M. M. Hanson

New Galactic star clusters discovered in the VVV survey

Context. VISTA Variables in the V´oa Lactea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. Aims. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. Methods. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated KS-band tile images for stellar overdensities. Subsequently, we examined the composite JHKS and ZJKS color images of each candidate. PSF photometry of 15 × 15 arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. Results. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar- metallicity Padova isochrones to the color-magnitude diagrams.

A Magnetically Supported Photodissociation Region in M17

The southwestern (SW) part of the Galactic H II region M17 contains an obscured ionization front that is most easily seen at infrared and radio wavelengths. It is nearly edge-on, thus offering an excellent opportunity to study the way in which the gas changes from fully ionized to molecular as radiation from the ionizing stars penetrates into the gas. M17 is also one of the very few H II regions for which the magnetic field strength can be measured in the photodissociation region ( PDR) that forms the interface between the ionized and molecular gas. Here we model an observed line of sight through the gas cloud, including the H+, H0 (PDR), and molecular layers, in a fully self-consistent single calculation. An interesting aspect of the M17 SW bar is that the PDR is very extended. We show that the strong magnetic field that is observed to be present inevitably leads to a very deep PDR, because the structure of the neutral and molecular gas is dominated by magnetic pressure, rather than by gas pressure, as previously had been supposed. We also show that a wide variety of observed facts can be explained if a hydrostatic geometry prevails, in which the gas pressure from an inner X-ray hot bubble and the outward momentum of the stellar radiation field compress the gas and its associated magnetic field in the PDR, as has already been shown to occur in the Orion Nebula. The magnetic field compression may also amplify the local cosmic-ray density. The pressure in the observed magnetic field balances the outward forces, suggesting that the observed geometry is a natural consequence of the formation of a star cluster within a molecular cloud.

L-band spectroscopy of Galactic OB-stars

Context. Mass-loss, occurring through radiation driven supersonic winds, is a key issue throughout the evolution of massive stars. Two outstanding problems are currently challenging the theory of radiation-driven winds: wind clumping and the weak-wind problem . Aims. We seek to obtain accurate mass-loss rates of OB stars at different evolutionary stages to constrain the impact of both problems in our current understanding of massive star winds. Methods. We perform a multi-wavelength quantitative analysis of a sample of ten Galactic OB-stars by means of the atmospheric code cmfgen, with special emphasis on the L -band window. A detailed investigation is carried out on the potential of Br α and Pf γ as mass-loss and clumping diagnostics. Results. For objects with dense winds, Br α samples the intermediate wind while Pf γ maps the inner one. In combination with other indicators (UV, H α , Br γ ) these lines enable us to constrain the wind clumping structure and to obtain “true” mass-loss rates. For objects with weak winds, Br α emerges as a reliable diagnostic tool to constrain Ṁ . The emission component at the line Doppler-core superimposed on the rather shallow Stark absorption wings reacts very sensitively to mass loss already at very low Ṁ values. On the other hand, the line wings display similar sensitivity to mass loss as H α , the classical optical mass loss diagnostics. Conclusions. Our investigation reveals the great diagnostic potential of L -band spectroscopy to derive clumping properties and mass-loss rates of hot star winds. We are confident that Br α will become the primary diagnostic tool to measure very low mass-loss rates with unprecedented accuracy.

ORBITAL DYNAMICS OF CYGNUS X-3

Orbital-phase-resolved infrared spectra of Cygnus X-3 in outburst and quiescence, including tomographic analysis, are presented. We confirm the phasing of broad He II and N V lines in quiescence, such that maximum blueshift corresponds to the X-ray minimum at Φ = 0.00 ± 0.04. In outburst, double-peaked He I structures show a similar phasing with two significant differences: (1) although varying in relative strength, there is continuous line emission in blue and red peaks around the orbit; and (2) an absorption component, ~ of an orbit out of phase with the emission features, is discerned. Doppler tomograms of the double-peaked profiles are consistent with a disk-wind geometry, rotating at velocities of 1000 km s-1. Regrettably, the tomography algorithm will produce a similar ring structure from alternative line sources if contaminated by overlying P Cygni profiles. This is certainly the case in the strong 2.0587 μm He I line, leading to an ambiguous solution for the nature of double-peaked emission. The absorption feature, detected of an orbit out of phase with the emission features, is consistent with an origin in the He star wind and yields for the first time a plausible radial velocity curve for the system. We directly derive the mass function of the system, 0.027 M☉. If we assume a neutron star accretor and adopt a high orbital inclination, i > 60°, we obtain a mass range for the He star of 5 M☉ MWR 11 M☉. Alternatively, if the compact object is a black hole, we estimate MBH 10 M☉. We discuss the implications of these masses for the nature and size of the binary system.

MASSCLEANage—STELLAR CLUSTER AGES FROM INTEGRATED COLORS

We present the recently updated and expanded MASSCLEANcolors, a database of 70 million Monte Carlo models selected to match the properties (metallicity, ages, and masses) of stellar clusters found in the Large Magellanic Cloud (LMC). This database shows the rather extreme and non-Gaussian distribution of integrated colors and magnitudes expected with different cluster age and mass and the enormous age degeneracy of integrated colors when mass is unknown. This degeneracy could lead to catastrophic failures in estimating age with standard simple stellar population models, particularly if most of the clusters are of intermediate or low mass, like in the LMC. Utilizing the MASSCLEANcolors database, we have developed MASSCLEANage, a statistical inference package which assigns the most likely age and mass (solved simultaneously) to a cluster based only on its integrated broadband photometric properties. Finally, we use MASSCLEANage to derive the age and mass of LMC clusters based on integrated photometry alone. First, we compare our cluster ages against those obtained for the same seven clusters using more accurate integrated spectroscopy. We find improved agreement with the integrated spectroscopy ages over the original photometric ages. A close examination of our results demonstrates the necessity of solving simultaneously for mass and age to reduce degeneracies in the cluster ages derived via integrated colors. We then selected an additional subset of 30 photometric clusters with previously well-constrained ages and independently derive their age using the MASSCLEANage with the same photometry with very good agreement. The MASSCLEANage program is freely available under GNU General Public License.

AGE AND MASS FOR 920 LARGE MAGELLANIC CLOUD CLUSTERS DERIVED FROM 100 MILLION MONTE CARLO SIMULATIONS

We present new age and mass estimates for 920 stellar clusters in the Large Magellanic Cloud (LMC) based on previously published broadband photometry and the stellar cluster analysis package, MASSCLEANage. Expressed in the generic fitting formula, d 2 N/dMdtM α t β, the distribution of observed clusters is described by α = –1.5 to –1.6 and β = –2.1 to –2.2. For 288 of these clusters, ages have recently been determined based on stellar photometric color-magnitude diagrams, allowing us to gauge the confidence of our ages. The results look very promising, opening up the possibility that this sample of 920 clusters, with reliable and consistent age, mass, and photometric measures, might be used to constrain important characteristics about the stellar cluster population in the LMC. We also investigate a traditional age determination method that uses a χ2 minimization routine to fit observed cluster colors to standard infinite-mass limit simple stellar population models. This reveals serious defects in the derived cluster age distribution using this method. The traditional χ2 minimization method, due to the variation of U, B, V, R colors, will always produce an overdensity of younger and older clusters, with an underdensity of clusters in the log (age/yr) = [7.0, 7.5] range. Finally, we present a unique simulation aimed at illustrating and constraining the fading limit in observed cluster distributions that includes the complex effects of stochastic variations in the observed properties of stellar clusters.

New galactic star clusters discovered in the VVV survey. Candidates projected on the inner disk and bulge

I.N. and A.M. acknowledge support for this work by the Spanish Government Ministerio de Ciencia e Innovacion (MICINN) through grant AYA2012-39364-C02-02.

MASSCLEAN—MASSIVE CLUSTER EVOLUTION AND ANALYSIS PACKAGE: DESCRIPTION AND TESTS

We present MASSCLEAN, a new, sophisticated and robust stellar cluster image and photometry simulation package. This visualization tool is able to create color-magnitude diagrams (CMDs) and standard FITS images in any of the traditional optical and near-infrared bands based on cluster characteristics input by the user, including but not limited to distance, age, mass, radius, and extinction. At the limit of very distant, unresolved clusters, we have checked the integrated colors created in MASSCLEAN against those from other simple stellar population models with consistent results. We have also tested models that provide a reasonable estimate of the field star contamination in images and CMDs. We demonstrate the package by simulating images and CMDs of well-known massive Milky Way clusters and compare their appearance to real data. Because the algorithm populates the cluster with a discrete number of tenable stars, it can be used as part of a Monte Carlo Method to derive the probabilistic range of characteristics (integrated colors, for example) consistent with a given cluster mass and age. Our simulation package is available for download and will run on any standard desktop running UNIX/Linux. Full documentation on installation and its use is also available. Finally, a Web-based version of MASSCLEAN, which can be immediately used and is sufficiently adaptable for most applications, is available through a Web interface.

MASSCLEANCOLORS—MASS-DEPENDENT INTEGRATED COLORS FOR STELLAR CLUSTERS DERIVED FROM 30 MILLION MONTE CARLO SIMULATIONS

We present Monte Carlo models of open stellar clusters with the purpose of mapping out the behavior of integrated colors with mass and age. Our cluster simulation package allows for stochastic variations in the stellar mass function to evaluate variations in integrated cluster properties. We find that UBVK colors from our simulations are consistent with simple stellar population (SSP) models, provided the cluster mass is large, M cluster ≥ 106 M ☉. Below this mass, our simulations show two significant effects. First, the mean value of the distribution of integrated colors moves away from the SSP predictions and is less red, in the first 107 to 108 years in UBV colors, and for all ages in (V – K). Second, the 1σ dispersion of observed colors increases significantly with lower cluster mass. We attribute the former to the reduced number of red luminous stars in most of the lower mass clusters and the latter to the increased stochastic effect of a few of these stars on lower mass clusters. This latter point was always assumed to occur, but we now provide the first public code able to quantify this effect. We are completing a more extensive database of magnitudes and colors as a function of stellar cluster age and mass that will allow the determination of the correlation coefficients among different bands, and improve estimates of cluster age and mass from integrated photometry.

Obscured clusters - III. Follow-up observations of Mercer 23

Context. New infrared surveys have revealed over 1000 new open cluster candidates in the Milky Way, but these candidates need to be confirmed with follow up observations. Of particular interest are young, massive star clusters because they serve as nearby analogues to the distant super star clusters studied as point sources in other galaxies. Aims. We determine the physical parameters and investigate the high-mass stellar content of the infrared star cluster Mercer 23, situated near the Galactic plane (l = 53. ◦ 772, b =+ 0. ◦ 164). Methods. Our analysis is based on new Baade/PANIC JHKS and ISAAC/VLT imaging of Mercer 23 and ISAAC/VLT moderate resolution (R ≈ 4000) spectroscopy of the brightest cluster members in the H -a ndK-bands. The cluster age is determined from isochrone main-sequence (MS) and pre-MS fitting. We derive stellar parameters for eight of the stellar members, using a full non-LTE modeling of the obtained spectra. Results. Mercer 23 is a very young cluster, with age of t = 2–4 Myr. The cluster suffers reddening of E(J−KS) = 1.35, AV = 7. 2m ag. The derived distance is d = 6.5 ± 0.3 kpc. Our spectral modeling allows us to conclude that the three most luminous member are evolved highly massive stars: a WR star, and two mid-O supergiant stars, based on their derived luminosity. Conclusions. Mercer 23 is not a super-massive cluster such as those recently recognized to exist in the Milky Way. However, its mass