THE ASTRONOMICAL JOURNAL      VOLUME 105, NUMBER 3, PAGE 938         MARCH 1993

               DYNAMICS OF THE YOUNG BINARY LMC CLUSTER NGC 1850

                     PHILIPPE FISCHER AND DOUGLAS L. WELCH

  Department of Physics and Astronomy, McMaster University, Hamilton, Ontario
                                L8S 4M1, Canada

                                  MARIO MATEO

 The Observatories of the Carnegie Institute of Washington, 813 Santa Barbara
                      Street, Pasadena, California 91101


                                   ABSTRACT

    In this paper we have examined the age and internal dynamics of the
    young binary LMC cluster NGC 1850 using BV CCD images and echelle
    spectra of 52 supergiants.  Isochrone fits to a BV color-magnitude
    diagram revealed that the primary cluster has an age of Tau = 90 +/- 30
    Myr while the secondary member has Tau = 6 +/- 5 Myr.  The reddening
    was found to be E(B-V) = 0.17 mag.  BV surface brightness profiles were
    constructed out to R > 40 pc, and single-component King-Michie (KM)
    models were applied.  The total cluster luminosity varied from L(B) =
    2.60-2.65 x 10^6 L(Bsun) and L(V) = 1.25-1.35 x 10^6 as the anisotropy
    radius varied from infinity to three times the scale radius with the
    isotropic models providing the best agreement with the data.  Simple
    tests were made to check for tidal truncation in the profiles and we
    concluded that there was slight evidence favoring truncation.  The
    bright background and binary nature of NGC 1850 render this conclusion
    somewhat uncertain.  Of the 52 stars with echelle spectra, a subset of
    36 was used to study the cluster dynamics.  The KM radial velocity
    distributions were fitted to these velocities yielding total cluster
    masses of 5.4-5.9 +/- 2.4 x 10^4 Msun corresponding to M/L(B) = 0.02
    +/- 0.01 Msun/L(Bsun) or M/L(V) = 0.05 +/- 0.02 Msun/L(Vsun).  A
    rotational signal in the radial velocities has been detected at the 93%
    confidence level implying a rotation axis at a position angle of 100
    deg.  A variety of rotating models were fit to the velocity data
    assuming cluster ellipticities of Epsilon = 0.1-0.3.  These models
    provided slightly better agreement with the radial velocity data than
    the KM models and had masses that were systematically lower by a few
    percent.  Values for the slope of the mass function were determined
    using the derived M/L, theoretical mass-luminosity relationships, and
    several forms for the IMF.  The preferred value for the slope of the
    power-law IMF is a relatively shallow, x = 0.29 [+0.3,-0.8] assuming
    the B band M/L or x = 0.71 [+0.2,-0.4] for the V band.