THE CCD PHOTOMETRY OF THE GLOBULAR-CLUSTER PALOMAR-1

A CCD photometry of the halo cluster Palomar 1 is presented in the Thu an-Gunn photometric system. The principal sequences of the color-magni tude diagrams are delineated in different spectral bands. The color-ma gnitude diagrams of the cluster show a well defined red horizontal bra nch, a subgiant branch and a main-sequence down to about two magnitude s below the main sequence turnoff. The giant branch is absent and the brightest stars are the horizontal branch stars. The age of the cluste r determined by comparison with the isochrones of Bell and Vanden Berg (1987) is consistent with an age in the interval 12 - 14 Gyr. A dista nce modulus of (m - M)(90) = 15.38+/-0.15 magnitude and E(g - r) = 0.1 6 has been derived. An estimate of the cluster structural parameters s uch as core radius and concentration parameter gives tau(c) = 1.5 pc a nd c = 1.46. A mass estimate of 1.1 10(3) M(circle dot) and a mass-to- light ratio of 1.79 have been obtained using King's (1966) method. The morphology of color-magnitude diagrams allows Pal 1 to be interpreted as probably a globular cluster rather than an old open one.