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We report observations of SN 2004dt obtained with the Very Large Telescope ofthe European Southern Observatory on August 13.30, 2004 when the supernova wasmore 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 velocitiesof absorption minimum around 17,000 \kms. The line profiles show materialmoving at velocities as high as 25,000 \kms in these lines. The observationsalso reveal absorption lines from \ion{S}{2} and \ion{Si}{3} with a velocity ofonly 11,000 \kms. The highest velocity in the \ion{S}{2} features can be tracedno higher than 15,000 \kms, much lower than those of O, Mg, Si, and Ca. SN2004dt has a polarization spectrum unlike any previously observed. Thevariation 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 burnedsilicon layer with substantial departure from spherical symmetry. A geometrythat would account for the observations is one in which the distribution ofoxygen is essentially spherically symmetric, but with bubbles ofintermediate-mass elements with significant opacity within the oxygensubstrate.Comment: Submitted to Ap

Premaximum Spectropolarimetry of the Type Ia SN 2004dt