New Candidates for Planetary-mass Brown Dwarfs in IC 348
  We have used infrared images obtained with the Wide Field Camera 3 on board
the Hubble Space Telescope to search for planetary-mass brown dwarfs in the
star-forming cluster IC 348. In those images, we have identified 12 objects
that have colors indicative of spectral types later than M8, corresponding to
masses of <=30 Jupiter masses at the age of IC 348. The four brightest
candidates have been observed with spectroscopy, all of which are confirmed to
have late types. Two of those candidates appear to be young, and thus are
likely members of the cluster, while the ages and membership of the other two
candidates are uncertain. One of the former candidates is the faintest known
member of IC 348 in extinction-corrected K_s and is expected to have a mass of
4-5 Jupiter masses based on evolutionary models and an assumed age of 3 Myr.
Four of the remaining eight candidates have ground-based photometry that
further supports their candidacy as brown dwarfs, some of which are fainter
(and potentially less massive) than the known members.
