The luminosity, mass, and age distributions of compact star clusters in M83 based on Hubble Space Telescope/Wide Field Camera 3 observations
Astrophysical Journal 719:1 (2010) 966-978
Abstract:
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multiband images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL ∝ Lα, with α =-2.04 ± 0.08, down to MV ≈-5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI, Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power law, dN/dτ ∝ τγ, with γ =-0.9 ± 0.2, for M ≳ few × 103 M⊙ and τ ≲ 4 × 108 yr. This indicates that clusters are disrupted quickly, with ≈80%-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM ∝ Mβ, with β =-1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, MC, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters. © 2010. The American Astronomical Society. All rights reserved.A 43-GHz VLA survey in the ELAIS N2 area
\mnras 408 (2010) 657-668
The chemical signatures of the first star clusters in the universe
Astrophysical Journal 721:1 (2010) 582-596
Abstract:
The chemical abundance patterns of the oldest stars in the Galaxy are expected to contain residual signatures of the first stars in the early universe. Numerous studies attempt to explain the intrinsic abundance scatter observed in some metal-poor populations in terms of chemical inhomogeneities dispersed throughout the early Galactic medium due to discrete enrichment events. Just how the complex data and models are to be interpreted with respect to "progenitor yields" remains an open question. Here we showthat stochastic chemical evolutionmodels to date have overlooked a crucial fact. Essentially, all stars today are born in highly homogeneous star clusters and it is likely that this was also true at early times. When this ingredient is included, the overall scatter in the abundance plane [Fe/H] versus [X/Fe] (C-space), where X is a nucleosynthetic element, can be much less than derived from earlier models. Moreover, for moderately flat clustermass functions (γ ≲ 2), and/or formass functions with a highmass cutoff (M max ≳ 105M⊙), stars exhibit a high degree of clumping in C-space that can be identified even in relatively small data samples. Since stellar abundances can be modified by mass transfer in close binaries, clustered signatures are essential for deriving the yields of the first supernovae.We present a statistical test to determine whether a given set of observations exhibit such behavior. Our initial work focuses on two dimensions in C-space, but we show that the clustering signal can be greatly enhanced by additional abundance axes. The proposed experiment will be challenging on existing 8-10 m telescopes, but relatively straightforward for a multi-object echelle spectrograph mounted on a 25-40 m telescope. © 2010. The American Astronomical Society. All rights reserved.Progressive star formation in the young galactic super star cluster NGC 3603
Astrophysical Journal 720:2 (2010) 1108-1117
Abstract:
Early Release Science observations of the cluster NGC 3603 with the WFC3 on the refurbished Hubble Space Telescope allow us to study its recent star formation history. Our analysis focuses on stars with Hαexcess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Hαexcess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.UV-dropout galaxies in the goods-south field from WFC3 Early Release Science observations
Astrophysical Journal 720:2 (2010) 1708-1716