Slow and fast components in the X-ray light curves of Gamma-Ray Bursts

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. We present a temporal and spectral study of the light curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the Wide Field Cameras on board BeppoSAX. For some events the time profiles are characterized by peaks superposed on a slowly evolving pedestal, which in some cases becomes less apparent at higher energies. We describe this behaviour with the presence of two components (slow and fast) having different variability time scales. We modelled the time evolution of slow components by means of an analytical function able to describe asymmetric rising and decaying profiles. The residual light curves, after the subtraction of the slow components, generally show structures more similar to the original curves in the highest energy band. Spectral study of these two components was performed evaluating their hardness ratios, used also to derive photon indices. Slow components are found generally softer than the fast ones suggesting that their origin is likely different. Being typical photon indices lower than those of the afterglows there is no evidence that the emission processes are similar. Another interesting possibility is that slow components can be related to the presence of a hot photosphere having a thermal spectrum with kT around a few keV superposed to a rapid variable non-thermal emission of the fast component.Comment: 16 pages, 20 figures (18 color, 2 B&W), accepted for publication in Astronomy and Astrophysic

The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391

We have obtained new photometry and intermediate resolution ($\Delta \lambda = 2.7$ \AA\ ) spectra of 19 of these objects (14.9 $\le$ $I_c$ $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their $VRI$ photometry, spectral types, radial velocity, and H$\alpha$ emission strengths. Higher S/N spectra were obtained for 8 of these probable cluster members in order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain an estimate of the cluster's age. One of these 8 stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium depletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical model predictions, we derive an age for IC~2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster ($\sim$35 Myr) this result for IC~2391 is comparable those recently derived for the Pleiades and Alpha Persei clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.Comment: ApJ Letters, acccepte

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 $\sim$0.06 mag and a second one, made of about 10% of stars, a little fainter (by $\sim$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 $\Delta$Y$\sim$0.027. 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 $\sim$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

Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars

Probing O-enrichment in C-rich dust planetary nebulae

The abundance of O in planetary nebulae (PNe) has been historically used as a metallicity indicator of the interstellar medium (ISM) where they originated; e.g., it has been widely used to study metallicity gradients in our Galaxy and beyond. However, clear observational evidence for O self enrichment in low-metallicity Galactic PNe with C-rich dust has been recently reported. Here we report asymptotic giant branch (AGB) nucleosynthesis predictions for the abundances of the CNO elements and helium in the metallicity range Zsun/4 < Z < 2Zsun. Our AGB models, with diffusive overshooting from all the convective borders, predict that O is overproduced in low-Z low-mass (~1-3 Msun) AGB stars and nicely reproduce the recent O overabundances observed in C-rich dust PNe. This confirms that O is not always a good proxy of the original ISM metallicity and another chemical elements such as Cl or Ar should be used instead. The production of oxygen by low-mass stars should be thus considered in galactic-evolution models.Comment: Accepted for publication in MNRAS Letters (5 pages, 1 figure, and 1 table

The early evolution of Globular Clusters: the case of NGC 2808

Enhancement and spread of helium among globular cluster stars have been recently suggested as a way to explain the horizontal branch blue tails, in those clusters which show a primordial spread in the abundances of CNO and other elements involved in advanced CNO burning (D'Antona et al. 2002). In this paper we examine the implications of the hypothesis that, in many globular clusters, stars were born in two separate events: an initial burst (first generation), which gives origin to probably all high and intermediate mass stars and to a fraction of the cluster stars observed today, and a second, prolonged star formation phase (second generation) in which stars form directly from the ejecta of the intermediate mass stars of the first generation. In particular, we consider in detail the morphology of the horizontal branch in NGC 2808 and argue that it unveils the early cluster evolution, from the birth of the first star generation to the end of the second phase of star formation. This framework provides a feasible interpretation for the still unexplained dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the RR Lyr region to the gap in the helium content between the red clump, whose stars are considered to belong to the first stellar generation and have primordial helium, and the blue side of the horizontal branch, whose minimum helium content reflects the helium abundance in the smallest mass (~4Msun)contributing to the second stellar generation. This scenario provides constraints on the required Initial Mass Function, in a way that a great deal of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa

Algebraic Characterization of Vector Supersymmetry in Topological Field Theories

An algebraic cohomological characterization of a class of linearly broken Ward identities is provided. The examples of the topological vector supersymmetry and of the Landau ghost equation are discussed in detail. The existence of such a linearly broken Ward identities turns out to be related to BRST exact antifield dependent cocycles with negative ghost number.Comment: 30 pages, latex2e file, subm. to Journ. of Math. Phy

Galactic planetary nebulae with precise nebular abundances as a tool to understand the evolution of asymptotic giant branch stars

We present nucleosynthesis predictions (HeCNOCl) from asymptotic giant branch (AGB) models, with diffusive overshooting from all the convective borders, in the metallicity range Z/4 < Z < 2Zsun. They are compared to recent precise nebular abundances in a sample of Galactic planetary nebulae (PNe) that is divided among double-dust chemistry (DC) and oxygen-dust chemistry (OC) according to the infrared dust features. Unlike the similar subsample of Galactic carbon-dust chemistry PNe recently analysed by us, here the individual abundance errors, the higher metallicity spread, and the uncertain dust types/subtypes in some PNe do not allow a clear determination of the AGB progenitor masses (and formation epochs) for both PNe samples; the comparison is thus more focussed on a object-by-object basis. The lowest metallicity OC PNe evolve from low-mass (~1 Msun) O-rich AGBs, while the higher metallicity ones (all with uncertain dust classifications) display a chemical pattern similar to the DC PNe. In agreement with recent literature, the DC PNe mostly descend from high-mass (M > 3.5 Msun) solar/supersolar metallicity AGBs that experience hot bottom burning (HBB), but other formation channels in low-mass AGBs like extra mixing, stellar rotation, binary interaction, or He pre-enrichment cannot be disregarded until more accurate C/O ratios would be obtained. Two objects among the DC PNe show the imprint of advanced CNO processing and deep second dredge-up, suggesting progenitors masses close to the limit to evolve as core collapse supernovae (above 6 Msun). Their actual C/O ratio, if confirmed, indicate contamination from the third dredge-up, rejecting the hypothesis that the chemical composition of such high-metallicity massive AGBs is modified exclusively by HBB.Comment: Accepted for publication in MNRAS (11 pages, 3 figures, and 2 tables

APPLICAZIONE DI TECNICHE DI COLLASSAMENTO IPOCENTRALE A DUE SEQUENZE SISMICHE DI BASSA ENERGIA NELL’APPENNINO MERIDIONALE

In this study we analyse two recent low energy (Mdmax = 4.1) seismic sequences (1990-1997) that affected the northern sector (Sannio-Benevento area) of the Southern Apennines chain. We applied the Best Estimate Method to earthquake locations in order to constrain the position and geometry of the seismogenic structures. The most striking features of the obtained fault geometries indicate that earthquakes of the 1990 Benevento sequence align along a NW-SE striking structure, while the earthquakes of the 1997 Sannio sequence outline a NNE-SSW striking structure. The southernmost NW-SE structure dips towards the NE, is characterized by a conjugate fault arrangement and overlies the fault responsible for a larger historical earthquake (Iomax = XI MCS, 1688 earthquake). Available focal mechanisms from earthquakes that occurred on the recognized NW-SE and NE-SW faults are consistent with dip-slip normal solutions. This suggests the occurrence of coexisting NW-SE and NE-SW extension in the Southern Apennines

Group-theoretical approach to a non-central extension of the Kepler-Coulomb problem

Bound and scattering states of a non-central extension of the three-dimensional Kepler-Coulomb Hamiltonian are worked out analytically within the framework of the potential groups of the problem, SO(7) for bound states and SO(6,1) for scattering states. In the latter case, the S matrix is calculated by the method of intertwining operators.Comment: 12 pages, to appear in J. Phys. A : Math. Theo