2008CBET.1298....1C

R. Chornock, A. V. Filippenko, W. Li, and R. J. Foley, University of California, Berkeley, report that a spectrum of SN 2008ax (CBETs 1280, 1285) obtained by A. Stockton (University of Hawaii) using the Keck I 10-m telescope (+ LRIS; range 310-920 nm) on Mar. 9.6 UT reveals that the object has evolved into a type-IIb supernova. The spectrum is dominated by P-Cyg features of the hydrogen Balmer series, Fe II, and Ca II, with the absorption minimum of H-alpha blueshifted by roughly 17600 km/s. In addition, notches have appeared in the H-alpha emission profile near 645 and 682 nm, which are identified as the incipient He I 667.8- and 706.5-nm absorptions indicative of a type-IIb supernova. The overall spectrum bears a strong resemblance to the type-IIb supernova 1987K (Filippenko 1988, A.J. 96, 1941) near maximum light, but with higher expansion velocities. Furthermore, spectropolarimetry (range 415-695 nm) was obtained by E. C. Moran (Wesleyan University) and J. Hodge and K. Merriman (University of California, Davis) using the Kast spectrograph on the Shane 3-m telescope at Lick Observatory on Mar. 9.4. The polarization data show that SN 2008ax is strongly polarized, with a modulation of more than 3 percent across the H-alpha line. Other type-IIb supernovae observed in the past, such as 1993J (Hoeflich et al. 1996, Ap.J. 459, 307; Tran et al. 1997, PASP 109, 489) and 1996cb (Wang et al. 2001, Ap.J. 550, 1030) have also shown large intrinsic polarizations, while normal type-IIP supernovae usually show little polarization at this early age (e.g., Leonard et al. 2001, Ap.J. 553, 861). Further observations at all wavelengths are encouraged.