Pre-Maximum Spectropolarimetry of the Type Ia SN 2004dt
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abstract
We report observations of SN 2004dt obtained with the Very Large Telescope of the European Southern Observatory on August 13.30, 2004 when the supernova was more than a week before optical maximum. SN 2004dt showed strong lines of ion{O}{1}, ion{Mg}{2}, ion{Si}{2}, and ion{Ca}{2} with typical velocities of absorption minimum around 17,000 kms. The line profiles show material moving at velocities as high as 25,000 kms in these lines. The observations also reveal absorption lines from ion{S}{2} and ion{Si}{3} with a velocity of only 11,000 kms. The highest velocity in the ion{S}{2} features can be traced no higher than 15,000 kms, 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 ion{Si}{2} 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 richly-structured partially burned 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 bubbles of intermediate-mass elements with significant opacity within the oxygen substrate.
