The $Spitzer$ infrared spectrograph survey of protoplanetary disks in
  Orion A: I. disk properties
  We present our investigation of 319 Class II objects in Orion A observed by
$Spitzer$/IRS. We also present the follow-up observation of 120 of these Class
II objects in Orion A from IRTF/SpeX. We measure continuum spectral indices,
equivalent widths, and integrated fluxes that pertain to disk structure and
dust composition from IRS spectra of Class II objects in Orion A. We estimate
mass accretion rates using hydrogen recombination lines in the SpeX spectra of
our targets. Utilizing these properties, we compare the distributions of the
disk and dust properties of Orion A disks to those of Taurus disks with respect
to position within Orion A (ONC and L1641) and to the sub-groups by the
inferred radial structures, such as transitional disks vs. radially continuous
full disks. Our main findings are as follows. (1) Inner disks evolve faster
than the outer disks. (2) Mass accretion rate of transitional disks and that of
radially continuous full disks are statistically significantly displaced from
each other. The median mass accretion rate of radially continuous disks in ONC
and L1641 is not very different from that in Taurus. (3) Less grain processing
has occurred in the disks in ONC compared to those in Taurus, based on analysis
of the shape index of the 10 $\mu$m silicate feature ($F_{11.3}/F_{9.8}$). (4)
The 20-31 $\mu$m continuum spectral index tracks the projected distance from
the most luminous Trapezium star, $\theta^{1}$ Ori C. A possible explanation is
the effect of UV ablation of the outer part of the disks.
