Category Archives: ApJ

Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tuc…

47 Tucanae (NGC 104) :: New published paper

The Astrophysical Journal Letters
Volume 647, Number 1
(August 10 2006)

F. R. Ferraro, E. Sabbi, R. Gratton, G. Piotto, B. Lanzoni, E. Carretta, R. T. Rood, Alison Sills, F. Fusi Pecci, S. Moehler, G. Beccari, S. Lucatello, and N. Compagni
“Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process”

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We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

Table of Contents of ApL Volume 647, Number 1:

On the primordial scenario for abundance variations within GCs…

New paper (ApJ Vol. 645):

Salaris, Maurizio; Weiss, Achim; Ferguson, Jason W.; Fusilier, David J.

On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globular clusters. We have tested, with fully consistent stellar evolution calculations, whether theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the color-magnitude diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations and varying initial He mass fractions. Comparisons with “normal” α-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the red giant branch tip are also very weakly affected, apart from-depending on the He content of the polluting material-the red giant branch bump region. We also study the distribution of stars along the zero-age horizontal branch and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.

Publication: The Astrophysical Journal, Volume 645, Issue 2, pp. 1131-1137.

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A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in 47 Tuc

New article on ApJ:

Heinke, Craig O.; Rybicki, George B.; Narayan, Ramesh; Grindlay, Jonathan E.
“A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae”

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities, as well as effective temperatures, which we make available to the scientific community. We present multiepoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 Msolar is assumed, our spectral fits require that its radius be in the range Rns=14.5+1.8-1.6 km (90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of Mns=2.20+0.03-0.16 Msolar is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

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Merging of Globular Clusters in Inner Galactic Regions. I.

    Miocchi, P.; Capuzzo Dolcetta, R.; Di Matteo, P.; Vicari, A.

Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?

The Astrophysical Journal, Volume 644, Issue 2, pp. 940-953.

The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial, very massive (of the order of 107 Msolar) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a timescale much shorter than their internal relaxation time….

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