ASASSN-14ms: The Most Energetic Known Explosion of a Type Ibn Supernova and Its Physical Origin

ASASSN-14ms may represent the most luminous Type Ibn supernova (SN Ibn) ever detected, with an absolute U-band magnitude brighter than -22.0 mag and a total bolometric luminosity >1.0 × 10⁴⁴ erg s^-1 near maximum light. The early-time spectra of this SN are characterized by a blue continuum on which are superimposed narrow P Cygni profile lines of He I, suggesting the presence of slowly moving (~1000 km s^-1), He-rich circumstellar material (CSM). At 1-2 months after maximum brightness, the He I line profiles become only slightly broader, with blueshifted velocities of 2000-3000 km s^-1, consistent with the CSM shell being continuously accelerated by the SN light and ejecta. Like most SNe Ibn, the light curves of ASASSN-14ms show rapid post-peak evolution, dropping by ~7 mag in the V band over three months. Such a rapid post-peak decline and high luminosity can be explained by interaction between SN ejecta and helium-rich CSM of 0.9 M_⊙ at a distance of ~10¹⁵ cm. The CSM around ASASSN-14ms is estimated to originate from a pre-explosion event with a mass-loss rate of 6.7 M_⊙ yr^-1 (assuming a velocity of ~1000 km s^-1), which is consistent with abundant He-rich material violently ejected during the late Wolf-Rayet (WN9-11 or Opfe) stage. After examining the light curves for a sample of SNe Ibn, we find that the more luminous ones tend to have slower post-peak decline rates, reflecting that the observed differences may arise primarily from discrepancies in the CSM distribution around the massive progenitors.