SN 2003du: Signatures of the Circumstellar Environment in a Normal Type Ia Supernova?

We present observations of the Type Ia supernova 2003du and report thedetectionof an unusual, high-velocity component in the Ca II infrared triplet,similar tofeatures previously observed in SN 2000cx and SN 2001el. This featureexhibits a large expansion velocity (~18,000 km/s) which is nearly constantbetween -7 and +2 days relative to maximum light, and disappears shortlythereafter. Otherthan this feature, the spectral evolution and light curveresemble those of a normal SN Ia. We find that the Ca II feature can plausibly be caused by a dense shellformed when circumstellar material of solar abundance is overrun by the rapidlyexpanding outermost layers of the SN ejecta. Model calculations show that theoptical and infrared spectra are remarkably unaffected by the circumstellarinteraction. In particular, no hydrogen lines are detectable in eitherabsorption or emission. The only qualitatively different features are thestrong, high-velocity feature in the Ca II IR-triplet, and a somewhat weaker OI feature near 7,300 AA. The morphology and time evolution of these featuresprovide an estimate for the amount of accumulated matter and an indication ofthe mixing in the dense shell. We apply these diagnostic tools to SN 2003du andinfer that about 2 x 10^{-2} M_sun of solar abundance material may haveaccumulated in a circumstellar shell prior to the observations. Furthermore,the early light curve data imply that the circumstellar material was originallyvery close to the progenitor system, perhaps from an accretion disk, Roche lobeor common envelope.Comment: 35 Pages, 11 Figures, to appear in ApJ. Resubmission includes expanded discussion & new figures to match with accepted journal versio