Electronic Telegram No. 1285
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)
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SUPERNOVA 2008ax IN NGC 4490
      S. Blondin, Harvard-Smithsonian Center for Astrophysics (CfA), on
behalf of the CfA Supernova Group; A. V. Filippenko, R. J. Foley, and W. Li,
University of California, Berkeley; and L. Dessart, Steward Observatory,
University of Arizona, report that a spectrum (range 350-740 nm) of the
possible supernova announced on CBET 1280, obtained on Mar. 4.37 UT by A.
Vaz with the F. L. Whipple Observatory 1.5-m telescope (+ FAST), shows it to
be a young, reddened, peculiar, type-II supernova.  The spectrum is dominated
by broad P-Cyg profiles of the Balmer series with a broad absorption around
545 nm due to He I (rest 587.6 nm).  The spectrum is similar to an early
spectrum of the peculiar type-II supernova 1987A taken on 1987 Feb. 25 UT
(i.e., roughly two days past the neutrino outburst and 82 days before V-band
maximum light; Phillips et al. 1988, A.J. 95, 1087), although the line widths
and blueshifts are greater in 2008ax.  Adopting a recession velocity of 589
km/s for the host galaxy of 2008ax (from The Updated Zwicky Catalogue,
http://tdc-www.harvard.edu/uzc/), the maximum absorptions in the H_beta (rest
486.1 nm), He I (rest 587.6 nm), and H_alpha (rest 656.3 nm) lines are
blueshifted by roughly 23500, 22800, and 26600 km/s, respectively (cf. 16000,
14900, and 18500 km/s for the aforementioned 1987A spectrum).  The peak
emissions also display significant blueshifts (6700 km/s for H_alpha), as
observed in the early spectra of type-II supernovae (cf. Dessart and Hillier
2005, A.Ap. 437, 667).  Also, interstellar Na I D absorption at the redshift
of the host galaxy is visible in the spectrum, suggesting some reddening
towards 2008ax.  In the comparison plot (visible at website URL
http://www.cfa.harvard.edu/supernova/spectra/SNngc4490_comp.gif), the spectrum
of 2008ax is dereddened by E(B-V) = 0.6 mag to reproduce the overall spectral
energy distribution of the 1987A comparison spectrum.  Assuming that the host
galaxy is in the Hubble flow and the supernova suffers from A_V about 2 mag of
visual extinction, the discovery magnitude of 16.1 corresponds to an absolute
magnitude of roughly -15.4, comparable to 1987A in its early stages [M_V = -14
on 1987 Feb. 27, adopting the apparent V-band magnitude of 4.45 reported by
Hamuy et al. (1988, A.J. 95, 63) and a distance modulus of -18.5 to the LMC].


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
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                         (C) Copyright 2008 CBAT
2008 March 5                     (CBET 1285)              Daniel W. E. Green