We present observations of the Type Ia supernova 2003du obtained with the Hobby ^ { * } Eberly Telescope ( HET ) 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 ( \approx 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 8,000 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 ( \approx 5 \times 10 ^ { -2 } M _ { \odot } ) , the high-velocity component of the Ca ii line merges with the photospheric line forming a broad feature .
A cut-off of the blue wings of strong , un-blended 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 due to Thomson scattering , resulting in larger ( B - V ) colors by up to 0.3 ^ { m } .

We apply these diagnostic tools to SN 2003du and infer that about 2 \times 10 ^ { -2 } M _ { \odot } 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 can not 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 SNe Ia and may have eluded discovery because optical spectra often do not extend significantly beyond 7500 Å .