Central Bureau Electronic Telegrams, 1290, 1 (2008/March-0)
Supernova 2008ax in NGC 4490.
LI W., VAN DYK S.D., CUILLANDRE J.-C., FILIPPENKO A.V., FOLEY R.J., CHORNOCK R., SMITH N., WANG X., WOLFE A.M., JORGENSON R. and RAFELSKI M.
Abstract (from CDS):
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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