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

TL;DR

SN 2022acko is a nearby low-luminosity Type IIP that exhibits early circumstellar interaction, evidenced by a broad 4600 Å ledge feature and a transient narrow H$\alpha$ line. By combining high-cadence photometry, UV data, and spectra with distance and metallicity estimates, the authors constrain a low-mass progenitor ($M_{ZAMS} \sim 9-10\,M_{\odot}$) and a confined CSM with radius $\sim 2\times 10^{14}$ cm, corresponding to a pre-explosion mass loss rate of $\sim 5\times 10^{-4}\,M_{\odot}\,\rm yr^{-1}$. The Ni mass is found to be $M_{Ni} \approx 0.017^{+0.009}_{-0.007}\,M_{\odot}$, and hydrodynamical/analytic modeling indicates a modest explosion energy ($\sim (0.26-0.50)\times 10^{51}$ erg) and an ejecta mass of a few $M_{\odot}$. The study highlights the prevalence of early CSI signatures in SNe II and supports a link between LL SNe II and low-mass RSG progenitors, with broader implications for mass loss history and explosion physics in the final stages of massive-star evolution.

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 days after the explosion, while the light curve was observed up to $\sim$ 300 days. 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 $9-10M$_{\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 Ålines arising from the ionized ejecta. Moreover, a possible flash-ionized (FI) emission line of H$α$ (FWHM $\sim 1100\ \rm km \ s^{-1}$) was superposed on the broad emission component of H$α$ 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$α$ emission line within two days suggests highly confined CSM within $\sim \rm 2\times10^{14}\, cm$. Assuming a spherically symmetric CSM, the mass loss rate within this radius is estimated to be $\rm \sim 5 \times 10^{-4} M_{\odot} \ year^{-1}$ 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 (CSI) is more common than previously thought.

Figures (15)

• Figure 1: $g$-band image of SN 2022acko in NGC 1300, taken on 2022 Dec. 15, with the Lijiang 2.4-m telescope. North is up and east is to the right. The locations of the supernova and local reference stars are marked by white tick marks and circles, respectively.
• Figure 2: The UV and optical light curves of SN 2022acko. The light curves of different bands have been vertically shifted for clarity.
• Figure 3: The position of SN 2022acko in the SN II family in terms of various photometric and spectroscopic indicators, including the V-band absolute magnitude ($M^{50}_V$), expansion velocity of Fe ii$\lambda$5169 measured at 50 d after the explosion ($v_{\rm exp}^{50}$), the $^{56}$Ni mass, the maximum V-band absolute magnitude($M_{V}^{\rm max}$) and the decline rate of the plateau ($s_2$). The SNe II sample in 2003ApJ...582..905H2014ApJ...786...67A2014MNRAS.439.2873S2015MNRAS.448.2608V together with SN 2008in, SN 2009N, SN 2005cs and SN 1999em are shown for comparison.
• Figure 4: $uvw2$-band (top-panel), V-band (mid-panel) and $r$-band (bottom-panel) light-curve comparisons between SN 2022acko and some well-studied SNe II, including the typical SN IIP 1999em 2001ApJ...558..615H2002PASP..114...35L2003MNRAS.338..939E and 2017eaw 2019ApJ...875..136V2019ApJ...876...19S, the low-luminosity SN 2005cs 2006MNRAS.370.1752P2009MNRAS.394.2266P, SN 2016bkv 2018ApJ...859...78N2018ApJ...861...63H, SN 2018lab 2023ApJ...945..107P, SN 2008in 2011ApJ...736...76R and SN 2009N 2011ApJ...736...76R, the FI object SN 2018zd 2020MNRAS.498...84Z and the ledge object SN 2021yja 2022ApJ...935...31H
• Figure 5: The $uvw2-v$ and B-V color curve evolution of SN 2022acko, along with that of other well-studied SNe II. All the colours have been corrected for both the Galactic and host-galaxy reddening.