Discovery of Variability of the Progenitor of SN 2011dh in M51 Using the Large Binocular Telescope

We show that the candidate progenitor of the core-collapse SN 2011dh in M51 (8 Mpc away) was fading by 0.039 +- 0.006 mag/year during the three years prior to the supernova, and that this level of variability is moderately unusual for other similar stars in M 51. While there are uncertainties about whether the true progenitor was a blue companion to this candidate, the result illustrates that there are no technical challenges to obtaining fairly high precision light curves of supernova progenitors using ground based observations of nearby (<10 Mpc) galaxies with wide field cameras on 8m-class telescopes. While other sources of variability may dominate, it is even possible to reach into the range of evolution rates required by the quasi-static evolution of the stellar envelope. For M 81, where we have many more epochs and a slightly longer time baseline, our formal 3 sigma sensitivity to slow changes is presently 3 millimag/year for a M_V ~= -8 mag star. In short, there is no observational barrier to determining whether the variability properties of stars in their last phases of evolution (post Carbon ignition) are different from earlier phases.