Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]

TL;DR

This study precisely measures the absolute space velocity of globular cluster NGC6397 using HST astrometry and VLT radial velocities, enabling orbit calculations within the Galactic potential.

Contribution

It provides the first accurate 3D velocity components of NGC6397 through combined astrometric and spectroscopic data, refining its orbital parameters.

Findings

Measured proper motion: [+3.39, -17.55] mas/yr

Radial velocity: +18.36 km/s

Calculated cluster orbit in Galactic potential

Abstract

In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component has been obtained from radial velocities measured on spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39 +/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances, and briefly discuss the implications.