A far ultraviolet study of the globular clusters NGC 1851 & NGC 6681

Abstract

I present a far-ultraviolet study, taken with the Hubble Space Telescope (HST), of the globular clusters NGC 1851 and NGC 6681. My focus is primarily on the time-domain, and I present searches for – and classification of - far-ultraviolet variables and transients in my data. The far-ultraviolet sources have been cross-identified with objects detected at near-ultraviolet and optical wavelengths in archival HST imaging. The combined photometric catalogs were used to identify several core populations, including some that are likely to be binaries containing a hot component. In NGC 1851, I identified 36 variable candidates in the 273 exposures taken over 3 HST visits of 4 orbits each. Twenty five of the variable candidates are distributed along the horizontal branch, 12 are likely RR Lyraes and 13 are on the blue horizontal branch. Five variable candidates are located among the blue straggler populations with one of them a likely SX Phoenicis pulsator. The remaining six variable candidates have photometric properties that point to white dwarf + main sequence binary systems. One of the far-ultraviolet variables in NGC 1851, FUV1, was found to have a period of 18 minutes in our far-ultraviolet time-series photometry. The two possibilities for the nature of this far-ultraviolet variable are: (i) it may be an intermediate polar (i.e. acompact binary containing an accreting magnetic white dwarf), or (ii) it may be an AM CVn (i.e. and interacting double-white dwarf system). The AM CVn interpretation is favoured because of the lack of an X-ray detection in ≅ 65 ksec of Chandra imaging. I also present a search for transients (e.g. dwarf novae) in 20 years of far-ultraviolet imaging of the core of globular cluster NGC 6681. No such transients were found in the 80 epochs of observations. If all of these epochs can be considered independent, the search should be nearly 100% complete for dwarf novae with duty cycles > 5%, and still ≅ 60% complete for systems with duty cycles of ∼ 1%. The detection of zero outbursts allows me to place a 2 - σ upper limit of 3 on the mean number that might have been detected in similar surveys. This places strongish constraints on models for the cataclysmic variable populations.

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