Hard Cosmic Ray Sea in the Galactic Center: a consistent interpretation of H.E.S.S. and Fermi-LAT γ\gamma-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

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 $10^{-3} \leq Z \leq 4\times 10^{-3}$ and mass $0.9 M\odot < M < 8M\odot$, 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 ($M < 2 M\odot$) 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 $\sim 6 M\odot$, 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

Normal state electronic properties of LaO1−x_{1-x}Fx_{x}BiS2_{2} superconductors

A good description of the electronic structure of BiS$_{2}$-based superconductors is essential to understand their phase diagram, normal state and superconducting properties. To describe the first reports of normal state electronic structure features from angle resolved photoemission spectroscopy (ARPES) in LaO$_{1-x}$F$_{x}$BiS$_{2}$, we used a minimal microscopic model to study their low energy properties. It includes the two effective tight-binding bands proposed by Usui et al [Phys.Rev.B 86, 220501(R)(2012)], and we added moderate intra- and inter-orbital electron correlations related to Bi-($p_{Y}$, $p_{X}$) and S-($p_{Y}$, $p_{X}$) orbitals. We calculated the electron Green's functions using their equations of motion, which we decoupled in second-order of perturbations on the correlations. We determined the normal state spectral density function and total density of states for LaO$_{1-x}$F$_{x}$BiS$_{2}$, focusing on the description of the k-dependence, effect of doping, and the prediction of the temperature dependence of spectral properties. Including moderate electron correlations, improves the description of the few experimental ARPES and soft X-ray photoemission data available for LaO$_{1-x}$F$_{x}$BiS$_{2}$. Our analytical approximation enabled us to calculate the spectral density around the conduction band minimum at $\vec{k}_{0}=(0.45\pi,0.45\pi)$, and to predict the temperature dependence of the spectral properties at different BZ points, which might be verified by temperature dependent ARPES.Comment: 9 figures. Manuscript accepted in Physica B: Condensed Matter on Jan. 25, 201

On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars

The O-rich Asymptotic Giant Branch (AGB) stars experience strong mass loss with efficient dust condensation and they are major sources of dust in the interstellar medium. Alumina dust (Al$_2$O$_3$) is an important dust component in O-rich circumstellar shells and it is expected to be fairly abundant in the winds of the more massive and O-rich AGB stars. By coupling AGB stellar nucleosynthesis and dust formation, we present a self-consistent exploration on the Al$_2$O$_3$ production in the winds of AGB stars with progenitor masses between $\sim$3 and 7 M$_{\odot}$ and metallicities in the range 0.0003 $\le$ Z $\le$ 0.018. We find that Al$_2$O$_3$ particles form at radial distances from the centre between $\sim2$ and 4 R$_*$ (depending on metallicity), which is in agreement with recent interferometric observations of Galactic O-rich AGB stars. The mass of Al$_2$O$_3$ dust is found to scale almost linearly with metallicity, with solar metallicity AGBs producing the highest amount (about 10$^{-3}$ M$_{\odot}$) of alumina dust. The Al$_2$O$_3$ grain size decreases with decreasing metallicity (and initial stellar mass) and the maximum size of the Al$_2$O$_3$ grains is $\sim$0.075 $\mu m$ for the solar metallicity models. Interestingly, the strong depletion of gaseous Al observed in the low-metallicity HBB AGB star HV 2576 seems to be consistent with the formation of Al$_2$O$_3$ dust as predicted by our models. We suggest that the content of Al may be used as a mass (and evolutionary stage) indicator in AGB stars experiencing HBB.Comment: 13 pages, 8 figures, accepted for publication in MNRA

The two gap transitions in Ge1−x_{1-x}Snx_x: effect of non-substitutional complex defects

The existence of non-substitutional $\beta$-Sn defects in Ge$_{1-x}$Sn$_{x}$ was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B 81, 155204 (2010)], which established that although most Sn enters substitutionally ($\alpha$-Sn) in the Ge lattice, a second significant fraction corresponds to the Sn-vacancy defect complex in the split-vacancy configuration ( $\beta$-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 Ge$_{1-x}$Sn$_{x}$, including substitutional $\alpha$-Sn as well as non-substitutional $\beta$-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 $\beta$-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 Ge$_{1-x}$Sn$_{x}$ as a function of the total Sn-concentration: namely from an indirect to a direct gap, first, and the metallization transition at higher $x$. They also highlight the role of $\beta$-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