The Globular Clusters NGC 2808 and NGC 1851

A UB V photoelectric investigation on the previously unresearched southern globular clusters NGC 2808 and NGC 1851 was carried out at the Cerro Tololo Inter-American Observatory with the 16-inch and 36-inch reflectors. Fifty stars in NGC 2808 were observed an average of 1.7 times to apparent magni- tude V = 15.3, and forty-eight stars in NGC 1851 were observed an average of 2.4 times to magnitude V = 15.7. At V = 15.7 the 36-inch was used to the limit of the instrumentation in spite of excellent photometric conditions. On NGC 2808 it was not possible to reach the horizontal branch in the color-magnitude diagram. On NGC 1851 four stars are suspected of belonging to the horizontal branch from their position in the C-M diagram at V = 15.65. The reddening of NGC 2808 is estimated to be E(B - V) = 0.28. The apparent visual distance modu- lus is (m - M)app = 15.85, the true distance modulus is (m - = 15.01 if the visual absorption is A v = 3E(B - V). The distance is 10 kpc. On NGC 1851 the reddening is estimated to be E(B - V) = 0.24. The apparent visual distance modulus is (m - M)app = 15.15, the true distance modulus is (m - = 14.43. The distance is 7.69 kpc. I, OBSERVATIONAL PROCEDURE Direct photographic plates for printing finding charts were taken the nights of De- cember 21 and 22, 1967. The 36-inch was used to photograph the cluster itself. The Schmidt camera was used to photograph the cluster with the neighboring region. The photoelectric work was done on eleven photometric nights between January 7 and January 17, 1968, the first four nights with the 36-inch and the last seven nights with the 16-inch No. 1. During this period both objects were in good position to be ob- served from the latitude of the observatory, 3°0 S. NGC 1851 was observed the first half of the night; NGC 2808, the second. The 36-inch was used to measure the cluster stars plus the brighter reference stars for each object. The reference stars were usually measured three times, at the beginning, middle, and end of the cluster's respective half-night of observation. The 16-inch was used to measure the three reference stars for each object and the standard stars in the UBV system taken from the photoelectric sequence of the southern hemisphere of A. Gutierrez-Moreno et al. (1969). Sixteen standard stars were observed an average of six times each. Extinction coefficients were evaluated by observing each standard star twice every 4 hours, with air masses usually ranging from 1.0 to 2.0. Out of thirty pairs of observations a mean extinction coefficient was adopted for the observing interval. The photometer at both telescopes used the same 1P21 photomultiplier and the same set of standard UBV Corning filters (V #3384, B #5030, U #9863). Direct-current readout was employed. The reductions were made by measuring the Brown chart deflections by hand and feeding all the entering information (namely, cluster, star, gain step, deflection in three colors, hour angle, declination) into a computer. The program was designed by Luis Petit-Laurent of IBM in Santiago, Chile; the computer used was a 360 IBM of the University of Chile. The mean extinction co- efficients adopted for the period were <a(v)) = 0.120, (a(b -v)> = 0.100, <<a(u-b)> = 0.257. * Contribuh~ons from the Cerro Tololo Inter-A meric~n Observatory, No. 64. The Cerro Tololo Inter- American Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 85