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We present observations of the Type Ia supernova 2003du obtained with the Hobby-Eberly Telescope and report the detection of a high-velocity component in the Ca II infrared triplet near 8000 Å, similar to features previously observed in SN 2000cx and SN 2001el. This feature exhibits a large expansion velocity (≈18,000 km s-1), which is nearly constant between -7 and +2 days relative to maximum light and disappears shortly thereafter. Other than this feature, the spectral evolution and light curve of SN 2003du resemble those of a normal SN Ia. We consider a possible origin for this high-velocity Ca II line in the context of a self-consistent spherical delayed-detonation model for the supernova. We find that the Ca II feature can be caused by a dense shell formed when circumstellar material of solar abundance is overrun by the rapidly expanding outermost layers of the SN ejecta. Model calculations show that the optical and infrared spectra are remarkably unaffected by the circumstellar interaction and the resulting shell. In particular, no hydrogen lines are detectable in either absorption or emission after the phase of dynamic interaction. The only qualitatively different features in the model spectra are the strong, high-velocity feature in the Ca II IR triplet around 8000 Å and a somewhat weaker O I feature near 7,300 Å. The Doppler shift and time evolution of these features provides an estimate for the amount of accumulated matter (decreasing Doppler shift with increasing shell mass) and also an indication of the mixing within the dense shell. For high shell masses (≈5 × 10-2 M⊙), the high-velocity component of the Ca II line merges with the photospheric line forming a broad feature. A cutoff of the blue wings of strong, unblended lines (particularly the Si II feature at about 6,000 Å) may also be observable for larger shell masses. The model SN Ia light curves are little effected except at very early times when the shell is partially optically thick because of Thomson scattering, resulting in larger (B-V) colors by up to 0.3 mag. We apply these diagnostic tools to SN 2003du and infer that about 2 × 10-2 M⊙ of solar abundance material may have accumulated in a shell prior to the observations. Furthermore, in this interpretation, the early light-curve data imply that the circumstellar material was originally very close to the progenitor system, perhaps from an accretion disk, Roche lobe, or common envelope. Because of the observed confinement of Ca II in velocity space and the lack of ongoing interaction inferred from the light curve, the matter cannot be placed in the outer layers of the exploding white dwarf star or related to a recent period of high mass loss in the progenitor system prior to the explosion. We note that the signatures of circumstellar interaction could be rather common in Type Ia supernovae and may have eluded discovery because optical spectra often do not extend significantly beyond 7500 Å.

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Astrophysical Journal

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