Disappearance of the progenitor star of supernova 2011dh

The Type IIb supernova (SN) 2011dh exploded in M51 2011 May 31 and was one of the brightest and most nearby SNe in recent years. Based on images obtained by the Hubble Space Telescope (HST) before the explosion Maund et al. (2011, ApJ, 739, L37) found an extended yellow supergiant coincident with the SN and proposed it to be the progenitor star. However, both Arcavi et al. (2011, ApJ, 742, L18) and Soderberg et al. (2012, ApJ, 752, 78) presented arguments against an extended progenitor, suggesting the actual progenitor to be a more compact and bluer star, possibly the companion of the yellow supergiant. On the other hand, the more detailed modeling by Bersten et al. (2012, ApJ, 757, 31) showed that the early optical observations could be well explained by an extended progenitor star as the one proposed.

As Type IIb SNe fade rather quickly there was a good chance to detect the disappearance (or not) of the yellow supergiant in just 1-2 years after the explosion and finally settle the issue. Observations of the SN site by the Hubble Space Telescope (HST) and Nordic Optical Telescope (NOT) were recently presented by Van Dyk et al. (ATel, 4850) and Ergon et al. (arXiv 1305.1851, ATel, 4912), respectively. Colour images made from the NOT B, V and r observations is presented here. Above we show M51 before the explosion and a comparison animation of the SN site before and one and a half year after the explosion. Below we show the same but with the comparison images of the SN site displayed separately (post-explosion image on top of the pre-explosion image). The quality of the pre-explosion comparison image has been slightly degraded to match the post-explosion image.

As seen in the images, the flux at the position of the yellow supergiant is now considerably reduced as compared to before the explosion. Combining the observations made by Ergon et al. (arXiv 1305.1851, ATel, 4912) and Van Dyk et al. (ATel, 4850) the flux is reduced with at least 60-75 percent in all optical filters (except U for which there is no observations). The 0.76 mag decline between 2013 Jan 20 and Apr 14, measured in the NOT V-band observations, is consistent with the remaining flux being emitted by the fading SN. It is therefore most likely that the yellow supergiant was indeed the progenitor of SN 2011dh as proposed by Maund et al. (2011, ApJ, 739, L37).

The colour images shown here were constructed from B,V and r observations where the RGB values have been scaled to match the number of photons per wavelength, the contrast of small and medium scale features boosted my median filtering and the image finally transformed to a logarithmic scale.