* Astronomy

Blobrana · L

RE: 47 Tucanae

Blobrana · L

Title: Medium-resolution s-process Element Survey of 47 Tuc giant stars
Authors: C.C. Worley, P.L. Cottrell

Medium-resolution (R~6,500) spectra of 97 giant stars in the globular cluster 47 Tucanae (47 Tuc) have been used to derive the C and N abundance sensitive index, deltaC, and to infer abundances of several key elements, Fe, Na, Si, Ca, Zr and Ba for a sample of 13 of these stars with similar Teff and log g. These stars have stellar properties similar to the well-studied 47 Tuc giant star, Lee 2525, but with a range of CN excess (deltaC) values which are a measure of the CN abundance. The deltaC index is shown to be correlated with Na abundance for this sample, confirming previous studies. The Fe, Ca, Si and the light- and heavy-s process (slow neutron capture) elements, Zr and Ba respectively, have a narrow range of abundance values in these stars, indicative of a homogeneous abundance within this population of stars. The constancy of many element abundances (Fe, Si, Ca, Zr, Ba) and the deltaC and Na abundance correlation could imply that there has been a second era of star formation in this cluster that has revealed the products of CNO cycle burning via hot bottom burning (depletion of C, enhancement of N and the production of Na for high deltaC population). But there is no overall metallicity change across the range of deltaC values at a given position in the HR diagram that has been seen in some other globular clusters.

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Title: The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster
Authors: Kristin A. Woodley (1), Ryan Goldsbury (1), Jason Kalirai (2,3), Harvey Richer (1), Pier-Emmanuel Tremblay (4), Jay Anderson (2), Pier Bergeron (5), Aaron Dotter (2), Lisa Esteves (6), Greg Fahlman (7), Brad Hansen (8), Jeremy Heyl (1), Jarrod Hurley (9), R. Michael Rich (8), Michael Shara (10), Peter Stetson (7) ((1) University of British Columbia, (2) STScI, (3) John Hopkins University, (4) Universitat Heidelberg, (5) Universite de Montreal, (6) University of Guelph, (7) Herzberg Institute of Astrophysics, (8) University of California at Los Angeles, (9) Swinburne University of Technology, (10) American Museum of Natural History)

We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs to pure hydrogen atmosphere white dwarf models. Our photometric dataset is obtained from a 121 orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 square arcmins and extend over a radial range of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood analysis, we obtain a best fitting unreddened distance modulus of (m - M)o=13.36±0.02±0.06 corresponding to a distance of 4.70±0.04±0.13 kpc, where the first error is random and the second is systematic. We also search the white dwarf photometry for infrared excess in the F160W filter, indicative of debris disks or low mass companions, and find no convincing cases within our sample.

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Blobrana · L

Title: The VISIR@VLT Mid-IR view of 47Tuc: A further step in solving the puzzle of RGB mass loss
Authors: Y. Momany, I. Saviane, A. Smette, A. Bayo, L. Girardi, G. Marconi, A. P. Milone, A. Bressan

There is an ongoing debate regarding the onset luminosity of dusty mass loss in population-II red giant stars. In this paper we present VISIR@VLT MIR 8.6 micron imaging of 47Tuc, centre of attention of a number of space-based Spitzer observations and studies. The VISIR high resolution (diffraction limited) observations allow excellent matching to existing optical Hubble space telescope catalogues. The optical-MIR coverage of the inner 1.15 arcmin of the cluster provide the cleanest possible, blending-free, sampling of the upper 3 magnitudes of the giant branch. Our diagrams show no evidence of faint giants with MIR-excess. A combined NIR-MIR diagram further confirms the near absence of dusty red giants. Dusty red giants and asymptotic giant stars are confined to the 47Tuc long period variables population. In particular, dusty red giants are limited to the upper one 8.6 micron magnitude below the giant branch tip. This particular luminosity level corresponds to ~1000 solar luminosity, suggested in previous determinations to mark the onset of dusty mass-loss. Interestingly, at this luminosity level, we detect a small deviation between the colours of red giants and the theoretical isochrones.

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Title: The origin of an extended X-ray emission apparently associated with the globular cluster 47 Tucanae
Authors: Takayuki Yuasa, Kazuhiro Nakazawa, Kazuo Makishima

Using the Suzaku X-ray Imaging Spectrometer, we performed a 130 ks observation of an extended X-ray emission, which was shown by ROSAT and Chandra observations to apparently associate with the globular cluster 47 Tucanae. The obtained 0.5-6 keV spectrum was successfully fitted with a redshifted thin thermal plasma emission model whose temperature and redshift are 2.2^{+0.2}_{-0.3} keV (at the rest frame) and 0.34 ±0.02, respectively. Derived parameters, including the temperature, redshift, and luminosity, indicate that the extended X-ray source is a background cluster of galaxies, and its projected location falls, by chance, on the direction of the proper motion of 47 Tucanae.

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47 Tucanae: The Sky's Best Globular
Today, a fine globular cluster. It's not the brightest, it's not the biggest, but it may very well be the prettiest. 47 Tucanae lies deep in the southern skies and, in a perfect world, is something we all see through a telescope at least once in our lives.

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Title: Stellar Exotica in 47 Tucanae
Authors: Christian Knigge (1), Andrea Dieball (1), Jesus Maiz Apellaniz (2), Knox S. Long (3), David R. Zurek (4), Michael M. Shara (4) ((1) University of Southampton, (2) IAA-CSIC, (3) STSCI, (4) AMNH)

We present far-UV spectroscopy obtained with 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 FUV-NIR SEDs. Based on our analysis of this extensive data set, we report the following main results. (1) We spectroscopically confirm 3 previously known or suspected CVs via the detection of emission lines and find new evidence for dwarf nova eruptions in two of these. (2) Only one other source in our spectroscopic sample exhibits marginal evidence for line emission, but predicted and observed CV numbers still 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 is probably the remnant of a subgiant that has been stripped of its envelope and may represent the ``smoking gun'' of a recent dynamical encounter. (4) We have found a Helium WD, the second to be optically detected in 47 Tuc, and the first outside a millisecond-pulsar system. (5) We have discovered a BS-WD binary system, the first known in any globular cluster. (6) We have found two additional candidate WD binary systems with putative main sequence and subgiant companions. (7) We estimate the WD binary fraction in the core of 47 Tuc to be 15 +17/-9 (stat) +8/-7 (sys). (8) One BS in our sample may exceed twice the cluster turn-off mass, but the uncertainties are large. 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.

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Title: An Eclipsing "Blue Straggler" V228 from 47 Tuc: Evolutionary Consideration
Authors: Marek J. Sarna

We perform evolutionary calculations of binary stars to find progenitors of system with parameters similar to the eclipsing binary system V228. We show that a V228 binary system may be formed starting with an initial binary system which has a low main sequence star as an accretor. We also show that the best fitting model implies loss of about 50 per cent of initial total orbital momentum but only 5 per cent of initial total mass.

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Title: Deep near-infrared photometry of the globular cluster 47 Tucanae. Reconciling theory and observations
Authors: M. Salaris, E. V. Held, S. Ortolani, M. Gullieuszik, and Y. Momany

The Galactic globular cluster 47 Tucanae is central to studies of Galaxy formation, and a test-bed for theoretical models, distance determination and extragalactic age-dating techniques. Independent parallax-based distance determinations in the optical spectral range provide discrepant results; also, star counts along the Red Giant Branch from optical data have disclosed a worrying disagreement with theoretical predictions, that impacts not only the theory of red giant stars, but also the calibration of the age scale of extragalactic systems.
Aims. Our new near-infrared data for 47 Tuc set constraints on its distance and test the reliability of theoretical red giant branch star counts, independently of previous conclusions from optical work.
Methods. We have obtained deep near-infrared imaging of 47 Tuc using SOFI at the ESO New Technology Telescope. Colour-magnitude diagrams, isochrones and synthetic horizontal branch modelling have been used to determine the distance of 47 Tuc and constrain its age. We have also constructed a luminosity function of red giant stars, which has been compared with theoretical predictions of stellar evolution models.
Results. We obtain a distance (m-M)₀ = 13.18 $\pm$ 0.03 (random) $\pm$ 0.04 (systematic), for [Fe/H] = -0.7 $\pm$ 0.1 and E(B-V) = 0.04 $\pm$ 0.02. This supports the shorter end of the range of distances obtained from optical studies. The mean horizontal branch star mass is between 0.65 and 0.66 solar mases, and its 1 $\sigma$ Gaussian dispersion is between 0.010 and 0.012 solar masses. The cluster age can only be approximately estimated from the data, and is between ~10 and ~13 Gyr. The luminosity function of red giant branch (and early-asymptotic giant branch) stars does not show a statistically significant discrepancy with theory. The brightness of the red giant branch bump in the near-infrared is possibly fainter than the models, although the uncertainty on the spectroscopic metallicity and age prevents to reach a firm conclusion on this issue.

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-- Edited by Blobrana at 09:28, 2007-12-28