SN 2006bp: Probing the Shock Breakout of a Type II-P Supernova

HET optical spectroscopy and unfiltered ROTSE-III photometry spanning the first 11 months since explosion of the Type II-P SN 2006bp are presented. The data suggest that the supernova was first detected just hours after shock breakout. Optical spectra obtained about 2 days after breakout exhibit narrow emission lines corresponding to He II λ4200, He II λ4686, and C IV λ5805 in the rest frame. These emission features persist in a second observation obtained 5 hr later but are not detected the following night or in subsequent observations. These lines probably emanate from material close to the explosion site, possibly in the outer layers of the progenitor that have been ionized by the high-energy photons released at shock breakout. A P Cygni profile is observed around 4450 A in the +2 and +3 day spectra. We propose that this line is due to He II λ4687 rather than high-velocity Hβ, as previously suggested. Further HET spectra cover the evolution across the photometric plateau up to 73 days after breakout and during the nebular phase around day +340. Expansion velocities are derived for key features. The measured decay slope for the unfiltered light curve is 0.0073 ± 0.0004 mag day-1 between days +121 and +335, which is significantly slower than the decay of rate 56Co. We present a quasi-bolometric light curve through day +60. We see a slow cooling over the first 25 days but no sign of an early sharp peak; any such feature from the shock breakout must have lasted less than ~1 day.

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