Superlinear Type II Superluminous Supernovae 2017fck and 2019cmv: A Possible Origin from Interacting Thermonuclear Supernovae

Additional power sources to traditional supernovae (SNe) are necessary to account for the extreme luminosities of superluminous SNe (SLSNe). A main power source for hydrogen-rich SLSNe (SLSNe-II) is thought to be circumstellar material (CSM) interaction. However, the nature of underlying SNe and their progenitor systems remain elusive as they are hidden below strong CSM signatures. Here, we present optical photometry and spectroscopy of SLSNe-II 2017fck and 2019cmv. They are characterized by post-maximum "superlinear" light curves which we also identify in a sample of Type Ia SNe interacting with CSM (SNe Ia-CSM), along with their light-curve correlations and spectral similarities. Thus, we compute a numerical light-curve model grid of SNe Ia-CSM with various SN Ia subtypes and CSM distributions. Our model grid spans the observed parameter space of SNe Ia-CSM in terms of their rise times, peak luminosities, and decline rates with a wide CSM mass range of $\sim2-11$ M$_\odot$, indicating the diversity in their progenitor systems. For SNe 2017fck and 2019cmv, we infer high CSM masses of $\sim11$ and $6$ M$_\odot$, respectively, which might be produced during the common envelope evolution of a white dwarf and massive ($\geq8$ M$_\odot$) or intermediate-mass ($<8$ M$_\odot$) companion. Together with the proposed connection of SLSN-II 2006gy to SNe Ia-CSM, SNe 2017fck and 2019cmv may offer a possible thermonuclear origin for superlinear SLSNe-II with peak optical luminosities up to $10^{44}$ erg s$^{-1}$.