Electron-capture supernovae exploding within their progenitor wind

¹ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: moriyatk@astro.uni-bonn.de
² Department of Physics, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Kobe, 658-8501 Hyogo, Japan
³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583 Chiba, Japan
⁴ Institute for Theoretical and Experimental Physics, Bolshaya Cheremushkinskaya 25, 117218 Moscow, Russia
⁵ Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, Universitetski pr. 13, 119992 Moscow, Russia

Received: 23 May 2014
Accepted: 17 July 2014

Abstract

The most massive stars on the asymptotic giant branch (AGB), or the so-called super-AGB stars, are thought to produce supernovae triggered by electron captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to have slow winds with high mass-loss rates, so their circumstellar density is high. The explosions of super-AGB stars are therefore presumed to occur in this dense circumstellar environment. We provide the first synthetic light curves for such events by exploding realistic electron-capture supernova progenitors within their super-AGB winds. We find that the early light curve – that is, before the recombination wave reaches the bottom of the hydrogen-rich envelope of supernova ejecta (the plateau phase) – is not affected by the dense wind. However, after the luminosity drop following the plateau phase, the luminosity remains much higher when the super-AGB wind is taken into account. We compare our results to the historical light curve of SN 1054, the progenitor of the Crab Nebula, and show that the explosion of an electron-capture supernova within an ordinary super-AGB wind can explain the observed light curve features. We conclude that SN 1054 could have been a Type IIn supernova without any extra extreme mass loss, which was previously suggested to be necessary to account for its early high luminosity. We also show that our light curves match Type IIn supernovae with an early plateau phase or the so-called Type IIn-P supernovae, and suggest that they are electron-capture supernovae within super-AGB winds. Although some electron-capture supernovae can be bright in the optical spectral range due to the large progenitor radius, their X-ray luminosity from the interaction does not necessarily get as bright as other Type IIn supernovae whose optical luminosities are also powered by the interaction. Thus, we suggest that optically bright X-ray-faint Type IIn supernovae can emerge from electron-capture supernovae. Optically faint Type IIn supernovae, such as SN 2008S, can also originate from electron-capture supernovae if their hydrogen-rich envelope masses are small. We argue that some of them can be observed as Type IIn-b supernovae due to the small hydrogen-rich envelope mass.