Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era

Sollima, A.; Baumgardt, Holger; Zocchi, Alice; Balbinot, E.; Gieles, M.; Hénault-Brunet, V.; Varri, Anna Lisa

doi:10.1093/mnras/stv1079

Cited by 45 publications

(69 citation statements)

References 55 publications

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“…Finally, the derived central velocity dispersions agree 13 Dark remnants are white dwarf stars, neutron stars, and black holes. Recently, comparable fractions of dark remnants were obtained with state-ofthe-art cluster modeling by Sollima et al (2015) and Sollima et al (2016). well with the predictions by King (1966), where the total mass is obtained by summing the masses of all individual stars.…”

Section: D Simulated Clusterssupporting

confidence: 68%

“…(ii) Two past studies dealt with the determination of masses in GCs with Gaia data (An, Evans, & Deason 2012;Sollima et al 2015) yielding somewhat conflicting results. The main problem is to break the mass-anisotropy degeneracy, with the anisotropy signal being stronger outside the central regions where relaxation has the highest effect.…”

Section: Measurement Of Derived Quantitiesmentioning

confidence: 99%

“…And even in the case of heavy background contamination, proper motions will be only mildly affected (Figure 10). The assumption by Sollima et al (2015) that one can only use stars contaminated by less than 10% of the flux by objects more distant than 3.54", is far too pessimistic. The simulations by An, Evans, & Deason (2012), prove that 10% mass estimates can be provided for GCs if one can count on at least 100 tracers (we can use red giants) with proper motion errors <100 µas.…”

Section: Measurement Of Derived Quantitiesmentioning

confidence: 99%

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Globular clusters with Gaia

Pancino

Bellazzini

Giuffrida

et al. 2017

Mon. Not. R. Astron. Soc.

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The treatment of crowded fields in Gaia data will only be a reality in a few years from now. In particular, for globular clusters, only the end-of-mission data (public in 2022-2023) will have the necessary full crowding treatment and will reach sufficient quality for the faintest stars. As a consequence, the work on the deblending and decontamination pipelines is still ongoing. We describe the present status of the pipelines for different Gaia instruments, and we model the end-of-mission crowding errors on the basis of available information. We then apply the nominal post-launch Gaia performances, appropriately worsened by the estimated crowding errors, to a set of 18 simulated globular clusters with different concentration, distance, and field contamination. We conclude that there will be 10 3 -10 4 stars with astrometric performances virtually untouched by crowding (contaminated by <1 mmag) in the majoritiy of clusters. The most limiting factor will be field crowding, not cluster crowding: the most contaminated clusters will only contain 10-100 clean stars. We also conclude that: (i) the systemic proper motions and parallaxes will be determined to 1% or better up to 15 kpc, and the nearby clusters will have radial velocities to a few km s −1 ; (ii) internal kinematics will be of unprecendented quality, cluster masses will be determined to 10% up to 15 kpc and beyond, and it will be possible to identify differences of a few km s −1 or less in the kinematics (if any) of cluster sub-populations up to 10 kpc and beyond; (iii) the brightest stars (V 17 mag) will have space-quality, wide-field photometry (mmag errors), and all Gaia photometry will have 1-3% errors on the absolute photometric calibration.

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Section: D Simulated Clusterssupporting

confidence: 68%

Section: Measurement Of Derived Quantitiesmentioning

confidence: 99%

Section: Measurement Of Derived Quantitiesmentioning

confidence: 99%

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Globular clusters with Gaia

Pancino

Bellazzini

Giuffrida

et al. 2017

Mon. Not. R. Astron. Soc.

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“…For all the potentials the bound stars are consistent with isotropy and the anisotropy that develops is contained mostly in the PEs. It has been shown that simulations of GCs with dense starting conditions develop radial anisotropy (Sollima et al 2015;Zocchi et al 2016) but those with larger initial r hm /rJ, similar to our initial conditions, do not develop any radial anisotropy and instead show tangential anisotropy near the tidal radius (Baumgardt & Makino 2003). This is thought to be due to the balance between the preferential production and preferential loss of radial orbits: two-body interactions predominantly scatter stars outwards on radial orbits, and these stars then escape more easily than those on other orbits (Takahashi, Lee & Inagaki 1997, Tiongco, Vesperini & Varri 2016a.…”

Section: Anisotropy Of the Dispersion And Rotationmentioning

confidence: 99%

“…DR1 provided proper motions of many field stars in the Milky Way and also included open cluster stars, and future releases will provide proper motions of stars in the outer regions of GCs. Therefore, understanding the effects of PEs on the kinematics is paramount to correctly interpreting the new data, as current models have been shown to still have large biases when comparing to projected data from simulations (Shanahan & Gieles 2015;Sollima et al 2015), and will also help to develop a prescription for including their effects in a selfconsistent model.…”

Section: Introductionmentioning

confidence: 99%

The properties of energetically unbound stars in stellar clusters

Claydon¹,

Gieles²,

Zocchi³

2017

Mon. Not. R. Astron. Soc.

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Several Milky Way star clusters show a roughly flat velocity dispersion profile at large radii, which is not expected from models with a tidal cut-off energy. Possible explanations for this excess velocity include: the effects of a dark matter halo, modified gravity theories and energetically unbound stars inside of clusters. These stars are known as potential escapers (PEs) and can exist indefinitely within clusters which are on circular orbits. Through a series of N -body simulations of star cluster systems, where we vary the galactic potential, orbital eccentricity and stellar mass function, we investigate the properties of the PEs and their effects on the kinematics. We derive a prediction for the scaling of the velocity dispersion at the Jacobi surface due to PEs, as a function of cluster mass, angular velocity of the cluster orbit, and slope of the mass profile of the host galaxy. We see a tentative signal of the mass and orbital velocity dependence in kinematic data of globular clusters from literature. We also find that the fraction of PEs depends sensitively on the galactic mass profile, reaching as high as 40% in the cusp of a Navarro-Frenk-White profile and as the velocity anisotropy also depends on the slope of the galactic mass profile, we conclude that PEs provide an independent way of inferring the properties of the dark matter mass profile at the galactic radius of (globular) clusters in the Gaia era.

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Inverting the dynamical evolution of globular clusters: clues to their origin

Gieles

Alexander

2015

Proc. IAU

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Abstract. Scaling relations for globular clusters (GCs) differ from scaling relations for pressure supported (elliptical) galaxies. We show that two-body relaxation is the dominant mechanism in shaping the bivariate dependence of density on mass and Galactocentric distance for Milky Way GCs with masses 10 6 M , and it is possible, but not required, that GCs formed with similar scaling relations as ultra-compact dwarf galaxies. We use a fast cluster evolution model to fit a parameterised model for the initial properties of Milky Way GCs to the observed present-day properties. The best-fit cluster initial mass function is substantially flatter (power-law index α = −0.6 ± 0.2) than what is observed for young massive clusters (YMCs) forming in the nearby Universe (α −2). A slightly steeper CIMF is allowed when considering the metal-rich GCs separately (α −1.2 ± 0.4). If stellar mass loss and two-body relaxation in the Milky Way tidal field are the dominant disruption mechanisms, then GCs formed differently from YMCs.

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Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era

Cited by 45 publications

References 55 publications

Globular clusters with Gaia

Globular clusters with Gaia

The properties of energetically unbound stars in stellar clusters

Inverting the dynamical evolution of globular clusters: clues to their origin

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