Electronic structure calculations and molecular dynamics simulations with linear system-size scaling

We present a method for total energy minimizations and molecular dynamics simulations based either on tight-binding or on Kohn-Sham hamiltonians. The method leads to an algorithm whose computational cost scales linearly with the system size. The key features of our approach are (i) an orbital formulation with single particle wavefunctions constrained to be localized in given regions of space, and (ii) an energy functional which does not require either explicit orthogonalization of the electronic orbitals, or inversion of an overlap matrix. The foundations and accuracy of the approach and the performances of the algorithm are discussed, and illustrated with several numerical examples including Kohn-Sham hamiltonians. In particular we present calculations with tight-binding hamiltonians for diamond, graphite, a carbon linear chain and liquid carbon at low pressure. Even for a complex case such as liquid carbon -- a disordered metallic system with differently coordinated atoms -- the agreement between standard diagonalization schemes and our approach is very good. Our results establish the accuracy and reliability of the method for a wide class of systems and show that tight binding molecular dynamics simulations with a few thousand atoms are feasible on small workstations

La Linguistica Computazionale nell'analisi dei documenti tecnico-scientifici

L'elaborato sviluppato ha come scopo l'illustrazione dell'applicazione di tecniche e metodi di Linguistica Computazionale applicati all'analisi di testi tecnico-scientifici come brevetti o pubblicazioni. L'utilizzo di techiche di analisi automatica dei testi permette di estrapolare informazioni dagli stessi in maniera efficace ed efficiente. Sono stati applicati strumenti e metodologie di Linguistica Computazionale in vari casi applicativi, tra cui un'analisi del settore Biomedicale attraverso l'analisi di brevetti contenenti informazioni inerenti tale settore multidisciplinare

Post-T Tauri stars: a false problem

We consider the problem of the apparent lack of old T Tauri stars in low-mass star forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetime and ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. We conclude that the probability of finding a large population of old stars in a star forming region is intrinsically very small and that the post-T Tauri problem is by and large not existent.Comment: 6 pages (LaTeX), no Figures to be published in The Astrophysical Journal Letter

Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A

Magnetic fields are believed to play a crucial role in the process of star formation. We compare high-angular resolution observations of the submillimeter polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a low-mass protostar, with models of the collapse of magnetized molecular cloud cores. Assuming a uniform dust alignment efficiency, we computed the Stokes parameters and synthetic polarization maps from the model density and magnetic field distribution by integrations along the line-of-sight and convolution with the interferometric response. The synthetic maps are in good agreement with the data. The best-fitting models were obtained for a protostellar mass of 0.8 solar masses, of age 9e4 yr, formed in a cloud with an initial mass-to-flux ratio ~2 times the critical value. The magnetic field morphology in NGC 1333 IRAS 4A is consistent with the standard theoretical scenario for the formation of solar-type stars, where well-ordered, large-scale, rather than turbulent, magnetic fields control the evolution and collapse of the molecular cloud cores from which stars form.Comment: 4 pages, 5 figures. Accepted by Astronomy and Astrophysic

Non-thermal photons and H2 formation in the early Universe

The cosmological recombination of H and He at z \sim 1000 and the formation of H2 during the dark ages produce a non-thermal photon excess in the Wien tail of the cosmic microwave background (CMB) blackbody spectrum. Here we compute the effect of these photons on the H- photodetachment and H2+ photodissociation processes. We discuss the implications for the chemical evolution of the Universe in the post-recombination epoch, emphasizing how important a detailed account of the full vibrational manifold of H2 and H2+ in the chemical network is. We find that the final abundances of H2, H2+, H3+ and HD are significantly smaller than in previous calculations that neglected the effect of non-thermal photons. The suppression is mainly caused by extra hydrogen recombination photons and could affect the formation rate of first stars. We provide simple analytical approximations for the relevant rate coefficients and briefly discuss the additional effect of dark matter annihilation on the considered reaction rates.Comment: 10 pages, 12 figures, 1 table; accepted for publication in MNRA

Massive black hole factories: Supermassive and quasi-star formation in primordial halos

Supermassive stars and quasi-stars (massive stars with a central black hole) are both considered as potential progenitors for the formation of supermassive black holes. They are expected to form from rapidly accreting protostars in massive primordial halos. We explore how long rapidly accreting protostars remain on the Hayashi track, implying large protostellar radii and weak accretion luminosity feedback. We assess the potential role of energy production in the nuclear core, and determine what regulates the evolution of such protostars into quasi-stars or supermassive stars. We follow the contraction of characteristic mass scales in rapidly accreting protostars, and infer the timescales for them to reach nuclear densities. We compare the characteristic timescales for nuclear burning with those for which the extended protostellar envelope can be maintained. We find that the extended envelope can be maintained up to protostellar masses of 3.6x10^8 \dot{m}^3 solar, where \dot{m} denotes the accretion rate in solar masses per year. We expect the nuclear core to exhaust its hydrogen content in 7x10^6 yrs. If accretion rates \dot{m}>>0.14 can still be maintained at this point, a black hole may form within the accreting envelope, leading to a quasi-star. Alternatively, the accreting object will gravitationally contract to become a main-sequence supermassive star. Due to the limited gas reservoir in dark matter halos with 10^7 solar masses, the accretion rate onto the central object may drop at late times, implying the formation of supermassive stars as the typical outcome of direct collapse. However, if high accretion rates are maintained, a quasi-star with an interior black hole may form.Comment: 9 pages, 4 figures, submitted to A&A. Comments are welcom