Constraints on the Binary Companion to the SN Ic 1994I Progenitor
  Core-collapse supernovae (SNe), marking the deaths of massive stars, are
among the most powerful explosions in the Universe, responsible, e.g., for a
predominant synthesis of chemical elements in their host galaxies. The majority
of massive stars are thought to be born in close binary systems. To date,
putative binary companions to the progenitors of SNe may have been detected in
only two cases, SNe 1993J and 2011dh. We report on the search for a companion
of the progenitor of the Type Ic SN 1994I, long considered to have been the
result of binary interaction. Twenty years after explosion, we used the Hubble
Space Telescope to observe the SN site in the ultraviolet (F275W and F336W
bands), resulting in deep upper limits on the expected companion: F275W > 26.1
mag and F336W > 24.7 mag. These allows us to exclude the presence of a main
sequence companion with a mass >~ 10 Msun. Through comparison with theoretical
simulations of possible progenitor populations, we show that the upper limits
to a companion detection exclude interacting binaries with semi-conservative
(late Case A or early Case B) mass transfer. The limits tend to favor systems
with non-conservative, late Case B mass transfer with intermediate initial
orbital periods and mass ratios. The most likely mass range for a putative main
sequence companion would be ~5--12 Msun, the upper end of which corresponds to
the inferred upper detection limit.
