Bibcode
Riaz, B.; Martín, E. L.; Tata, R.; Monin, J.-L.; Phan-Bao, N.; Bouy, H.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 419, Issue 3, pp. 1887-1912.
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1
2012
Citations
9
Refereed citations
9
Description
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 centre, along with several massive O-, B- and A-type
stars in the cluster. We present optical VRI and near-infrared JHK
photometric observations, complemented with Spitzer/Infrared Array
Camera archival data, with an aim to identify the young low-mass
population and the disc candidates in this region. 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 have classified the sources based on
the [4.5] - [8] colour, which is least affected by extinction. We find a
˜20 per cent fraction of Class I/Class II young stellar objects
(YSOs) in the cluster, while a large 80 per cent fraction of the sources
have a Class III classification. We have made use of the INT Photometric
Hα Survey (IPHAS) in order to probe the strength in Hα
emission for this large population of Class III sources. Nearly all of
the Class III objects have photospheric (r'-Hα) colours, implying
an absence of Hα in emission. This large population of Class III
sources is thus likely the extinct field star population rather than the
discless 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 disc 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 disc sources with ages between ˜1 and 5 Myr, and
at lower masses of ˜0.1-0.4 M&sun;. 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 disc
sources in NGC 6823 indicates similar star formation properties in the
outer regions of the Galaxy as observed for young clusters in the solar
neighbourhood.