Premaximum Spectropolarimetry of the Type Ia SN 2004dt**Based on observations collected at the European Southern Observatory (ESO), Chile [ESO program 073.D-0565(A)].
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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-1. The line profiles show material moving at velocities as high as 25,000 km s-1 in these lines. The observations also reveal absorption lines from S II and Si III with a velocity of only 11,000 km s-1. The highest velocity in the S II features can be traced to about 15,000 km s-1, 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 la 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. 2006. The American Astronomical Society. All rights reserved.