Origins of Type Ibn SNe 2006jc/2015G in interacting binaries and
  implications for pre-SN eruptions
  Type Ibn supernovae (SNe Ibn) are intriguing stellar explosions whose spectra
exhibit narrow helium lines with little hydrogen. They trace the presence of
circumstellar material (CSM) formed via pre-SN eruptions of their
stripped-envelope progenitors. Early work has generally assumed that SNe Ibn
come from massive Wolf-Rayet (WR) stars via single star evolution. In this
paper, we report ultraviolet (UV) and optical observations of two nearby Type
Ibn SNe 2006jc and 2015G conducted with the Hubble Space Telescope (HST) at
late times. A point source is detected at the position of SN 2006jc, and we
confirm the conclusion of Maund et al. that it is the progenitor's binary
companion. Its position on the Hertzsprung-Russell (HR) diagram corresponds to
a star that has evolved off the main sequence (MS); further analysis implies a
low initial mass for the companion star ($M_2$ $\le$ 11.9$^{+1.2}_{-0.8}$
$M_\odot$) and a secondary-to-primary initial mass ratio very close to unity
($q$ = $M_2/M_1$ $\sim$ 1); the SN progenitor's hydrogen envelope had been
stripped through binary interaction. We do not detect the binary companion of
SN 2015G. For both SNe, the surrounding stellar populations have relatively old
ages and argue against any massive WR stars as their progenitors. These results
suggest that SNe Ibn may have lower-mass origins in interacting binaries. As a
result, they also provide evidence that the giant eruptions commonly seen in
massive luminous blue variables (LBVs) can also occur in much lower-mass,
stripped-envelope stars just before core collapse.
