A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope I: Is the Massive Star Cluster Double?

We present first results from a high resolution multi-band survey of the Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged Westerlund 2 with the Advanced Camera for Surveys through the $F555W$, $F814W$, and $F658N$ filters and with the Wide Field Camera 3 in the $F125W$, $F160W$, and $F128N$ filters. We derive the first high resolution pixel-to-pixel map of the color excess $E(B-V)_g$ of the gas associated with the cluster, combining the H$\alpha$ ($F658N$) and Pa$\beta$ ($F128N$) line observations. We demonstrate that, as expected, the region is affected by significant differential reddening with a median of $E(B-V)_g=1.87$~mag. After separating the populations of cluster members and foreground contaminants using a $(F814W-F160W)$ vs. $F814W$ color-magnitude diagram, we identify a pronounced pre-main-sequence population in Westerlund 2 showing a distinct turn-on. After dereddening each star of Westerlund 2 individually in the color-magnitude diagram we find via over-plotting PARSEC isochrones that the distance is in good agreement with the literature value of $\sim4.16 \pm 0.33$~kpc. With zero-age-main-sequence fitting to two-color-diagrams, we derive a value of total to selective extinction of $R_V=3.95 \pm 0.135$. A spatial density map of the stellar content reveals that the cluster might be composed of two clumps. We estimate the same age of 0.5-2.0 Myr for both clumps. While the two clumps appear to be coeval, the northern clump shows a $\sim 20 \%$ lower stellar surface density.Comment: 24 pages, 27 figures, 7 tables; Accepted for publication to The Astronomical Journa

Present-day Mass Function of Six Small Magellanic Cloud Intermediate-age and Old Star Clusters

We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC339, NGC416, and Lindsay 38 and the old star cluster NGC121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to 0.6 M with a power-law exponent α ranging from 1.51 0.11 (Lindsay 1) to 2.29 0.15 (NGC339). We derive total stellar masses of 105 M , except for Lindsay 38, whose mass is of the order of 104 M. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the α versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between α and the cluster core and tidal radii (rc and rt , respectively), the half-light radii rh , age, central surface brightness, metallicity, and galactocentric radius rgc. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time tr, h, which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the rh, m/rt versus r0/rh, m plane imply that all of the clusters are tidally filled. Our SMC clusters with projected distances larger than 3kpc from the SMC center should have Jacobi radii significantly larger than their observed King tidal radii. The clusters also have higher mean densities than the estimated central density of the SMC. It is possible that these clusters formed in a denser overall environment of the younger SMC, or that the cluster structures were unusually strongly influenced by encounters with giant molecular clouds

Structural Parameters of Seven SMC Intermediate-Age and Old Star Clusters

We present structural parameters for the seven intermediate-age and old star clusters NGC121, Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud. We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than 1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post core collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes.Comment: 16 pages, 13 figure

An Open System for Social Computation

Part of the power of social computation comes from using the collective intelligence of humans to tame the aggregate uncertainty of (otherwise) low veracity data obtained from human and automated sources. We have witnessed a surge in development of social computing systems but, ironically, there have been few attempts to generalise across this activity so that creation of the underlying mechanisms themselves can be made more social. We describe a method for achieving this by standardising patterns of social computation via lightweight formal specifications (we call these social artifacts) that can be connected to existing internet architectures via a single model of computation. Upon this framework we build a mechanism for extracting provenance meta-data across social computations

The young massive star cluster Westerlund 2 observed with MUSE. II. MUSEpack -- a Python package to analyze the kinematics of young star clusters

We mapped the Galactic young massive star cluster Westerlund 2 (Wd2) with the integral field spectrograph MUSE (spatial resolution: 0.2arcsec/px, spectral resolution: $\Delta \lambda$ = 1.25A, wavelength range 4600-9350A) mounted on the VLT, as part of an on-going study to measure the stellar and gas kinematics of the cluster region. In this paper we present the fully reduced dataset and introduce our new Python package "MUSEpack", which we developed to measure stellar radial velocities with an absolute precision of 1-2km/s without the necessity of a spectral template library. This novel method uses the two-dimensional spectra and an atomic transition line library to create templates around strong absorption lines for each individual star. The code runs fully automatically on multi-core machines, which makes it possible to efficiently determine stellar radial velocities of a large number of stars with the necessary precision to measure the velocity dispersion of young star clusters. MUSEpack also provides an enhanced method for removing telluric lines in crowded fields without sky exposures and a Python wrapper for ESO's data reduction pipeline. We observed Wd2 with a total of 11 short and 5 long exposures to cover the bright nebular emission and OB stars, as well as the fainter pre-main sequence stars down to ~1M$_\odot$. The survey covers an area of ~11arcmin$^2$ (15.8pc$^2$). In total, we extracted 1,725 stellar spectra with a mean S/N>5 per pixel. A typical radial velocity (RV) uncertainty of 4.78km/s, 2.92km/s, and 1.1km/s is reached for stars with a mean S/N>10, S/N>20, S/N>50 per pixel, respectively. Depending on the number of spectral lines used to measure the RVs, it is possible to reach RV accuracies of 0.9km/s, 1.3km/s, and 2.2km/s with $\geq5$, 3-4, and 1-2 spectral lines, respectively. The combined statistical uncertainty on the radial velocity measurements is 1.10km/s.Comment: accepted to AJ; 19 pages, 10 figures, 4 tables; MUSEpack is available from https://github.com/pzeidler89/MUSEpack; The documentation can be found on: https://musepack.readthedocs.io/en/latest/index.htm

Age Determination of Six Intermediate-age SMC Star Clusters with HST/ACS

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419 shows several main-sequence turn-offs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally concentrated blue straggler star candidates in NGC416, while for the other clusters these are not present. Using the red clump magnitudes, we find that the closest cluster, NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative distance of ~17kpc, which confirms the large depth of the SMC.Comment: 25 pages, 45 Figure

The Resolved Stellar Population of the Post-Starburst Galaxy NGC 1569

We present WFPC2-HST photometry (in the F439W and F555W bands) of the resolved stars in NGC 1569. The derived color-magnitude diagram (CMD) contains 2800 objects down to the 26th mag, and is complete down to the 23rd. Adopting the literature distance modulus and reddening, the CMD samples stars more massive than 4 Mo, corresponding to a look-back time of 0.15 Gyr approximately. The data are compared to synthetic CMDs to derive the recent star formation history in NGC 1569. It is found that the observed field has experienced a global burst of star formation (SF) lasting at least 0.1 Gyr, and ended a few Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than 10 Myr. The level of the SF rate was very high: 0.5 Mo/yr for a Salpeter IMF, in stars with mass ranging from 0.1 to 120 Mo. When scaled for the surveyed area, the SF rate is approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, or slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.Comment: 63 pages including 2 tables and 12 figures, to be published in The Astrophysical Journa