[IAUC] CBET 1290: 20080306 : SUPERNOVA 2008ax IN NGC 4490

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                                                  Electronic Telegram No. 1290
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
     W. Li, University of California, Berkeley (UCB); S. D. Van Dyk, Spitzer
Science Center, California Institute of Technology; J.-C. Cuillandre,
Canada-France-Hawaii Telescope (CFHT) Corporation; and A. V. Filippenko, R.
J. Foley, R. Chornock, N. Smith, and X. Wang, UCB, report that they have
isolated a possible progenitor for the young type-II supernova 2008ax in
Hubble Space Telescope images of NGC 4490 (Snapshot program 5446; GO/DD
program 9042) taken with the Wide Field Planetary Camera 2 (WFPC2).
Geometrical transformation between a deep CFHT r' image taken on Mar. 5.47 UT
and the HST/WFPC2 images yields a precision of 0.5 WFPC2 pixel (0".05) for
the supernova location on the WFPC2 images.  Within the uncertainty of the
supernova position, an object is clearly detected in the F450W image, with
a position of R.A. = 12h30m40s.82, Decl. = +41o38'15".2 (equinox 2000.0),
and a magnitude of B = 23.6.  The object is also present in the F606W image
(with V = 23.5) and F814W image (with I = 22.6, but it might be contaminated
by another source).  The nature of this object depends on the host-galaxy
reddening estimates.  From a Keck ESI spectrum of 2008ax taken on Mar. 5.48
by A. M. Wolfe and R. Jorgenson, University of California, San Diego; and M.
Rafelski, University of California, Los Angeles, Li et al. measured an
equivalent width of 0.10 nm for the narrow Na I D absorption lines,
suggesting an E(B-V) reddening of about 0.3 mag.  Assuming this reddening
and the Galactic reddening law, as well as a distance modulus to NGC 4490 of
29.5 mag (Tully 1988, *Nearby Galaxies Catalog*, Cambridge University Press),
the object has an absolute V magnitude of -6.9 and colors of B-V = -0.2 and
V-I = 0.4, consistent with blue/yellow supergiant stars.  Another
possibility is that the object is a compact star cluster, although the
point-spread function of the source appears stellar.  Note that the
progenitor of SN 1987A in the Large Magellanic Cloud (Gilmozzi et al. 1987,
Nature 328, 318; Sonneborn et al. 1987, Ap.J. 323, L35) was a blue
supergiant star with a zero-age main-sequence mass of about 20 solar masses,
and there is spectroscopic similarity between SN 2008ax and SN 1987A as
reported on CBET 1285.  Photometry and spectroscopy (low and high resolution)
of this peculiar type-II supernova are encouraged.


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                         (C) Copyright 2008 CBAT
2008 March 6                     (CBET 1290)              Daniel W. E. Green