The G305 star-forming complex: the central star clusters Danks 1 and Danks 2

The G305 star-forming complex: the central star clusters Danks 1 and Danks 2 Ben Davies 0 1 3 J. S. Clark 7 Christine Trombley 0 Donald F. Figer 0 Francisco Najarro 6 Paul A. Crowther 5 Rolf-Peter Kudritzki 4 9 Mark Thompson 8 James S. Urquhart 2 Luke Hindson 8 0 Center for Detectors, Rochester Institute of Technology , 54 Memorial Drive, Rochester, NY 14623 , USA 1 School of Physics & Astronomy, University of Leeds , Woodhouse Lane, Leeds LS2 9JT 2 Australia Telescope National Facility, CSIRO Astronomy and Space Science , PO Box 76, Epping, NSW 1710 , Australia 3 Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA 4 Institute for Astronomy, University of Hawaii , 2680 Woodlawn Drive, Honolulu, HI 96822 , USA 5 Department of Physics and Astronomy, University of Sheffield , Hounsfield Road, Sheffield S3 7RH 6 Centro de Astrobiolog a (CSIC-INTA) , Ctra. de Torrejo n a Ajalvir km-4, 28850 Torrejo n de Ardoz, Madrid , Spain 7 Department of Physics and Astronomy, The Open University , Walton Hall, Milton Keynes MK7 6AA 8 Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane , Hatfield AL10 9AB 9 Max Planck Institute for Astrophysics , Karl-Schwarzchild-Str. 1, 85748 Garching , Germany A B S T R A C T The G305 H II complex (G305.4+0.1) is one of the most massive star-forming structures yet identified within the Galaxy. It is host to many massive stars at all stages of formation and evolution, from embedded molecular cores to post-main-sequence stars. Here, we present a detailed near-infrared analysis of the two central star clusters Danks 1 and Danks 2, using Hubble Space Telescope+NICMOS imaging and Very Large Telescope+ISAAC spectroscopy. We find that the spectrophotometric distance to the clusters is consistent with the kinematic distance to the G305 complex, an average of all measurements giving a distance of 3.8 0.6 kpc. From analysis of the stellar populations and the pre-main-sequence stars, we find that Danks 2 is the elder of the two clusters, with an age of 3+31 Myr. Danks 1 is clearly younger with an age of 1.5+10..55 Myr, and is dominated by three very luminous H-rich Wolf-Rayet stars which may have masses 100 M . The two clusters have mass functions consistent with the Salpeter slope, and total cluster masses of 8000 1500 and 3000 800 M for Danks 1 and Danks 2, respectively. Danks 1 is significantly the more compact cluster of the two, and is one of the densest clusters in the Galaxy with log (/M pc3) = 5.5+00..54. In addition to the clusters, there is a population of apparently isolated Wolf-Rayet stars within the molecular cloud's cavity. Our results suggest that the star-forming history of G305 began with the formation of Danks 2, and subsequently Danks 1, with the origin of the diffuse evolved population currently uncertain. Together, the massive stars at the centre of the G305 region appear to be clearing away what is left of the natal cloud, triggering a further generation of star formation at the cloud's periphery. stars; formation - stars; Wolf-Rayet - ISM; clouds - H II regions - open clusters and associations; general - open clusters and associations; individual; Danks 1; Danks 2 1 I N T R O D U C T I O N Massive stars have a profound effect on their wider Galactic environment, via the production of copious quantities of ionizing radiation, and from the input of mechanical energy and chemically processed matter into the interstellar medium. For these reasons, an understanding of their life cycle is of importance to many areas of astronomy. Unfortunately, a number of questions regarding this still remain unanswered, with the nature of their formation mechanism(s) being particularly opaque. While growing observational evidence suggests that stars between 20 and 40 M may form via disc-mediated accretion in a manner analogous to their lower mass counterparts (e.g. W33A and W51N; see Zapata et al. 2009; Davies et al. 2010) it is still not clear how more massive stars form (Zinnecker & Yorke 2007), despite compelling observational evidence for stars with masses significantly in excess of 40 M (WR20a, NGC 3603-A1 and R145; Bonanos et al. 2004; Rauw et al. 2005; Schnurr et al. 2008, 2009). Stellar hierarchies appear to be a signature of star formation, with stars predominantly forming in clusters and, in turn, clusters forming in larger complexes (e.g. Larsen 2004). Such structures, spanning tens to hundreds of parsecs, are most readily identifiable in external star-forming galaxies such as M51 (Bastian et al. 2005). While ages for individual clusters within the M51 complexes are difficult to determine, it appears that they are likely to be rather youthful (e.g. <10 Myr) and massive (330 104 M ). Similar complexes are seen within interacting, starbursting galaxies such as the Antennae, where star formation rates are an order of a magnitude higher than in M51, in turn yielding individual clusters with masses >106 M (Bastian et al. 2005, 2006). A precise understanding of the nature of such complexes would be invaluable for the following reasons: (i) they appear to represent a ubiquitous mode of star formation in starburst galaxies and (ii) by virtue of their masses they provide a statistically well-sampled stellar mass function. Unfortunately, the distances of their host galaxies and compact nature conspire to make the determination of the properties of individual clusters let alone stars observationally challenging. Therefore, one might ask whether such structures are present within our own Galaxy. The recent detection of a number of massive ( 104 M ) red supergiant (RSG) dominated clusters at the base of the Scutum-Crux arm is suggestive of such a complex (Figer et al. 2006; Davies et al. 2007; Clark et al. 2009b; Negueruela et al. 2010, 2011), although their spatial extent (100 pc) and age spread (1020 Myr) currently preclude an unambiguous association with a single, physically distinct structure. One observational approach to overcome such uncertainties is to search for young, massive clusters still embedded in their natal giant molecular cloud (GMC) and/or associated giant H II region. Such a strategy guarantees the youth of such a complex, potentially enabling individual examples of massive young stellar objects (MYSOs) to be identified, and ultimately the global star formation history from a spatially resolved census of the (proto) stellar populations. The latter goal is particularly important, since the limited spatial resolution of such objects in external galaxies precludes a detailed analysis of the processes by which the GMC is converted into stars and star clusters. A number of GMCs which appear to contain both massive (>103 M ), young clusters as well as deeply embedded MYSOs have been identified (e.g. W49A, W51 and the Carina nebula; Alves & Homeier 2003; Kumar, Kamath & Davis 2004; Smith & Brooks 2007). Another such region is the G305 star-forming complex (l = 305.4, (...truncated)