A Deep Optical Luminosity Function of NGC 6712 with the VLT: Evidence for Severe Tidal Disruption

TL;DR

This study used deep VLT observations to derive the luminosity and mass functions of NGC 6712, providing strong evidence that severe tidal disruption has stripped the cluster of its lower-mass stars.

Contribution

It presents the deepest main sequence luminosity function for NGC 6712 and links the high-mass peak to tidal stripping effects due to its orbit.

Findings

Luminosity function peaks at M_R~4.5, corresponding to ~0.75 M_solar.

Mass function shows a significant deficiency of low-mass stars (~0.25 M_solar) compared to other clusters.

Evidence suggests tidal forces have heavily stripped the cluster of low-mass stars.

Abstract

The VLT on Cerro Paranal was used to observe four fields located at 2.3' from the center of the Galactic globular cluster NGC 6712 in the V and R bands. The resulting color-magnitude diagram shows a well defined main sequence reaching down to the 5 sigma detection limit at V~25, R~23.5 or approximately 4 mag below the main sequence turn-off, the deepest obtained so far on this cluster. This yields a main sequence luminosity function that peaks at M_R~4.5 and drops down to the 50% completeness limit at M_R~8.5. Transformation to a mass function via the latest mass-luminosity relation appropriate to this object indicates that the peak of the luminosity function corresponds to ~0.75 Msolar, a value significantly higher than the ~0.25 Msolar measured for most other clusters observed so far. Since this object, in its Galactic orbit, penetrates very deeply into the Galactic bulge with perigalactic distance of ~0.3 kpc, this result is the first strong evidence that tidal forces have stripped this cluster of a substantial portion of its lower mass star population all the way down to its half-light radius and possibly beyond.