3,244 research outputs found
Models for the lithium abundances of multiple populations in globular clusters and the possible role of the Big Bang lithium
Globular cluster stars show chemical abundance patterns typical of hot-CNO
processing. Lithium is easily destroyed by proton capture in stellar
environments, so its abundance may be crucial to discriminate among different
models proposed to account for multiple populations. In order to reproduce the
observed O-Na anticorrelation and other patterns typical of multiple
populations, the formation of second generation stars must occur from the
nuclearly processed stellar ejecta, responsible of the chemical anomalies,
diluted with pristine gas having the composition of first generation stars. The
lithium abundance in the unprocessed gas -which is very likely to be equal to
the lithium abundance emerging from the Big Bang- affects the lithium chemical
patterns among the cluster stars. This paper focuses on a scenario in which
processed gas is provided by asymptotic giant branch (AGB) stars. We examine
the predictions of this scenario for the lithium abundances of multiple
populations. We study the role of the non-negligible lithium abundance in the
ejecta of massive AGB (A(Li)~2), and, at the same time, we explore how our
models can constrain the extremely large ---and very model dependent--- lithium
yields predicted by recent super--AGB models. We show that the super--AGB
yields may be tested by examining the lithium abundances in a large set of blue
main sequence stars in wCen and/or NGC2808. In addition, we examine the
different model results obtained by assuming for the pristine gas either the
Big Bang abundance predicted by the standard models (A(Li)=2.6-2.7), or the
abundance detected at the surface of population II stars (A(Li)=2.2-2.3). Once
a chemical model is well constrained, the O--Li distribution could perhaps be
used to shed light on the primordial lithium abundance
Variations in the lithium abundances of turn off stars in the globular cluster 47 Tuc
aims: Our aim is to determine Li abundances in TO stars of the Globular
Cluster 47 Tuc and test theories about Li variations among TO stars. method: We
make use of high resolution (R~ 43000), high signal-to-noise ratio (S/N=50--70)
spectra of 4 turn off (TO) stars obtained with the UVES spectrograph at the
8.2m VLT Kueyen telescope. results: The four stars observed, span the range
1.6<~A(Li)} <~ 2.14, providing a mean A(Li) = 1.84 with a standard deviation of
0.25 dex. When coupled with data of other two TO stars of the cluster,
available in the literature, the full range in Li abundances observed in this
cluster is 1.6<~A(Li)<~ 2.3. The variation in A(Li) is at least 0.6 dex (0.7
dex considering also the data available in the literature) and the scatter is
six times larger than what expected from the observational error. We claim that
these variations are real. A(Li) seems to be anti-correlated with A(Na) exactly
as observed in NGC 6752. No systematic error in our analysis could produce such
an anti-correlation. conclusions: Na production through p captures on 22Ne at
temperatures in excess of 3x10^7 K and the contemporary Li destruction could
result in this anti-correlation. However such nuclear processing cannot have
taken place in the stars themselves, which do not reach such high temperatures,
even at their centre. This points towards the processing in a previous
generation of stars. The low N/O ratios in the observed stars and the apparent
lack of correlation between N an Li abundances, place a strong constraint on
the properties of this previous generation. Our results indicate a different
behaviour among the Globular Clusters so far studied as far as the abundance
patterns are concerned.Comment: recommended for publication in A&A by the managing associate Edito
Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
A recent survey of a large sample of Galactic intermediate-mass (>3 Msun)
asymptotic giant branch (AGB) stars shows that they exhibit large
overabundances of rubidium (Rb) up to 100--1000 times solar. These observations
set constraints on our theoretical notion of the slow neutron capture process
(s process) that occurs inside intermediate-mass AGB stars. Lithium (Li)
abundances are also reported for these stars. In intermediate-mass AGB stars,
Li can be produced by proton captures occuring at the base of the convective
envelope. For this reason the observations of Rb, Zr, and Li set complementary
constraints on different processes occurring in the same stars. We present
predictions for the abundances of Rb, Zr, and Li as computed for the first time
simultaneously in intermediate-mass AGB star models and compare them to the
current observational constraints. We find that the Rb abundance increases with
increasing stellar mass, as is inferred from observations but we are unable to
match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing
zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source
yields significant enhancements in the Rb abundance. However this leads to Zr
abundances that exceed the upper limits of the current observational
constraints. If the third dredge-up (TDU) efficiency remains as high during the
final stages of AGB evolution as during the earlier stages, we can match the
lowest values of the observed Rb abundance range. We predict large variations
in the Li abundance, which are observed. Finally, the predicted Rb production
increases with decreasing metallicity, in qualitative agreement with
observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008
and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the
observed abundances.Comment: 11 pages, 7 figures, accepted for publication on Astronomy &
Astrophysic
Lithium during the AGB evolution in young open clusters of the Large Magellanic cloud
We present the results of mid-resolution spectroscopy in the LiI 6708 AA
spectral region of Asymptotic Giant Branch (AGB) stars belonging to young open
clusters of the Large Magellanic Cloud. Most stars belong to the clusters NGC
1866 and NGC 2031, which have an age of ~ 150 Myr. Lithium lines of different
strength are detected in the spectra of stars evolving along the AGB, not
always in agreement with theoretical predictions. We also analyze the infrared
luminosities (ISOCAM data) of these stars, to discuss if their evolutionary
phase precedes or follows the lithium production stage.Comment: 10 pages, 6 figure
Models of hydrostatic magnetar atmospheres at high luminosities
We investigate the possibility of Photospheric Radius Expansion (PRE) during
magnetar bursts. Identification of PRE would enable a determination of the
magnetic Eddington limit (which depends on field strength and neutron star mass
and radius), and shed light on the burst mechanism. To do this we model
hydrostatic atmospheres in a strong radial magnetic field, determining both
their maximum extent and photospheric temperatures. We find that
spatially-extended atmospheres cannot exist in such a field configuration:
typical maximum extent for magnetar-strength fields is ~10 m (as compared to
200 km in the non-magnetic case). Achieving balance of gravitational and
radiative forces over a large range of radii, which is critical to the
existence of extended atmospheres, is rendered impossible in strong fields due
to the dependence of opacities on temperature and field strength. We conclude
that high luminosity bursts in magnetars do not lead to expansion and cooling
of the photosphere, as in the non-magnetic case. We also find the maximum
luminosity that can propagate through a hydrostatic magnetar atmosphere to be
lower than previous estimates. The proximity and small extent of the
photospheres associated with the two different polarization modes also calls
into question the interpretation of two blackbody fits to magnetar burst
spectra as being due to extended photospheres.Comment: Accepted for publication in MNRAS. 14 pages, 6 figures, 2 table
Individual Tree Detection in Large-Scale Urban Environments using High-Resolution Multispectral Imagery
We introduce a novel deep learning method for detection of individual trees
in urban environments using high-resolution multispectral aerial imagery. We
use a convolutional neural network to regress a confidence map indicating the
locations of individual trees, which are localized using a peak finding
algorithm. Our method provides complete spatial coverage by detecting trees in
both public and private spaces, and can scale to very large areas. We performed
a thorough evaluation of our method, supported by a new dataset of over 1,500
images and almost 100,000 tree annotations, covering eight cities, six climate
zones, and three image capture years. We trained our model on data from
Southern California, and achieved a precision of 73.6% and recall of 73.3%
using test data from this region. We generally observed similar precision and
slightly lower recall when extrapolating to other California climate zones and
image capture dates. We used our method to produce a map of trees in the entire
urban forest of California, and estimated the total number of urban trees in
California to be about 43.5 million. Our study indicates the potential for deep
learning methods to support future urban forestry studies at unprecedented
scales
Baby-Led Weaning: The Evidence to Date
Purpose of ReviewInfants are traditionally introduced to solid foods using spoon-feeding of specially prepared infant foods.Recent FindingsHowever, over the last 10–15 years, an alternative approach termed ‘baby-led weaning’ has grown in popularity. This approach involves allowing infants to self-feed family foods, encouraging the infant to set the pace and intake of the meal. Proponents of the approach believe it promotes healthy eating behaviour and weight gain trajectories, and evidence is starting to build surrounding the method. This review brings together all empirical evidence to date examining behaviours associated with the approach, its outcomes and confounding factors.SummaryOverall, although there is limited evidence suggesting that a baby-led approach may encourage positive outcomes, limitations of the data leave these conclusions weak. Further research is needed, particularly to explore pathways to impact and understand the approach in different contexts and populations
Lithium abundance in the globular cluster M4: from the Turn-Off to the RGB Bump
We present Li and Fe abundances for 87 stars in the GC M4,obtained with
GIRAFFE high-resolution spectra. The targets range from the TO up to the RGB
Bump. The Li abundance in the TO stars is uniform, with an average value
A(Li)=2.30+-0.02 dex,consistent with the upper envelope of Li content measured
in other GCs and in the Halo stars,confirming also for M4 the discrepancy with
the primordial Li abundance predicted by WMAP+BBNS. The iron content of M4 is
[Fe/H]=-1.10+-0.01 dex, with no systematic offsets between dwarf and giant
stars.The behaviour of the Li and Fe abundance along the entire evolutionary
path is incompatible with models with atomic diffusion, pointing out that an
additional turbulent mixing below the convective region needs to be taken into
account,able to inhibit the atomic diffusion.The measured A(Li) and its
homogeneity in the TO stars allow to put strong constraints on the shape of the
Li profile inside the M4 TO stars. The global behaviour of A(Li) with T_{eff}
can be reproduced with different pristine Li abundances, depending on the kind
of adopted turbulent mixing.One cannot reproduce the global trend starting from
the WMAP+BBNS A(Li) and adopting the turbulent mixing described by Richard et
al.(2005) with the same efficiency used by Korn et al.(2006) to explain the Li
content in NGC6397. Such a solution is not able to well reproduce
simultaneously the Li abundance observed in TO and RGB stars.Otherwise,
theWMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing
able to reach deeper stellar regions where the Li is burned. The cosmological
Li discrepancy cannot be easily solved with the present,poor understanding of
the turbulence in the stellar interiors and a future effort to well understand
the true nature of this non-canonical process is needed.Comment: Accepted for publication in the MNRA
The Palomar/Keck Adaptive Optics Survey of Young Solar Analogs: Evidence for a Universal Companion Mass Function
We present results from an adaptive optics survey for substellar and stellar
companions to Sun-like stars. The survey targeted 266 F5-K5 stars in the 3Myr
to 3Gyr age range with distances of 10-190pc. Results from the survey include
the discovery of two brown dwarf companions (HD49197B and HD203030B), 24 new
stellar binaries, and a triple system. We infer that the frequency of
0.012-0.072Msun brown dwarfs in 28-1590AU orbits around young solar analogs is
3.2% (+3.1%,-2.7%; 2sigma limits). The result demonstrates that the deficiency
of substellar companions at wide orbital separations from Sun-like stars is
less pronounced than in the radial velocity "brown dwarf desert." We infer that
the mass distribution of companions in 28-1590AU orbits around solar-mass stars
follows a continuous dN/dM_2 ~ M_2^(-0.4) relation over the 0.01-1.0Msun
secondary mass range. While this functional form is similar to that for
<0.1Msun isolated objects, over the entire 0.01-1.0Msun range the mass
functions of companions and of isolated objects differ significantly. Based on
this conclusion and on similar results from other direct imaging and radial
velocity companion surveys in the literature, we argue that the companion mass
function follows the same universal form over the entire range between 0-1590AU
in orbital semi-major axis and 0.01-20Msun in companion mass. In this context,
the relative dearth of substellar versus stellar secondaries at all orbital
separations arises naturally from the inferred form of the companion mass
function.Comment: Final version accepted by ApJ Supplements. 50 pages, including 12
tables + 16 figures. Version with full tables available at
http://www.astro.sunysb.edu/metchev/PUBLICATIONS/cmf.pd
High Resolution X-ray Spectroscopy of the Post-T Tauri Star PZ Tel
We present an analysis of the Chandra High Energy Transmission Grating
Spectrometer observation of the rapidly rotating P_(rot)=0.94 d post T Tauri
(~20 Myr old) star PZ Telescopii, in the Tucana association. Using two
different methods we have derived the coronal emission measure distribution,
em(T), and chemical abundances. The em(T) peaks at log T = 6.9 and exhibits a
significant emission measure at temperatures log T > 7. The coronal abundances
are generally ~0.5 times the solar photospheric values that are presumed fairly
representative of the composition of the underlying star. A minimum in
abundance is seen at a first ionization potential (FIP) of 7-8 eV, with
evidence for higher abundances at both lower and higher FIP, similar to
patterns seen in other active stars. From an analysis of the He-like triplet of
Mg XI we have estimated electron densities of ~10^(12)-10^(13) cm^(-3). All the
coronal properties found for PZ Tel are much more similar to those of AB Dor,
which is slightly older than PZ Tel, than to those of the younger T Tauri star
TW Hya. These results support earlier conclusions that the soft X-ray emission
of TW Hya is likely dominated by accretion activity rather than by a
magnetically-heated corona. Our results also suggest that the coronae of
pre-main sequence stars rapidly become similar to those of older active
main-sequence stars soon after the accretion stage has ended.Comment: 15 pages, 8 Postscript figures, accepted for publication in
Astrophysical Journa
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