2006ApJ...653..490W

We report observations of SN 2004dt obtained with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) on 2004 August 13.30, when the supernova was more than a week before optical maximum. SN 2004dt showed strong lines of O I, Mg II, Si II, and Ca II, with typical velocities of absorption minimum around 17,000 km/s. The line profiles show material moving at velocities as high as 25,000 km/s in these lines. The observations also reveal absorption lines from S II and Si III with a velocity of only 11,000 km/s. The highest velocity in the S II features can be traced to about 15,000 km/s, much lower than those of O, Mg, Si, and Ca. SN 2004dt has a polarization spectrum unlike any previously observed. The variation of the polarization across some Si II lines approaches 2%, making SN 2004dt the most highly polarized SN Ia ever observed. In contrast, the strong line of O I at 777.4 nm shows little or no polarization signature. The degree of polarization points to a silicon layer with substantial departure from spherical symmetry. A geometry that would account for the observations is one in which the distribution of oxygen is essentially spherically symmetric, but with protrusions of intermediate-mass elements within the oxygen-rich region.