A Subsolar Metallicity Progenitor for Cassiopeia A, the Remnant of a Type IIb Supernova

Abstract

We report, for the first time, the detection of the Mn-Kα line in the Type IIb supernova (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 solarmetallicity progenitor with a typical explosion energy (1 × 10 51 erg) fails to reproduce such a high electron fraction. In such models, the explosive Si-burning regime extends only to the Si/O layer established during the progenitor's hydrostatic evolution; the Y e in the Si/O layer is lower than the value required by our observational constraints. We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regime were 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 sub-solar metallicity progenitor (Z 0.5Z ) for Cassiopeia A. The low initial metallicity can be used to rule out a singlestar progenitor, leaving the possibility of a binary progenitor with a compact companion (white dwarf, neutron star or black hole). 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.

Progenitors of Type IIb Supernovae. II. Observable Properties

“…Interestingly, Weil et al (2020) found evidence for a WR-like wind phase close to CC for the progenitor of Cas A. This is in addition to a recent estimate showing the progenitor to have been sub-solar (Sato et al 2020). However, Schure et al (2008) constrained the maximum lifetime of a potential WR progenitor for Cas A to be 2000 yrs, making the progenitor incompatible with EB SNe IIb.…”

Chemical Abundances in Sgr A East: Evidence for a Type Iax Supernova Remnant

“…Ono et al (2019) reported Mn and Cr lines from Sgr A East using Suzaku data. Mn and Cr are more frequently observed in thermonuclear SNRs (see Yang et al 2013, and references therein), although they have also been found in the X-ray-luminous corecollapse SNR Cassiopeia A (e.g., Sato et al 2020b).…”

Formation pathway for lonely stripped-envelope supernova progenitors: implications for Cassiopeia A

“…We apply sub-solar metallicity (Z = 0.0055) for the models. This value is chosen because we have Cas A in mind as a possible target of application, which is suggested to have exploded from a sub-solar metallicity progenitor (Sato et al 2020). However, our results are not strongly dependent on metallicity since the structure of early RSG envelopes is not sensitive to metallicity variations other than slight differences in the final masses due to the metallicity dependent wind.…”

“…Origin of SESN without companions 11 ∼15-20 M . Recent modelling of X-ray spectra further suggest that the progenitor had a sub-solar metallicity (Sato et al 2020), meaning that the stellar wind would have been relatively weak. All of the above strengthens the case that the progenitor was unlikely to have formed through a single star channel.…”

“…In this paper we explore a new scenario that was proposed in Sato et al (2020) for producing stripped-envelope SN progenitors (Figure 1). In classical scenarios, the main driver of stellar mass loss are winds driven by radiative forces or mass transfer induced by gravitational forces in binary systems.…”