Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function

摘要

We present the CMD from deep HST imaging in the globular cluster NGC 6397.The ACS was used for 126 orbits to image a single field in two colors (F814W,F606W) 5 arcmin SE of the cluster center. The field observed overlaps that ofarchival WFPC2 data from 1994 and 1997 which were used to proper motion (PM)clean the data. Applying the PM corrections produces a remarkably clean CMDwhich reveals a number of features never seen before in a globular cluster CMD.In our field, the main sequence stars appeared to terminate close to thelocation in the CMD of the hydrogen-burning limit predicted by two independentsets of stellar evolution models. The faintest observed main sequence stars areabout a magnitude fainter than the least luminous metal-poor field halo starsknown, suggesting that the lowest luminosity halo stars still await discovery.At the bright end the data extend beyond the main sequence turnoff to well upthe giant branch. A populous white dwarf cooling sequence is also seen in thecluster CMD. The most dramatic features of the cooling sequence are its turn tothe blue at faint magnitudes as well as an apparent truncation near F814W = 28.The cluster luminosity and mass functions were derived, stretching from theturn off down to the hydrogen-burning limit. It was well modeled with either avery flat power-law or a lognormal function. In order to interpret these fitsmore fully we compared them with similar functions in the cluster core and witha full N-body model of NGC 6397 finding satisfactory agreement between themodel predictions and the data. This exercise demonstrates the important roleand the effect that dynamics has played in altering the cluster IMF.