A low mass red supergiant progenitor candidate for SN 2012aw

ATel #3994; M. Fraser, J. R. Maund, S. J. Smartt, R. Kotak (Queens University Belfast), J. Sollerman and M. Ergon (Stockholm University)
on 22 Mar 2012; 09:00 UT
Credential Certification: Morgan Fraser (mfraser02@qub.ac.uk)

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We obtained an unfiltered image of the Type IIP SN 2012aw (CBET 3054) from the Nordic Optical Telescope + ALFOSC on 2012 March 19.9 UT, under excellent (~0.5") seeing conditions. 6 sources were identified in both the the ALFOSC image, and a pre-explosion HST+WFPC2 F555W mosaic of M95 retrieved (together with F439W and F814W-filter images) from the Hubble Legacy Archive. Using a geometric transformation derived from the position of the fiducial sources in both images, SN 2012aw was located in the WFPC2 image to an accuracy of 0.24".

We find SN 2012aw to be coincident with a faint, red source in the pre-explosion WFPC data, which is the probable progenitor. Using HSTPhot (Dolphin, 2000, PASP, 112, 1383) to perform PSF-fitting photometry on the individual frames, we obtain a magnitude of F555W=26.70±0.06 and F814W=23.39±0.02 for the progenitor candidate. The source was not detected in the F439W filter image.

Freedman et al. (2001, ApJ, 553, 47) give a Cephid-derived distance modulus of 29.9 for M95; the foreground (Milky Way) extinction towards M95 is low (E(B-V)=0.03, Schlegel et al. 1998, ApJ, 500, 525). We hence determine an absolute magnitude for the progenitor of F555W (~V) = -3.3, which would make this star the reddest and probably coolest of the red supergiant progenitors of Type IIP SNe found so far. Synthetic photometry from MARCS model spectra (Gustafsson et al. 2008, A&A, 486, 951) provides a colour of F555W-F814W= 3.3 for a Teff=3400 red supergiant, and a bolometric correction of -3.24. This would give a progenitor luminosity of 4.5 dex, which in turn implies a progenitor ZAMS mass of ~8 solar masses (e.g. from Smartt et al. 2009, MNRAS, 395, 1409; Eldridge & Tout 2004, MNRAS, 353, 87).

An alternative scenario is that the progenitor is heavily extinguished by circumstellar (or host galaxy) dust, although we note that the early spectra are blue, and do not show strong absorption in the NaD lines which could be associated with a large extinction. There is no source detection in Spitzer images at all wavelengths between 3.6 - 24 microns, which will provide further constraints on circumstellar dust and optical obscuration.

Further analysis of the stellar SED and more precise astrometry is ongoing.