13,248 research outputs found
Hard Cosmic Ray Sea in the Galactic Center: a consistent interpretation of H.E.S.S. and Fermi-LAT -ray data
We present a novel interpretation of the gamma-ray diffuse emission measured
by H.E.S.S. in the Galactic Center (GC) region and the Galactic ridge. Our
starting base is an updated analysis of PASS8 Fermi-LAT data, which allows to
extend down to few GeV the spectra measured by H.E.S.S. and to infer the
primary CR radial distribution above 100 GeV. We compare those results with a
CR transport model assuming a harder scaling of the diffusion coefficient with
rigidity in the inner Galaxy. Such a behavior reproduces the radial dependence
of the CR spectral index recently inferred from Fermi-LAT measurements in the
inner GP. We find that, in this scenario, the bulk of the Galactic ridge
emission can be naturally explained by the interaction of the diffuse,
steady-state Galactic CR sea interacting with the gas present in the Central
molecular zone. The evidence of a GC PeVatron is significantly weaker than that
inferred adopting a conventional (softer) CR sea.Comment: Oral contribution to the International Cosmic Ray Conference (ICRC
2017), 12-20 July 2017, Bexco, Busan, Kore
The two gap transitions in GeSn: effect of non-substitutional complex defects
The existence of non-substitutional -Sn defects in GeSn
was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B
81, 155204 (2010)], which established that although most Sn enters
substitutionally (-Sn) in the Ge lattice, a second significant fraction
corresponds to the Sn-vacancy defect complex in the split-vacancy configuration
( -Sn ), in agreement with our previous theoretical study [Ventura et
al., Phys. Rev. B 79, 155202 (2009)]. Here, we present our electronic structure
calculation for GeSn, including substitutional -Sn as
well as non-substitutional -Sn defects. To include the presence of
non-substitutional complex defects in the electronic structure calculation for
this multi-orbital alloy problem, we extended the approach for the purely
substitutional alloy by Jenkins and Dow [Jenkins and Dow, Phys. Rev. B 36, 7994
(1987)]. We employed an effective substitutional two-site cluster equivalent to
the real non-substitutional -Sn defect, which was determined by a
Green's functions calculation. We then calculated the electronic structure of
the effective alloy purely in terms of substitutional defects, embedding the
effective substitutional clusters in the lattice. Our results describe the two
transitions of the fundamental gap of GeSn as a function of the
total Sn-concentration: namely from an indirect to a direct gap, first, and the
metallization transition at higher . They also highlight the role of
-Sn in the reduction of the concentration range which corresponds to the
direct-gap phase of this alloy, of interest for optoelectronics applications.Comment: 11 pages, 9 Figure
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
The helium spread in the Globular cluster 47 Tuc
Spectroscopy has shown the presence of the CN band dicothomy and the Na-O
anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc,
otherwise considered a "normal" prototype of high metallicity clusters from the
photometric analysis. Very recently, the re-analysis of a large number of
archival HST data of the cluster core has been able to put into evidence the
presence of structures in the Sub Giant Branch: it has a brighter component
with a spread in magnitude by 0.06 mag and a second one, made of about
10% of stars, a little fainter (by 0.05 mag). These data also show that
the Main Sequence of the cluster has an intrinsic spread in color which, if
interpreted as due to a small spread in helium abundance, suggests
Y0.027. In this work we examine in detail whether the Horizontal
Branch morphology and the Sub Giant structure provide further independent
indications that a real --although very small-helium spread is present in the
cluster. We re--analyze the HST archival data for the Horizontal Branch of 47
Tuc, obtaining a sample of 500 stars with very small photometric errors,
and build population synthesis based on new models to show that its particular
morphology can be better explained by taking into account a spread in helium
abundance of 2% in mass. The same variation in helium is able to explain the
spread in luminosity of the Sub Giant Branch, while a small part of the second
generation is characterized by a small C+N+O increase and provides an
explanation for the fainter Sub Giant Branch. We conclude that three
photometric features concur to form the paradigm that a small but real helium
spread is present in a cluster that has no spectacular evidence for multiple
populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010
May 19; in original form 2010 February 9. 7 pages and 3 figures. No table
A test for asymptotic giant branch evolution theories: Planetary Nebulae in the Large Magellanic Cloud
We used a new generation of asymptotic giant branch (AGB) stellar models that
include dust formation in the stellar winds to find the links between
evolutionary models and the observed properties of a homogeneous sample of
Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the
evolutionary yields of elements such as CNO and the corresponding observed
chemical abundances is a powerful tool to shed light on evolutionary processes
such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the
occurrence of HBB is needed to interpret the nitrogen-enriched (log(N/H)+12>8)
PNe. In particular, N-rich PNe with the lowest carbon content are nicely
reproduced by AGB models of mass M >=6 Mo, whose surface chemistry reflects the
pure effects of HBB. PNe with log(N/H)+12<7.5 correspond to ejecta of stars
that have not experienced HBB, with initial mass below about 3 Mo. Some of
these stars show very large carbon abundances, owing to the many TDU episodes
experienced. We found from our LMC PN sample that there is a threshold to the
amount of carbon accumulated at AGB surfaces, log(C/H)+12<9. Confirmation of
this constraint would indicate that, after the C-star stage is reached,AGBs
experience only a few thermal pulses, which suggests a rapid loss of the
external mantle, probably owing to the effects of radiation pressure on
carbonaceous dust particles present in the circumstellar envelope. The
implications of these findings for AGB evolution theories and the need to
extend the PN sample currently available are discussed.Comment: 12 pages, 4 figures, 1 table, accepted for publication in MNRAS (2015
July 13; in original form 2015 June 9
Planetary Nebulae in the Small Magellanic Cloud
We analyse the planetary nebulae (PNe) population of the Small Magellanic
Cloud (SMC), based on evolutionary models of stars with metallicities in the
range and mass , evolved through the asymptotic giant branch (AGB) phase. The models
used account for dust formation in the circumstellar envelope. To characterise
the PNe sample of the SMC, we compare the observed abundances of the various
species with the final chemical composition of the AGB models: this study
allows us to identify the progenitors of the PNe observed, in terms of mass and
chemical composition. According to our interpretation, most of the PNe descend
from low-mass () stars, which become carbon rich, after
experiencing repeated third dredge-up episodes, during the AGB phase. A
fraction of the PNe showing the signature of advanced CNO processing are
interpreted as the progeny of massive AGB stars, with mass above , undergoing strong hot bottom burning. The differences with the
chemical composition of the PNe population of the Large Magellanic Cloud (LMC)
is explained on the basis of the diverse star formation history and
age-metallicity relation of the two galaxies. The implications of the present
study for some still highly debated points regarding the AGB evolution are also
commented.Comment: Accepted for publication in MNRAS, 11 pages, 4 figure
Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally-pulsing phase
We report the first spectroscopic identification of massive Galactic
asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP)
phase. These stars are the most Li-rich massive AGBs found to date, super
Li-rich AGBs with logE(Li)~3-4. The high Li overabundances are accompanied by
weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our
observations with the most recent hot bottom burning (HBB) and s-process
nucleosynthesis models confirms that HBB is strongly activated during the first
TPs but the 22Ne neutron source needs many more TP and third dredge-up episodes
to produce enough Rb at the stellar surface. We also show that the short-lived
element Tc, usually used as an indicator of AGB genuineness, is not detected in
massive AGBs which is in agreement with the theoretical predictions when the
22Ne neutron source dominates the s-process nucleosynthesis.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures and 1 table); final version (language corrected
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