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Register a new userBiases in the determination of dynamical parameters of star clusters: today and in the Gaia era
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The structural and dynamical properties of star clusters are generally derived by means of the comparison between steady-state analytic models and the available observables. With the aim of studying the biases of this approach, we fitted different analytic models to simulated observations obtained from a suite of direct N-body simulations of star clusters in different stages of their evolution and under different levels of tidal stress to derive mass, mass function and degree of anisotropy. We find that masses can be under/over-estimated up to 50% depending on the degree of relaxation reached by the cluster, the available range of observed masses and distances of radial velocity measures from the cluster center and the strength of the tidal field. The mass function slope appears to be better constrainable and less sensitive to model inadequacies unless strongly dynamically evolved clusters and a non-optimal location of the measured luminosity function are considered. The degree and the characteristics of the anisotropy developed in the N-body simulations are not adequately reproduced by popular analytic models and can be detected only if accurate proper motions are available. We show how to reduce the uncertainties in the mass, mass-function and anisotropy estimation and provide predictions for the improvements expected when Gaia proper motions will be available in the near future.
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Stellar clusters are important for astrophysics in many ways, for instance as optimal tracers of the Galactic populations to which they belong or as one of the best test bench for stellar evolutionary models. Gaia DR1, with TGAS, is just skimming the wealth of exquisite information we are expecting from the more advanced catalogues, but already offers good opportunities and indicates the vast potentialities. Gaia results can be efficiently complemented by ground-based data, in particular by large spectroscopic and photometric surveys. Examples of some scientific results of the Gaia-ESO survey are presented, as a teaser for what will be possible once advanced Gaia releases and ground-based data will be combined.
Classical Cepheids in open clusters are key ingredients for stellar population studies and the characterization of variable stars, as they are tracers of young and massive populations and of recent star formation episodes. Cluster Cepheids are of particular importance since they can be age dated by using the clusters stellar population to obtain the Cepheid period-luminosity-age relation. In this contribution, we present the preliminary results of an all-sky search for classical Cepheids in Galactic open clusters by taking advantage of the unprecedented astrometric precision of the second data release of the Gaia satellite. To do this, we determined membership probabilities by performing a Bayesian analysis based on the spatial distribution of Cepheids and clusters, and their kinematics. Here we describe our adopted methodology.
We present wide-field, ground-based Johnson-Cousins UBVRI photometry for 48 Galactic globular clusters based on almost 90000 public and proprietary images. The photometry is calibrated with the latest transformations obtained in the framework of our secondary standard project, with typical internal and external uncertainties of order a few millimagnitudes. These data provide a bridge between existing small-area, high-precision HST photometry and all sky-catalogues from large surveys like Gaia, SDSS, or LSST. For many clusters, we present the first publicly available photometry in some of the five bands (typically U and R). We illustrate the scientific potential of the photometry with examples of surface density and brightness profiles and of colour-magnitude diagrams, with the following highlights: (i) we study the morphology of NGC 5904, finding a varying ellipticity and position angle as a function of radial distance; (ii) we show U-based colour-magnitude diagrams and demonstrate that no cluster in our sample is free from multiple stellar populations, with the possible exception of a few clusters with high and differential reddening or field contamination, for which more sophisticated investigations are required. This is true even for NGC 5694 and Terzan 8, that were previously considered as (mostly) single-population candidates.
The orbits, atmospheric parameters, chemical abundances, and ages of individual stars in the Milky Way provide the most comprehensive illustration of galaxy formation available. The Tycho-Gaia Astrometric Solution (TGAS) will deliver astrometric parameters for the largest ever sample of Milky Way stars, though its full potential cannot be realized without the addition of complementary spectroscopy. Among existing spectroscopic surveys, the RAdial Velocity Experiment (RAVE) has the largest overlap with TGAS ($gtrsim$200,000 stars). We present a data-driven re-analysis of 520,781 RAVE spectra using The Cannon. For red giants, we build our model using high-fidelity APOGEE stellar parameters and abundances for stars that overlap with RAVE. For main-sequence and sub-giant stars, our model uses stellar parameters from the K2/EPIC. We derive and validate effective temperature $T_{rm eff}$, surface gravity $log{g}$, and chemical abundances of up to seven elements (O, Mg, Al, Si, Ca, Fe, Ni). We report a total of 1,685,851 elemental abundances with a typical precision of 0.07 dex, a substantial improvement over previous RAVE data releases. The synthesis of RAVE-on and TGAS is the most powerful data set for chemo-dynamic analyses of the Milky Way ever produced.
We use the framework developed as part of the MESA Isochrones and Stellar Tracks (MIST) project to assess the utility of several types of observables in jointly measuring the age and 1D stellar model parameters in star clusters. We begin with a pedagogical overview summarizing the effects of stellar model parameters, such as the helium abundance, mass-loss efficiency, and the mixing length parameter, on observational diagnostics including the color-magnitude diagram, mass-radius relation, and surface abundances, amongst others. We find that these parameters and the stellar age influence observables in qualitatively distinctive, degeneracy-breaking ways. To assess the current state of affairs, we use the recent Gaia Data Release 2 (DR2) along with data from the literature to investigate three well-studied old open clusters---NGC6819, M67, NGC6791---as case studies. Although there is no obvious tension between the existing observations and the MIST models for NGC6819, there are interesting discrepancies in the cases of M67 and NGC6791. At this time, parallax zero point uncertainties in Gaia DR2 remain one of the limiting factors in the analysis of these clusters. With a combination of exquisite photometry, parallax distances, and cluster memberships from Gaia at the end of its mission, we anticipate precise and accurate ages for these and other star clusters in the Galaxy.
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