3D Rotation of the Open Cluster NGC 2516

Abstract

We have combined Gaia astrometry with Gaia-ESO Survey radial velocities to measure the 3D rotation of the open cluster NGC 2516. We compiled a sample of 430 members with astrometry and spectroscopy and use these to determine a distance to the cluster of 406.3 +/- 0.8 pc, which we then use to infer the 3D positions and velocities of all stars in the cluster using a Bayesian model. We identify the axis of maximum cluster rotation and measure a median rotational velocity of 0.12 +/- 0.02 km/s. We find the axis of maximum cluster rotation to be 74 +/- 17 degrees to the plane of our galaxy. We compare this rotation rate to measurements of cluster rotation in other open clusters and find that it is inconsistent with the expected dependences on cluster age and mass.

Figures (3)

• Figure 1: Properties of our cluster members sample in NGC 2516 (red) compared to the members from hunt24. The top row show the observed plane of the sky distribution (top left), proper motion distribution (top centre), and position in the Gaia colour-magnitude diagram (top right). The middle row shows histograms of the observed distribution of the kinematic measurements $\mu_\alpha$ (middle left), $\mu_\delta$ (middle centre) and RV (middle right). The bottom row shows the reprojected distribution of members in the 3D Galactic Cartesian coordinate system $XYZ$. The extent covered by the three $XYZ$ axes is the same in all three plots, 16 pc.
• Figure 2: Angular dependence of the median rotational velocity, $v_\varphi$, for a rotation axis orientated by the angles ($\theta, \phi$) relative to the Galactic cartesian coordinate system. The black lines show the median value of $v_\varphi$ at each angle, with the grey shaded area showing the uncertainty. In the left-hand panel, where $\theta$ is allowed to vary, $\phi$ is set to the value of maximum rotation of $\phi = 164^\circ$. In the right-hand panel, where $\phi$ is allowed to vary, $\theta$ is set to the value of maximum rotation of $\theta = 109^\circ$. The red line shows the fitted sinusoidal functions.
• Figure 3: Rotational velocity, $v_\varphi$, plotted against radius (in the plane perpendicular to the rotation axis) for all stars in our sample. The red dashed line shows the best-fitting linear relationship with a gradient of $-0.070 \pm 0.037$ km s$^{-1}$ pc$^{-1}$.