NGC 6441: another indication of very high helium content in GGCs stars

NGC 6441 :: new paper

Caloi, V.; D’Antona, F.

NGC 6441: another indication of very high helium content in globular cluster stars

The metal-rich bulge globular cluster NGC 6441 shows a well developed blue horizontal branch (Rich et al.), together with a strong slope upward from the red clump to the blue of the RR Lyrae region. Both features, the former corresponding to the well-known second parameter problem, are not explained by conventional evolutionary models. Helium self-enrichment is proposed as a possible solution to both questions, a mechanism already invoked for the interpretation of the peculiarities in NGC 2808 and M13. We make use of horizontal branch simulations, covering a wide range in main sequence helium abundance, to investigate whether the main features of NGC 6441 horizontal branch population, including the RR Lyrae variables period, can be reproduced. To describe the horizontal branch of NGC 6441, the helium content Y in the red clump must reach at least 0.35; values up to Y~0.37 are necessary to populate the RR Lyr region, reproducing also the observed mean long periods; depending on the dispersion in mass loss assumed in the simulations, values up to Y~0.38–0.40 are necessary to populate the blue HB. The total self–enriched population amounts to ~60% of the whole stellar content. Self-enrichment and multiple star formation episodes in the early evolution of globular clusters appear more and more able to account for many of the chemical and population peculiarities observed in these systems. The very large helium abundances (Y>0.35) required for ~14% of the horizontal branch population pose some problem on the enrichment mechanisms.
Astronomy and Astrophysics, Volume 463, Issue 3, March I 2007, pp.949-955 (A&A Homepage)

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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 M_solar is assumed, our spectral fits require that its radius be in the range R_ns=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 M_ns=2.20^+0.03_-0.16 M_solar 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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Abundance anomalies in hot HB stars of NGC 2808

[Paper]

Pace, G.; Recio-Blanco, A.; Piotto, G.; Momany, Y.

Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808

Astronomy and Astrophysics, Volume 452, Issue 2, June III 2006, pp.493-501

Aims.We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808.
Methods: . Our measurements are based on moderate-resolution spectra taken with the multi-object fiber facility FLAMES-UVES, mounted on Kueyen at the Very Large Telescope.
Results: . We confirm that stars hotter than a threshold temperature have super-solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. [Fe/H]≃-1.1. The threshold temperature is estimated to be about 12 000 K, corresponding to the so called u-jump, and coincides with the sudden departure of the cluster horizontal branch from the models. The metallicity increases with temperature for star hotter than the jump, confirming the hypothesis that the process responsible for this abrupt metallic enhancement is the levitation due to the strong radiation field in absence of a significative convective envelope. A metallicity dependence of the abundance enhancement is also suggested, with more metal poor clusters having a higher increase in metal content.
Conclusions: .The slope in the temperature vs. abundance diagram is higher than the errors involved, and the metal content of the cluster plays possibly a role in determining the amplitude of the jump (more metal poor clusters show more enhancement after the jump), although other parameters, such as clusters' characteristics and even the atomic species involved, may also someway contribute.

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