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

Abstract: We present observations of SN 2023xgo, a transitional Type Ibn/Icn SN, from $-5.6$ to 63 days relative to $r$-band peak. Early spectra show CIII $\lambda$5696 emission like Type Icn SNe, shifting to Type Ibn features. The He I velocities (1800-10000 km s$^{-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, matching SNe Ibn/Icn but slower than fast transients. SN 2023xgo is the faintest in our SN Ibn sample (M${r}$ = -17.65 $\pm$ 0.04) but shows typical colour and host properties. Semi-analytical modelling of the light curve suggests a compact CSM shell (10$^{12}$-10$^{13}$ cm), mass-loss rate between 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, spectral modelling favours a $\sim$3 M${\odot}$ helium star progenitor with extended ($\sim$10$^{15}$ cm), stratified CSM (density exponent of 2.9) and 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 changes in the progenitors mass loss over time. This mass-loss behavior fits both binary and single-star evolution. Early Icn-like features stem from hot carbon ionization, fading to Ibn-like with cooling. SN 2023xgo thus offers rare insight into the connection between SNe Icn, Ibn, and SNe Ibn with ejecta signatures.