SN 2023xgo: Helium-rich Type Icn or Carbon-Flash Type Ibn supernova?

Abstract

We present observations of SN~2023xgo, a transitional Type Ibn/Icn supernova, from $-5.6$ to $+63$~days relative to the $r$-band peak. Early spectra show C~III $\lambda 5696$ emission reminiscent of Type~Icn SNe, which later gives way to Type~Ibn features. The He~I velocities ($1800$--$10{,}000$~km~s$^{-1}$) and pseudo-equivalent widths are among the highest in the Ibn/Icn class. The light curve declines at $0.14$~mag~d$^{-1}$ until $+30$~days, consistent with SNe~Ibn/Icn and slower than fast transients. SN~2023xgo is the faintest in our SN~Ibn sample ($M_r=-17.65\pm0.04$) but shows typical color and host properties. Semi-analytical modeling of the light curve suggests a compact CSM shell ($\sim 10^{12}$--$10^{13}$~cm) and a mass-loss rate of $10^{-4}$--$10^{-3}$~$M_{\odot}$~yr$^{-1}$, with CSM and ejecta masses of $\sim 0.22$ and $0.12$~$M_{\odot}$, respectively. Post-maximum light-curve and spectral modeling favor a $\sim 3$~$M_{\odot}$ helium-star progenitor with extended ($\sim 10^{15}$~cm), stratified CSM (density exponent $n=2.9$) and a mass-loss rate of $0.1$--$2.7$~$M_{\odot}$~yr$^{-1}$. These two mass-loss regimes imply a radially varying CSM, shaped by asymmetry or temporal changes in the progenitor's mass loss. This behavior is compatible with both binary and single-star evolution. We argue that the early Icn-like features arise from hot carbon ionization and fade to Ibn-like signatures as the ejecta and CSM cool, making SN~2023xgo a rare probe of the connection between SNe~Icn, SNe~Ibn, and Ibn events with ejecta signatures.