On the density profile of the globular cluster M92

TL;DR

This study provides detailed density and brightness profiles of globular cluster M92, comparing models and revealing that Wilson models fit better than King models, with implications for understanding star distribution without requiring an extra-tidal halo.

Contribution

The paper offers new observational profiles of M92 and demonstrates that Wilson models better fit the data than King models, especially in outer regions, without needing an extra-tidal halo.

Findings

Wilson models better fit the outer cluster regions

No need for an extra-tidal halo to explain star distribution

Profiles differ based on stellar tracers used

Abstract

We present new number density and surface brightness profiles for the globular cluster M92 (NGC 6341). These profiles are calculated from optical images collected with the CCD mosaic camera MegaCam at the Canada-France-Hawaii-Telescope and with the Advanced Camera for Surveys on the Hubble Space Telescope. The ground-based data were supplemented with the Sloan Digital Sky Survey photometric catalog. Special care was taken to discriminate candidate cluster stars from field stars and to subtract the background contamination from both profiles. By examining the contour levels of the number density, we found that the stellar distribution becomes clumpy at radial distances larger than about 13 arcminutes, and there is no preferred orientation of contours in space. We performed detailed fits of King and Wilson models to the observed profiles. The best-fit models underestimate the number density inside the core radius. Wilson models better represent the observations, in particular in the outermost cluster regions: the good global agreement of these models with the observations suggests that there is no need to introduce an extra-tidal halo to explain the radial distribution of stars at large radial distances. The best-fit models for the number density and the surface brightness profiles are different, even though they are based on the same observations. Additional tests support the evidence that this fact reflects the difference in the radial distribution of the stellar tracers that determine the observed profiles (main sequence stars for the number density, bright evolved stars for the surface brightness).