On the Explosion Geometry of Red Supergiant Stars

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From progenitor studies, type II-Plateau supernovae (SNe II-P) have been decisively and uniquely determined to arise from isolated red supergiant stars, establishing the most homogeneous --- and well understood --- progenitor class of any type of core-collapse supernova. The precise nature of the mechanism responsible for the stellar explosion, however, remains the subject of considerable debate. A fundamental clue to the nature of the explosion mechanism is explosion geometry: In short, are supernovae round? Because young supernova atmospheres are electron-scattering dominated, their net linear polarization provides a direct probe of early-time supernova geometry, with higher degrees of polarization generally indicating greater departures from spherical symmetry.
Here we present spectropolarimetry data for the most well-sampled SN II-P to date, SN 2008bk, and compare (and contrast) the results with those obtained for SN 2004dj, the only other SN II-P for which spectropolarimetry data were obtained with similar fine temporal sampling before, during, and after the fall off of the photometric plateau (Leonard et al. 2006). Both objects are polarized, indicating departures from spherical symmetry, although the timing of the onset -- as well as the persistence -- of the polarization differ between the two objects. Curiously, the detailed spectropolarimetric characteristics of the two objects at the epochs of recorded maximum polarization are extremely similar, feature by feature, suggesting a common cause --- or, at least, geometry. We interpret the data in light of non-Local-Thermodynamic Equilibrium, time-dependent radiative-transfer simulations specifically crafted for SN II-P ejecta.
DCL acknowledges support from NSF grant AST-1009571, under which part of this research was carried out. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under observing programs 081.D-0128, 082.D-0151, and 085.D-0391 (PI: Dessart).

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