  We have obtained millimeter wavelength photometry, high-resolution optical
spectroscopy and adaptive optics near-infrared imaging for a sample of 26
Spitzer-selected transition circumstellar disks. All of our targets are located
in the Ophiuchus molecular cloud (d ~125 pc) and have Spectral Energy
Distributions (SEDs) suggesting the presence of inner opacity holes. We use
these ground-based data to estimate the disk mass, multiplicity, and accretion
rate for each object in our sample in order to investigate the mechanisms
potentially responsible for their inner holes. We find that transition disks
are a heterogeneous group of objects, with disk masses ranging from < 0.6 to 40
M_JUP and accretion rates ranging from <1E-11 1E-7 M_solar/yr, but most tend to
have much lower masses and accretion rates than "full disks" (i.e., disks
without opacity holes). Eight of our targets have stellar companions: 6 of them
are binaries and the other 2 are triple systems. In four cases, the stellar
companions are close enough to suspect they are responsible for the inferred
inner holes. We find that 9 of our 26 targets have low disk mass (< 2.5 M_JUP)
and negligible accretion (< 1E-11) M_solar/yr, and are thus consistent with
photoevaporating (or photoevaporated) disks. Four of these 9 non-accreting
objects have fractional disk luminosities < 1E-3 and could already be in a
debris disk stage. Seventeen of our transition disks are accreting. Thirteen of
these accreting objects are consistent with grain growth. The remaining 4
accreting objects have SEDs suggesting the presence of sharp inner holes, and
thus are excellent candidates for harboring giant planets.
