A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere

We derive a fully relativistic, self-similar solution to describe the propagation of a shock along an exponentially decreasing atmosphere, in the limit of very large Lorentz factor. We solve the problem in planar symmetry and compute the acceleration of the shock in terms of the density gradient crossed during its evolution. We apply our solution to the acceleration of shocks within the atmosphere of a HyperNova, and show that velocities consistent with the requirements of GRB models can be achieved with exponential atmospheres spanning a wide density range.Comment: ApJL in pres

Extinction properties of the X-ray bright/optically faint afterglow of GRB 020405

We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral ``mismatch'' by testing several types of extinction curves. For the first time we test extinction curves computed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly depen dent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.Comment: 6 pages, 5 figures, accepted for publication on A&

Dynamics and molecular evolution of HIV-1 strains in Sicily among antiretroviral na\uefve patients.

HIV-1 subtype B is the most frequent strain in Sicily. To date, there is no available data about the genetic diversity of HIV-1 viral strains circulating in Sicily among antiretroviral (ARV) na\uefve subjects and the role of immigration as potential determinant of evolutionary dynamics of HIV-1 molecular epidemiology. For this purpose, HIV-1 polymerase (pol) sequences obtained from 155 ARV na\uefve individuals from 2004 to 2009 were phylogenetically analysed. The overall rate of HIV-1 non-B infections was 31.0% (n = 48/155), increasing from 7.8% in 2004\u20132006 to 40.9% in 2009, and about one-third were identified as unique recombinant forms. CRF02_AG was the prevalent non-B clade (n = 28/48, 58.3%), while subtype C-related strains were responsible for about 30% HIV-1 infections. Non-B viruses strictly associated with heterosexual transmission (85.4%) and were mostly found among immigrants (77.1%). Phylogenetic analysis of non-B sequences found in foreign-born subjects was geographically correlated to the respective country of origin. Moreover, the detection of non-B viral variants in the autochthonous population may support an increasing genetic diversity in Sicily as well as a local circulation of HIV strains also uncommon in our country. In Sicily, HIV-1 epidemic is still mostly attributable to the B subtype. Nevertheless, migration and population movements are progressively introducing novel HIV-1 subtypes causing a continuous increase of HIV-1 molecular dynamic at local level. Molecular surveillance is needed to monitor the genetic evolution of HIV-1 epidemic

Low delta-V near-Earth asteroids: A survey of suitable targets for space missions

In the last decades Near-Earth Objects (NEOs) have become very important targets to study, since they can give us clues to the formation, evolution and composition of the Solar System. In addition, they may represent either a threat to humankind, or a repository of extraterrestrial resources for suitable space-borne missions. Within this framework, the choice of next-generation mission targets and the characterisation of a potential threat to our planet deserve special attention. To date, only a small part of the 11,000 discovered NEOs have been physically characterised. From ground and space-based observations one can determine some basic physical properties of these objects using visible and infrared spectroscopy. We present data for 13 objects observed with different telescopes around the world (NASA-IRTF, ESO-NTT, TNG) in the 0.4 - 2.5 um spectral range, within the NEOSURFACE survey (http://www.oa-roma.inaf.it/planet/NEOSurface.html). Objects are chosen from among the more accessible for a rendez-vous mission. All of them are characterised by a delta-V (the change in velocity needed for transferring a spacecraft from low-Earth orbit to rendez-vous with NEOs) lower than 10.5 km/s, well below the Solar System escape velocity (12.3 km/s). We taxonomically classify 9 of these objects for the first time. 11 objects belong to the S-complex taxonomy; the other 2 belong to the C-complex. We constrain the surface composition of these objects by comparing their spectra with meteorites from the RELAB database. We also compute olivine and pyroxene mineralogy for asteroids with a clear evidence of pyroxene bands. Mineralogy confirms the similarity with the already found H, L or LL ordinary chondrite analogues.Comment: 9 pages, 7 figures, to be published in A&A Minor changes by language edito

Colors and taxonomy of Centaurs and Trans-Neptunian Objects

The study of the surface properties of Centaurs and Trans-Neptunian Objects (TNOs) provides essential information about the early conditions and evolution of the outer Solar System. Due to the faintness of most of these distant and icy bodies, photometry currently constitutes the best technique to survey a statistically significant number of them. Our aim is to investigate color properties of a large sample of minor bodies of the outer Solar System, and set their taxonomic classification. We carried out visible and near-infrared photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and ISAAC instruments at the Very Large Telescope (European Southern Observatory). Using G-mode analysis, we derived taxonomic classifications according to the Barucci et al. (2005a) system. We report photometric observations of 31 objects, 10 of them have their colors reported for the first time ever. 28 Centaurs and TNOs have been assigned to a taxon. We combined the entire sample of 38 objects taxonomically classified in the framework of our programme (28 objects from this work; 10 objects from DeMeo et al. 2009a) with previously classified TNOs and Centaurs, looking for correlations between taxonomy and dynamics. We compared our photometric results to literature data, finding hints of heterogeneity for the surfaces of 4 objects.Comment: 7 pages, 4 figures. To be published in Astronomy and Astrophysic

Renormalization of Coulomb interactions in s-wave superconductor Nax_xCoO2_2

We study the renormalized Coulomb interactions due to retardation effect in Na$_x$CoO$_2$. Although the Morel-Anderson's pseudo potential for $a_{1g}$ orbital $\mu^*_{a1g}$ is relatively large because the direct Coulomb repulsion $U$ is large, that for interband transition between $a_{1g}$ and $e_g'$ orbitals $\mu^*_{a1g,eg'}$ is very small since the renormalization factor for pair hopping $J$ is square of that for $U$. Therefore, the s-wave superconductivity due to valence-band Suhl-Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na$_x$CoO$_2$.Comment: 2pages, 2figures, Proceedings of ICM in Kyoto, 200

UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers

We study the Gamma Ray Burst (GRB) environment and intervening absorbers by analyzing the optical absorption features produced by gas surrounding the GRB or along its line of sight. We analyzed high resolution spectroscopic observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which has at least four components in the main absorption system. We detect strong FeII, SiII, and NiII excited absorption lines associated with the bluemost component only. In addition to the host galaxy, at least two more absorbers lying along the line of sight to the afterglow have been detected in the redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost component in the host galaxy, we derive information about its distance from the site of the GRB explosion. We do so by assuming that the excited absorption lines are produced by indirect UV pumping, and compare the data with a time dependent photo-excitation code. The distance of this component is found to be 280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify two additional MgII absorbers, one of them with a rest frame equivalent width larger than 1A. The distance between the GRB and the absorber measured in this paper confirms that the power of the GRB radiation can influence the conditions of the interstellar medium up to a distance of at least several hundred pc. For the intervening absorbers, we confirm the trend that on average one strong intervening system is found per afterglow, as has been noted in studies exhibiting an excess of strong MgII absorbers along GRB sightlines compared to quasars.Comment: 8 Pages, 7 ps figures, A&A in pres

Blowin' in the wind: both `negative' and `positive' feedback in an obscured high-z Quasar

Quasar feedback in the form of powerful outflows is invoked as a key mechanism to quench star formation in galaxies, preventing massive galaxies to over-grow and producing the red colors of ellipticals. On the other hand, some models are also requiring `positive' AGN feedback, inducing star formation in the host galaxy through enhanced gas pressure in the interstellar medium. However, finding observational evidence of the effects of both types of feedback is still one of the main challenges of extragalactic astronomy, as few observations of energetic and extended radiatively-driven winds are available. Here we present SINFONI near infrared integral field spectroscopy of XID2028, an obscured, radio-quiet z=1.59 QSO detected in the XMM-COSMOS survey, in which we clearly resolve a fast (1500 km/s) and extended (up to 13 kpc from the black hole) outflow in the [OIII] lines emitting gas, whose large velocity and outflow rate are not sustainable by star formation only. The narrow component of Ha emission and the rest frame U band flux from HST-ACS imaging enable to map the current star formation in the host galaxy: both tracers independently show that the outflow position lies in the center of an empty cavity surrounded by star forming regions on its edge. The outflow is therefore removing the gas from the host galaxy (`negative feedback'), but also triggering star formation by outflow induced pressure at the edges (`positive feedback'). XID2028 represents the first example of a host galaxy showing both types of feedback simultaneously at work.Comment: 9 pages, 5 figures, accepted for publication in Ap

On the use of shape memory alloys for deployable passive heat radiators in space satellites

The present work presents a multifunctional structure for space engineering application part of the TOPDESS project, funded by ESA. The main aim of the project is the design of a thermal control device able to deploy through passive actuation. A combined device has been designed, made up of a Pulsating Heat Pipe (PHP) foldable heat exchanger and Shape Memory Alloy (SMA) wire. The deployment of the SMA wire is conceived to be controlled by thermal contact with the heat source and by conduction along the wire. Since the heat sources are lumped and the wire is subject to convection, a temperature gradient develops along the wire. A monodimensional mode able to predict the behavior of an SMA wire subjected to a spatial temperature gradient, is presented in this paper. The results show that the system can carry out folding and unfolding cycles with rotation angles greater than 80° only if the wire is subjected to uniform temperature distribution; in the case of temperature gradient, the achievable rotation angle is about 20°. The analysis states the feasibility of the actuation system, highlighting the critical technological aspects, to lay the groundwork for the future development of the whole system