Abstract
We present adaptive optics imaging of the core-collapse supernova (SN) 2009md, which we use together with archival Hubble Space Telescope data to identify a coincident progenitor candidate. We find the progenitor to have an absolute magnitude of V=−4.63+0.3−0.4 mag and a colour of V−I= 2.29+0.25−0.39 mag, corresponding to a progenitor luminosity of log L/L⊙∼ 4.54 ± 0.19 dex. Using the stellar evolution code STARS, we find this to be consistent with a red supergiant progenitor with M= 8.5+6.5−1.5 M⊙. The photometric and spectroscopic evolution of SN 2009md is similar to that of the class of sub-luminous Type IIP SNe; in this paper we compare the evolution of SN 2009md primarily to that of the sub-luminous SN 2005cs. We estimate the mass of 56Ni ejected in the explosion to be (5.4 ± 1.3) × 10−3 M⊙ from the luminosity on the radioactive tail, which is in agreement with the low 56Ni masses estimated for other sub-luminous Type IIP SNe. From the light curve and spectra, we show the SN explosion had a lower energy and ejecta mass than the normal Type IIP SN 1999em. We discuss problems with stellar evolutionary models, and the discrepancy between low observed progenitor luminosities (log L/L⊙∼4.3–5 dex) and model luminosities after the second dredge-up for stars in this mass range, and consider an enhanced carbon burning rate as a possible solution. In conclusion, SN 2009md is a faint SN arising from the collapse of a progenitor close to the lower mass limit for core collapse. This is now the third discovery of a low-mass progenitor star producing a low-energy explosion and low 56Ni ejected mass, which indicates that such events arise from the lowest end of the mass range that produces a core-collapse SN (7–8 M⊙).
Highlights
Core-collapse SNe mark the endpoint of stellar evolution for stars more massive than ∼8 M⊙ (Poelarends et al 2008; Siess 2007; Heger et al 2003; Eldridge & Tout 2004)
There has been some debate in the literature as to the precise nature of these events, and whether they represent the collapse of a low mass (∼9 M⊙) star with a O-Ne-Mg core (Kitaura, Janka & Hillebrandt 2006), or a high mass (∼25 M⊙) star with the formation of a black hole, possibly by the fallback of material onto an accreting proto-neutron star (Turatto et al 1998; Zampieri et al 2003)
A campaign of follow-up observations8 was initiated for SN 2009md shortly after discovery, with data obtained from the Liverpool Telescope (LT) + RATCam and SupIRCam, the Faulkes Telescope North (FTN) + EM01, the New Technology Telescope (NTT) + EFOSC2 and SOFI, the Nordic Optical Telescope (NOT) + ALFOSC, the Telescopio Nazionale Galileo (TNG) + NICS, the Calar Alto 2.2-m telescope + CAFOS, and the Wise Observatory Telescope + PI and LAIWO
Summary
Core-collapse SNe mark the endpoint of stellar evolution for stars more massive than ∼8 M⊙ (Poelarends et al 2008; Siess 2007; Heger et al 2003; Eldridge & Tout 2004). The fact that the progenitors of core-collapse SNe are massive, and luminous, gives a realistic prospect of successfully recovering them in high resolution archival data (see Smartt 2009 for a review). The detection of the progenitor of another sub-luminous Type IIP, coupled with the detailed monitoring and follow-up observations needed to understand the explosion, is of considerable interest. We verify the distance to NGC 3389 with the standard candle method for Type IIP SNe, discuss how changes to stellar evolutionary models may help ameliorate the discrepancy between the observed low luminosity of the progenitor of SN 2009md and the final luminosities found from models, estimate the explosion energy, and discuss the properties of the family of sub-luminous Type IIP SNe
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