SCYON Abstract

Received on April 30 2008

Stellar Exotica in 47 Tucanae

Authors	Christian Knigge (¹), Andrea Dieball (¹), Jesus Maiz Apellaniz (²), Knox S. Long (³), David R. Zurek (⁴), and Michael M. Shara (⁴)
Affiliation	(¹) University of Southampton (²) IAA-CSIC (³) STSCI (⁴) AMNH
Accepted by	Astrophysical Journal
Contact	christian@astro.soton.ac.uk
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Abstract

We present far-ultraviolet (FUV) spectroscopy obtained with the Hubble Space Telescope (HST) for 48 blue objects in the core of 47 Tuc. Based on their position in a FUV-optical colour-magnitude diagram, these were expected to include cataclysmic variables (CVs), blue stragglers (BSs), white dwarfs (WDs) and other exotic objects. For a subset of these sources, we also construct broad-band, FUV through near-infrared spectral energy distributions. Based on our analysis of this extensive data set, we report the following main results. (1) We detect emission lines in three previously known or suspected CVs and thus spectroscopically confirm the status of these systems. We also detect new dwarf nova eruptions in two of these CVs. (2) Only one other source in our spectroscopic sample exhibits marginal evidence for line emission. Thus CVs are not the only class of objects found in the gap between the WD and main sequences, nor are they common amongst objects near the top of the WD cooling sequence. Nevertheless, predicted and observed numbers of CV agree to within a factor of about 2-3. (3) We have discovered a hot (T_eff = 8700 K), low-mass (M = 0.05 M_sun) secondary star in a previously known 0.8-day binary system. This exotic object completely dominates the binary's FUV-NIR output and is probably the remnant of a subgiant that has been stripped of its envelope. Since this object must be in a short-lived evolutionary state, it may represent the ``smoking gun'' of a recent dynamical encounter. (4) We have found a Helium WD, only the second such object to be optically detected in 47 Tuc, and the first outside a millisecond pulsar system. (5) We have discovered a bright BS with a young WD companion, the only BS-WD binary known in any GC. (6) We have found two additional candidate WD binary systems, one containing a MS companion, the other containing a subgiant. (7) We estimate the WD binary fraction in the core of 47 Tuc to be 15 +17/-9 (stat) +8/-7 (sys) per cent. (8) The mass of the optically brightest BS in our sample may exceed twice the cluster turn-off mass, but the uncertainties are too large for this to be conclusive. Thus there is still no definitive example of such a ``supermassive'' BS in any GC. Taken as a whole, our study illustrates the wide range of stellar exotica that are lurking in the cores of GCs, most of which are likely to have undergone significant dynamical encounters.