Multiple populations in globular clusters Gratton, Raffaele G.; Carretta, Eugenio; Bragaglia, Angela
The Astronomy and astrophysics review,
10/2012, Letnik:
20, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Recent progress in studies of globular clusters has shown that they are not simple stellar populations, but rather are made up of multiple generations. Evidence stems both from photometry and ...spectroscopy. A new paradigm is arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second-parameter problem, it also opens new perspectives on the relation between globular clusters and the halo of our Galaxy, and by extension on all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focussing on the most recent studies. Several points remain to become properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.
We observed a sample of 90 red giant branch (RGB) stars in NGC 2808 using FLAMES/GIRAFFE and the high resolution grating with the set-up HR21. These stars have previous accurate atmospheric ...parameters and abundances of light elements. We derived aluminium abundances for them from the strong doublet Al i 8772–8773 Å as in previous works of our group. In addition, we were able to estimate the relative CN abundances for 89 of the stars from the strength of a large number of CN features. When adding self-consistent abundances from previous UVES spectra analysed by our team, we gathered Al/Fe ratios for a total of 108 RGB stars in NGC 2808. The full dataset of proton-capture elements is used to explore in detail the five spectroscopically detected discrete components in this globular cluster. We found that various classes of polluters are required to reproduce (anti)-correlations among all proton-capture elements in the populations P2, I1, and I2 with intermediate composition. This is in agreement with the detection of lithium in lower RGB second generation stars, requiring at least two kind of polluters. For chemically homogeneous populations, the best subdivision of our sample is into six components as derived from statistical cluster analysis. By comparing different diagrams element/Fe versus element/Fe, we show for the first time that a simple dilution model is not able to reproduce all the subpopulations in this cluster. Polluters of different masses are required. NGC 2808 is confirmed to be a tough challenge to any scenario for globular cluster formation.
Abundance Variations within Globular Clusters Gratton, Raffaele; Sneden, Christopher; Carretta, Eugenio
Annual review of astronomy and astrophysics,
01/2004, Letnik:
42, Številka:
1
Journal Article
Recenzirano
▪ Abstract Abundance variations within globular clusters (GCs), and of GC stars with respect to field stars, are important diagnostics of a variety of physical phenomena, related to the evolution of ...individual stars, mass transfer in binary systems, and chemical evolution in high density environments. The broad astrophysical implications of GCs as building blocks of our knowledge of the Universe make a full understanding of their history and evolution basic in a variety of astrophysical fields. We review the current status of the research in this field, comparing the abundances in GCs with those obtained for field stars, discussing in depth the evidence for H-burning at high temperatures in GC stars, describing the process of self-enrichment in GCs with particular reference to the case of the most massive Galactic GC (ω Cen), and discussing various classes of cluster stars with abundance anomalies. Whereas the overall pattern might appear very complex at first sight, exciting new scenarios are opening where the interplay between GC dynamical and chemical properties are closely linked with each other.
Celotno besedilo
Dostopno za:
CMK, DOBA, FMFMET, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Abstract
Radial velocity surveys suggest that the Solar System may be unusual and that Jupiter-like planets have a frequency < 20% around solar-type stars. However, they may be much more common in ...one of the closest associations in the solar neighbourhood. Young moving stellar groups are the best targets for direct imaging of exoplanets and four massive Jupiter-like planets have been already discovered in the nearby young
β
Pic Moving Group (BPMG) via high-contrast imaging, and four others were suggested via high precision astrometry by the European Space Agency’s Gaia satellite. Here we analyze 30 stars in BPMG and show that 20 of them might potentially host a Jupiter-like planet as their orbits would be stable. Considering incompleteness in observations, our results suggest that Jupiter-like planets may be more common than previously found. The next Gaia data release will likely confirm our prediction.
Convergent lines of evidence suggest that globular clusters host multiple stellar populations. It has been shown that in order to reproduce the abundance pattern found in M4, Li production must occur ...within the polluters, favoring the AGB scenario. Here, we present Li and Al abundances for a large sample of RGB stars in M12 and M5. These clusters have a very similar metallicity, while demonstrating differences in several cluster properties. Our results indicate that the first-generation and second-generation stars share the same Li content in M12; we recover an abundance pattern similar to that observed in M4. This is the first time a statistically significant correlation between the Li spread within a GC and its luminosity has been demonstrated. Finally, although Li-producing polluters are required to account for the observed pattern, other mechanisms, such as MS depletion, might have played a role in contributing to the Li internal variation, though at a relatively low level.
We discuss radial velocities for a sample of carbon-enhanced, s-process-rich, very metal-poor (CEMP-s) stars, analyzed with high-resolution spectroscopy obtained over multiple epochs. We find that ...similar to 68% of the stars in the sample show evidence of radial velocity variations. The expected detection fraction for these stars, adopting the measured binary fraction in the field ( similar to 60%) and assuming that they share the same period and eccentricity distribution, is similar to 22%. Even if one assumes that the true binary fraction of these stars is 100%, the expected detection percentage is similar to 36%. These values indicate that the binary fraction among CEMP-s stars is higher than the field binary fraction, suggesting that all of these objects are in double (or multiple) systems. The fact that the observed frequency of velocity variation exceeds the expected detection fraction in the case of an assumed binary fraction of 100% is likely due to a more restricted distribution of orbital periods for these objects, as compared to normal field binaries. Our results indicate that CEMP-s stars are the metal-poor analogs of classical CH stars.
We present the abundances of N in a sample of 62 stars on the red giant branch (RGB) in the peculiar globular cluster NGC 1851. The values of N/Fe ratio were obtained by comparing the flux measured ...in the observed spectra with that from synthetic spectra for up to about 15 features of CN. This is the first time that N abundances are obtained for such a large sample of RGB stars from medium-resolution spectroscopy in this cluster. With these abundances, we provide chemical tagging of the split RGB found from several studies in NGC 1851. The secondary reddest sequence on the RGB is populated almost exclusively by N-rich stars, confirming our previous suggestion based on Strömgren magnitudes and colours. These giants are also, on average, enriched in s-process elements such as Ba, and are likely the results of pollution from low-mass stars that experienced episodes of third dredge-up in the asymptotic giant branch phase.
Integral field spectroscopy represents a powerful technique for the detection and characterization of extrasolar planets through high-contrast imaging since it allows us to obtain simultaneously a ...large number of monochromatic images. These can be used to calibrate and then to reduce the impact of speckles, once their chromatic dependence is taken into account. The main concern in designing integral field spectrographs for high-contrast imaging is the impact of the diffraction effects and the noncommon path aberrations together with an efficient use of the detector pixels. We focus our attention on integral field spectrographs based on lenslet arrays, discussing the main features of these designs: the conditions of appropriate spatial and spectral sampling of the resulting spectrograph's slit functions and their related cross-talk terms when the system works at the diffraction limit. We present a new scheme for the integral field unit based on a dual-lenslet device (BIGRE), that solves some of the problems related to the classical Traitement Integral des Galaxies par l'Etude de leurs Rays (TIGER) design when used for such applications. We show that BIGRE provides much lower cross-talk signals than TIGER, allowing a more efficient use of the detector pixels and a considerable saving of the overall cost of a lenslet-based integral field spectrograph.
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