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On May 31, 2011 UT a supernova (SN) exploded in the nearby galaxy M51 (theWhirlpool Galaxy). We discovered this event using small telescopes equippedwith CCD cameras, as well as by the Palomar Transient Factory (PTF) survey, andrapidly confirmed it to be a Type II supernova. Our early light curve andspectroscopy indicates that PTF11eon resulted from the explosion of arelatively compact progenitor star as evidenced by the rapid shock-breakoutcooling seen in the light curve, the relatively low temperature in early-timespectra and the prompt appearance of low-ionization spectral features. Thespectra of PTF11eon are dominated by H lines out to day 10 after explosion, butinitial signs of He appear to be present. Assuming that He lines continue todevelop in the near future, this SN is likely a member of the cIIb (compactIIb; Chevalier and Soderberg 2010) class, with progenitor radius larger thanthat of SN 2008ax and smaller than the eIIb (extended IIb) SN 1993J progenitor.Our data imply that the object identified in pre-explosion Hubble SpaceTelescope images at the SN location is possibly a companion to the progenitoror a blended source, and not the progenitor star itself, as its radius (~10^13cm) would be highly inconsistent with constraints from our post-explosionphotometric and spectroscopic data.

PTF11eon/SN2011dh: Discovery of a Type IIb Supernova From a Compact Progenitor in the Nearby Galaxy M51