On the Disappearance of the Supergiant Progenitor of SN 2011dh in M51.

ATel #4912; Mattias Ergon, Jesper Sollerman (OKC, Stockholm), Tapio Pursimo, Thomas Augusteijn, John Telting, Olesja Smirnova (NOT, La Palma), Erkki Kankare, Seppo Mattila (Tuorla Observatory, Turku), Justyn Maund, Morgan Fraser (Queen's University, Belfast)
on 24 Mar 2013; 22:56 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Jesper Sollerman (jesper@astro.su.se)

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We report on high quality pre- and post-explosion B, V and r band imaging obtained with the 2.56 m Nordic Optical Telescope (NOT). Difference imaging reveals a reduction of 45-60 percent in flux at the position of the yellow supergiant coincident with SN 2011dh and proposed as the progenitor by Maund et al. (2011, ApJ, 739, L37). The pre-explosion imaging was obtained on May 26 2008 (B) and May 29 2011 (V and r), the latter just 2 days before explosion. The post-explosion imaging was obtained on Jan 20 2013 (V and r) and Mar 19 2013 (B). Our results thus confirm the results presented by Van Dyk et al. (ATEL #4850) based on F555W and F814W HST observations and adds further constraints on the remaining luminosity. We have used the HOTPANTS package to perform subtraction of the pre- and post-explosion images and aperture photometry to measure the magnitudes of the residuals to B = 23.00 +/- 0.10, V = 22.73 +/- 0.07 and r = 22.22 +/- 0.05 mag in the natural Vega (BV) and AB (r) systems of the NOT. The positions of the residuals in all bands are within 0.15 arcsec from the position of the SN. Using PSF photometry we measure the magnitudes of the yellow supergiant in the pre-explosion images to B = 22.41 +/- 0.12, V = 21.89 +/- 0.04 and r = 21.67 +/- 0.03 mag. The residuals then corresponds to a reduction of the flux with 58 +- 8 , 46 +- 3 and 60 +-3 percent in the B, V and r bands. The remaining flux, at least partly emitted by the SN, corresponds to B = 23.35 +/- 0.32, V = 22.56 +/- 0.10 and r = 22.67 +/- 0.11 mag. Our V and r band observations were obtained ~40 days before the HST observations presented by Van Dyk et al. (ATEL #4850) so the 0.64 mag difference between the V and F555W observations is consistent with a decline in the SN luminosity. The theoretically expected bolometric decline (Arnett, 1996, Supernovae And Nucleosynthesis, p 439) in ~40 days for a typical Type IIb SNe at ~600 days is 0.5-0.6 mag in good agreement with the measured difference. Combining the constraints presented here with those in Van Dyk et al. (ATEL #4850) the reduction of the pre-explosion yellow supergiant flux is at least 58 +- 3, 71 +- 1, 60 +- 3 and 69 +- 1 percent in the B, F555W, r and F814W bands. Hence we find it most likely that the yellow supergiant was indeed the progenitor of SN 2011dh as suggested by Maund et al. (2011). A colour image showing the disappearance is provided here.