SN 2011dh: DISCOVERY OF A TYPE IIb SUPERNOVA FROM A COMPACT PROGENITOR IN THE NEARBY GALAXY M51

Abstract

On 2011 May 31 UT a supernova (SN) exploded in the nearby galaxy M51 (the Whirlpool Galaxy). We discovered this event using small telescopes equipped with CCD cameras and also detected it with the Palomar Transient Factory survey, rapidly confirming it to be a Type II SN. Here, we present multi-color ultraviolet through infrared photometry which is used to calculate the bolometric luminosity and a series of spectra. Our early-time observations indicate that SN 2011dh resulted from the explosion of a relatively compact progenitor star. Rapid shock-breakout cooling leads to relatively low temperatures in early-time spectra, compared to explosions of red supergiant stars, as well as a rapid early light curve decline. Optical spectra of SN 2011dh are dominated by H lines out to day 10 after explosion, after which He i lines develop. This SN is likely a member of the cIIb (compact IIb) 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 spectra.

THE TYPE IIb SUPERNOVA 2013df AND ITS COOL SUPERGIANT PROGENITOR

“…What is most notable from the comparison is that the post-shockbreakout cooling of SN 2013df occurred at a later epoch in all bands, relative to that of SN 1993J. The post-breakout decline of SN 2011dh occurred at an even earlier relative epoch (e.g., Arcavi et al 2011;Bersten et al 2012).…”

“…14 We compare these curves with those at BVRIJH for SN 1993J (Richmond et al , 1996Matthews et al 2002) and for SN 2011dh (Arcavi et al 2011;Van Dyk et al 2013b;Ergon et al 2013). We also compare the z-band light curve of SN 2008ax (Pastorello et al 2008).…”

“…We did not yet possess template images of the host galaxy (i.e., prior to the SN discovery or when the SN has faded to invisibility) to subtract from the images with the SN present in (Richmond et al , 1996Matthews et al 2002;dashed lines) and of SN 2011dh (Arcavi et al 2011;Van Dyk et al 2013b;Ergon et al 2013;dot-dashed each band. Consequently, the photometry presented here should be considered preliminary; however, the SN is far from the main light from the host galaxy, so results including template subtraction prior to photometry might not substantially differ from what we present here.…”

“…SNe IIb are intrinsically rare (∼10%-11% of all core-collapse SNe; Smith et al 2011;Li et al 2011). The first identified case was SN 1987K (Filippenko 1988); wellstudied examples are the nearby SN 1993J in Messier 81 (e.g., Richmond et al 1996;Matheson et al 2000) and SN 2011dh in Messier 51 (e.g., Arcavi et al 2011;Ergon et al 2013). The progenitor channel for SN 1993J that has garnered most favor 10 Hubble fellow.…”

“…Although rarely seen owing to its very short duration early in the evolution of an SN, a rapid decline after an initial peak has been observed among SNe IIb (e.g., Richmond et al 1994Richmond et al , 1996Roming et al 2009;Arcavi et al 2011). This is interpreted as adiabatic cooling after the SN shock has broken out through the star's surface; the duration of this cooling is governed primarily by the radius of the progenitor (Chevalier & Fransson 2008;Nakar & Sari 2010;Rabinak & Waxman 2011;Bersten et al 2012).…”