A study of rotating globular clusters

Abstract

A study of rotating globular clusters ABSTRACTContext. NGC 4372 is a poorly studied old, very metal-poor globular cluster (GC) located in the inner Milky Way halo. Aims. We present the first in-depth study of the kinematic properties and derive the structural parameters of NGC 4372 based on the fit of a Plummer profile and a rotating, physical model. We explore the link between internal rotation to different cluster properties and together with similar studies of more GCs, we put these in the context of globular cluster formation and evolution. Methods. We present radial velocities for 131 cluster member stars measured from high-resolution FLAMES/GIRAFFE observations. Their membership to the GC is additionally confirmed from precise metallicity estimates. We build a velocity dispersion profile and a systemic rotation curve using this kinematic data set. Additionally, we obtain an elliptical number density profile of NGC 4372 based on optical images using a Markov chain Monte Carlo fitting algorithm. From this, we derive the cluster's half-light radius and ellipticity as r h = 3.44 ± 0.04 and = 0.08 ± 0.01. Finally, we give a physical interpretation of the observed morphological and kinematic properties of this GC by fitting an axisymmetric, differentially rotating, dynamical model. Results. Our results show that NGC 4372 has an unusually high ratio of rotation amplitude to velocity dispersion (1.2 vs. 4.5 km s −1 ) for its metallicity. This puts it in line, however, with two other exceptional, very metal-poor GCs: M 15 and NGC 4590. We also find a mild flattening of NGC 4372 in the direction of its rotation. Given its old age, this suggests that the flattening is indeed caused by the systemic rotation rather than tidal interactions with the Galaxy. Additionally, we estimate the dynamical mass of the GC M dyn = 2.0 ± 0.5 × 10 5 M based on the dynamical model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3 M /L , representative of an old, purely stellar population.

Differences in the rotational properties of multiple stellar populations in M13: a faster rotation for the ‘extreme’ chemical subpopulation

“…The position of a cluster in the diagram is often compared to the relation between V rot max /σ 0 and expected for oblate rotators viewed "edge-on" (Binney 2005;Cappellari et al 2007). Assuming an average ellipticity of ∼ 0.9 for M13 (see above), and estimating V rot max /σ 0 0.4 ± 0.1 from the values of Table 2 for the whole sample (and bearing in mind inclination effects), we find that M13 is in good agreement with the expected relation for an isotropic oblate rotator (comparing for example with figures from Bianchini et al 2013;Kacharov et al 2014). This suggests that the flattening is consistent with being caused by internal rotation.…”

TheGaia-ESO Survey: Kinematics of seven Galactic globular clusters

“…For example, while M 15 has been extensively studied presented a detailed analysis of this cluster based on nearly two thousand V r and proper motions), for NGC 5927 no velocity dispersion profile and no estimate of the central velocity dispersion are available in the literature (Simmerer et al 2013 provided only an estimate of the overall dispersion). For several clusters the samples presented in the literature are smaller than (NGC 6752, NGC 1851; Lane et al 2010b;Scarpa et al 2011;Carretta et al 2010Carretta et al , 2011 or similar to (NGC 4833, NGC 4372;Carretta et al 2014;Kacharov et al 2014) those considered here. An independent check of the Upper panels: we show the distribution of velocity differences with respect to the velocity measured with HR 10 for all the stars observed (from left to right, with HR 21, HR 11 and HR 13) and estimated uncertainties on V r ≤ 1 km s −1 for each measurement.…”

“…Different physical mechanisms may determine these deviations from the perfect sphere: velocity anisotropies, tidal stresses, and internal rotation (Goodwin 1997;Gnedin et al 1999;van den Bergh 2008;Bianchini et al 2013;Kacharov et al 2014). The idea that internal rotation plays a fundamental part in determining the morphology of GCs was formulated some 50 years ago (King 1961).…”

TheGaia-ESO Survey: Detailed abundances in the metal-poor globular cluster NGC 4372

“…They explain the broadening of the CMD sequences as the consequence of the high differential reddening probably due to the nearby Coal-sack Nebulae. A dereddened, narrower CMD has recently been constructed by Kacharov et al (2014). Carretta et al (2009a).…”