Observatory; K. Takats and T. Szalai, University of Szeged; and W. Zheng, University of Michigan, report that a spectrogram of SN 2011go (cf. CBET 2849) was obtained on Oct. 2.42 UT with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph; wavelength range 430-1000 nm, resolution 2 nm) at McDonald Observatory by M. Shetrone. The spectrum contains broad P-Cyg Balmer-lines, superimposed on a reddened continuum, plus a strong narrow H-alpha emission peak on top of the broad H-alpha emission component, as reported on CBET 2849. However, inspection of the 2D spectral image reveals that the source of the narrow emission component is the host galaxy, not the supernova. Thus, SN 2011go is probably not a super-luminous SN-2006gy-like event having both broad and narrow P-Cyg line profiles, contrary to the previous classification.

Features of He I 587.8-nm and O I 777.3-nm are also present in the spectrum. Comparison with supernova templates via the SNID code (Blondin and Tonry 2007, Ap.J. 666, 1024) gives early-phase spectra of the normal type-II-P supernovae 1999em and 2006bp as the best resembling templates. The redshift, estimated from the narrow H-alpha emission peak, is z = 0.02, in perfect agreement with the value listed in NED (Theureau et al. 1998, A.Ap. Suppl. 130, 333). The slope of the continuum suggests interstellar reddening of at least E(B-V) about 0.7 mag, which is mostly intrinsic within the host galaxy (PGC 19876 = MCG +07-15-2 ), since the galactic E(B-V) component is only 0.085 mag (Schlegel et al. 1998, Ap.J. 500, 525). Correcting for E(B-V) = 0.7 mag, the spectrum becomes similar to the early-phase spectra of SN 2011dh (Maund et al. 2011, Ap.J. 739, L37). Thus, it is more likely that SN 2011go is a heavily reddened, but otherwise normal, type-II-P supernova close to maximum light, although a type-IIb classification may also be possible.

Simultaneous BVRI photometry obtained with the 1.0-m RCC telescope at Konkoly Observatory (Hungary) on Oct. 2.1 UT under photometric conditions gives B = 18.88 ± 0.17, V = 18.19 ± 0.06, R = 17.43 ± 0.09, and I = 16.74 ± 0.09 mag (tied to Landolt photometric sequences). Adopting the luminosity distance to the host galaxy from NED (D about 87 Mpc) and the interstellar extinction estimated above [E(B-V) about 0.7 mag, implying A_V about 2.2 mag) the absolute magnitude of SN 2011go in the V-band is about -18.7, similar to other type-II supernovae. This supports the conclusion drawn above that SN 2011go is a normal type-II supernova, and not a super-luminous SN-2006gy-like event.