* Astronomy

Title: VLT Spectropolarimetry of the Type Ia SN2005ke - A step towards understanding subluminous events
Authors: F. Patat, P. Hoeflich, D. Baade, J. R. Maund, L. Wang, J. C. Wheeler

We performed optical spectropolarimetric observations of the subluminous Type Ia SN2005ke at 3 epochs (days -8, -7, and +76). The explosion properties are derived by comparing the data to explosion and radiation transfer models. The SN shows polarimetric properties that are very similar to the only other subluminous event for which spectropolarimetry is available, i.e. SN1999by. The data present a very marked dominant axis, which is shared by both the continuum and lines such as SiII 6355, suggesting that the relatively large, global asymmetry is common to the photosphere and the line-forming region. The maximum polarisation degree observed in the SiII 6355 absorption reaches 0.39±0.08%. At variance with what is seen in core-normal Type Ia, SN2005ke displays significant continuum polarisation, which grows from the blue to the red and peaks at about 7000 A, reaching ~0.7%. The properties of the polariaation and flux spectra can be understood within the framework of a subluminous delayed-detonation (DD), or pulsating DD scenario, or WD mergers. The difference in appearance with respect to core-normal SNe Ia is caused by low photospheric temperatures in combination with layers of unburned C, and more massive layers of the products of explosive C and O burning. The comparatively high level of continuum polarisation is explained in terms of a significant global asymmetry (~15%), which is well reproduced by an oblate ellipsoidal geometry. Our results suggest that SN2005ke arose either from a single-degenerate system in which the WD is especially rapidly rotating, close to the break-up velocity, or from a double-degenerate merger. Based on the current polarisation data, we cannot distinguish between these two possibilities.

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