* Astronomy

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OCL 854

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RE: NGC 3603

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Title: A perfect starburst cluster made in one go: the NGC 3603 young cluster
Author: Sambaran Banerjee, Pavel Kroupa

Understanding how distinct, near-spherical gas-free clusters of very young, massive stars shape out of vast, complex clouds of molecular hydrogen is one of the biggest challenges in astrophysics. A popular thought dictates that a single gas cloud fragments into many new-born stars which, in turn, energize and rapidly expel the residual gas to form a gas-free cluster. This study demonstrates that the above classical paradigm remarkably reproduces the well-observed central, young cluster (HD 97950) of the Galactic NGC 3603 star-forming region, in particular, its shape, internal motion and the mass distribution of stars, naturally and consistently follow from a single model calculation. Remarkably, the same parameters (star formation efficiency, gas expulsion time scale and delay) reproduce HD 97950 as were found to reproduce the Orion Nebula Cluster, Pleiades and R136. The present results thereby provide intriguing evidences of formation of star clusters through single-starburst events followed by significant residual gas expulsion.

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Collinder 244

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RE: NGC 3603

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Title: On the Origin of Mass Segregation in NGC 3603
Authors: Xiaoying Pang, Eva K. Grebel, Richard J. Allison, Simon P. Goodwin, Martin Altmann, Daniel Harbeck, Anthony F. J. Moffat, Laurent Drissen

We present deep Hubble Space Telescope/Wide Field and Planetary Camera 2 photometry of the young HD 97950 star cluster in the giant H{ii} region NGC 3603. The data were obtained in 1997 and 2007 permitting us to derive membership based on proper motions of the stars. Our data are consistent with an age of 1 Myr for the HD 97950 cluster. A possible age spread, if present in the cluster, appears to be small. The global slope of the incompleteness-corrected mass function for member stars within 60" is rm \Gamma=-0.88 ±0.15, which is flatter than the value of a Salpeter slope of -1.35. The radially varying mass function shows pronounced mass segregation ranging from slopes of -0.26 ±0.32 in the inner 5" to -0.94 ±0.36 in the outermost annulus (40" -- 60"). Stars more massive than 50 solar masses are found only in the cluster center. The \Lambda minimum spanning tree technique confirms significant mass segregation down to 30 solar masses. The dependence of \Lambda on mass, i.e., that high-mass stars are more segregated than low mass stars, and the (weak) dependence of the velocity dispersion on stellar mass might imply that the mass segregation is dynamical in origin. While primordial segregation cannot be excluded, the properties of the mass segregation indicate that dynamical mass segregation may have been the dominant process for segregation of high-mass stars.

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Title: Star Formation in the Starburst Cluster in NGC 3603
Authors: Matteo Correnti, Francesco Paresce, Rossella Aversa, Giacomo Beccari, Guido De Marchi, Marcella Di Criscienzo, Xiaoying Pang, Loredana Spezzi, Elena Valenti, Paolo Ventura

We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (~ 1 - 3 solar masses) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~ 30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20-30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East - North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction.

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Title: A Two-dimensional Map of Colour Excess in NGC 3603
Authors: Xiaoying Pang, Anna Pasquali, Eva K. Grebel

Using archival HST/WFC3 images centred on the young HD 97950 star cluster in the giant HII region NGC 3603, we computed the pixel-to-pixel distribution of the colour excess, E(B-V)g, of the gas associated with this cluster from its H_alpha/Pa_beta flux ratio. At the assumed distance of 6.9 kpc, the resulting median colour excess within 1 pc from the cluster center is E(B-V)g =1.51 ± 0.04 mag. Outside the cluster (at r > 1 pc), the colour excess is seen to increase with cluster-centric distance towards both North and South, reaching a value of about 2.2 mag at r = 2 pc from the cluster center. The radial dependence of E(B-V)g westward of the cluster appears rather flat at about 1.55 mag over the distance range 1.2 pc < r < 3 pc. In the eastern direction, E(B-V)g steadily increases from 1.5 mag at r = 1 pc to 1.7 mag at r = 2 pc, and stays nearly constant at 1.7 mag for 2 pc < r < 3 pc. The different radial profiles and the pixel-to-pixel variations of E(B-V)g clearly indicate the presence of significant differential reddening across the 4.9 pc x 4.3 pc area centred on the HD 97950 star cluster. We interpret the variations of E(B-V)g as the result of stellar radiation and stellar winds interacting with an inhomogeneous dusty local interstellar medium (ISM) whose density varies spatially. From the E(B-V)g values measured along the rims of the prominent pillars MM1 and MM2 in the southwest and southeast of the HD 97950 cluster we estimate an H2 column density of log10(N(H2))=21.7 and extrapolate it to log10(N(H2))=23 in the pillars' interior. We find the pillars to be closer to us than the central ionising cluster and suggest that star formation may be occurring in the pillar heads.

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Hubble captures odd star motion

Using the Hubble Space Telescope, astronomers have managed to measure the motion of stars in a stellar cluster.
Most stars are formed in these clusters, and this particular one, in the nebula NGC 3603, is one of the largest and most dense in the Milky Way.
The scientists discovered that stars in the cluster were not moving in the way they anticipated.
The discovery sheds new light how these stellar nurseries evolve and disperse.
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Hubble catches stars on the move
Surprising signs of unrest in massive star cluster

With a mass of more than 10 000 suns packed into a volume with a diameter of a mere three light-years, the massive young star cluster in the nebula NGC 3603 is one of the most compact stellar clusters in the Milky Way and an ideal place to test theories for their formation.
A team of astronomers from the Max-Planck Institute for Astronomy in Heidelberg and the University of Cologne led by Wolfgang Brandner (MPIA) wanted to track the movement of the cluster's many stars. Such a study could reveal whether the stars were in the process of drifting apart, or about to settle down.
The cluster, formally known as the NGC 3603 Young Cluster, is about 20 000 light-years from the Sun which makes these measurements extraordinarily difficult. It is necessary to compare images that were made years or even decades apart. The telescope and camera used must give very sharp images and be extremely stable over long periods.
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