Far-ultraviolet radiation from disk globular clusters

We present IUE spectra obtained in a survey of the metal-rich disk system of globular clusters. The five clusters with new SWP (1200-2000 Å) spectra all lie in the blue region of the van Albada, de Boer, & Dickens (1981) ultraviolet two-color diagram; none lie near the red position of 47 Tuc. In NGC 6624, 6637, and 6441 (all of which have [Fe/H] > -0.6), we find a previously unknown ultraviolet (UV) bright stellar population, which has a mild spectroscopic upturn at wavelengths below 2000 Å, similar to the UV rising branch seen in some metal-poor globular clusters and galaxies. In these cases, the SWP spectrum has a smaller spatial width than the long-wavelength spectrum and may be due to a small number of unresolved sources. Post-asymptotic giant branch stars would be too luminous to explain the far-UV flux of these clusters, but the luminosities and spectra are possibly consistent with evolved early asymptotic giant granch (AGB) stars, or the AGB-like evolutionary phases of extreme hot horizontal branch stars. Both types of evolved stars potentially are present in metal-rich populations (Greggio & Renzini 1990). NGC 6388, observed for the first time and well detected, differs substantially in having a flat, spatially resolved far-UV spectrum. We suggest that blue stragglers (which have been observed in 47 Tuc) probably account for UV flux down to 2000 Å, but that another source (different from those in 6624, 6637, and 6441) must be present below 2000 Å, perhaps extreme horizontal branch stars or cataclysmic variables. NGC 6624 is the only post-core-collapse cluster in the sample. While the far-UV flux is potentially linked to core binary populations rather than the stellar evolution of the cluster population, we emphasize that the UV rising flux of NGC 6624 is clearly detected in six IUE spectra from 1979 to 1989. If this source is a cataclysmic variable or related system, it has had far-UV emission over this 10 year period. The three clusters with UV rising fluxes - NGC 6624, 6637, and 6441 - have L_UV/L_TOT ≈ 0.02, similar to that of the galaxies. NGC 6388 has a UV fraction an order of magnitude lower. Follow-up UV imaging with the Hubble Space Telescope will provide the luminosities and lifetimes necessary to determine the nature of these hot cluster stars.