* Astronomy

Title: Cluster-formation in the Rosette molecular cloud at the junctions of filaments
Authors: N. Schneider (1), T. Csengeri (2), M. Hennemann (1), F. Motte (1), P. Didelon (1), C. Federrath (3,4), S. Bontemps (5), J. Di Francesco (6), D. Arzoumanian (1), V. Minier (1), Ph. André (1), T. Hill (1), A. Zavagno (7), Q. Nguyen-Luong (1), M. Attard (1), J.-Ph. Bernard (8), D. Elia (9), C. Fallscheer (6), M. Griffin (10), J. Kirk (10), R. Klessen (4), V. Könyves (1), P. Martin (11), A. Men'shchikov (1), P. Palmeirim (1), N. Peretto (1), M. Pestalozzi (9), D. Russeil (7), S. Sadavoy (12), T. Sousbie (13), L. Testi (14), P. Tremblin (1), D. Ward-Thompson (10), G. White (15,16) ((1) IRFU/SAp CEA/DSM, Laboratoire AIM CNRS - Université Paris Diderot, France, (2) Max-Planck Institut für Radioastronomie, Bonn, Germany, (3) Monash Centre for Astrophysics (MoCA), School of Mathematical Sciences, Monash University, Australia, (4) Zentrum für Astronomie der Universität Heidelberg, Inst. für Theor. Astrophysik, Heidelberg, Germany, (5) OASU/LAB-UMR5804, CNRS, Université Bordeaux 1, Floirac, France, (6) National Research Council of Canada, Herzberg Institute of Astrophysics, Victoria, Canada, (7) Laboratoire d'Astrophysique de Marseille, CNRS/INSU - Université de Provence, Marseille, France (8) Université de Toulouse, UPS, CESR, Toulouse, France, (9) IAPS-INAF, Roma, Italy, (10) Cardiff University School of Physics and Astronomy, Cardiff, UK, (11) CITA & Dep. of Astronomy and Astrophysics, University of Toronto, Canada, (12) Department of Physics and Astronomy, University of Victoria, Canada (13) IAP, UPMC, CNRS, Paris, France, (14) ESO, Garching, Germany, (15) Department of Physics & Astronomy, The Open University, UK, (16) RAL, Didcot, UK)

For many years feedback processes generated by OB-stars in molecular clouds, including expanding ionisation fronts, stellar winds, or UV-radiation, have been proposed to trigger subsequent star formation. However, hydrodynamic models including radiation and gravity show that UV-illumination has little or no impact on the global dynamical evolution of the cloud. The Rosette molecular cloud, irradiated by the NGC2244 cluster, is a template region for triggered star-formation, and we investigated its spatial and density structure by applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and probability density functions (PDFs) on Herschel column density maps, obtained within the HOBYS key program. The analysis reveals not only the filamentary structure of the cloud but also that all known infrared clusters except one lie at junctions of filaments, as predicted by turbulence simulations. The PDFs of sub-regions in the cloud show systematic differences. The two UV-exposed regions have a double-peaked PDF we interprete as caused by shock compression. The deviations of the PDF from the log-normal shape typically associated with low- and high-mass star-forming regions at Av~3-4m and 8-10m, respectively, are found here within the very same cloud. This shows that there is no fundamental difference in the density structure of low- and high-mass star-forming regions. We conclude that star-formation in Rosette - and probably in high-mass star-forming clouds in general - is not globally triggered by the impact of UV-radiation. Moreover, star formation takes place in filaments that arose from the primordial turbulent structure built up during the formation of the cloud. Clusters form at filament mergers, but star formation can be locally induced in the direct interaction zone between an expanding HII--region and the molecular cloud.

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Hershel captures the Rosette Nebula

The Rosette Nebula is a cloud of dust, about 5500 light years away, containing enough gas and dust to make about 10,000 stars like our Sun. In the centre of the nebula, and off to the right hand side of this image, is a cluster of hot, bright young stars. These are warming up the surrounding gas and dust, making it appear bluer. The small, bright white regions are cocoons of dust in which huge stars are currently being born.
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Big Babies in the Rosette Nebula

This image from the Herschel Space Observatory shows most the cloud associated with the Rosette nebula, a stellar nursery about 5,000 light-years from Earth in the Monoceros, or Unicorn, constellation. Herschel collects the infrared light given out by dust. The bright smudges are dusty cocoons containing massive embryonic stars, which will grow up to 10 times the mass of our sun. The small spots near the centre of the image are lower mass stellar embryos. The Rosette nebula itself, and its massive cluster of stars, is located to the right of the picture.
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Title: A Chandra Study of the Rosette Star-Forming Complex. I. The Stellar Population and Structure of the Young Open Cluster NGC 2244
Authors: Junfeng Wang, Leisa K. Townsley, Eric D. Feigelson, Patrick S. Broos, Konstantin V. Getman (1), Carlos Roman-Zuniga (2,3), Elizabeth Lada (3) ((1) Penn State (2) Harvard-Smithsonian Centre for Astrophysics (3) University of Florida)

We present the first high spatial resolution Chandra X-ray study of NGC 2244, the 2 Myr old stellar cluster immersed in the Rosette Nebula. Over 900 X-ray sources are detected; 77% have optical or FLAMINGOS near-infrared (NIR) stellar counterparts and are mostly previously uncatalogued young stellar cluster members. All known OB stars with spectral type earlier than B1 are detected and the X-ray selected stellar population is estimated to be nearly complete between 0.5 and 3 Msun. The X-ray luminosity function (XLF) ranges from 29.4<logLx<32 ergs/s in the hard (2-8keV) band. By comparing the NGC 2244 and Orion Nebula Cluster XLFs, we estimate a total population of 2000 stars in NGC 2244. A number of further results emerge from our analysis: The XLF and the associated K-band luminosity function indicate a normal Salpeter initial mass function (IMF) for NGC 2244. This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of <4Msun stars. The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster's stellar radial density profile shows two distinctive structures. This double structure, combined with the absence of mass segregation, indicates that this cluster is not in dynamical equilibrium. The spatial distribution of X-ray selected K-excess disk stars and embedded stars is asymmetric with an apparent deficit towards the north. The fraction of X-ray-selected cluster members with K-band excesses caused by inner protoplanetary disks is 6%, slightly lower than the 10% disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample. This is due to the high efficiency of X-ray surveys in locating disk-free T Tauri stars.

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