1,723,153 research outputs found
AGB subpopulations in the nearby globular cluster NGC 6397
Get PDFIt has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used
The space motion of the globular cluster NGC 6397
No full textAs a by-product of high-precision, ultradeep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly pointlike. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu(alpha) cos delta = 3.56 +/- 0.04 mas yr(-1) and mu(delta) = -17.34 +/- 0.04 mas yr(-1). We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way and find that the cluster has made frequent passages through the Galactic disk
Decomposition of the Globular Cluster NGC 6397
No full textThe kinematics and white dwarf distribution have been studied for the Globular Cluster NGC 6397. The
data was obtained from NASA’s Hubble Space Telescope in 2005. In particular, we used the images of a
field 5’ Southeast of the core of NGC 6397 from Advanced Camera for Surveys to conduct our analyses.
The first part of the study is about the kinematics of the globular cluster. Isotropy of velocity distribution
and cluster rotation have been considered. As anticipated, this relaxed cluster exhibited no strong signs of
anisotropy. However, there appears to be some level of rotation. The rotational motion turns out to be
mu sub alpha cos(delta) = 3.88 ± 1.41 mas yr −1 and mu sub delta = −14.83 ± 0.58 mas yr −1. This result is not entirely
expected and deserves further investigation in future studies.
The second of the thesis is based on white dwarf populations in the globular cluster and the Galactic
Bulge. As a first glance, there appears to be a lacking of white dwarfs at the age of approximately
0.6 Gyr. Further investigation reveals this to be statistically insignificant. Through this analysis, another
pattern of white dwarf abundance is discovered. There appeared to be much more stars at the age between
0.9 − 2.0 Gyr. This could be a manifestation of modeling error. As the final consideration of this thesis,
white dwarf candidates in the Galactic Bulge are illustrated. Approximately 10 candidates are found at the
most probable location of stars in the Bulge.
The analyses conducted in this thesis set stage for further development in understanding of globular
clusters. In particular, the rotation analysis raises curiosity about the dynamics of NGC 6397 in the plane
of the sky. Moreover, the velocity distribution analysis confirms properties and theories pertaining to
globular clusters.Science, Faculty ofPhysics and Astronomy, Department ofUnreviewedUndergraduat
The white dwarf population of NGC 6397
No full textContext. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.; Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions.; Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.; Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8(-0.75)(+0.50) Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.; Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.Peer Reviewe
The white dwarf population of NGC 6397
Get PDFContext. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.
Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions.
Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.
Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8+0.50-0.75 Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.
Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
The white dwarf population of NGC 6397
Get PDFContext. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.; Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions.; Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.; Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8(-0.75)(+0.50) Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.; Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.Peer ReviewedPostprint (published version
No Black Holes in NGC 6397
Get PDFRecently, \citet{vitral2021does} detected a central concentration of dark objects in the core-collapsed globular cluster NGC 6397, which could be interpreted as a subcluster of stellar-mass black holes. However, it is well established theoretically that any significant number of black holes in the cluster would provide strong dynamical heating and is fundamentally inconsistent with this cluster\u27s core-collapsed profile. Claims of intermediate-mass black holes in core-collapsed clusters should similarly be treated with suspicion, for reasons that have been understood theoretically for many decades. Instead, the central dark population in NGC 6397 is exactly accounted for by a compact subsystem of white dwarfs, as we demonstrate here by inspection of a previously published model that provides a good fit to this cluster. These central subclusters of heavy white dwarfs are in fact a generic feature of core-collapsed clusters, while central black hole subclusters are present in all {\em non\/}-collapsed clusters.3 pages, 1 figure, to submit to AAS journal
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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