Limits on ionized gas in globular clusters

Globular clusters are expected to accumulate interstellar gas due to mass loss from their stars. A sensitive search of six clusters for free-free emission from ionized gas at 8.4 GHz using the Very Large Array failed to detect any gas, with limits well below the expected accumulation from mass loss if the gas remains in the cluster. However, simple models of the equilibrium distribution of photoionized gas show that the velocity dispersion of the gas is greater than that of the stars, and beyond a critical radius the gas flows out of the cluster as a wind. The expected amounts of accumulated gas are then below the detection threshold of the observations. Fifteen radio point sources were detected in the six globular cluster fields. The majority are likely to be background sources, but five are associated with objects which are known cluster members: four probable low-mass X-ray binaries in Pal 2, NGC 6440, NGC 6624 and NGC 7078, and the planetary nebula K648 in NGC 7078. Two other radio sources in the field of NGC 7078 may also belong to this cluster.