Interstellar dust charging in dense molecular clouds: cosmic ray effects

The local cosmic-ray (CR) spectra are calculated for typical characteristic regions of a cold dense molecular cloud, to investigate two so far neglected mechanisms of dust charging: collection of suprathermal CR electrons and protons by grains, and photoelectric emission from grains due to the UV radiation generated by CRs. The two mechanisms add to the conventional charging by ambient plasma, produced in the cloud by CRs. We show that the CR-induced photoemission can dramatically modify the charge distribution function for submicron grains. We demonstrate the importance of the obtained results for dust coagulation: While the charging by ambient plasma alone leads to a strong Coulomb repulsion between grains and inhibits their further coagulation, the combination with the photoemission provides optimum conditions for the growth of large dust aggregates in a certain region of the cloud, corresponding to the densities $n(\mathrm{H_2})$ between $\sim10^4$ cm$^{-3}$ and $\sim10^6$ cm$^{-3}$. The charging effect of CR is of generic nature, and therefore is expected to operate not only in dense molecular clouds but also in the upper layers and the outer parts of protoplanetary discs.Comment: accepted by Ap

Petrarca e la Spagna

Spain in Petrarch is a geographical space confused with the myth and the history, above all the Roman history. From one hand the contemporary chronicle, with the military glory of Aragon and Castile, is dissolved in the memory of Rome; on the other hand a legendary past unites Spain to the Pillars of Hercules, with the ‘unintentional’ memory of the insane flight of Ulysses in the Divine Comedy. So Petrarch points at ancient Spain, in connection with Rome and the great Latin culture.L’immagine della Spagna in Petrarca è quella di uno spazio geografico che si confonde con il mito e la storia, soprattutto la storia romana. Da un lato la cronaca contemporanea, legata alla gloria militare di Aragona e Castiglia, si stempera nel ricordo grandioso di Roma; dall’altro un passato leggendario accomuna la Spagna alle colonne d’Ercole, con la memoria ‘involontaria’ del folle volo dell’Ulisse dantesco. Petrarca non può che pensare all’antica Spagna, al suo nesso con Roma e alla grande cultura latina

Peccati e peccatori

Il tema del peccato è tanto diffuso nell'opera volgare e latina di Petrarca da rendere necessario un energico taglio interpretativo, una qualunque delimitazione di campo. Proprio questa ampiezza è però il segno di un eclettismo che mette a confronto linguaggi e specialismi diversi, a volte implicati fra loro sino alla reciproca indistinzione. Quanto segue è un primo percorso fra i peccati di Petrarca, con soste che testimoniano livelli diversi di esercizio delle parole del peccato: un primo livello, interdiscorsivo, di appropriazione terminologica all'interno di un orizzonte, quello lirico del Canzoniere, che lascia affiorare il tema della colpa correlandolo alla fabula dell'amore; e un secondo livello, di sviluppo di campi semantici legati al tema del peccato, sullo sfondo delle opere latine.The topic of sin is so widespread in the vulgar and Latin work of Petrarch that it requires energy with an interpretative edge, one without delimitation of field. This expanse alone is but a sign of an eclecticism which draws comparisons between diverse languages and specialisms, occasionally intertwined to the point of reciprocal indifference. The following is an early journey through the sins of Petrarch, with breaks which witness the different levels of exercise of the words of sin: first, an interdiscursive level on the terminological appropriation within a horizon, such as the verse of the Canzoniere, which delved into the emotion of guilt, in correlation with the fable of love; and second, on the development of semantic fields in relation to sin, on the backdrop of Latin works

Cosmic-ray ionisation in circumstellar discs

Galactic cosmic rays are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions and charged dust grains in molecular clouds and circumstellar discs. We model the propagation of different components of Galactic cosmic rays versus the column density of the gas. Our study is focussed on the propagation at high densities, above a few g cm$^{-2}$, especially relevant for the inner regions of collapsing clouds and circumstellar discs. The propagation of primary and secondary CR particles (protons and heavier nuclei, electrons, positrons, and photons) is computed in the continuous slowing down approximation, diffusion approximation, or catastrophic approximation, by adopting a matching procedure for the different transport regimes. A choice of the proper regime depends on the nature of the dominant loss process, modelled as continuous or catastrophic. The CR ionisation rate is determined by CR protons and their secondary electrons below $\approx 130$ g cm$^{-2}$ and by electron/positron pairs created by photon decay above $\approx600$ g cm$^{-2}$. We show that a proper description of the particle transport is essential to compute the ionisation rate in the latter case, since the electron/positron differential fluxes depend sensitively on the fluxes of both protons and photons. Our results show that the CR ionisation rate in high-density environments, like, e.g., the inner parts of collapsing molecular clouds or the mid-plane of circumstellar discs, is larger than previously assumed. It does not decline exponentially with increasing column density, but follows a more complex behaviour due to the interplay of different processes governing the generation and propagation of secondary particles.Comment: 19 pages, 11 figures, accepted by A&

Il teatro politico di David Fennario

Il cap. 4 del mio libro tratta della play "Banana Boots" (1998) che ripropone una versione drammatizzata del suo più grande successo, Balconville (1979), quando venne rappresentato a Belfast, Irlanda del Nord

"Abolitionism and Black Consciousness: Common Roots in the Eighteenth Century"

This essay concentrates on Eighteenth Century abolitionist debate both in England and France, through a comparative analysis of Enlightenment thought and the first organized societies for slave emancipation. It quotes extensively from Olaudauh Equiano’s Narrative (1789) and other important documents from the period