53 Publications of the Astronomical Society of the Pacific, 110:53–59, 1998 January  1998. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Frequency of Binaries in the Open Cluster Trumpler 14 Beatriz Garcı ´a, 1,2 Stella Malaroda, 3 and Hugo Levato 1 Complejo Astrono ´mico El Leoncito, CC 467, 5400 San Juan, Argentina; Cricyt@lanet.losandes.com.ar, malaroda@castec.edu.ar, levato@castec.edu.ar Nidia Morrell 1,4,5 Facultad de Ciencias Astrono ´micas y Geofı ´sicas de la U.N.L.P. Paseo del Bosque s/n. 1900 La Plata, Argentina; nidia@facglp.fcaglp.unlp.edu.ar and Mo ´ nica Grosso 6 Complejo Astrono ´mico El Leoncito, CC 467, 5400 San Juan, Argentina; mgrosso@castec.edu.ar Received 1995 June 8; accepted 1997 November 11 ABSTRACT. We present new spectroscopic data for nine objects among the brightest stars in the field of the open cluster Trumpler 14. Radial velocities were measured from around 80 new spectrograms in order to provide more information about the binary nature of these objects. From this material, we conclude that at least two of the stars in the sample are radial-velocity variables, one of them showing double He i features. 1. INTRODUCTION An OB association is a large clustering of stars that includes B-type stars and some, or at least a few, O-type stars. The nebula NGC 3372, which surrounds the luminous blue variable (LBV) h Car, is one of the most studied H ii regions in the southern hemisphere. The study of this nebula and of the open clusters related to it or located in its neighborhood is a subject of interest for the analysis of some astrophysical problems, especially those related to star formation and massive star evolution. Many astronomical papers have been devoted to the study of the h Car region. Our own research group contributed some of them (Levato & Malaroda 1981, 1982; Morrell, Garcı ´a, & Levato 1988; Levato et al. 1991a, 1991b) dealing with spec- troscopic observations, and many references to previous work can be found therein. The open cluster Trumpler 14 in the h Car complex contains a large number of massive O-type stars. Levato et al. (1991a) and Penny et al. (1993) presented discrepant results about the 1 Member of the Carrera del Investigador Cientı ´fico del Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas de la Repu ´blica Argentina. 2 Now at the Centro Regional de Investigaciones Cientı ´ficas y Te ´cnicas, CC 131, 5500 Mendoza, Argentina. 3 Member of the Carrera del Investigador Cientı ´fico de la Comisio ´n de Investigaciones Cientı ´ficas de la Provincia de Buenos Aires. 4 Visiting Astronomer, Cerro Tololo Inter-American Observatory. CTIO is operated by AURA, Inc., under contract to the National Science Foundation. 5 Visiting Astronomer, Complejo Astrono ´mico El Leoncito. CASLEO is operated under agreement of the Universidad Nacional de La Plata, the Univ- ersidad Nacional de Co ´rdoba, the Universidad Nacional de San Juan, and CONICET. 6 Member of the Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas. binary nature of some of the brightest members of this cluster: for seven stars they have in common, three of them are spec- troscopic binaries according to Levato et al. (1991a), while the same stars present constant radial velocities in Penny et al. (1993). One of the points that is not yet clear is the presence of “bound” OB clusters within OB associations, as is the case of Trumpler 14. As Penny et al. (1993) have pointed out, there are only three ways to explain the expansion of the subgroups in an OB association: (1) dispersal of the original gas produced by the photoionizing flux of the massive stars, (2) mass loss by stellar winds and supernovae, and (3) dynamical exchange with binaries. In the case of Tr 14 the two first processes can be rejected, and only the dynamical exchange involving bi- naries remains as responsible for the expansion (see Penny et al. 1993). Morrell et al. (1988) suggested that the compact appearance of Tr 14 could be also explained if the distance to Tr 14 is greater than the distance to Tr 16, which appears less compact. They suggested two possible distances to Tr 14, 2.8 and 3.45 kpc. Using the results of Morrell et al. (1988), Penny et al. (1993) derived, for Trumpler 14, (1) the cluster radius, 1.0 to 1.2 pc; (2) the crossing time for the cluster, to 5 1.1 # 10 yr; and (3) using theoretical evolutionary tracks, an 5 1.4 # 10 age of about yr. Massey & Johnson (1993) found 5 5.5 # 10 from extensive CCD photometry that (1) the clusters Tr 14 and Tr 16 are at the same distance, and (2) this distance is 3.2 kpc (close to the larger value proposed by Morrell et al. 1988). If their results are correct, then the fact that Tr 14 presents a compact aspect could be explained by the anomalously low binary frequency. Thus, it is very important to know this fre- quency among the members of Tr 14.