NGC 2298: a globular cluster on its way to disruption

Abstract

We have studied the stellar main sequence (MS) of the globular cluster NGC 2298 using deep HST/ACS observations in the F606W and F814W bands covering an area of 3.4 arcmin × 3.4 arcmin around the cluster centre or about twice the cluster's half-mass radius. The colour-magnitude diagram that we derive in this way reveals a narrow and well defined MS extending down to the 10σ detection limit at m_606 ≃ 26.5, m_814 ≃ 25, corresponding to stars of 0.2 {M}_⊙. The luminosity function (LF) obtained with these data, once corrected for the limited effects of photometric incompleteness, reveals a remarkable deficiency of low-mass stars as well as a radial gradient, in that the LF becomes progressively steeper with radius. Using the mass-luminosity relation appropriate for the metallicity of NGC 2298, we derive the cluster's global mass function (GMF) by using a multi-mass Michie-King model. Over the range 0.8-0.2 {M}_⊙, the number of stars per unit mass decreases following a power-law distribution of the type dN/dm ∝ m^0.5, where, for comparison, typical halo clusters have dN/dm ∝ m^-1.5. If the IMF of NGC 2298 was similar to that of other metal poor halo clusters, like e.g. NGC 6397, the present GMF that we obtain implies that this object must have lost of the order of 85% of its original mass, at a rate much higher than that suggested by current models based on the available cluster orbit. The latter may, therefore, need revision.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.