131 research outputs found
Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen
Horizontal Branch stars belong to an advanced stage in the evolution of the
oldest stellar galactic population, occurring either as field halo stars or
grouped in globular clusters. The discovery of multiple populations in these
clusters, that were previously believed to have single populations gave rise to
the currently accepted theory that the hottest horizontal branch members (the
blue hook stars, which had late helium-core flash ignition, followed by deep
mixing) are the progeny of a helium-rich "second generation" of stars. It is
not known why such a supposedly rare event (a late flash followed by mixing) is
so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal
branch stars 10 , or why the blue hook luminosity range in this massive cluster
cannot be reproduced by models. Here we report that the presence of helium core
masses up to \sim 0.04 solar masses larger than the core mass resulting from
evolution is required to solve the luminosity range problem. We model this by
taking into account the dispersion in rotation rates achieved by the
progenitors, whose premain sequence accretion disc suffered an early disruption
in the dense environment of the cluster's central regions where
second-generation stars form. Rotation may also account for frequent
late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201
An empirical calibration of Lick indices using Milky Way Globular Clusters
To provide an empirical calibration relation in order to convert Lick indices
into abundances for the integrated light of old, simple stellar populations for
a large range in the observed [Fe/H] and [alpha/Fe]. This calibration
supersedes the previously adopted ones because it is be based on the real
abundance pattern of the stars instead of the commonly adopted metallicity
scale derived from the colours. We carried out a long-slit spectroscopic study
of 23 Galactic globular cluster for which detailed chemical abundances in stars
have been recently measured. The line-strength indices, as coded by the Lick
system and by Serven et al. (2005), were measured in low-resolution integrated
spectra of the GC light. The results were compared to average abundances and
abundance ratios in stars taken from the compilation by Pritzl et al. (2005) as
well as to synthetic models. Fe-related indices grow linearly as a function of
[Fe/H] for [Fe/H]>-2. Mg-related indices respond in a similar way to [Mg/H]
variations, however Mgb turns out to be a less reliable metallicity indicator
for [Z/H]<-1.5 . Despite the known Mg overabundance with respect to Fe in GC
stars, it proved impossible to infer a mean [Mg/Fe] for integrated spectra that
correlates with the resolved stars properties, because the sensitivity of the
indices to [Mg/Fe] is smaller at lower metallicities. We present empirical
calibrations for Ca, TiO, Ba and Eu indices as well as the measurements of
H_alpha and NaD.Comment: 16 pages, 15 figures, to appear on A&
A New View of the Circumstellar Environment of SN 1987A
We summarize the analysis of a uniform set of both previously-known and
newly-discovered scattered-light echoes, detected within 30" of SN 1987A in ten
years of optical imaging, and with which we have constructed the most complete
three-dimensional model of the progenitor's circumstellar environment.
Surrounding the SN is a richly-structured bipolar nebula. An outer,
double-lobed ``peanut,'' which we believe is the contact discontinuity between
the red supergiant and main sequence winds, is a prolate shell extending 28 ly
along the poles and 11 ly near the equator. Napoleon's Hat, previously believed
to be an independent structure, is the waist of this peanut, which is pinched
to a radius of 6 ly. Interior, the innermost circumstellar material lies along
a cylindrical hourglass, 1 ly in radius and 4 ly long, which connects to the
peanut by a thick equatorial disk. The nebulae are inclined 41o south and 8o
east of the line of sight, slightly elliptical in cross section, and marginally
offset west of the SN. The 3-D geometry of the three circumstellar rings is
studied, suggesting the equatorial ring is elliptical (b/a<0.98), and spatially
offset in the same direction as the hourglass. Dust-scattering models suggest
that between the hourglass and bipolar lobes: the gas density drops from 1--3
cm^{-3} to >0.03 cm^{-3}; the maximum dust-grain size increases from ~0.2
micron to 2 micron; and the Si:C dust ratio decreases. The nebulae have a total
mass of ~1.7 Msun, yielding a red-supergiant mass loss around 5*10^{-6} Msun
yr^{-1}.Comment: Accepted for publication in ApJ 2/14/05. 16 pages in emualteapj
forma
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
The Shaqadud Archaeological Project (Sudan): exploring prehistoric cultural adaptations in the Sahelian hinterlands
The authors present preliminary results from a new research project based in Jebel Shaqadud, Sudan. Their findings highlight the potential for this region's archaeological record to expand our understanding of the adaptation strategies used by human groups in arid north-east African environments away from rivers and lakes during the Holocene. Furthermore, they present exceptionally early radiocarbon dates that push postglacial human occupation in the eastern Sahel back to the twelfth millennium BP
Social approach in genetically engineered mouse lines relevant to autism
Profound impairment in social interaction is a core symptom of autism, a severe neurodevelopmental disorder. Deficits can include a lack of interest in social contact and low levels of approach and proximity to other children. In this study, a three-chambered choice task was used to evaluate sociability and social novelty preference in five lines of mice with mutations in genes implicated in autism spectrum disorders. Fmr1tm1Cgr/Y (Fmr1â/y) mice represent a model for fragile X, a mental retardation syndrome that is partially co-morbid with autism. We tested Fmr1â/y mice on two genetic backgrounds, C57BL/6J and FVB/N-129/OlaHsd (FVB/129). Targeted disruption of Fmr1 resulted in low sociability on one measure, but only when the mutation was expressed on FVB/129. Autism has been associated with altered serotonin levels and polymorphisms in SLC6A4 (SERT), the serotonin-transporter gene. Male mice with targeted disruption of Slc6a4 displayed significantly less sociability than wildtype controls. Mice with conditional overexpression of Igf-1 (Insulin-like growth factor-1) offered a model for brain overgrowth associated with autism. Igf-1 transgenic mice engaged in levels of social approach similar to wildtype controls. Targeted disruption in other genes of interest, En2 (Engrailed 2) and Dhcr7, was carried on genetic backgrounds that demonstrated low levels of exploration in the choice task, precluding meaningful interpretations of social behavior scores. Overall, results show that loss of Fmr1 or Slc6a4 gene function can lead to deficits in sociability. Findings from the fragile X-model suggest that the FVB/129 background confers enhanced susceptibility to consequences of Fmr1 mutation on social approach
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&
Luminosity and mass functions of the three main sequences of the globular cluster NGC 2808
High-precision HST photometry has recently revealed that the globular cluster
(GC) NGC 2808 hosts a triple main sequence (MS) corresponding to three stellar
populations with different helium abundances. We carried out photometry on
ACS/WFC HST images of NGC 2808 with the main purpose of measuring the
luminosity function (LF) of stars in the three different MSs, and the binary
fraction in the cluster. We used isochrones to transform the observed LFs into
mass functions (MFs). We estimated that the fraction of binary systems in NGC
2808 is f_bin ~0.05, and find that the three MSs have very similar LFs. The
slopes of the corresponding MFs are alpha=-1.2+/-0.3 for the red MS, alpha
=-0.9+/-0.3 for the middle MS, and alpha = -0.9+/-0.4 for the blue one, the
same, to within the errors. There is marginal evidence of a MF flattening for
masses M<~0.6 M_SUN for the the reddest (primordial) MS. These results
represent the first direct measurement of the present day MF and LF in distinct
stellar populations of a GC, and provide constraints on models of the formation
and evolution of multiple generations of stars in these objects.Comment: 16 pages, 17 figures, accepted for pubblication in A&
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then 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 about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
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.Comment: In press (The Astronomy and Astrophysics Review
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