Multicolor CCD Photometry of the Poorly Studied Globular Cluster NGC 6541

Abstract

We have carried out the first accurate multicolor (UBVRI) CCD photometry for more than 3000 stars (12^m.3<V<21^m.0) in the field of the globular cluster NGC 6541. This cluster was neglected in photometric studies after two photographic investigations by Alcaino (1971, A&A;, 13, 399; 1979, A&AS, 35, 233). The color-magnitude diagrams show close morphological similarities with M13 and M79, with the horizontal branch extending over 3 mag in V. We find the main-sequence turnoff at V_TO= 18.9±0.1 The V magnitude difference between the main sequence turn-off and the horizontal branch may be as great as 3^m.7. Four probable blue stragglers have been found. Comparing the cluster's CMD with that for the well-studied and little reddened cluster M79, we derive the estimates of metallicity ([Fe/H] ≡ 1.8 ± 0.2) and of reddening (E(B - V) = 0.15±0.02). We note considerable discrepancy in the positions of the horizontal branch in NGC 6541 and M79 in the U- (B - V) diagram and in the (U-B) - (B - V) two-color diagram. Our comparison of the color-magnitude diagrams with oxygen-enhanced isochrones shows that the cluster is very old; its age is at least 16 to 17 Gyr from the V- (B - V) diagram and perhaps older according to the other of our color-magnitude diagrams. (We assume here and for now no downward adjustments of age based on recent HIPPARCOS data.)