DUST FORMATION AND He II λ4686 EMISSION IN THE DENSE SHELL OF THE PECULIAR TYPE Ib SUPERNOVA 2006jc

We present evidence for the formation of dust grains in an unusual Type Ib supernova (SN) based on late-time spectra of SN 2006jc. The progenitor suffered an outburst qualitatively similar to those of luminous blue variables (LBVs) just 2 yr prior to the SN, and we propose that the dust formation is a consequence of the SN blast wave overtaking that LBV-like shell. The key evidence for dust formation is (a) the appearance of a red/near-infrared continuum emission source that can be fit by T ≈ 1600 K graphite grains, and (b) fading of the redshifted sides of intermediate-width He i emission lines, yielding progressively more asymmetric blueshifted lines as dust obscures receding material. This provides the strongest case yet for dust formation in any SN Ib/c. Both developments occurred between 51 and 75 d after peak brightness, while the few other SNe observed to form dust did so after a few hundred days. Geometric considerations indicate that dust formed in the dense swept-up shell between the forward and reverse shocks, and not in the freely expanding SN ejecta. The rapid cooling leading to dust formation may have been aided by extremely high shell densities of 10 cm, indicated by He i line ratios. The brief epoch of dust formation is accompanied by He ii λ4686 emission and enhanced X-ray emission, suggesting a common link. These clues suggest that the unusual dust formation in this object was not attributable to properties of the SN itself, but instead — like most peculiarities of SN 2006jc — was a consequence of interaction with the dense environment created by an LBV-like eruption 2 yr before the SN. Subject headings: dust, extinction — stars: mass loss — stars: winds, outflows — stars: Wolf-Rayet — supernovae: individual (SN 2006jc)