M. Fang, R. van Boekel, R. R. King, Th. Henning, J. Bouwman, Y. Doi, Y. K. Okamoto, V. Roccatagliata, A. Sicilia-Aguilar
(abridged) We investigate the properties of young stars and their disks in
the NGC 6357 complex, concentrating on the most massive star cluster within the
complex: Pismis 24. We discover two new young clusters in the NGC 6357 complex.
We give a revised distance estimate for Pismis 24 of 1.7+-0.2 kpc. We find that
the massive star Pis 24-18 is a binary system, with the secondary being the
main X-ray source of the pair. We derive the cluster mass function and find
that up to the completeness limit at low masses it agrees well with the IMF of
the Trapezium cluster. We derive a median age of 1 Myr for the Pismis 24
cluster members. We find five proplyds in HST archival imaging of the cluster,
four of which are newly found. In all cases the proplyd tails are pointing
directly away from the massive star system Pis 24-1. One proplyd shows a second
tail, pointing away from Pis 24-2, suggesting this object is being
photoevaporated from two directions simultaneously. We find that the global
disk frequency (~30%) in Pismis 24 is much lower than some other clusters of
similar age, such as the Orion Nebula Cluster. When comparing the disk
frequencies in 19 clusters/star-forming regions of various ages and different
(massive) star content, we find that the disks in clusters harboring extremely
massive stars (typically earlier than O5), like Pismis 24, are dissipated
roughly twice as quickly as in clusters/star-forming regions without extremely
massive stars. Within Pismis 24, we find that the disk frequency within a
projected distance of 0.6 pc from Pis 24-1 is substantially lower than at
larger radii (~19% vs. ~37%). We argue for a combination of photoevaporation
and irradiation with ionizing UV photons from nearby massive stars, causing
increased MRI-induced turbulence and associated accretion activity, to play an
important role in the dissipation of low-mass star disks in Pismis 24.
View original:
http://arxiv.org/abs/1201.0833
No comments:
Post a Comment