The globular cluster ngc 6540

Abstract

We have carried out U BV I and Gunn z CCD photometry and integrated spectroscopy in the range λλ3700-10 000 Å of NGC 6540, which was previously classified as an open cluster. The observations indicate that we are dealing with a globular cluster of metallicity [Fe/H] ≈ -1.0 with an extended blue horizontal branch but some stars are also present on the red side. We derive a reddening of E(B- V)= 0.60 ± 0.10 and a distance d = 3.5 kpc from the Sun. The cluster giant branch is very poorly populated like in some Palomar clusters; however it has a very concentrated nucleus. Bright giants, extended in the E-W direction, are outside the nucleus. Therefore the cluster has many characteristics in common with post core collapse clusters. A structural analysis by means of stellar isopleths reveals that the cluster is elongated in the N-S direction in the outer parts, whereas the inner contours are progressively twisted towards E-W.