Palomar 1: Another Young Galactic Halo Globular Cluster?

Abstract

?????Deep V and I CCD images of the loosely populated Galactic globular cluster Pal 1 and the surrounding field have been obtained with the Isaac Newton Telescope. A color-magnitude diagram (CMD) down to Vlim ~ 24 and a luminosity function down to Vlim = 23.25 have been constructed. Adopting a reddening E(V - I) = 0.20 ? 0.04 and comparing the CMD of Pal 1 with that of 47 Tucanae, we obtain a distance modulus (m - M)0 = 15.25 ? 0.25, indicating that Pal 1 is located 3.7 ? 0.4 kpc above the Galactic disc and 17.3 ? 1.6 kpc from the Galactic center. Comparison with 47 Tuc and M71 shows that Pal 1 must be significantly younger. The best-fitting isochrones, as listed by Bertelli et al., yield an age between 6.3 and 8 Gyr, which would make Pal 1 the youngest Galactic globular cluster identified thus far, casting some doubt on the real nature of this object. The luminosity function shows some evidence of mass segregation, consistent with the very short relaxation time. The global mass function can be fitted with a power law of slope x = -1.4 ? 0.7. This mass function is anomalously flat, suggesting either a strong dynamical evolution or an initial mass function significantly different from most other halo globular clusters. A discussion of the possible nature of Pal 1 is presented.