  NGC 6823 is a young open cluster that lies at a distance of ~2 kpc in the
Vulpecula OB1 association. Previous studies using CCD photometry and
spectroscopy have identified a Trapezium system of bright O- and B-type stars
at its center. We present optical, near-infrared and Spitzer photometric
observations of the cluster. Our survey reaches down to I~22 mag and Ks~18 mag.
There is significant differential reddening within the cluster. We find a
bimodal distribution for Av, with a peak at ~3 mag and a broader peak at ~10
mag. We find a ~20% fraction of Class I/Class II young stellar objects (YSOs)
in the cluster, while a large 80% fraction of the sources have a Class III
classification. We have made use of the IPHAS survey in order to probe the
strength in Halpha emission for this large population of Class III sources.
Nearly all of the Class III objects have photospheric (r'-Halpha) colors,
implying an absence of Halpha in emission. This large population of Class III
sources is thus likely the extincted field star population rather than the
diskless YSOs in the cluster. There is a higher concentration of the Class I/II
systems in the eastern region of the cluster and close to the central
Trapezium. The western part of the cluster mostly contains Class III/field
stars and seems devoid of disk sources. We find evidence of a pre-main sequence
population in NGC 6823, in addition to an upper main-sequence population. The
pre-main sequence population mainly consists of young disk sources with ages
between ~1-5 Myr, and at lower masses of ~0.1-0.4 Msun. There may be a possible
mass dependent age spread in the cluster, with the older stars being more
massive than the younger ones. The presence of young disk sources in NGC 6823
indicates similar star formation properties in the outer regions of the Galaxy
as observed for young clusters in the solar neighborhood.
