A Subsolar Metallicity Progenitor for Cassiopeia A, the Remnant of a Type IIb Supernova
Toshiki Sato, Takashi Yoshida, Hideyuki Umeda, Shigehiro Nagataki, M. Ono, Keiichi Maeda, Ryosuke Hirai, John P. Hughes, Brian J. Williams, Yoshitomo Maeda
Abstract
Abstract We report the detection of the Mn-K α line in the SN-IIb remnant, Cassiopeia A. Manganese ( 55 Mn after decay of 55 Co), a neutron-rich element, together with chromium ( 52 Cr after decay of 52 Fe), is mainly synthesized in core-collapse supernovae at the explosive incomplete Si-burning regime. Therefore, the Mn/Cr mass ratio with its neutron excess reflects the neutronization at the relevant burning layer during the explosion. Chandra's deep archival X-ray data of Cassiopeia A indicate a low Mn/Cr mass ratio with values in the range 0.10–0.66, which, when compared to one-dimensional SN explosion models, requires that the electron fraction be 0.4990 ≲ Y e ≲ 0.5 at the incomplete Si-burning layer. An explosion model assuming a solar-metallicity progenitor with a typical explosion energy (1 × 10 51 erg) fails to reproduce such a high electron fraction. We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regime was to extend into the O/Ne hydrostatic layer, which has a higher Y e . This would require an energetic (>2 × 10 51 erg) and/or asymmetric explosion of a subsolar-metallicity progenitor ( Z ≲ 0.5 Z ⊙ ) for Cassiopeia A. The low initial metallicity can be used to rule out a single-star progenitor, leaving the possibility of a binary progenitor with a compact companion. We discuss the detectability of X-rays from Bondi accretion onto such a compact companion around the explosion site. We also discuss other possible mass-loss scenarios for the progenitor system of Cassiopeia A.