Sparse Aperture Masking Observations of the FL Cha Pre-transitional Disk
  We present deep Sparse Aperture Masking (SAM) observations obtained with the
ESO Very Large Telescope of the pre-transitional disk object FL Cha (SpT=K8,
d=160 pc), the disk of which is known to have a wide optically thin gap
separating optically thick inner and outer disk components. We find non-zero
closure phases, indicating a significant flux asymmetry in the K-band emission
(e.g., a departure from a single point source detection). We also present
radiative transfer modeling of the SED of the FL Cha system and find that the
gap extends from ~0.06 to ~8.3 AU. We demonstrate that the non-zero closure
phases can be explained almost equally well by starlight scattered off the
inner edge of the outer disk or by a (sub)stellar companion. Single-epoch,
single-wavelength SAM observations of transitional disks with large cavities
that could become resolved should thus be interpreted with caution, taking the
disk and its properties into consideration. In the context of a binary model,
the signal is most consistent with a high-contrast (delta_K ~4.8 mag) source at
a ~40 mas (6 AU) projected separation. However, the flux ratio and separation
parameters remain highly degenerate and a much brighter source (deta_K ~1 mag)
at 15 mas (2.4 AU) can also reproduce the signal. Second-epoch,
multi-wavelength observations are needed to establish the nature of the SAM
detection in FL Cha.
