A joint JWST and HST view of Omega Centauri: Multiple stellar populations and their kinematics

TL;DR

This work tackles the internal kinematics of multiple stellar populations in Omega Centauri by combining JWST/NIRCam ($F115W$, $F277W$) data with archival HST observations to study stars in the intermediate radial range around the cluster center. Chromosome maps identify lower-stream (LS) and upper-stream (US) populations, including metal-poor and metal-rich subgroups, and a final sample of 3252 LS and 4712 US stars is analyzed for kinematics. The authors measure velocity dispersions, anisotropy $\beta(R)$, angular-momentum dispersion $\sigma_{L_z}$, and high-order moments $h_3$, $h_4$, as well as the degree of energy equipartition $\eta$ for masses $\sim0.15$–$0.7\,M_\odot$, finding US more radially anisotropic than LS, LS with higher $σ_{L_z}$, and a low, radially declining equipartition. These results provide constraints on the formation and dynamical evolution of Type II globular clusters and demonstrate the efficacy of JWST+HST synergy for probing complex stellar populations in GCs.

Abstract

We combine F115W and F277W images collected with the Near Infrared Camera of the James Webb Space Telescope (JWST) with multi-band, multi-epoch Hubble Space Telescope (HST) observations of Omega Centauri to investigate its multiple stellar populations and internal kinematics. Our study focuses on a region spanning $\sim$0.9 to $\sim$2.3 half-light radii from the cluster center, largely unexplored by HST and JWST. Using chromosome maps, we identify the principal populations along the upper main sequence and among M-dwarfs, distinguishing lower-stream (LS) stars, chemically akin to first-generation globular cluster stars with similar metallicities, and upper-stream (US) stars, enriched in helium and nitrogen but oxygen-poor. Both streams also host subpopulations with varying metallicities. We find radially anisotropic motions, with US stars exhibiting significantly stronger anisotropy than LS stars. Subdividing the US into extreme and intermediate light-element populations reveals a gradient in anisotropy, with intermediate stars lying between the LS and extreme US populations. However, metal-rich and metal-poor stars within each stream show moderate kinematic differences. The LS stars show higher angular momentum and dispersion compared to US stars, and also exhibit stronger systemic rotation and tangential proper-motion skewness, further highlighting their kinematic divergence. Finally, leveraging a mass range of $\sim$0.15 - 0.7 solar masses, we detect a low degree of energy equipartition for all cluster stars, which decreases with radial distance from the cluster center.

Figures (14)

• Figure 1: Footprints of the JWST (green) and HST (orange) observations used in this work. North is at the top, and east is to the left. Yellow circles indicate multiples of the half-light radius. The yellow cross indicates the cluster center at coordinates ($\alpha$, $\delta$) = (201.697°, $-47.480^\circ$).
• Figure 2: Distribution of proper motions measured in the cluster reference frame. The inset panel displays the projected proper motions in tangential and radial directions. Pink contours indicate probability density distributions derived from Gaussian mixture model fitting.
• Figure 3: $m_{\rm F277W}$ vs. $m_{\rm F115W}-m_{\rm F277W}$ Hess diagram for stars at $r_h > 1.5$ that according to the criteria of Section \ref{['sec:data']} have high-quality photometry (left). The right panel shows the corresponding CMD for proper-motion selected cluster members.
• Figure 4: Collection of photometric diagrams corrected for differential reddening that highlight the multiple MSs of $\omega$ Centauri proper-motion selected members.
• Figure 5: $\Delta_{C {\rm F275W,F336W,F438W}}$ vs. $\Delta_{\rm F275W,F277W}$ ChM for bright MS stars (top) and $\Delta_{\rm F115W,F277W}$ vs. $\Delta_{\rm F475W,F814W}$ for M dwarfs (bottom). The insets show the corresponding Hess diagrams. The arrows show the effect of changing, one at time, the abundances of helium mass fraction Y, [N/Fe], [O/Fe], and [Fe/H] by 0.05, 1.2, $-$0.5, and 0.3 dex with respect to a reference stellar population with [Fe/H]=$-$1.7, [$\alpha$/Fe]=0.4 and solar carbon and nitrogen contents milone2018amarino2019a.