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

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

Teoria di Gauge Abeliana di Chern-Simons, Integrale Funzionale e Invarianti di 3-varieta'

In questa tesi mi occupo della teoria di campo di Chern-Simons (CS), una teoria di gauge che non descrive un insieme di particelle, e le osservabili corrispondono ad invarianti topologici. La teoria di CS può essere risolta esattamente non solo in R3 (o S 3 ), ma in qualsiasi 3-varietà M chiusa ed orientata. Questo risultato può essere ottenuto in due differenti modi. Nel primo si utilizza un metodo combinatorico basato sulla chirurgia, attraverso il quale è possibile ricostruire i valori di aspettazione in M a partire dai risultati noti in R3 . Il secondo è l'approccio fisico standard, nel quale si danno i campi, l'azione e le regole per l'integrazione funzionale in M. Mostreremo che i risultati sono equivalenti. Nel primo capitolo introduttivo vengono discussi alcuni aspetti generali della teoria come le proprietà dell'azione, contenuto in particelle, anticipando alcuni risultati sui valori d'aspettazione. Il secondo capitolo ha lo scopo di introdurre alcune nozioni di base di teoria dei nodi che ritroveremo per tutto il resto del lavoro. Nel terzo capitolo viene studiata la teoria in R3 utilizzando i metodi standard delle teorie di campo; in particolare sono calcolati i propagatori e i funzionali generatori per le funzioni di correlazione dei campi A e della curvatura F. Il significato delle osservabili è inoltre discusso, e ne viene calcolato il valore d'aspettazione sia utilizzando uno sviluppo perturbativo, che un approccio non perturbativo. A partire dal quarto capitolo, viene discussa l'estensione della teoria in varietà M differenti da R3 (utilizzando la chirurgia di Dehn). In particolare, viene mostrato brevemente come come sia possibile risolvere la teoria in M utilizzando i risultati noti in R3 (o S 3 ). Il quinto capitolo è interamente dedicato allo studio delle connessioni di gauge A in M mediante l'utilizzo della nozione di fibrato. Alla fine, si mostra che è possibile esprimere A in termini di classi di Deligne-Beilinson. Il sesto capitolo rappresenta la parte più importante di questo lavoro di tesi. Il funzionale d'azione di CS viene esteso da R3 ad una 3-varietà generica M.Viene studiata la struttura dello spazio delle configurazioni, ovvero delle orbite di gauge, per le connessioni U(1). L'integrazione funzionale è modellata sulla struttura delle orbite di gauge e si mostra che l'integrale funzionale assume la forma di una somma di integrali funzionali sulle 1-forme in presenza di configurazioni di background, che sono in corrispondenza biunivoca con gli elementi del primo gruppo di omologia di M. Viene calcolato esplicitamente sia le funzioni di partizioni normalizzate che i valori di aspettazione utilizzando unicamente l'integrazione funzionale in una generica varietà. In particolare si mostra come sia possibile ottenere invarianti per 3-varietà direttamente dall'integrazione funzionale. L'ultimo capitolo è destinato agli esempi, e vengono confrontati i risultati ottenuti con l'integrazione funzionale e ottenuti mediante metodi combinatorici

The dawn of the chemical evolution of the Universe

A recently published review paper summarizes our current understanding of the origin of the simplest chemical species in the Universe, synthesized soon after the epoch of recombination about 400,000 years after the Big Bang. These primitive molecules constituted the cooling agents that allowed the primordial gas to form the first stars, in turn producing the heavy elements on which our life is based.Un articolo di rassegna recentemente pubblicato fa il punto sull’origine delle prime molecole, formate nell’Universo successivamente all’epoca della ricombinazione avvenuta circa 400,000 anni dopo il Big Bang. Queste molecole primitive svolsero il ruolo di agenti di raffreddamento del gas primordiale, permettendo la formazione delle prime stelle, che, a loro volta, generarono gli elementi pesanti alla base della vita sulla Terra

On the relative abundance of LiH and LiH+ molecules in the early universe: new results from quantum reactions

The relative efficiencies of the chemical pathways that can lead to the destruction of LiH and LiH+ molecules, conjectured to be present in the primordial gas and to control molecular cooling processes in the gravitational collapse of the post-recombination era, are revisited by using accurate quantum calculations for the several reactions involved. The new rates are employed to survey the behavior of the relative abundance of these molecules at redshifts of interest for early universe conditions. We find significant differences with respect to previous calculations, the present ones yielding LIH abundances higher than LiH+ at all redshifts.Comment: The Astrophysical Journal, in pres

Star formation in disk galaxies driven by primordial H_2

We show that gaseous \HI disks of primordial composition irradiated by an external radiation field can develop a multiphase medium with temperatures between 10^2 and 10^4 K due to the formation of molecular hydrogen. For a given \HI column density there is a critical value of the radiation field below which only the cold \HI phase can exist. Due to a time decreasing quasar background, the gas starts cooling slowly after recombination until the lowest stable temperature in the warm phase is reached at a critical redshift $z=z_{cr}$. Below this redshift the formation of molecular hydrogen promotes a rapid transition towards the cold \HI phase. We find that disks of protogalaxies with 10^{20}\simlt N_{HI}\simlt 10^{21} cm^{-2} are gravitationally stable at $T\sim 10^4$ K and can start their star formation history only at z \simlt z_{cr}\sim 2, after the gas in the central portion of the disk has cooled to temperatures T\simlt 300 K. Such a delayed starbust phase in galaxies of low gas surface density and low dynamical mass can disrupt the disks and cause them to fade away. These objects could contribute significantly to the faint blue galaxy population.Comment: 16 pages (LaTeX), 2 Figures to be published in Astrophysical Journal Letter

The link of biocompatibility to cytokine production

The link of biocompatibility to cytokine production. Recent studies suggest that chronic inflammation plays a role in the pathogenesis of cardiovascular disease. Cytokines released from jeopardized tissues stimulate the liver to synthesize acute phase proteins, including C-reactive protein (CRP). Baseline levels of CRP in apparently healthy persons or in persons with unstable angina constitute an independent risk factor for cardiovascular events. More recently, it has been suggested that CRP is useful not only as a marker of the acute phase response, but is also involved in the pathogenesis of the disease. CRP may, in fact, directly interact with the atherosclerotic vessels or ischemic myocardium by activation of the complement system, thereby promoting inflammation and thrombosis. Several studies in uremic patients have implicated CRP as a marker of malnutrition, resistance to erythropoietin, and chronic stimulation in hemodialysis. An increased cytokine production secondary to blood interaction with bioincompatible dialysis components has been reported by several studies; interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), and mainly IL-6 are the three proinflammatory cytokines involved in the pathogenesis of hemodialysis-related disease. We have provided evidence for the occurrence of high CRP and IL-6 levels in chronic dialytic patients exposed to contaminate dialysate and suggest that backfiltration may induce a chronic, slowly developing inflammatory state that may be abrogated by avoiding backfiltration of contaminate dialysate. Therefore, CRP is implicated as a marker linking bioincompatibility associated with backfiltration and increased cytokine production with a clinical state of chronic inflammation