218 research outputs found
Blue Straggler Stars in Galactic Open Clusters and the effect of field star contamination
We investigate the distribution of Blue Straggler stars in the field of three
open star clusters. The main purpose is to highlight the crucial role played by
general Galactic disk fore-/back-ground field stars, which are often located in
the same region of the Color Magnitude Diagram as Blue Straggler stars. We
analyze photometry taken from the literature of 3 open clusters of
intermediate/old age rich in Blue Straggler stars, and which are projected in
the direction of the Perseus arm, and study their spatial distribution and the
Color Magnitude Diagram. As expected, we find that a large portion of the Blue
Straggler population in these clusters are simply young field stars belonging
to the spiral arm. This result has important consequences on the theories of
the formation and statistics of Blue Straggler stars in different population
environments: open clusters, globular clusters or dwarf galaxies. As previously
emphasized by many authors, a detailed membership analysis is mandatory before
comparing the Blue Straggler population in star clusters against theoretical
models. Moreover, these sequences of young field stars (blue plumes) are
potentially powerful tracers of Galactic structure which require further
consideration.Comment: 11 pages, 4 figurs, in press as Research Note in A&
The Gaia-ESO Survey: Detailed Abundances in the Metal-poor Globular Cluster NGC 4372
We present the abundance analysis for a sample of 7 red giant branch stars in
the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part
of the Gaia-ESO Survey. This is the first extensive study of this cluster from
high resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca,
Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19
0.03 and find no evidence for a metallicity spread. This metallicity
makes NGC 4372 one of the most metal-poor galactic globular clusters. We also
find an {\alpha}-enhancement typical of halo globular clusters at this
metallicity. Significant spreads are observed in the abundances of light
elements. In particular we find a Na-O anti-correlation. Abundances of O are
relatively high compared with other globular clusters. This could indicate that
NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al
spread is also present which spans a range of more than 0.5 dex in Al
abundances. Na is correlated with Al and Mg abundances at a lower significance
level. This pattern suggests that the Mg-Al burning cycle is active. This
behavior can also be seen in giant stars of other massive, metal-poor clusters.
A relation between light and heavy s-process elements has been identified.Comment: 14 pages, 14 figures, accepted for publication in A&
Radial distribution of the multiple stellar populations in omega Centauri
We present a detailed study of the radial distribution of the multiple
populations identified in the Galactic globular cluster omega Cen. We used both
space-based images (ACS/WFC and WFPC2) and ground-based images (FORS1@VLT and
[email protected] ESO telescopes) to map the cluster from the inner core to the
outskirts (~20 arcmin). These data sets have been used to extract high-accuracy
photometry for the construction of color-magnitude diagrams and astrometric
positions of ~900 000 stars. We find that in the inner ~2 core radii the blue
main sequence (bMS) stars slightly dominate the red main sequence (rMS) in
number. At greater distances from the cluster center, the relative numbers of
bMS stars with respect to rMS drop steeply, out to ~8 arcmin, and then remain
constant out to the limit of our observations. We also find that the dispersion
of the Gaussian that best fits the color distribution within the bMS is
significantly greater than the dispersion of the Gaussian that best fits the
color distribution within the rMS. In addition, the relative number of
intermediate-metallicity red-giant-branch stars which includes the progeny of
the bMS) with respect to the metal-poor component (the progeny of the rMS)
follows a trend similar to that of the main-sequence star-count ratio
N_bMS/N_rMS. The most metal-rich component of the red-giant branch follows the
same distribution as the intermediate-metallicity component. We briefly discuss
the possible implications of the observed radial distribution of the different
stellar components in omega Cen.Comment: 16 pages, 14 figures (6 in low resolution), 3 tables. Accepted for
publication in Astronomy and Astrophysics on 23 September 200
Formation of Multiple Populations in Globular Clusters: Another Possible Scenario
While chemical composition spreads are now believed to be a universal
characteristic of globular clusters (GCs), not all of them present multiple
populations in their color-magnitude diagrams (CMDs). Here we present a new
scenario for the formation of GCs, in an attempt to qualitatively explain this
otherwise intriguing observational framework. Our scenario divides GCs into
three groups, depending on the initial mass (M_I) of the progenitor structure
(PS), as follows. i) Massive PSs can retain the gas ejected by massive stars,
including the ejecta of core-collapse SNe. ii) Intermediate-mass PSs can retain
at least a fraction of the fast winds of massive stars, but none of the
core-collapse SNe ejecta. iii) Low-mass PSs can only retain the slow winds of
intermediate-mass stars. Members of the first group would include omega
Centauri (NGC 5139), M54 (NGC 6715), M22 (NGC 6656), and Terzan 5, whereas NGC
2808 (and possibly NGC 2419) would be members of the second group. The
remaining GCs which only present a spread in light elements, such as O and Na,
would be members of the third group. According to our scenario, the different
components in omega Cen should not display a sizeable spread in age. We argue
that this is consistent with the available observations. We give other simple
arguments in favor of our scenario, which can be described in terms of two main
analytical relations: i) Between the actual observed ratio between first and
second generation stars (R_SG^FG) and the fraction of first generation stars
that have been lost by the GC (S_L); and ii) Between S_L and M_I. We also
suggest a series of future improvements and empirical tests that may help
decide whether the proposed scenario properly describes the chemical evolution
of GCs.Comment: Accepted for publication in Astronomy and Astrophysic
A photometric study of the old open clusters Berkeley 73, Berkeley 75 and Berkeley 25
CCD BVI photometry of the faint open clusters Berkeley~73, Berkeley~75 and
Berkeley~25 are presented. The two latter are previously unstudied clusters to
our knowledge. While Berkeley~73 is found to be of intermediate-age (about 1.5
Gyr old), Berkeley~75 and Berkeley~25 are two old clusters, with ages larger
than 3.0 Gyr. We provide also estimates of the clusters size. Very
interestingly, all these clusters turn out to lie far away from the Galactic
Center, at 16 kpc, and quite high onto the Galactic plane, at
kpc. They are therefore important targets to probe the
properties of the structure of the Galaxy in this direction, where the Canis
Major over-density has been recently discovered to be located.Comment: 8 pages, 10 figures, in press in A&
C and N abundances of MS and SGB stars in NGC 1851
We present the first chemical analysis of stars on the double subgiant branch
(SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra
of subgiants and fainter stars covering the spectral region between
3650-6750\AA, to derive C and N abundances from the spectral features at
4300\AA (G-band) and at ~ 3883\AA (CN). We added to our sample ~ 45 unvevolved
stars previously observed with FORS2 at the VLT. These two datasets were
homogeneously reduced and analyzed. We derived abundances of C and N for a
total of 64 stars and found considerable star-to-star variations in both [C/H]
and [N/H] at all luminosities extending to the red giant branch (RGB) base
(V~18.9). These abundances appear to be strongly anticorrelated, as would be
expected from the CN-cycle enrichment, but we did not detect any bimodality in
the C or N content. We used HST and ground-based photometry to select two
groups of faint- and bright-SGB stars from the visual and Str\"omgren
color-magnitude diagrams. Significant variations in the carbon and nitrogen
abundances are present among stars of each group, which indicates that each SGB
hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in
the total C+N content, where the fainter SGB have about 2.5 times the C+N
content of the brighter ones. Coupling our results with literature photometric
data and abundance determinations from high-resolution studies, we identify the
fainter SGB with the red-RGB population, which also should be richer on average
in Ba and other s-process elements, as well as in Na and N, when compared to
brighter SGB and the blue-RGB population.Comment: 17 pages, 3 tables, 16 figures; typos corrected, added checks on
temperature scale; A&A accepted (in press
Reconstructing Neutrino Properties from Collider Experiments in a Higgs Triplet Neutrino Mass Model
We extend the minimal supersymmetric standard model with bilinear R-parity
violation to include a pair of Higgs triplet superfields. The neutral
components of the Higgs triplets develop small vacuum expectation values (VEVs)
quadratic in the bilinear R-parity breaking parameters. In this scheme the
atmospheric neutrino mass scale arises from bilinear R-parity breaking while
for reasonable values of parameters the solar neutrino mass scale is generated
from the small Higgs triplet VEVs. We calculate neutrino masses and mixing
angles in this model and show how the model can be tested at future colliders.
The branching ratios of the doubly charged triplet decays are related to the
solar neutrino angle via a simple formula.Comment: 19 pages, 4 figures; one formula corrected, two author's names
corrected; some explanatory comments adde
On the use of the main sequence knee (saddle) to measure globular cluster ages
In this paper we review the operational definition of the so-called main sequence knee (MS-knee), a feature in the color-magnitude diagram (CMD) occurring at the low-mass end of the MS. The magnitude of this feature is predicted to be independent of age at fixed chemical composition. For this reason, its difference in magnitude with respect to the MS turn-off (MS-TO) point has been suggested as a possible diagnostic to estimate absolute globular cluster (GC) ages. We first demonstrate that the operational definition of the MS-knee currently adopted in the literature refers to the inflection point of the MS (that we here more appropriately named MS-saddle), a feature that is well distinct from the knee and that cannot be used as its proxy. The MS-knee is only visible in near-infrared CMDs, while the MS-saddle can be also detected in optical-NIR CMDs. By using different sets of isochrones we then demonstrate that the absolute magni- tude of the MS-knee varies by a few tenths of a dex from one model to another, thus showing that at the moment stellar models may not capture the full systematic error in the method. We also demonstrate that while the absolute magnitude of the MS-saddle is almost coincident in different models, it has a systematic dependence on the adopted color combinations which is not predicted by stellar models. Hence, it cannot be used as a reliable reference for absolute age determination. Moreover, when statistical and systematic uncertainties are properly taken into ac- count, the difference in magnitude between the MS-TO and the MS-saddle does not provide absolute ages with better accuracy than other methods like the MS-fitting
The Gaia-ESO Survey : The analysis of high-resolution UVES spectra of FGK-type stars
Date of Acceptance: 01/09/2014Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 105 stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.Peer reviewe
The two metallicity groups of the globular cluster M22: a chemical perspective
We present a detailed chemical composition analysis of 35 red giant stars in
the globular cluster M22. High resolution spectra for this study were obtained
at five observatories, and analyzed in a uniform manner. We have determined
abundances of representative light proton-capture, alpha, Fe-peak and
neutron-capture element groups. Our aim is to better understand the peculiar
chemical enrichment history of this cluster, in which two stellar groups are
characterized by a different content in iron, neutron capture elements Y, Zr
and Ba, and alpha element Ca. The principal results of this study are: (i)
substantial star-to-star metallicity scatter (-2.0<[Fe/H]<-1.6); (ii)
enhancement of s-process/r-process neutron-capture abundance ratios in a
fraction of giants, positively correlated with metallicity; (iii) sharp
separation between the s-process rich and s-process poor groups by [La/Eu]
ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H],
suggesting that this element also has a significant s-process component; and
(v) presence of Na-O and C-N anticorrelations in both the stellar groups.Comment: 26 pages, 19 figures. Accepted for publication in A&
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