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

We present optical-ultraviolet photometry and optical spectra for the type II supernova (SN) 2022acko. The spectroscopic observations span phases from ~1.5 to ~60 d after the explosion, while the light curve was observed up to ~300 d. The V-band peak is -15.5 ± 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 (MZAMS ~9–10 M). 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 Hα (FWHM ~ 1100\ km \ s⁻¹) was superposed on the broad emission component of Hα P-Cgyni profile in the $t=1.5$ d spectrum. Assuming an ejecta velocity of 12000\ km\ s⁻¹, the rapid disappearance of this narrow Hα emission line within 2 d suggests highly confined circumstellar material (CSM) within ~ 2× 10¹⁴\, cm. Assuming a spherically symmetric CSM, the mass loss rate within this radius is estimated to be ~ 5 × 10⁻⁴ M \ 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.