Dust Formation and He<scp>ii</scp>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 SN\nbased on late-time spectra of SN 2006jc. The progenitor suffered an LBV-like\noutburst just 2 yr earlier, and we propose that the dust formation is a\nconsequence of the SN blast wave overtaking that LBV-like shell. The key\nevidence for dust formation is (a) the appearance of a red/near-IR continuum\nsource fit by 1600 K graphite grains, and (b) fading of the redshifted sides of\nHe I emission lines, yielding progressively more asymmetric blueshifted lines\nas dust obscures receding material. This provides the strongest case yet for\ndust formation in any SN Ib/c. Both developments occurred between 51 and 75 d\nafter peak, while other SNe observed to form dust did so after a few hundred\ndays. Geometric considerations indicate that dust formed in the dense swept-up\nshell between the forward and reverse shocks, and not in the freely expanding\nSN ejecta. Rapid cooling leading to dust formation may have been aided by\nextremely high shell densities, as indicated by He I line ratios. The brief\nepoch of dust formation is accompanied by He II 4686 emission and enhanced\nX-ray emission. These clues suggest that the unusual dust formation in this\nobject was not due to properties of the SN itself, but instead -- like most\npeculiarities of SN 2006jc -- was a consequence of the dense environment\ncreated by an LBV-like eruption 2 yr before the SN.\n