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Triggered Star Formation in a Turbulent ISM
Proceedings IAU Symposium No. 237, 2006
B. G. Elmegreen & J. Palous, eds.
c
# 2006 International Astronomical Union
DOI: 00.0000/X000000000000000X
The History of Star Formation in the
Galactic Young Open Cluster NGC 6231
Mario E. van den Ancker 1
1 European Southern Observatory, Karl­Schwarzschild­Strasse 2, D­85748 Garching, Germany
e­mail: mvandena@eso.org
Abstract. We study the star formation history of the galactic young open cluster NGC 6231
using new, deep, wide­field BVRI imaging. Contrary to previous suggestions, we do not find
a lack of low­mass cluster members; our derived mass function is compatible with a Salpeter
IMF. The star formation history of NGC 6231 appears to be bi­modal, with a first wave of star
formation activity 3--5 Myr ago, followed by a new generation of stars forming # 1 Myr ago.
Keywords. Star Formation, Pre­main sequence Stars, Open clusters and associations
The star formation history of the rich open cluster NGC 6231 has been hotly debated
in the literature ever since the suggestion by Eggen (1976) that a violent process must
have triggered star formation in the region. In the largest photometric study of the region
to date, Sung et al. (1998) suggested an abrupt decrease in the number of low­mass stars
in NGC 6231 ­ giving further credibility to a scenario in which star formation in this
region may have been triggered. Interestingly, Reed & Cudworth (2003), recently found
that the trajectory of the globular cluster NGC 6397 intersected that of the natal cloud
of NGC 6231 around five million years ago ­ close to the estimated age of NGC 6231 --
thus providing a plausible candidate for the triggering mechanism.
New BV RI images of a region of 30 # centered on the core of NGC 6231 were obtained
with the Wide Field Imager (WFI) on the ESO/MPG 2.2m telescope at La Silla, Chile.
Photometry in all three broadband filters was extracted for all stars down to V = 21 and
combined with JHK photometry from 2MASS whenever available.
A fit of isochrones to the background­corrected colour­magnitude diagram of NGC
6231 confirms its young age (# 3 Myr). We have detected 19 intermediate­mass stars
with large amounts of near­infrared excess radiation. They are most likely young stars
surrounded by disks (i.e. Herbig Ae/Be and classical T Tauri stars). We do not find
evidence for a lack of low­mass cluster members, as suggested by Sung et al.; within
our completeness limit, the cluster luminosity function appears to be compatible with
a Salpeter IMF. Interestingly enough, we do find strong evidence for the existence of a
significant age spread within our cluster members. We conclude that if star formation
in NGC 6231 is indeed triggered -- either by a recent supernova or by the passage of
NGC 6397 through the galactic plane -- this did not simple result in one single burst
of star formation activity. Rather star formation activity in this region appears to have
started gradually around 5 Myr ago, reached a local maximum around 3 Myr ago, and
experienced another burst of star forming activity around 1 million years ago.
References
Eggen, O.J. 1976, QJRAS 17, 472
Reed R.F., Cudworth K.M. 2003, AAS 203, 1006
Sung H., Bessell, M.S., Lee, S.­W. 1998, AJ 115, 734
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