On the progenitor of SN 2008ax in NGC 4490

ATel #1417; Weidong Li
on 6 Mar 2008; 23:58 UT
Credential Certification: Weidong Li (weidong@astron.berkeley.edu)

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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 (all UCB), report that they have isolated a possible progenitor for the young type-II SN 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 SN 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 brightness of B = 23.6 mag. The object is also present in the F606W image (with V = 23.5 mag), and F814W image (with I = 22.6 mag, 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 SN 2008ax taken on Mar 5.48 UT by A. M. Wolfe, R. Jorgenson (University of California, San Diego), and M. Rafelski (University of California, Los Angeles), we measured an equivalent width of 1.0 Ang 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), the object has an absolute V magnitude of -6.9 and colors of B-V = -0.2 mag and V-I = 0.4 mag, 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. We note that the progenitor of SN 1987A in the Large Magellanic Cloud (Gilmozzi et al. 1987, Nature 328, 318; Sonneborn et al. 1987, ApJ 323, L35) is a blue supergiant star with zero age main sequence mass of about 20 solar mass, and there is spectroscopic similarity between SN 2008ax and SN 1987A as reported in CBET 1285. Photometry and spectroscopy (low and high resolution) of this peculiar type II supernova are encouraged.