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

TL;DR

This paper reports the discovery and analysis of a Type IIb supernova in M51, revealing a compact progenitor star through early light curve and spectral data, and discusses the progenitor's nature and possible companion.

Contribution

It provides detailed early observations of PTF11eon/SN2011dh, identifying it as a compact Type IIb supernova and constraining the progenitor's properties and environment.

Findings

Supernova PTF11eon is a compact Type IIb with a small progenitor radius.

Early spectra show hydrogen lines and initial signs of helium.

Pre-explosion images suggest a possible companion or blended source, not the progenitor.

Abstract

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