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Title: Analysis of the CN and CH molecular band strengths in stars of the open cluster NGC 6791
Authors: R. Carrera (1,2) ((1) Instituto de Astrofísica de Canarias, Spain, (2) Departamento de Astrofísica, Universidad de La Laguna, Spain)

Low resolution SDSS/SEGUE spectra have been used to study the behaviour of the strengths of the CN and CH molecular bands in stars at different evolutionary stages of the open cluster NGC 6791. We find a significant spread in the strengths of the CN bands, more than twice that expected from the uncertainties, although the bimodalities observed in globular clusters are not clearly observed here. This behaviour, is observed not only among red clump objects but also in unevolved stars such as those in the main sequence and lower red giant branch. In contrast, not all the stars studied show significant scatter in their CH strengths.

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Title: Age and helium content of the open cluster NGC 6791 from multiple eclipsing binary members. II. age dependencies and new insights
Authors: K. Brogaard, D. A. VandenBerg, H. Bruntt, F. Grundahl, S. Frandsen, L. R. Bedin, A. P. Milone, A. Dotter, G. A. Feiden, P. B. Stetson, E. Sandquist, A. Miglio, D. Stello, J. Jessen-Hansen

Models of stellar structure and evolution can be constrained by measuring accurate parameters of detached eclipsing binaries in open clusters. Multiple binary stars provide the means to determine helium abundances in these old stellar systems, and in turn, to improve estimates of their age. In the first paper of this series, we demonstrated how measurements of multiple eclipsing binaries in the old open cluster NGC6791 sets tighter constraints on the properties of stellar models than has previously been possible, thereby potentially improving both the accuracy and precision of the cluster age. Here we add additional constraints and perform an extensive model comparison to determine the best estimates of the cluster age and helium content, employing as many observational constraints as possible. We improve our photometry and correct empirically for differential reddening effects. We then perform an extensive comparison of the CMDs and eclipsing binary measurements to Victoria and DSEP isochrones to estimate cluster parameters. We also reanalyse a spectrum of the star 2-17 to improve [Fe/H] constraints. We find a best estimate of the age of ~8.3 Gyr while demonstrating that remaining age uncertainty is dominated by uncertainties in the CNO abundances. The helium mass fraction is well constrained at Y = 0.30 ±0.01 resulting in dY/dZ ~ 1.4 assuming that such a relation exists. During the analysis we firmly identify blue straggler stars, including the star 2-17, and find indications for the presence of their evolved counterparts. Our analysis supports the RGB mass-loss found from asteroseismology and we determine precisely the absolute mass of stars on the lower RGB, 1.15±0.02 solar masses. This will be an important consistency check for the detailed asteroseismology of cluster stars.

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Title: The origin and orbit of the old, metal-rich, open cluster NGC 6791
Authors: L. Jilkova, G. Carraro, B. Jungwiert, I. Minchev

NGC6791 is a unique stellar system among Galactic open clusters being at the same time one of the oldest open clusters and the most metal rich. Combination of its properties is puzzling and poses question of its origin. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outwards to its current location. In this work we study the cluster's orbit and investigate the possible migration processes which might have displaced NGC6791 to its present-day position, under the assumption that it actually formed in the inner disk. To this aim we performed integrations of NGC6791's orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and disk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state-of-the art initial conditions for NGC6791, we calculate 1000 orbits back in time for about 1 Gyr turning on and off different non-axisymmetric components of the global potential. We then compare statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 10^4 test-particles originating close to the Galactic Center (having initial galocentric radii in the range of 3-5kpc) and undergoing radial migration during 8Gyr of forward integration. We find that a model which incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC6791 from the inner disk (galocentric radii of 3-5kpc) to its present-day location. Such a model can provide orbital parameters which are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low.

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Title: Stellar lifetime and ultraviolet properties of the old metal-rich Galactic open cluster NGC6791: a pathway to understand the UV upturn of elliptical galaxies
Authors: Alberto Buzzoni (1), Emanuele Bertone (2), Giovanni Carraro (3 and 4), Lucio Buson (5) ((1) INAF - Osservatorio Astronomico di Bologna, Italy, (2) INAOE - Inst. Nac. de Astrofisica Optica y Electronica, Tonantzintla, Pue, Mexico, (3) ESO - European Southern Obs., Santiago, Chile, (4) Dip. Astronomia Universita' di Padova, Italy, (5) INAF - Osservatorio Astronomico di Padova, Italy)

The evolutionary properties of the old metal-rich Galactic open cluster NGC6791 are assessed, based on deep UB photometry and 2Mass JK data. For 4739 stars in the cluster, bolometric luminosity and effective temperature have been derived from theoretical (U-B) and (J-K) colour fitting. The derived H-R diagram has been matched with the UVBLUE grid of synthetic stellar spectra to obtain the integrated SED of the system, together with a full set UV (Fanelli) and optical (Lick) narrow-band indices. The cluster appears to be a fairly good proxy of standard elliptical galaxies, although with significantly bluer infrared colours, a shallower 4000A Balmer break, and a lower Mg2 index. The confirmed presence of a dozen hot stars, along their EHB evolution, leads the cluster SED to consistently match the properties of the most active UV-upturn galaxies, with 1.7±0.4% of the total bolometric luminosity emitted shortward of 2500A.
The cluster Helium abundance results Y=0.30 ±0.04, while the Post-MS implied stellar lifetime from star number counts fairly agrees with the theoretical expectations from both the Padova and BASTI stellar tracks. A Post-MS fuel consumption of 0.43 ±0.01 solar masses is found for NGC6791 stars, in close agreement with the estimated mass of cluster He-rich white dwarfs. Such a tight figure may lead to suspect that a fraction of the cluster stellar population does actually not reach the minimum mass required to effectively ignite He in the stellar core.

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Calculen amb precisió l'edat de les estrelles

Un grup de científics, encapçalat per la UPC, determina que l'edat de les estrelles nanes blanques és de 8.000 milions d'anys. Els resultats de la recerca es publicaran a la prestigiosa revista científica 'Nature'.
Un equip de científics de la Universitat Politècnica de Catalunya (UPC), l'Institut d'Estudis Espacials de Catalunya, l'Institut de Ciències de l'Espai  del Consejo Superior de Investigaciones Científicas (CSIC), la Universidad Nacional de La Plata (Argentina) i la Liverpool John Moores University (Regne Unit), encapçalat per l'investigador Enrique García-Berro, del Departament de Física Aplicada de la UPC, ha demostrat que l'edat de les estrelles nanes blanques del cúmul galàctic NGC 6791 és de 8.000 milions d'anys i no de 6.000 milions, tal i com es creia fins ara. Aquesta recerca obre nous camins per aprofundir en el coneixement de l'origen de l'Univers. Els resultats de la recerca es publiquen a la revista científica Nature, el 13 de maig.

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Imagine having three clocks in your house, each chiming at a different time. Astronomers have found the equivalent of three out-of-sync "clocks" in the ancient open star cluster NGC 6791. The dilemma may fundamentally challenge the way astronomers estimate cluster ages, researchers said.
Using NASA's Hubble Space Telescope to study the dimmest stars in the cluster, astronomers uncovered three different age groups. Two of the populations are burned-out stars called white dwarfs. One group of these low-wattage stellar remnants (red circles) appears to be 6 billion years old, another (blue circles) appears to be 4 billion years old. The ages are out of sync with those of the cluster's normal stars, which are 8 billion years old.

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Position(2000): R.A. 19h 20m 53s.00, Dec. +37° 46' 18".00

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Title: Spitzer Space Telescope evidence in NGC 6791: no super-mass-loss at super-solar metallicity to explain helium white dwarfs?
Authors: Jacco Th. van Loon (Keele University, UK), Martha L. Boyer (University of Minnesota, USA), Iain McDonald (Keele University, UK)

We use archival Spitzer Space Telescope photometry of the old, super-solar metallicity massive open cluster NGC 6791 to look for evidence of enhanced mass loss, which has been postulated to explain the optical luminosity function and low white dwarf masses in this benchmark cluster. We find a conspicuous lack of evidence for prolificacy of circumstellar dust production that would have been expected to accompany such mass loss. We also construct the optical and infrared luminosity functions, and demonstrate that these fully agree with theoretical expectations. We thus conclude that there is no evidence for the mass loss of super-solar metallicity red giants to be sufficiently high that they can avoid the helium flash at the tip of the red giant branch.

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A new survey suggests that low-mass 'corpses' of stars are much more common in the universe than previously thought.
The finding is detailed in an upcoming issue of the Astrophysical Journal, and it could give planet hunters a new lead in their search for alien worlds.
These stellar runts are the product of a process that reduces metal-rich suns into skeletons of their former selves, but until now was thought only to kick-in in the distant future.

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Title: Reaching the End of the White Dwarf Cooling Sequence in NGC 6791
Authors: L. R. Bedin (1), I. R. King (2), J. Anderson (1), G. Piotto (3), M. Salaris (4), S. Cassisi (5), A. Serenelli (6) ((1) STScI, (2) Univ. of Washington Seattle, (3) Univ. Padova, (4) J. Moores Univ. Liverpool, (5) INAF-Obs. Teramo, (6) IAS Princeton)

We present new observations of the white dwarf sequence of the old open cluster NGC 6791. The brighter peak previously observed in the white dwarf luminosity function (WDLF) is now better delineated, and the second, fainter peak that we suggested earlier is now confirmed. A careful study suggests that we have reached the end of the white dwarf sequence. The WDs that create the two peaks in the WDLF show a significant turn to the blue in the colour-magnitude diagram. The discrepancy between the age from the WDs and that from the main sequence turnoff remains, and we have an additional puzzle in the second peak in the WDLF. Canonical WD models seem to fail --at least at ~25%-level-- in reproducing the age of clusters of this metallicity. We discuss briefly possible ways of arriving at a theoretical understanding of the WDLF.

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Title: Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
Authors: Jasonjot S. Kalirai, P. Bergeron, Brad M. S. Hansen, Daniel D. Kelson, David B. Reitzel, R. Michael Rich, Harvey B. Richer
(Version v3)

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ 0.4) open clusters in our Galaxy, and has a colour-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS spectra, suggests that most of these stars are undermassive, <M> = 0.43 ± 0.06 Msun, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash, and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen core stars and therefore the corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

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