Electronic Telegram No. 1298 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://www.cfa.harvard.edu/iau/cbat.html SUPERNOVA 2008ax IN NGC 4490 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. Chornock et al. also report an improved estimate for the equivalent width of the narrow Na I D absorption lines in the Keck ESI spectrum as reported on CBET 1290. The new measurement is about 0.18 nm, suggesting an E(B-V) reddening of about 0.5 mag. Adopting this new reddening and the other information reported on CBET 1290, the possible progenitor of SN 2008ax has an absolute magnitude of M(V) = -7.5 and colors of B-V = -0.4 and V-I = 0.1, consistent with a bright blue supergiant star. Note that the progenitor of SN 1993J (Aldering et al. 1994, A.J. 107, 662; Van Dyk et al. 2002, PASP 114, 1322) has M(V) = -7.0 and is a yellow K2 Ia supergiant, so the possible progenitor of SN 2008ax is more luminous and has a bluer color than that of SN 1993J, if the assumed reddening is correct. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2008 CBAT 2008 March 12 (CBET 1298) Daniel W. E. Green