Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions

The specific mechanisms which leads to the formation of fractal nanostructures by pulsed laser deposition remain elusive despite intense research efforts, motivated mainly by the technological interest in obtaining tailored nanostructures with simple and scalable production methods. Here we focus on fractal nanostructures of titanium dioxide, $TiO_2$, a strategic material for many applications, obtained by femtosecond laser ablation at ambient conditions. We model the fractal formation through extensive Monte Carlo simulations based on a set of minimal assumptions: irreversible sticking and size independent diffusion. Our model is able to reproduce the fractal dimensions and the area distributions of the nanostructures obtained in the experiments for different densities of the ablated material. The comparison of theory and experiment show that such fractal aggregates are formed after landing of the ablated material on the substrate surface by a diffusive mechanism. Finally we discuss the role of the thermal conductivity of the substrate and the laser fluence on the properties of the fractal nanostructures. Our results represent an advancement towards controlling the production of fractal nanostructures by pulsed laser deposition.Comment: 21 page

Thermo-mechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near infrared pump-probe diffraction experiments

The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of metallic nano-disks has been studied by near infrared pump-probe diffraction measurements, over a temporal range spanning from 100 fs to several nanoseconds. The experiments demonstrate that, in these systems, a two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the reciprocal periodicity of the array, can be excited by ~120 fs Ti:sapphire laser pulses. In order to clarify the interaction between the nanodisks and the substrate, numerical calculations of the elastic eigenmodes and simulations of the thermodynamics of the system are developed through finite-element analysis. At this light, we unambiguously show that the observed 2DSAW velocity shift originates from the mechanical interaction between the 2DSAWs and the nano-disks, while the correlated 2DSAW damping is due to the energy radiation into the substrate.Comment: 13 pages, 10 figure

Metal production in M33: space and time variations

Nearby galaxies are ideal places to study in detail metallicity gradients and their time evolution. We consider chemical abundances of a new sample of \hii\ regions complemented with previous literature data-sets. We compare \hii\ region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we follow the time evolution of the baryonic components and chemical abundances in the disk of M33, assuming that the galaxy is accreting gas from an external reservoir. Supported by a uniform sample of nebular spectroscopic observations, we conclude that: {\em i}) the metallicity distribution in M33 is very complex, showing a central depression in metallicity probably due to observational bias; {\em ii}) the metallicity gradient in the disk of M33 has a slope of -0.037$\pm$ 0.009 dex kpc$^{-1}$ in the whole radial range up to $\sim$8 kpc, and -0.044$\pm$ 0.009 dex kpc$^{-1}$ excluding the central kpc; {\em iii}) there is a small evolution of the slope with time from the epoch of PN progenitor formation to the present-time.}Comment: A&A accepted, 15 Pags, 13 Figs, language correctio

Galactic chemical evolution of heavy elements: from Barium to Europium

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable to reproduce a large set of Galactic (local and non local) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, it shows a sharp production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution given by the Galactic chemical evolution model at the epoch of the solar system formation. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars) showing that the observed trends can be understood in the light of the present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.Comment: 34 pages, 13 figures. accepted by Ap

Evolution of Li, Be and B in the Galaxy

In this paper we study the production of Li, Be and B nuclei by Galactic cosmic ray spallation processes. We include three kinds of processes: (i) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes); (ii) spallation by CNO cosmic ray nuclei impinging on interstellar p and 4He (inverse processes); and (iii) alpha-alpha fusion reactions. The latter dominate the production of 6Li and 7Li. We calculate production rates for a closed-box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multi-zone multi-population model we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem, but instead are the consequences of inaccurate treatments of Galactic evolution.Comment: 26 pages, 5 figures, LaTeX. The Astrophysical Journal, in pres

The building up of the disk galaxy M33 and the evolution of the metallicity gradient

The evolution of radial gradients of metallicity in disk galaxies and its relation with the disk formation are not well understood. Theoretical models of galactic chemical evolution make contrasting predictions about the time evolution of metallicity gradients. To test chemical evolution models and trace the star formation and accretion history of low luminosity disk galaxies we focus on the Local Group galaxy M33. We analyze O/H and S/H abundances in planetary nebulae, H{\sc ii} regions, and young stars, together with known [Fe/H] abundances in the old stellar population of M33. With a theoretical model, we follow the time evolution of gas (diffuse and condensed in clouds), stars, and chemical abundances in the disk of M33, assuming that the galaxy is accreting gas from an external reservoir. Our model is able to reproduce the available observational constraints on the distribution of gas and stars in M33 and to predict the time evolution of several chemical abundances. In particular, we find that a model characterized by a continuous infall of gas on the disk, at a rate of $\dot M_{\rm inf}\approx 1$ $M_\odot$ yr$^{-1}$, almost constant with time, can also account for the relatively high rate of star formation and for the shallow chemical gradients. Supported by a large sample of high resolution observations for this nearby galaxy, we conclude that the metallicity in the disk of M33 has increased with time at all radii, with a continuous flattening of the gradient over the last $\sim 8$ Gyr.Comment: 16 pages, 11 figures, A&A accepte

Modeling the radial abundance distribution of the transition galaxy ngc 1313

NGC 1313 is the most massive disk galaxy showing a flat radial abundance distribution in its interstellar gas, a behavior generally observed in magellanic and irregular galaxies. We have attempted to reproduce this flat abundance distribution using a multiphase chemical evolution model, which has been previously used sucessfully to depict other spiral galaxies along the Hubble morphological sequence. We found that it is not possible to reproduce the flat radial abundance distribution in NGC 1313, and at the same time, be consistent with observed radial distributions of other key parameters such the surface gas density and star formation profiles. We conclude that a more complicated galactic evolution model including radial flows, and possibly mass loss due to supernova explosions and winds, is necessary to explain the apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri

Infrared, UV/VIS and Raman Spectroscopy of Comet Wild-2 Samples Returned by the Stardust Mission

Results from the preliminary examination of Stardust samples obtained using various spectroscopic methods will be presented

The stellar populations of spiral disks.II Measuring and modeling the radial distribution of absorption spectral indices

The radial distributions of the Mg2 and Fe5270 Lick spectral indices have been measured to large radial distances on the disks of NGC 4303 and NGC 4535 using an imaging technique based on interference filters. These data, added to those of NGC 4321 previously published in Paper I of this series are used to constraint chemical (multiphase) evolutionary models for these galaxies. Because the integrated light of a stellar disk is a time average over the history of the galaxy weighted by the star formation rate, these constraints complement the information on chemical gradients provided by the study of HII regions which, by themselves, can only provide the alpha-elements abundance accumulate over the life of the galaxy. The agreement between the observations and the model predictions shown here lends confidence to the models which are then used to describe the time evolution of galaxy parameters such as star formation rates, chemical gradients, and gradients in the mean age of the stellar population.Comment: to be published in Astrophysical Journa