The Size Scale of Star Clusters

Madrid, Juan P.; Hurley, Jarrod R.; Sippel, Anna C.

doi:10.1088/0004-637x/756/2/167

Cited by 53 publications

(53 citation statements)

References 69 publications

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“…But in this case the radii of star clusters should eventually reflect their local tidal gravitational fields that usually decrease with increasing galactocentric radius. Our results suggest that clusters across the range of mass, age, and environment covered by our samples do not show effects of tidal truncations in their half-light radii; otherwise, a stronger increase of cluster size with galactocentric distance is expected (e.g., Madrid et al 2012;Webb et al 2016).…”

Section: 0mentioning

confidence: 69%

“…Consistent with this expectation, we observe an overall increase in median radius as our clusters age from a few to 200Myr old, suggesting that many of the star clusters in our samples are still expanding to fill their tidal radii. Similarly, we also would expect to see a stronger correlation between effective radius and galactocentric distance if star clusters were significantly affected by the tidal field of their host galaxy (e.g., Madrid et al 2012;Puzia et al 2014).…”

Section: Discussionmentioning

confidence: 98%

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Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗

Ryon

Gallagher

Smith

et al. 2017

ApJ

109

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We present a study of the effective (half-light) radii and other structural properties of a systematically selected sample of young, massive star clusters (5×10 3  M and 200Myr) in two nearby spiral galaxies, NGC628 and NGC1313. We use Hubble Space Telescope (HST) WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS), an HST Treasury Program. We measure effective radii with GALFIT, a two-dimensional image-fitting package, and with a new technique to estimate effective radii from the concentration index of observed clusters. The distribution of effective radii from both techniques spans ∼0.5-10pc and peaks at 2-3pc for both galaxies. We find slight positive correlations between effective radius and cluster age in both galaxies, but no significant relationship between effective radius and galactocentric distance. Clusters in NGC1313 display a mild increase in effective radius with cluster mass, but the trend disappears when the sample is divided into age bins. We show that the vast majority of the clusters in both galaxies are much older than their dynamical times, suggesting they are gravitationally bound objects. We find that about half of the clusters in NGC628 are underfilling their Roche lobes, based on their Jacobi radii. Our results suggest that the young, massive clusters in NGC628 and NGC1313 are expanding, due to stellar mass loss or two-body relaxation, and are not significantly influenced by the tidal fields of their host galaxies.

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Section: 0mentioning

confidence: 69%

Section: Discussionmentioning

confidence: 98%

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗

Ryon

Gallagher

Smith

et al. 2017

ApJ

109

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“…The slope in the mass–age plane is similar to that of the K200 model and distinct from that of the K100 model with R gc = 8.5 kpc. An investigation of the mass‐loss rates and dissolution times of star clusters as a function of orbit within the Galaxy will be the subject of an upcoming paper (Madrid, Hurley & Sippel ).…”

Section: General Evolutionmentioning

confidence: 99%

A direct N-body model of core-collapse and core oscillations

Hurley

Shara

2012

Monthly Notices of the Royal Astronomical Society

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We report on the results of a direct N-body simulation of a star cluster that started with N = 200 000, comprising 195 000 single stars and 5000 primordial binaries. The code used for the simulation includes stellar evolution, binary evolution, an external tidal field and the effects of two-body relaxation. The model cluster is evolved to 12 Gyr, losing more than 80 per cent of its stars in the process. It reaches the end of the main core-collapse phase at 10.5 Gyr and experiences core oscillations from that point onwards -direct numerical confirmation of this phenomenon. However, we find that after a further 1 Gyr the core oscillations are halted by the ejection of a massive binary comprised of two black holes from the core, producing a core that shows no signature of the prior core-collapse. We also show that the results of previous studies with N ranging from 500 to 100 000 scale well to this new model with larger N. In particular, the time-scale to core collapse (in units of the relaxation time-scale), mass segregation, velocity dispersion and the energies of the binary population all show similar behaviour at different N.Key words: methods: numerical -binaries: close -stars: evolution -stars: kinematics and dynamics -globular clusters: general -galaxies: star clusters: general. I N T RO D U C T I O NThe increasing ability of the direct N-body method to provide reliable models of the dynamical evolution of star clusters has closely mirrored increases in computing power (Heggie 2011). The community has progressed from small-N models performed on workstations (e.g. In this paper we present an N-body simulation of star cluster evolution that begins with N = 200 000 stars and binaries. This extends the N parameter space covered by direct N-body models and performs two important functions. First it provides a new calibration point for the MC method -this statistical method is increasingly valid for increasing N, so calibrations at higher N are more reliable. It also allows us to further develop our theoretical understanding of star cluster evolution and investigate how well inferences drawn from models of smaller N scale to larger values. The latter is the focus of this paper. A good example of the small-N models that we wish to compare with is the comprehensive study of star cluster evolution presented by Giersz & Heggie (1997) using models that included a mass function, stellar evolution and the tidal field of a point mass galaxy, albeit starting with 500 stars instead of 200 000. More recent examples for comparison include Baumgardt & Makino (2003) and Küpper, Kroupa & Baumgardt (2008). We were also motivated to produce a model that exhibited core collapse close to a Hubble time without dissolving by that time. What we find when interpreting this model is that much of the behaviour reported previously for smaller N-body models stands up well in comparison but that the actions of a binary comprised of two black holes (BHs) provide a late twist to the evolution of the cluster core.In Section 2 we describ...

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“…This is because star clusters lose more mass during pericentric passages on eccentric orbits, and therefore undergo stronger expansion due to some internal dynamical mechanism such as primordial mass segregation and dynamical relaxation which are dominant processes in the weaker tidal field at larger Galactocentric radii (Madrid, Hurley, & Sippel 2012;Haghi et al 2014) . It is therefore likely that an eccentric cluster orbit like the one we suggest has had an important influence on Pal 4's evolution.…”

Section: Discussionmentioning

confidence: 99%

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

Zonoozi

Haghi

Kroupa

et al. 2017

Mon. Not. R. Astron. Soc.

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Palomar 4 is a low-density globular cluster with a current mass ≈ 30000 M ⊙ in the outer halo of the Milky Way with a two-body relaxation time of the order of a Hubble time. Yet, it is strongly mass segregated and contains a stellar mass function depleted of low-mass stars. Pal 4 was either born this way or it is a result of extraordinary dynamical evolution. Since two-body relaxation cannot explain these signatures alone, enhanced mass loss through tidal shocking may have had a strong influence on Pal 4. Here, we compute a grid of direct N-body simulations to model Pal 4 on various eccentric orbits within the Milky Way potential to find likely initial conditions that reproduce its observed mass, half-light radius, stellar MF-slope and line-of-sight velocity dispersion. We find that Pal 4 is most likely orbiting on an eccentric orbit with an eccentricity of e ≈ 0.9 and pericentric distance of R p ≈ 5 kpc. In this scenario, the required 3D half-mass radius at birth is similar to the average sizes of typical GCs (R h ≈ 4−5 pc), while its birth mass is about M 0 ≈ 10 5 M ⊙ . We also find a high degree of primordial mass segregation among the cluster stars, which seems to be necessary in every scenario we considered. Thus, using the tidal effect to constrain the perigalactic distance of the orbit of Pal 4, we predict that the proper motion of Pal 4 should be in the range −0.52 µ δ −0.38 mas yr −1 and −0.30 µ α cos δ −0.15 mas yr −1 .

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The Size Scale of Star Clusters

Cited by 53 publications

References 69 publications

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗

A direct N-body model of core-collapse and core oscillations

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

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The Size Scale of Star Clusters

Cited by 53 publications

References 69 publications

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies∗

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies∗

A direct N-body model of core-collapse and core oscillations

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

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Product

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Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗

Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies^∗