SN 2022acko: a low-luminosity SNe IIP with signs of early circumstellar interaction

ABSTRACT We present optical-ultraviolet photometry and optical spectra for the type II supernova (SN) 2022acko. The spectroscopic observations span phases from $\sim$1.5 to $\sim$60 d after the explosion, while the light curve was observed up to $\sim$300 d. The V-band peak is $-15.5 \pm 0.3$ mag, suggesting that SN 2022acko is a low-luminosity SN II (LLSN). The overall observed properties of SN 2022acko are consistent with those produced by a lower mass progenitor ($\rm M_{ZAMS} \sim$9–10 M$_{\odot }$). The spectra at $t=1.5$ d and $t=2.5$ d exhibit a broad emission feature peaking near 4600 Å (the ‘ledge’ feature), which we interpret as blueshifted He ii 4686 Å lines arising from the ionized ejecta. Moreover, a possible flash-ionized (FI) emission line of H$\alpha$ (FWHM $\sim 1100\ \rm km \ s^{-1}$) was superposed on the broad emission component of H$\alpha$ P-Cgyni profile in the $t=1.5$ d spectrum. Assuming an ejecta velocity of $\rm 12000\ km\ s^{-1}$, the rapid disappearance of this narrow H$\alpha$ emission line within 2 d suggests highly confined circumstellar material (CSM) within $\sim \rm 2\times 10^{14}\, cm$. Assuming a spherically symmetric CSM, the mass loss rate within this radius is estimated to be $\rm \sim 5 \times 10^{-4} {\rm M}_{\odot } \ yr^{-1}$ based on our hybrid light curve model. The early ‘ledge’ feature observed in SN 2022acko have also been observed in other SNe II, suggesting that early-phase circumstellar interaction is more common than previously thought.