562 results on '"Raffaele, G."'
Search Results
2. Is the topological surface state floating on top of a thick lead layer? The case of the Pb/Bi2Se3 interface
- Author
-
De Luca, Oreste, Shvets, Igor A., Eremeev, Sergey V., Aliev, Ziya S., Kopciuszynski, Marek, Barinov, Alexey, Ronci, Fabio, Colonna, Stefano, Chulkov, Evgueni V., Agostino, Raffaele G., Papagno, Marco, and Flammini, Roberto
- Subjects
Condensed Matter - Materials Science ,Condensed Matter - Other Condensed Matter - Abstract
The puzzling question about the floating of the topological surface state on top of a thick Pb layer, has now possibly been answered. A study of the interface made by Pb on Bi2Se3 for different temperature and adsorbate coverage condition, allowed us to demonstrate that the evidence reported in the literature can be related to the surface diffusion phenomenon exhibited by the Pb atoms, which leaves the substrate partially uncovered. Comprehensive density functional theory calculations show that despite the specific arrangement of the atoms at the interface, the topological surface state cannot float on top of the adlayer but rather tends to move inward within the substrate., Comment: 9 pages, 5 figures
- Published
- 2023
3. Unveiling tumor invasiveness: enhancing cancer diagnosis with phase-contrast microtomography for 3D virtual histology
- Author
-
Donato, Sandro, Agostino, Raffaele G., Arana Peña, Lucia M., Arfelli, Fulvia, Brombal, Luca, Longo, Renata, Martellani, Fulvia, Romano, Andrea, Rosano, Ida, Saccomano, Giulia, Tromba, Giuliana, and Bonazza, Deborah
- Published
- 2024
- Full Text
- View/download PDF
4. Hourly operation of a regulated lake via Model Predictive Control
- Author
-
Cestari, Raffaele G., Castelletti, Andrea, and Formentin, Simone
- Subjects
Electrical Engineering and Systems Science - Systems and Control - Abstract
The optimal operation of regulated lakes is a challenging task involving conflicting objectives, ranging from controlling lake levels to avoid floods and low levels to water supply downstream. The traditional approach to operation policy design is based on an offline optimization, where a feedback control rule mapping lake storage into daily release decisions is identified over a set of observational data. In this paper, we propose a receding-horizon policy for a more frequent, online regulation of the lake level, and we discuss its tuning as compared to benchmark approaches. As side contributions, we provide a daily alternative based on the same rationale, and we show that this is still valid under some assumptions on the water inflow. Numerical simulations are used to show the effectiveness of the proposed approach. We demonstrate the approach on the regulated lake Como, Italy., Comment: 6 pages, 5 figures, IFAC 2nd-CMWRS 2022, Milan
- Published
- 2022
5. X-ray computed $\mu$-tomography for the characterization of optical fibers
- Author
-
Ferraro, Mario, Crocco, Maria C., Mangini, Fabio, Jonard, Maxime, Sangiovanni, Francesco, Zitelli, Mario, Filosa, Raffaele, Beltrano, Joseph J., De Luca, Antonio, Barberi, Riccardo C., Agostino, Raffaele G., Couderc, Vincent, Wabnitz, Stefan, and Formoso, Vincenzo
- Subjects
Physics - Optics ,Condensed Matter - Materials Science - Abstract
In spite of their ubiquitous applications, the characterization of glass fibers by means of all-optical techniques is still facing some limitations. Recently, X-ray absorption has been proposed as a method for visualizing the inner structure of both standard and microstructure optical fibers. Here, we exploit X-ray absorption as nondestructive technique for the characterization of optical glass fibers. Starting from absorption contrast X-ray computed micro-tomography measurements, we obtain information about the spatial profile of the fiber refractive index at optical frequencies. We confirm the validity of our approach by comparing its results with complementary characterization techniques, based on electron spectroscopy or multiphoton microscopy.
- Published
- 2022
6. Recent advances in sustainable and efficient hydrogen storage nanomaterials
- Author
-
Attia, Nour F., Elashery, Sally E.A., Nour, Mohamed A., Policicchio, Alfonso, Agostino, Raffaele G., Abd-Ellah, Marwa, Jiang, Saihua, and Oh, Hyunchul
- Published
- 2024
- Full Text
- View/download PDF
7. The efficiency of bio-char as bitumen modifier
- Author
-
Valeria Loise, Pietro Calandra, Alfonso Policicchio, Luigi Madeo, Cesare Oliviero Rossi, Michele Porto, Abraham Abe, Raffaele G. Agostino, and Paolino Caputo
- Subjects
Biocompatible additive ,Chemical structure ,Bitumen ,Char ,Activated carbon ,Rheology ,Science (General) ,Q1-390 ,Social sciences (General) ,H1-99 - Abstract
Improving the mechanical properties of bitumen is an important goal for road pavements design. For this reason, new compounds are now being sought for testing as bitumen modifiers.In this work, the authors studied the effect that two different chars have on two 50/70 bitumens with different chemical and physical characteristics. A complete morphological, surface and bulk characterization of the two additives was carried out. In addition, rheology, Nuclear Magnetic Resonance (NMR) relaxometry and atomic force microscopy were used to analyze the effect that the two additives exert on the properties of the bitumens. According to the results, the char sample with high porosity could be used as a modifier of mechanical properties, while no rejuvenation effects were observed for either of the two additives tested. In addition, the two additives do not give rise to segregation phenomena.
- Published
- 2024
- Full Text
- View/download PDF
8. The efficiency of bio-char as bitumen modifier
- Author
-
Loise, Valeria, Calandra, Pietro, Policicchio, Alfonso, Madeo, Luigi, Oliviero Rossi, Cesare, Porto, Michele, Abe, Abraham, Agostino, Raffaele G., and Caputo, Paolino
- Published
- 2024
- Full Text
- View/download PDF
9. Nonlinear optical effects in natural topaz
- Author
-
Filosa, Raffaele, Elettivo, Giuseppe S., Ferraro, Mario, Procopio, Salvatore, Nicolino, Antonella, Crocco, Maria C., Beltrano, Joseph J., Barberi, Riccardo C., Formoso, Vincenzo, Guzzi, Rita, De Luca, Antonio, Agrosì, Giovanna, Tempesta, Gioacchino, and Agostino, Raffaele G.
- Published
- 2023
- Full Text
- View/download PDF
10. X-ray computed μ-tomography analysis to evaluate the crack growth in an additive manufactured Ti-6Al-4V alloy sample stressed with in-phase axial and torsional loading
- Author
-
Renzo, Danilo A., Crocco, Maria C., Maletta, Carmine, Pagnotta, Leonardo, Sgambitterra, Emanuele, Berto, Filippo, Furgiuele, Franco, Filosa, Raffaele, Beltrano, Joseph J., Barberi, Riccardo C., Agostino, Raffaele G., and Formoso, Vincenzo
- Published
- 2023
- Full Text
- View/download PDF
11. Hydrogen storage and mobility determined by NMR to various organically functionalized porous silica synthetized by using the post-grafting method
- Author
-
Conte, Giuseppe, Simari, Cataldo, Putz, Ana-Maria, Bonaventura, Carlo Poselle, Porto, Michele, Desiderio, Giovanni, Agostino, Raffaele G., and Policicchio, Alfonso
- Published
- 2023
- Full Text
- View/download PDF
12. Char from Pyrolysis of Waste Tires to Increase Bitumen Performances
- Author
-
Paolino Caputo, Pietro Calandra, Alfonso Policicchio, Giuseppe Conte, Raffaele G. Agostino, Mikolaj Pochylski, Abraham Abe, and Cesare Oliviero Rossi
- Subjects
activated carbon bitumen ,anti-aging effect ,char ,rheology ,waste pyrolysis ,Technology ,Engineering (General). Civil engineering (General) ,TA1-2040 ,Biology (General) ,QH301-705.5 ,Physics ,QC1-999 ,Chemistry ,QD1-999 - Abstract
Road pavement characteristics can be improved by modifying bitumens through addition of fine particles. To avoid environmental issues, attention is recently being paid to bio-materials. In this study, a 50/70 bitumen was modified through the addition of char obtained from the pyrolysis of waste tires. Char addition causes an increase in transition (gel to sol) temperature of up to 4 °C and an increase in rigidity under working conditions (50 °C) of up to about one order of magnitude. The effect of the gas type flowing under the pyrolysis process (CO2, N2 and CO2 + N2) on the mechanical characteristics of the bitumen was also investigated. More marked effects on the rheological performances were found if char coming from pyrolysis under CO2 was used (the gel-to-sol transition temperature is increased by about 4.5 °C) compared to that coming from pyrolysis under N2 (gel-to-sol transition temperature is increased by about 3 °C). The effect is at a maximum for char from CO2 at 3% wt/wt, whereas regarding char from N, a lesser effect is seen with a more uniform (flat) effect as a function of char% and with an increase in gel-to-sol transition temperature of about 3 °C. Coherently, char obtained from pyrolysis under CO2 was found to possess a higher surface area constituted by smaller particles than that coming from pyrolysis under N2. In fact, the BET-specific surface passes 79 m2/g for N2-char to 174 m2/g for CO2-char and the micropore volume fraction increases from 2 to 9. The observed differences can be attributed to the oxidizing environment (CO2) that is more aggressive and reactive in the synthesis phase compared to an inert environment (N2). Char also showed an anti-aging effect, hindering the increase in rigidity typically associated with the aging process. This effect was explained in terms of the compatibility of char with bitumen’s organic nature, and presumably its more effective hosting in a bituminous structure, which is presumably higher when char is produced under CO2 rather than N2. This study quantitatively indicates how a residue derived from the pyrolysis of a waste material can be efficiently re-used to increase the mechanical characteristics of bitumen, accomplishing the recent circular-based needs for environmental protection.
- Published
- 2023
- Full Text
- View/download PDF
13. Aluminium abundances in five discrete stellar populations of the globular cluster NGC 2808
- Author
-
Carretta, Eugenio, Bragaglia, Angela, Lucatello, Sara, Gratton, Raffaele G., D'Orazi, Valentina, and Sollima, Antonio
- Subjects
Astrophysics - Solar and Stellar Astrophysics ,Astrophysics - Astrophysics of Galaxies - Abstract
We observed a sample of 90 red giant branch (RGB) stars in NGC 2808 using FLAMES/GIRAFFE and the high resolution grating with the set up HR21. These stars have previous accurate atmospheric parameters and abundances of light elements. We derived aluminium abundances for them from the strong doublet Al I 8772-8773 Angstrom as in previous works of our group. In addition, we were able to estimate the relative CN abundances for 89 of the stars from the strength of a large number of CN features. When adding self consistent abundances from previous UVES spectra analysed by our team, we gathered [Al/Fe] ratios for a total of 108 RGB stars in NGC 2808. The full dataset of proton-capture elements is used to explore in details the five spectroscopically detected discrete components in this globular cluster. We found that different classes of polluters are required to reproduce the (anti)-correlations among all proton-capture elements in the populations P2, I1, and I2 with intermediate composition. This is in agreement with the detection of lithium in lower RGB second generation stars, requiring at least two kind of polluters. To have chemically homogeneous populations the best subdivision of our sample is into six components, as derived from statistical cluster analysis. By comparing different diagrams [element/Fe] vs [element/Fe] we show for the first time that a simple dilution model is not able to reproduce all the sub-populations in this cluster. Polluters of different masses are required. NGC 2808 is confirmed to be a tough challenge to any scenario for globular cluster formation., Comment: 15 pages, 15 figures, 6 tables; accepted for publication on Astronomy and Astrophysics
- Published
- 2018
- Full Text
- View/download PDF
14. Copper-doped activated carbon from amorphous cellulose for hydrogen, methane and carbon dioxide storage
- Author
-
Conte, Giuseppe, Policicchio, Alfonso, De Luca, Oreste, Rudolf, Petra, Desiderio, Giovanni, and Agostino, Raffaele G.
- Published
- 2022
- Full Text
- View/download PDF
15. Chemical characterization of the globular cluster NGC 5634 associated to the Sagittarius dwarf spheroidal galaxy
- Author
-
Carretta, Eugenio, Bragaglia, Angela, Lucatello, Sara, D'Orazi, Valentina, Gratton, Raffaele G., Donati, Paolo, Sollima, Antonio, and Sneden, Chris
- Subjects
Astrophysics - Solar and Stellar Astrophysics ,Astrophysics - Astrophysics of Galaxies - Abstract
As part of our on-going project on the homogeneous chemical characterization of multiple stellar populations in globular clusters (GCs), we studied NGC 5634, associated to the Sagittarius dwarf spheroidal galaxy, using high-resolution spectroscopy of red giant stars collected with FLAMES@VLT. We present here the radial velocity distribution of the 45 observed stars, 43 of which are member, the detailed chemical abundance of 22 species for the seven stars observed with UVES-FLAMES, and the abundance of six elements for stars observed with GIRAFFE. On our homogeneous UVES metallicity scale we derived a low metallicity [Fe/H]=-1.867 +/-0.019 +/-0.065 dex (+/-statistical +/-systematic error) with sigma=0.050 dex (7 stars). We found the normal anti-correlations between light elements (Na and O, Mg and Al), signature of multiple populations typical of massive and old GCs. We confirm the associations of NGC 5634 to the Sgr dSph, from which the cluster was lost a few Gyr ago, on the basis of its velocity and position and the abundance ratios of alpha and neutron capture elements., Comment: 16 pages, 10 figures, 11 tables; accepted for publication on Astronomy and Astrophysics
- Published
- 2017
- Full Text
- View/download PDF
16. Alkali Metals Adsorbed on Nanoporous Graphene: Charge Transfer and Metallic Phase.
- Author
-
Marchiani, Dario, Frisenda, Riccardo, Mariani, Carlo, Avila, José, Dudin, Pavel, Sueyoshi, Hayato, Jeong, Samuel, Ito, Yoshikazu, De Luca, Oreste, Macchi, Sara, Papagno, Marco, Agostino, Raffaele G., Pacilé, Daniela, and Betti, Maria Grazia
- Published
- 2024
- Full Text
- View/download PDF
17. Hydrogen storage performance of methyl-substituted mesoporous silica with tailored textural characteristics
- Author
-
Policicchio, Alfonso, Putz, Ana-Maria, Conte, Giuseppe, Stelitano, Sara, Bonaventura, Carlo Poselle, Ianăşi, Cătălin, Almásy, László, Wacha, András, Horváth, Zsolt Endre, and Agostino, Raffaele G.
- Published
- 2021
- Full Text
- View/download PDF
18. On the serendipitous discovery of a Li-rich giant in the globular cluster NGC 362
- Author
-
D'Orazi, Valentina, Gratton, Raffaele G., Angelou, George C., Bragaglia, Angela, Carretta, Eugenio, Lattanzio, John C., Lucatello, Sara, Momany, Yazan, and Sollima, Antonio
- Subjects
Astrophysics - Solar and Stellar Astrophysics ,Astrophysics - Astrophysics of Galaxies - Abstract
We have serendipitously identified the first lithium-rich giant star located close to the red giant branch bump in a globular cluster. Through intermediate-resolution FLAMES spectra we derived a lithium abundance of A(Li)=2.55 (assuming local thermodynamical equilibrium), which is extremely high considering the star's evolutionary stage. Kinematic and photometric analysis confirm the object as a member of the globular cluster NGC 362. This is the fourth Li-rich giant discovered in a globular cluster but the only one known to exist at a luminosity close to the bump magnitude. The three previous detections are clearly more evolved, located close to, or beyond the tip of their red giant branch. Our observations are able to discard the accretion of planets/brown dwarfs, as well as an enhanced mass-loss mechanism as a formation channel for this rare object. Whilst the star sits just above the cluster bump luminosity, its temperature places it towards the blue side of the giant branch in the colour-magnitude diagram. We require further dedicated observations to unambiguously identify the star as a red giant: we are currently unable to confirm whether Li production has occurred at the bump of the luminosity function or if the star is on the pre zero-age horizontal branch. The latter scenario provides the opportunity for the star to have synthesised Li rapidly during the core helium flash or gradually during its red giant branch ascent via some extra mixing process., Comment: Accepted for publication in The Astrophysical Journal Letters
- Published
- 2015
- Full Text
- View/download PDF
19. Hourly operation of a regulated lake via Model Predictive Control.
- Author
-
Raffaele G. Cestari, Andrea Castelletti, and Simone Formentin
- Published
- 2022
- Full Text
- View/download PDF
20. Refractive index profiling of multimode specialty optical fibers by absorption contrast X-ray computed microtomography
- Author
-
Ferraro Mario, Crocco Maria C., Mangini Fabio, Filosa Raffaele, Solano Andrea, Agostino Raffaele G., Barberi Riccardo C., Couderc Vincent, Klimczak Mariusz, Filipkowski Adam, Buczynski Ryszard, Wabnitz Stefan, and Formoso Vincenzo
- Subjects
Physics ,QC1-999 - Abstract
We report on successful refractive index profiling of commercially available step-index and in-house made graded-index multimode specialty optical fibers by means of X-ray computed microtomography. Our results demonstrate that the latter is an advantageous technique for characterizing large core optical fibers, which allows for retrieving information about the refractive index at optical frequencies by exploiting the absorption coefficient of X-rays.
- Published
- 2023
- Full Text
- View/download PDF
21. Different healing characteristics of thiol-bearing molecules on CVD-grown MoS2
- Author
-
Giovanna Feraco, Oreste De Luca, Ali Syari’ati, Sardar Hameed, Abdurrahman Ali El Yumin, Jianting Ye, Raffaele G Agostino, and Petra Rudolf
- Subjects
molybdenum disulphide ,functionalisation ,x-ray photoelectron spectroscopy ,photoluminescence ,Materials of engineering and construction. Mechanics of materials ,TA401-492 ,Physics ,QC1-999 - Abstract
Vacancies in atomically thin molybdenum disulphide play an essential role in controlling its optical and electronic properties, which are crucial for applications in sensorics, catalysis or electronics. For this reason, defect engineering employing thiol-terminated molecules is used to heal and/or functionalise defective nanosheets. In this work, chemical vapour deposition-grown MoS _2 with different defect densities was functionalised with three molecules: 4-aminothiophenol (ATP), biphenyl-4-thiol (BPT) and 4-nitrothiophenol (NTP). The molecules’ efficacy in functionalising MoS _2 was probed by x-ray photoelectron, Raman and photoluminescence (PL) spectroscopy. The results show that exposing a defective single layer of MoS _2 to either ATP, BPT or NTP molecules heals the defects, however the chemical structure of these molecules affects the optical response and only for BPT the PL intensity increases.
- Published
- 2023
- Full Text
- View/download PDF
22. Assessment of activated carbon fibers from commercial Kevlar® as nanostructured material for gas storage: Effect of activation procedure and adsorption of CO2 and CH4
- Author
-
Conte, Giuseppe, Stelitano, Sara, Policicchio, Alfonso, Minuto, Francesco Demetrio, Lazzaroli, Victor, Galiano, Francesco, and Agostino, Raffaele G.
- Published
- 2020
- Full Text
- View/download PDF
23. Hydrogen storage performances for mesoporous silica synthesized with mixed tetraethoxysilane and methyltriethoxysilane precursors in acidic condition
- Author
-
Policicchio, Alfonso, Conte, Giuseppe, Stelitano, Sara, Bonaventura, Carlo Poselle, Putz, Ana-Maria, Ianăşi, Cătălin, Almásy, László, Horváth, Zsolt Endre, and Agostino, Raffaele G.
- Published
- 2020
- Full Text
- View/download PDF
24. Posidonia Oceanica and Wood chips activated carbon as interesting materials for hydrogen storage
- Author
-
Pedicini, Rolando, Maisano, Susanna, Chiodo, Vitaliano, Conte, Giuseppe, Policicchio, Alfonso, and Agostino, Raffaele G.
- Published
- 2020
- Full Text
- View/download PDF
25. Floating of the topological surface state on top of a thick lead layer: The case of the Pb/Bi2Se3 interface
- Author
-
De Luca, Oreste, primary, Shvets, Igor A., additional, Eremeev, Sergey V., additional, Aliev, Ziya S., additional, Kopciuszynski, Marek, additional, Barinov, Alexey, additional, Ronci, Fabio, additional, Colonna, Stefano, additional, Chulkov, Evgueni V., additional, Agostino, Raffaele G., additional, Papagno, Marco, additional, and Flammini, Roberto, additional
- Published
- 2023
- Full Text
- View/download PDF
26. Experimental Study on the Mechanical Properties of Basalt FRCM Made of Various Matrices: Validation by X-Ray Microtomography
- Author
-
Crocco, Maria C., primary, Scuro, Carmelo, additional, Filosa, Raffaele, additional, Codispoti, Rosamaria, additional, Ferraro, Mario, additional, Solano, Andrea, additional, Agostino, Raffaele G., additional, Barberi, Riccardo C., additional, Olivito, Renato S., additional, and Formoso, Vincenzo, additional
- Published
- 2023
- Full Text
- View/download PDF
27. Lithium abundances in globular cluster giants: NGC 6218 (M12) and NGC 5904 (M5)
- Author
-
D'Orazi, Valentina, Angelou, George C., Gratton, Raffaele G., Lattanzio, John C., Bragaglia, Angela, Carretta, Eugenio, Lucatello, Sara, and Momany, Yazan
- Subjects
Astrophysics - Solar and Stellar Astrophysics - Abstract
Convergent lines of evidence suggest that globular clusters host multiple stellar populations. It appears that they experience at least two episodes of star formation whereby a fraction of first-generation stars contribute astrated ejecta to form the second generation(s). To identify the polluting progenitors we require distinguishing chemical signatures such as that provided by lithium. Theoretical models predict that lithium can be synthesised in AGB stars, whereas no net Li production is expected from other candidates. It has been shown that in order to reproduce the abundance pattern found in M4, Li production must occur within the polluters, favouring the AGB scenario. Here we present Li and Al abundances for a large sample of RGB stars in M12 and M5. These clusters have a very similar metallicity, whilst demonstrating differences in several cluster properties. Our results indicate that the first-generation and second-generation stars share the same Li content in M12; we recover an abundance pattern similar to that observed in M4. In M5 we find a higher degree of complexity and a simple dilution model fails in reproducing the majority of the stellar population. In both clusters we require Li production across the different stellar generations, but production seems to have occurred to different extents. We suggest that such a difference might be related to the cluster mass with the Li production being more efficient in less-massive clusters. This is the first time a statistically significant correlation between the Li spread within a GC and its luminosity has been demonstrated. Finally, although Li-producing polluters are required to account for the observed pattern, other mechanisms, such as MS depletion, might have played a role in contributing to the Li internal variation, though at relatively low level., Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 14 figures
- Published
- 2014
- Full Text
- View/download PDF
28. A sequence of nitrogen-rich very red giants in the globular cluster NGC 1851
- Author
-
Carretta, Eugenio, D'Orazi, Valentina, Gratton, Raffaele G., and Lucatello, Sara
- Subjects
Astrophysics - Galaxy Astrophysics ,Astrophysics - Solar and Stellar Astrophysics - Abstract
We present the abundances of N in a sample of 62 stars on the red giant branch (RGB) in the peculiar globular cluster NGC 1851. The values of [N/Fe] ratio were obtained by comparing the flux measured in the observed spectra with that from synthetic spectra for up to about 15 features of CN. This is the first time that N abundances are obtained for such a large sample of RGB stars from medium-resolution spectroscopy in this cluster. With these abundances we provide a chemical tagging of the split red giant branch found from several studies in NGC 1851. The secondary, reddest sequence on the RGB is populated almost exclusively by N-rich stars, confirming our previous suggestion based on Stromgren magnitudes and colours. These giants are also, on average, enriched in s-process elements such as Ba, and are likely the results of pollution from low mass stars that experienced episodes of third dredge-up in the asymptotic giant branch phase., Comment: Version to match the one in press on Astronomy and Astrophysics
- Published
- 2014
- Full Text
- View/download PDF
29. Terzan 8: a Sagittarius-flavoured globular cluster
- Author
-
Carretta, Eugenio, Bragaglia, Angela, Gratton, Raffaele G., D'Orazi, Valentina, Lucatello, Sara, and Sollima, Antonio
- Subjects
Astrophysics - Astrophysics of Galaxies - Abstract
Massive globular clusters (GCs) contain at least two generations of stars with slightly different ages and clearly distinct light elements abundances. The Na-O anticorrelation is the best studied chemical signature of multiple stellar generations. Low-mass clusters appear instead to be usually chemically homogeneous. We are investigating low-mass GCs to understand what is the lower mass limit where multiple populations can form, mainly using the Na and O abundance distribution. We used VLT/FLAMES spectra of giants in the low-mass, metal-poor GC Terzan 8, belonging to the Sagittarius dwarf galaxy, to determine abundances of Fe, O, Na, alpha-, Fe-peak, and neutron-capture elements in six stars observed with UVES and 14 observed with GIRAFFE. The average metallicity is [Fe/H]=-2.27+/-0.03 (rms=0.08), based on the six high-resolution UVES spectra. Only one star, observed with GIRAFFE, shows an enhanced abundance of Na and we tentatively assign it to the second generation. In this cluster, at variance with what happens in more massive GCs, the second generation seems to represent at most a small minority fraction. We discuss the implications of our findings, comparing Terzan 8 with the other Sgr dSph GCs, to GCs and field stars in the Large Magellanic Cloud, Fornax, and in other dwarfs galaxies., Comment: 15 pages, 12 figures, 10 tables; accepted for publication on Astronomy and Astrophysics
- Published
- 2013
- Full Text
- View/download PDF
30. Rubidium abundances in the globular clusters NGC 6752, NGC 1904 and NGC 104 (47 Tuc)
- Author
-
D'Orazi, Valentina, Lugaro, Maria, Campbell, Simon W., Bragaglia, Angela, Carretta, Eugenio, Gratton, Raffaele G., Lucatello, Sara, and D'Antona, Francesca
- Subjects
Astrophysics - Astrophysics of Galaxies ,Astrophysics - Solar and Stellar Astrophysics - Abstract
Large star-to-star variations of the abundances of proton-capture elements, such as Na and O, in globular clusters (GCs) are interpreted as the effect of internal pollution resulting from the presence of multiple stellar populations. To better constrain this scenario we investigate the abundance distribution of the heavy element rubidium (Rb) in NGC 6752, NGC 1904, and NGC 104 (47 Tuc). Combining the results from our sample with those in the literature, we found that Rb exhibits no star-to-star variations, regardless the cluster metallicity, with the possible intriguing, though very uncertain, exception of the metal-rich bulge cluster NGC 6388. If no star-to-star variations will be confirmed for all GCs, it implies that the stellar source of the proton-capture element variations must not have produced significant amounts of Rb. This element is observed to be enhanced at extremely high levels in intermediate-mass AGB (IM-AGB) stars in the Magellanic Clouds (i.e., at a metallicity similar to 47 Tuc and NGC 6388). This may present a challenge to this popular candidate polluter, unless the mass range of the observed IM-AGB stars does not participate in the formation of the second-generation stars in GCs. A number of possible solutions are available to resolve this conundrum, also given that the Magellanic Clouds observations are very uncertain and may need to be revised. The fast rotating massive stars scenario would not face this potential problem as the slow mechanical winds of these stars during their main-sequence phase do not carry any Rb enhancements; however, these candidates face even bigger issues such as the production of Li and the close over-imposition with core-collapse supernova timescales. Observations of Sr, Rb, and Zr in metal-rich clusters such as NGC 6388 and NGC 6441 are sorely needed to clarify the situation., Comment: Accepted for publication in The Astrophysical Journal
- Published
- 2013
- Full Text
- View/download PDF
31. Fluorine variations in the globular cluster NGC 6656 (M22): implications for internal enrichment timescales
- Author
-
D'Orazi, Valentina, Lucatello, Sara, Lugaro, Maria, Gratton, Raffaele G., Angelou, George, Bragaglia, Angela, Carretta, Eugenio, Alves-Brito, Alan, Ivans, Inese I., Masseron, Thomas, and Mucciarelli, Alessio
- Subjects
Astrophysics - Astrophysics of Galaxies - Abstract
Observed chemical (anti)correlations in proton-capture elements among globular cluster stars are presently recognized as the signature of self-enrichment from now extinct, previous generations of stars. This defines the multiple population scenario. Since fluorine is also affected by proton captures, determining its abundance in globular clusters provides new and complementary clues regarding the nature of these previous generations, and supplies strong observational constraints to the chemical enrichment timescales. In this paper we present our results on near-infrared CRIRES spectroscopic observations of six cool giant stars in NGC 6656 (M22): the main objective is to derive the F content and its internal variation in this peculiar cluster, which exhibits significant changes in both light and heavy element abundances. We detected F variations across our sample beyond the measurement uncertainties and found that the F abundances are positively correlated with O and anticorrelated with Na, as expected according to the multiple population framework. Furthermore, our observations reveal an increase in the F content between the two different sub-groups, s-process rich and s-process poor, hosted within M22. The comparison with theoretical models suggests that asymptotic giant stars with masses between 4 and 5 Msun are responsible for the observed chemical pattern, confirming evidence from previous works: the difference in age between the two sub-components in M22 must be not larger than a few hundreds Myr., Comment: Accepted for publication in The Astrophysical Journal, 10 pages, 10 figures
- Published
- 2012
- Full Text
- View/download PDF
32. Aluminum abundances of multiple stellar generations in the globular cluster NGC 1851
- Author
-
Carretta, Eugenio, D'Orazi, Valentina, Gratton, Raffaele G., and Lucatello, Sara
- Subjects
Astrophysics - Astrophysics of Galaxies ,Astrophysics - Solar and Stellar Astrophysics - Abstract
We study the distribution of aluminum abundances among red giants in the peculiar globular cluster NGC 1851. Aluminum abundances were derived from the strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We coupled these abundances with previously derived abundance of O, Na, Mg to fully characterize the interplay of the NeNa and MgAl cycles of H-burning at high temperature in the early stellar generation in NGC 1851. The stars in our sample show well defined correlations between Al,Na and Si; Al is anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily increases going from the first generation stars to the second generation populations with intermediate and extremely modified composition. We confirm on a larger database the results recently obtained by us (Carretta et al. 2011a): the pattern of abundances of proton-capture elements implies a moderate production of Al in NGC 1851. We find evidence of a statistically significant positive correlation between Al and Ba abundances in the more metal-rich component of red giants in NGC 1851., Comment: Astronomy and Astrophysics, in press
- Published
- 2012
- Full Text
- View/download PDF
33. The chemical composition of nearby young associations: s-process element abundances in AB Doradus, Carina-Near, and Ursa Major
- Author
-
D'Orazi, Valentina, Biazzo, Katia, Desidera, Silvano, Covino, Elvira, Andrievsky, Sergei M., and Gratton, Raffaele G.
- Subjects
Astrophysics - Solar and Stellar Astrophysics ,Astrophysics - Astrophysics of Galaxies - Abstract
Recently, several studies have shown that young, open clusters are characterised by a considerable over-abundance in their barium content. In particular, D'Orazi et al. (2009) reported that in some younger clusters [Ba/Fe] can reach values as high as ~0.6 dex. The work also identified the presence of an anti-correlation between [Ba/Fe] and cluster age. For clusters in the age range ~4.5 Gyr-500 Myr, this is best explained by assuming a higher contribution from low-mass asymptotic giant branch stars to the Galactic chemical enrichment. The purpose of this work is to investigate the ubiquity of the barium over-abundance in young stellar clusters. We analysed high-resolution spectroscopic data, focusing on the s-process elemental abundance for three nearby young associations, i.e. AB Doradus, Carina-Near, and Ursa Major. The clusters have been chosen such that their age spread would complement the D'Orazi et al. (2009) study. We find that while the s-process elements Y, Zr, La, and Ce exhibit solar ratios in all three associations, Ba is over-abundant by ~0.2 dex. Current theoretical models can not reproduce this abundance pattern, thus we investigate whether this unusually large Ba content might be related to chromospheric effects. Although no correlation between [Ba/Fe] and several activity indicators seems to be present, we conclude that different effects could be at work which may (directly or indirectly) be related to the presence of hot stellar chromospheres., Comment: Accepted for publication in MNRAS
- Published
- 2012
- Full Text
- View/download PDF
34. Diluting the material forming the second generation stars in Globular Clusters: the contribution by unevolved stars
- Author
-
Gratton, Raffaele G. and Carretta, Eugenio
- Subjects
Astrophysics - Astrophysics of Galaxies - Abstract
In this short communication we consider the possibility that stars less evolved than the polluters are the source of the dilution needed to explain the observed composition of second-generation globular cluster (GC) stars and the Na-O and Mg-Al anticorrelations. If these stars can lose 0.5-1% of their mass during the relevant epochs, there is enough diluting material to produce the observed anticorrelations. In this case, the original mass of proto-GCs was several tens times higher than the current mass of GCs. While not strictly impossible, this is a stringent hypothesis that needs more support. Should this scenario be found true, then the link between the primordial (first-generation) population in GC and the field population would be very strong., Comment: Accepted for publication on Astronomy and Astrophysics
- Published
- 2010
- Full Text
- View/download PDF
35. The second and third parameters of the Horizontal Branch in Globular Clusters
- Author
-
Gratton, Raffaele G., Carretta, Eugenio, Bragaglia, Angela, Lucatello, Sara, and D'Orazi, Valentina
- Subjects
Astrophysics - Solar and Stellar Astrophysics - Abstract
The Horizontal Branch (HB) second parameter of Globular Clusters (GCs) is a major open issue in stellar evolution. Large photometric and spectroscopic databases allow a re-examination of this issue. We derive median and extreme (90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB taking into account evolutionary effects, and compare these masses with those expected at the tip of the Red Giant Branch to derive the total mass lost by the stars. A simple linear dependence on metallicity of this total mass lost explains well the median colours of HB stars. Adopting this mass loss law as universal, we find that age is the main second parameter. However, at least a third parameter is clearly required. The most likely candidate is the He abundance, which might be different in GCs stars belonging to the different stellar generations whose presence was previously derived from the Na-O and Mg-Al anticorrelations. Variations in the median He abundance allow explaining the extremely blue HB of some GCs; such variations are correlated with the R-parameter. Suitable He abundances allow deriving ages from the HB which are consistent with those obtained from the Main Sequence. Small corrections to these latter ages are then proposed, producing a tight age-metallicity relation for disk and bulge GCs. Star-to-star variations in the He content explain the extension of the HB. There is a strong correlation between this extension and the interquartile of the Na-O anticorrelation. The main driver for the variations in the He-content within GCs seems the total cluster mass. 47 Tuc and M3 exhibit exceptional behaviours; however, they can be accommodated in a scenario for the formation of GCs that relates their origin to cooling flows generated after very large episodes of star formation., Comment: 30 pages, 31 figures. In press on Astronomy and Astrophysics, version after language editing
- Published
- 2010
- Full Text
- View/download PDF
36. BIGRE: a low cross-talk integral field unit tailored for extrasolar planets imaging spectroscopy
- Author
-
Antichi, Jacopo, Dohlen, Kjetil, Gratton, Raffaele G., Mesa, Dino, Claudi, Riccardo U., Giro, Enrico, Boccaletti, Anthony, Mouillet, David, Puget, Pascal, and Beuzit, Jean-Luc
- Subjects
Astrophysics - Instrumentation and Methods for Astrophysics ,Astrophysics - Earth and Planetary Astrophysics - Abstract
Integral field spectroscopy (IFS) represents a powerful technique for the detection and characterization of extrasolar planets through high contrast imaging, since it allows to obtain simultaneously a large number of monochromatic images. These can be used to calibrate and then to reduce the impact of speckles, once their chromatic dependence is taken into account. The main concern in designing integral field spectrographs for high contrast imaging is the impact of the diffraction effects and the non-common path aberrations together with an efficient use of the detector pixels. We focus our attention on integral field spectrographs based on lenslet-arrays, discussing the main features of these designs: the conditions of appropriate spatial and spectral sampling of the resulting spectrograph's slit functions and their related cross-talk terms when the system works at the diffraction limit. We present a new scheme for the integral field unit (IFU) based on a dual-lenslet device (BIGRE), that solves some of the problems related to the classical TIGER design when used for such applications. We show that BIGRE provides much lower cross-talk signals than TIGER, allowing a more efficient use of the detector pixels and a considerable saving of the overall cost of a lenslet-based integral field spectrograph., Comment: 17 pages, 18 figures, accepted for publication in ApJ
- Published
- 2009
- Full Text
- View/download PDF
37. Char from Pyrolysis of Waste Tires to Increase Bitumen Performances.
- Author
-
Caputo, Paolino, Calandra, Pietro, Policicchio, Alfonso, Conte, Giuseppe, Agostino, Raffaele G., Pochylski, Mikolaj, Abe, Abraham, and Oliviero Rossi, Cesare
- Subjects
CHAR ,WASTE tires ,COMBUSTION ,PYROLYSIS ,BITUMEN ,TRANSITION temperature - Abstract
Road pavement characteristics can be improved by modifying bitumens through addition of fine particles. To avoid environmental issues, attention is recently being paid to bio-materials. In this study, a 50/70 bitumen was modified through the addition of char obtained from the pyrolysis of waste tires. Char addition causes an increase in transition (gel to sol) temperature of up to 4 °C and an increase in rigidity under working conditions (50 °C) of up to about one order of magnitude. The effect of the gas type flowing under the pyrolysis process (CO
2 , N2 and CO2 + N2 ) on the mechanical characteristics of the bitumen was also investigated. More marked effects on the rheological performances were found if char coming from pyrolysis under CO2 was used (the gel-to-sol transition temperature is increased by about 4.5 °C) compared to that coming from pyrolysis under N2 (gel-to-sol transition temperature is increased by about 3 °C). The effect is at a maximum for char from CO2 at 3% wt/wt, whereas regarding char from N, a lesser effect is seen with a more uniform (flat) effect as a function of char% and with an increase in gel-to-sol transition temperature of about 3 °C. Coherently, char obtained from pyrolysis under CO2 was found to possess a higher surface area constituted by smaller particles than that coming from pyrolysis under N2 . In fact, the BET-specific surface passes 79 m2 /g for N2 -char to 174 m2 /g for CO2 -char and the micropore volume fraction increases from 2 to 9. The observed differences can be attributed to the oxidizing environment (CO2 ) that is more aggressive and reactive in the synthesis phase compared to an inert environment (N2 ). Char also showed an anti-aging effect, hindering the increase in rigidity typically associated with the aging process. This effect was explained in terms of the compatibility of char with bitumen's organic nature, and presumably its more effective hosting in a bituminous structure, which is presumably higher when char is produced under CO2 rather than N2 . This study quantitatively indicates how a residue derived from the pyrolysis of a waste material can be efficiently re-used to increase the mechanical characteristics of bitumen, accomplishing the recent circular-based needs for environmental protection. [ABSTRACT FROM AUTHOR]- Published
- 2024
- Full Text
- View/download PDF
38. Lattice Dynamics of Quinacridone Polymorphs: A Combined Raman and Computational Approach
- Author
-
Giunchi, Andrea, primary, Pandolfi, Lorenzo, additional, Della Valle, Raffaele G., additional, Salzillo, Tommaso, additional, Venuti, Elisabetta, additional, and Girlando, Alberto, additional
- Published
- 2023
- Full Text
- View/download PDF
39. Different healing characteristics of thiol-bearing molecules on CVD-grown MoS2
- Author
-
Feraco, Giovanna, primary, Luca, Oreste De, additional, Syari’ati, Ali, additional, Hameed, Sardar, additional, El Yumin, Abdurrahman Ali, additional, Ye, Jianting, additional, Agostino, Raffaele G, additional, and Rudolf, Petra, additional
- Published
- 2023
- Full Text
- View/download PDF
40. Properties of second generation stars in Globular Clusters
- Author
-
Carretta, Eugenio, Bragaglia, Angela, Gratton, Raffaele G., and Lucatello, Sara
- Subjects
Astrophysics - Abstract
We present the first results from the analysis of GIRAFFE spectra of more than 1200 red giants stars in 19 Galactic Globular Clusters (GCs), to study the chemical composition of second generation stars and their link with global cluster parameters. We confirm that the extension of the Na-O anticorrelation (the most striking signature of polluted, second generation populations) is strictly related to the very blue (and hot) extreme of the Horizontal Branch (HB). Long anticorrelations seem to require large mass and large-sized, eccentric orbits, taking the GCs far away from the central regions of the Galaxy. We can separate three populations in each cluster (primordial, intermediate and extreme) based on the chemical composition. In all GCs we observe a population of primordial composition, similar to field stars of similar metallicity. We find that in all GCs the bulk (from 50 to 70%) of stars belong to the intermediate component. Finally, the extreme, very oxygen-poor component is observed preferentially in massive clusters, but is not present in all massive GCs., Comment: Comments: 10 pages, 7 figures, uses aastex.cls. Written version of a contributed talk presented to the conference ``Chemical Evolution of Dwarf Galaxies and Stellar Clusters", Garching, Germany, July 21-25, 2008
- Published
- 2008
41. The link between chemical anomalies along the red giant branch and the horizontal branch extension in globular clusters
- Author
-
Carretta, Eugenio, Recio-Blanco, Alejandra, Gratton, Raffaele G., Piotto, Giampaolo, and Bragaglia, Angela
- Subjects
Astrophysics - Abstract
We find a strong correlation between the extension of the Na-O anticorrelation observed in red giant branch (RGB) stars and the high temperature extension of the horizontal branch (HB) blue tails of Galactic globular clusters (GCs). The longer is the O-depleted tail of the Na-O anticorrelation observed in the RGB stars, the higher is the maximum temperature reached by the bluest HB stars in the GC. This result provides a clear, empirical evidence of a link between the extension of the HB and the presence of star-to-star abundance variations of proton-capture elements in GC stars. We discuss the possible interpretation of this correlation., Comment: Comments: 6 pages, 1 figure, uses emulateapj.cls; accepted for publication in the Astrophysical Journal Letters
- Published
- 2007
- Full Text
- View/download PDF
42. Variations in the lithium abundances of turn off stars in the globular cluster 47 Tuc
- Author
-
Bonifacio, Piercarlo, Pasquini, Luca, Molaro, Paolo, Carretta, Eugenio, François, Patrick, Gratton, Raffaele G., James, Gael, Sbordone, Luca, Spite, François, and Zoccali, Manuela
- Subjects
Astrophysics - Abstract
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 Editor
- Published
- 2007
- Full Text
- View/download PDF
43. Beryllium abundance in turn-off stars of NGC 6752
- Author
-
Pasquini, Luca, Bonifacio, Piercarlo, Randich, Sofia, Galli, Daniele, Gratton, Raffaele G., and Wolff, B.
- Subjects
Astrophysics - Abstract
Aims: To measure the beryllium abundance in two TO stars of the Globular Cluster NGC 6752, one oxygen rich and sodium poor, the other presumably oxygen poor and sodium rich. Be abundances in these stars are used to put on firmer grounds the hypothesis of Be as cosmochronometer and to investigate the formation of Globular Clusters. Method:We present near UV spectra with resolution R$\sim 45000$ obtained with the UVES spectrograph on the 8.2m VLT Kueyen telescope, analysed with spectrum synthesis based on plane parallel LTE model atmospheres. Results:Be is detected in the O rich star with log(Be/H)=-12.04 $\pm$0.15, while Be is not detected in the other star for which we obtain the upper limit log(Be/H)$<$-12.2. A large difference in nitrogen abundance (1.6 dex) is found between the two stars. Conclusions:The Be measurement is compatible with what found in field stars with the same [Fe/H] and [O/H]. The 'Be age' of the cluster is found to be 13.3 Gyrs, in excellent agreement with the results from main sequence fitting and stellar evolution. The presence of Be confirms the results previously obtained for the cluster NGC 6397 and supports the hypothesis that Be can be used as a clock for the early formation of the Galaxy. Since only an upper limit is found for the star with low oxygen abundance, we cannot decide between competing scenarios of Globular Cluster formation, but we can exclude that 'polluted' stars are substantially younger than 'unpolluted' ones. We stress that the Be test might be the only measurement capable of distinguishing between these scenarios., Comment: Accepted for publication by Astronomy & Astrophysics
- Published
- 2006
- Full Text
- View/download PDF
44. Surface modification of molecular sieve fillers for mixed matrix membranes
- Author
-
Golemme, Giovanni, Policicchio, Alfonso, Sardella, Eloisa, De Luca, Giuseppina, Russo, Beatrice, Liguori, Paola F., Melicchio, Alessandro, and Agostino, Raffaele G.
- Published
- 2018
- Full Text
- View/download PDF
45. The Binary Frequency Among Carbon-Enhanced, s-Process Rich, Metal-Poor Stars
- Author
-
Lucatello, Sara, Tsangarides, Stelios, Beers, Timothy C., Carretta, Eugenio, Gratton, Raffaele G., and Ryan, Sean G.
- Subjects
Astrophysics - Abstract
We discuss radial velocities for a sample of carbon-enhanced, s-process rich, very metal-poor stars (CEMP-s hereafter), analyzed with high-resolution spectroscopy obtained over multiple epochs. We find that ~68% of the stars in the sample show evidence of radial velocity variations. The expected detection fraction for these stars, adopting the measured binary fraction in the field (~60%), and assuming that they share the same period and eccentricity distribution, is ~22%. Even if one assumes that the true binary fraction of these stars is 100%, the expected detection percentage is ~36%. These values indicate that the binary fraction among CEMP-s stars is higher than the field binary fraction, suggesting that all of these objects are in double (or multiple) systems. The fact that the observed frequency of velocity variation exceeds the expected detection fraction in the case of an assumed binary fraction of 100% is likely due to a more restricted distribution of orbital periods for these objects, as compared to normal field binaries. Our results indicate that CEMP-s stars are the metal-poor analogues of classical CH-stars., Comment: 22 pages, Accepted for publication on ApJ
- Published
- 2004
- Full Text
- View/download PDF
46. Observational evidence for a different IMF in the early Galaxy
- Author
-
Lucatello, Sara, Gratton, Raffaele G., Beers, Timothy C., and Carretta, Eugenio
- Subjects
Astrophysics - Abstract
The unexpected high incidence of carbon-enhanced, s-process enriched unevolved stars amongst extremely metal-poor stars in the halo provides a significant constraint on the Initial Mass Function (IMF) in the early Galaxy. We argue that these objects are evidence for the past existence of a large population of intermediate-mass stars, and conclude that the IMF in the early Galaxy was different from the present, and shifted toward higher masses., Comment: 14 pages, 1 color figure, accepted for publication on ApJ
- Published
- 2004
- Full Text
- View/download PDF
47. Polymorphism, phonon dynamics and carrier-phonon coupling in pentacene
- Author
-
Della Valle, Raffaele G., Brillante, Aldo, Farina, Luca, Venuti, Elisabetta, Masino, Matteo, and Girlando, Alberto
- Subjects
Condensed Matter - Materials Science - Abstract
The crystal structure and phonon dynamics of pentacene is computed with the Quasi Harmonic Lattice Dynamics (QHLD) method, based on atom-atom potential. We show that two crystalline phases of pentacene exist, rather similar in thermodynamic stability and in molecular density. The two phases can be easily distinguished by Raman spectroscopy in the 10-100 cm-1 spectral region. We have not found any temperature induced phase transition, whereas a sluggish phase change to the denser phase is induced by pressure. The bandwidths of the two phases are slightly different. The charge carrier coupling to low-frequency phonons is calculated., Comment: 6 pages, 3 figures. Presented at ICFPAM-7
- Published
- 2003
- Full Text
- View/download PDF
48. BEDT-TTF organic superconductors: the entangled role of phonons
- Author
-
Girlando, Alberto, Masino, Matteo, Brillante, Aldo, Della Valle, Raffaele G., and Venuti, Elisabetta
- Subjects
Condensed Matter - Superconductivity - Abstract
We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency., Comment: 4 pages, including 4 figures. Published version, with Ref. 17 corrected after publication
- Published
- 2002
- Full Text
- View/download PDF
49. An abundance analysis for four Red Horizontal Branch Stars in the extremely metal rich globular cluster NGC 6528
- Author
-
Carretta, Eugenio, Cohen, Judith G., Gratton, Raffaele G., and Behr, Bradford B.
- Subjects
Astrophysics - Abstract
We present the results of the first analysis of high dispersion spectra of four red HB stars in the metal rich globular cluster NGC 6528, located in Baade's Window. We find that the mean [Fe/H] for NGC 6528 is +0.07+-0.01 dex (error of the mean), with a star-to-star scatter of sigma = 0.02 dex (4 stars), although the total error is likely to be larger (~0.1 dex) due to systematic errors related to the effective temperature scale and to model atmospheres. This metallicity is somewhat larger than both the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and that found in our previous paper for NGC 6553. However, we find that the spectra of clump stars in NGC 6528 and NGC 6553 are very similar each other, the slightly different metal abundances found being possibly due to the different atmospheric parameters adopted in the two analyses. For NGC 6528 we find excesses for the alpha-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn is found to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O; however this is somewhat uncertain due to the dependence of the O abundance on the adopted atmospheric parameters and to coupling between C and O abundances in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~ +0.4) in all stars examined. Since the present analysis is based on higher quality material, we propose to revise our previous published metal abundance for NGC 6553 to [Fe/H]=$0.06+-0.15., Comment: accepted for publication in the September 2001 issue of The Astronomical Journal; 3 new figures and updated results and calibrations
- Published
- 2001
- Full Text
- View/download PDF
50. A scaling approximation for structure factors in the integral equation theory of polydisperse nonionic colloidal fluids
- Author
-
Gazzillo, Domenico, Giacometti, Achille, Della Valle, Raffaele G., Venuti, Elisabetta, and Carsughi, Flavio
- Subjects
Condensed Matter - Statistical Mechanics ,Condensed Matter - Soft Condensed Matter - Abstract
Integral equation of pure liquids, combined with a new "scaling approximation" based on a corresponding states treatment of pair correlation functions, is used to evaluate approximate structure factors for colloidal fluids constituted of uncharged particles with polydispersity in size and energy parameters. Both hard spheres and Lennard-Jones interactions are considered. For polydisperse hard spheres, the scaling approximation is compared to theories utilized by small angle scattering experimentalists (decoupling approximation, local monodisperse approximation)and to the van der Waals one-fluid theory. The results are tested against predictions from analytical expressions, exact within the Percus-Yevick approximation. For polydisperse Lennard-Jones particles, the scaling approximation combined with a "modified hypernetted chain" integral equation, is tested against molecular dynamics data generated for the present work. Despite ist simplicity, the scaling approximation exhibits a satisfactory performance for both potentials and represents a considerable improvement over the above mentioned theories. Shortcomings of the proposed theory, its applicability to the analysis of experimental scattering data, and its possible extensions to different potentials are finally discussed., Comment: 12 pages, 7 postscript figures (included), Latex 3.0, uses aps.sty
- Published
- 1999
- Full Text
- View/download PDF
Catalog
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.