[IAUC] CBET 1298: 20080312 : SUPERNOVA 2008ax IN NGC 4490

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                                                  Electronic Telegram No. 1298
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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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.


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                         (C) Copyright 2008 CBAT
2008 March 12                    (CBET 1298)              Daniel W. E. Green