The Physical Properties of PS1-12sk and the implications for its Progenitor System

PS1-12sk is a type Ibn supernova (SN) found in a host environment showing no obvious ongoing star formation, which challenges the massive star explosion scenario. We use the ejecta-circumstellar medium (CSM) interaction (CSI) and the CSI plus $^{56}$Ni models in the context of double white dwarf (WD) merger to fit the bolometric light curve (LC) of PS1-12sk, since the He emission lines at the photospheric phases indicated the interaction between the SN ejecta and He-rich CSM. We find that the CSI model failed to explain the LC, but the CSI plus $^{56}$Ni model can account for the bolometric LC. The derived masses of the two WDs and $^{56}$Ni are $\sim 0.70 M_\odot$, $\sim 0.40 M_\odot$, and $\sim 0.09\,M_\odot$, respectively. The facts that the ejecta mass ($\sim 0.984 M_\odot$) is well below the Chandrasekhar limit ($\sim 1.4 M_\odot$) and that the $^{56}$Ni mass is comparable to the $^{56}$Ni yields of the explosions of some sub-Chandrasekhar explosion models support the scenario that PS1-12sk might be from a sub-Chandrasekhar explosion induced by the merger of two low-mass WDs. The derived innermost radius ($\sim 13.81 \times 10^{12}$ cm) and the mass of the CSM ($\sim 0.116 M_\odot$) disfavor the possibility that the CSM was formed in the merger phase. We suggest that the flybys before the merger can account for the position and mass of the CSM.