The Crab nebula and the class of Type IIn-P supernovae caused by sub-energetic electron-capture explosions

• Published in: Monthly notices of the Royal Astronomical Society 2013-09, Vol.434 (1), p.102-113
• Main Author: Smith, Nathan
• Format: Article
• Language: English
• Subjects: Electrons; Kinetics; Luminosity; Nebulae; Neutron stars; Star & galaxy formation; Supernovae
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt1004

What sort of supernova (SN) gave rise to the Crab nebula? While there are several indications that the Crab arose from a sub-energetic explosion of an 8-10 M progenitor star, this would appear to conflict with the high luminosity indicated by historical observations. This paper shows that several well-known observed properties of the Crab and SN 1054 are well matched by a particular breed of Type IIn SN. The Crab's properties are best suited to the Type IIn-P subclass (Type IIn spectra with plateau light curves), exemplified by SNe 1994W, 2009kn and 2011ht. These events probably arise from relatively low energy (1050 erg) explosions with low 56Ni yield that may result from electron-capture SN (ecSN) explosions, but their high visual-wavelength luminosity and Type IIn spectra are dominated by shock interaction with dense circumstellar material (CSM) rather than the usual recombination photosphere. In this interaction, a large fraction of the 1050 erg of the total kinetic energy can be converted to visual-wavelength luminosity. After about 120 d, nearly all of the mass outside the neutron star in the CSM and ejecta ends up in a slowly expanding (1000-1500 km s−1) thin dense shell, which is then accelerated and fragmented by the growing pulsar wind nebula in the subsequent 1000 yr, producing the complex network of filaments seen today. There is no need to invoke the extended, invisible fast SN envelope hypothesized to reside outside the Crab. As differentiated from a normal SN II-P, SNe IIn-P provide a much better explanation for several observed features of the Crab: (1) no blast wave outside the Crab nebula filaments, (2) no rapidly expanding SN envelope outside the filaments, (3) a total mass of ∼5 M swept up in a thin slow shell, (4) a low kinetic energy of the Crab at least an order of magnitude below a normal core-collapse SN, (5) a high peak luminosity (−18 mag) despite the low kinetic energy, (6) chemical abundances consistent with an 8-10 M star and (7) a low 56Ni yield. A number of other implications are discussed, concerning other Crab-like remnants, the origin of dust in the Crab filaments, diversity in the initial masses of SNe IIn, and the putative association between ecSNe and SN impostors. This model predicts that if/when light echoes from SN 1054 are discovered, they will exhibit a Type IIn spectrum, probably similar to SNe 1994W and 2011ht.