635 research outputs found
Dust photophoretic transport in a disk irradiated by an evolving PMS star
No full textAs reported by Tielens et al. (2005) [14] the interstellar medium (ISM) is very poor in crystalline solids. For instance, Kemper et al. (2004) [7] well reproduce the interstellar absorption band using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, leading to a crystalline fraction of the interstellar silicates around 0.2% by mass. The proto-solar nebula is supposed to be formed from the material coming from the ISM. As a consequence, the primordial dust in the solar system should be composed of amorphous solids excepted grains which have undergone thermal annealing in high temperature regions (i.e. around 1000-1500 K) close to the star (i.e. r . 1 - 2 AU). Besides this, comets are presumed to have formed in the cold outer part of the solar nebula. Campins & Ryan (1989) have found that crystalline olivine is a major component of the silicates in comet Halley. Stardust samples of comet 81P/Wild 2 include large singleminerals crystals and X-raymicroscopic analysis leads to a crystal mass fraction fcryst larger than ~ 50% (Ogliore et al. 2009, Brownlee et al. 2006). More generally comets have a fcryst larger than ~ 20% (Lindsay et al. 2013, Kelley & Wooden, 2009). This discrepancy between ISMgrains crystallinity and comets one is the mark of a radial transport process occuring in the accretion disk. Several transport processes have been proposed to explain the presence of these refractory material in comets: annealing by shock waves in the outer solar nebula (Harker & Desch 2002) [3], radial mixing processes by turbulent diffusion (Gail, 2001; Wehrstedt & Gail, 2002; Bockelée-Morvan et al., 2002) or transport by photophoresis (Mousis et al., 2007 andMoudens et al., 2011). In this paper, we focus on this last process for which we employ a 1+1D accretion disk model irradiated by a pre-main sequence (PMS) star. The model of nebula and the model of star are both evolving. Concerning the photophoresis itself, essentially, we have followed the approach developped by other authors in the past (Krauss & Wurm, 2005, Krauss et al. 2007, Mousis et al., 2007 and Moudens et al., 2011) [5, 6, 9, 10]; but several crucial aspects have been improved: the estimation of the free mean path of the molecules of gases and the computation of the viscosity, the opacity the disk material is no longer taken constant but rather depends on the distance to the central star, while the opacity formulae come from the Bell & Lin (1994) [1]. We have developed a 1+1D irradiated disk model which evolution is based on the equation introduced by Lynden-Bell & Pringle (1974) and Pringle (1981) [8, 12] where Sigma-w is the photoevaporation and the turbulent viscosity computed in the frame of the Shakura & Sunayev (1973) [13] formalism. The vertical structure of the disk, required to derive the values of , has been obtained by integrating the equations used by Papaloizou & Terquem (1999) [11]. In addition, the irradiation by the central PMS star is included by modifying the temperature boundary condition of the vertical structure (Hueso & Guillot, 2005) [4]. The disk follows the stellar track (Teff, R⋆) computed by Tognelli et al. (2011) [15]. Both vertical structure model and disk temporal evolution programs have been implemented from scratch and parallelized using Open Multi-Processing (OpenMP). We have nicknamed the whole package EvAD1. In this paper we have study the influence of the PMS star luminosity on the disk structure and its evolution. The modeled T Tauri star has a phase with a luminosity significantly larger than the current solar one. Then, we discuss the necessity of hypothesizing a central hole in the accretion disk. Other effects are also discussed, like the influence of the thermal conductivity of dust aggregates
Standard pre-main sequence models of low-mass stars
No full textThe main characteristics of standard pre-main sequence (PMS) models are described. A discussion of the uncer-tainties affecting the current generation of PMS evolutionary tracks and isochrones is also provided. In particular, the impact of the uncertainties in the adopted equation of state, radiative opacity, nuclear cross sections, and initial chemical abundances are analysed
Accreting pre-main-sequence models and abundance anomalies in globular clusters
No full textWe investigated the possibility of producing helium-enhanced stars in globular clusters by accreting polluted matter during the pre-main-sequence phase. We followed the evolution of two different classes of pre-main-sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the protostellar evolution. The later is the beginning of the late accretion and the lower are both the central helium and the surface lithium abundances at the end of the accretion phase and in Zero AgeMain Sequence (ZAMS). In order to produce a relevant increase of the central helium content the accretion of polluted matter should start at ages lower than 1 Myr. The inclusion of the protostellar evolution has a strong impact on the ZAMS models too. The adoption of a very low seed mass (i.e. 0.001 M⊙) results in models with the lowest central helium and surface lithium abundances. The higher is the accretion rate and the lower is the final helium content in the core and the residual surface lithium. In the worst case - i.e. seed mass 0.001 M⊙ and accretion rate ≥10-5 M⊙ yr-1 - the central helium is not increased at all and the surface lithium is fully depleted in the first few million years
Cumulative theoretical uncertainties in lithium depletion boundary age
No full textWe performed a detailed analysis of the main theoretical uncertainties affecting the age at the lithium depletion boundary (LDB). To do that we computed almost 12 000 pre-mainsequence models with mass in the range [0.06, 0.4] M ̇ by varying input physics (nuclear reaction cross-sections, plasma electron screening, outer boundary conditions, equation of state, and radiative opacity), initial chemical elements abundances (total metallicity, helium and deuterium abundances, and heavy elements mixture), and convection efficiency (mixing length parameter, αML). As a first step, we studied the effect of varying these quantities individually within their extreme values. Then, we analysed the impact of simultaneously perturbing the main input/parameters without an a priori assumption of independence. Such an approach allowed us to build for the first time the cumulative error stripe, which defines the edges of the maximum uncertainty region in the theoretical LDB age. We found that the cumulative error stripe is asymmetric and dependent on the adopted mixing length value. For αML = 1.00, the positive relative age error ranges from 5 to 15 per cent, while for solarcalibrated mixing length, the uncertainty reduces to 5-10 per cent. A large fraction of such an error (≈40 per cent) is due to the uncertainty in the adopted initial chemical elements abundances
Effect of planet ingestion on low-mass stars evolution: The case of 2MASS J08095427-4721419 star in the Gamma Velorum cluster
No full textWe analysed the effects of planet ingestion on the characteristics of a pre-main-sequence star similar to the Gamma Velorum cluster member 2MASS J08095427-4721419 (#52). We discussed the effects of changing the age t0 at which the accretion episode occurs, the mass of the ingested planet and its chemical composition. We showed that the mass of the ingested planet required to explain the current [Fe/H]#52 increases by decreasing the age t0 and/or by decreasing the iron content of the accreted matter. We compared the predictions of a simplified accretion method - where only the variation of the surface chemical composition is considered - with that of a full accretion model that properly accounts for the modification of the stellar structure. We showed that the two approaches result in different convective envelope extension which can vary up to 10 per cent. We discussed the impact of the planet ingestion on a stellar model in the colour-magnitude diagram, showing that a maximum shift of about 0.06 dex in the colour and 0.07 dex in magnitude are expected and that such variations persist even much later the accretion episode. We also analysed the systematic bias in the stellar mass and age inferred by using a grid of standard non-accreting models to recover the characteristics of an accreting star. We found that standard non-accreting models can safely be adopted for mass estimate, as the bias is ≲6 per cent, while much more caution should be used for age estimate where the differences can reach about 60 per cent
ÆSOPUS 2.0: Low-temperature Opacities with Solid Grains
No full textIn this study we compute the equation of state and Rosseland mean opacity from temperatures of T ≃ 30,000 K down to T ≃ 400 K, pushing the capabilities of the ÆSOPUS code into the regime where solid grains can form. The GGchem code is used to solve the chemistry for temperatures less than ≃3000 K. Atoms, molecules, and dust grains in thermodynamic equilibrium are all included in the equation of state. To incorporate monochromatic atomic and molecular cross sections, an optimized opacity sampling technique is used. The Mie theory is employed to calculate the opacity of 43 grain species. Tables of Rosseland mean opacities for scaled-solar compositions are provided. Based on our computing resources, opacities for other chemical patterns, as well as various grain sizes, porosities, and shapes, can be easily computed upon user request to the corresponding author
7Li abundance in pre-main sequence stars. Testing theory against clusters and binary systems
No full textThe disagreement between theory and observations for surface 7Li abundance is a long standing problem which requires a quantitative analysis of the main uncertainties affecting theoretical models. We performed a detailed evaluation of the errors affecting 7Li abundance predictions coming from the chemical composition and the adopted physical inputs. Then, present 7Li abundances have been compared with the observations available for five young open clusters of different ages and chemical composition, namely, Ic 2602, α Per, Blanco1, Pleiades, and Ngc 2516, and for four detached double-lined eclipsing binary systems. We confirm, for standard models, the disagreement between theoretical predictions and 7Li observations for young open clusters if the same convection efficiency (mixing length parameter) during the pre-main sequence and the main sequence phase is adopted. However, if during the pre-main sequence a convection efficiency lower than the main sequence one is used, a satisfactory agreement with 7Li observations can be achieved. Such value is with a good approximation independent of the age and the metallicity, at least in the range covered by our analysis
7Li in young open clusters and binary systems
No full textThe disagreement between observations and theoretical predictions for surface 7Li abundances is still a challenging issue for stellar modellers. We re-examine the level of disagreement in the case of five young open clusters, namely Ic2602, alpha Per, Pleiades, Blanco 1, and Ngc 2516, and few double-lined eclipsing binaries. The calculations have been performed with our up-to-date standard evolutionary code, paying particular attention to the uncertainty evaluation on surface 7Li. We found that to reproduce the observed Li(T_eff) profile, a convection efficiency much lower than the one needed to fit MS stars colour of each cluster is required. Such value of the mixing length parameter results to be independent of the age and of the chemical composition of the cluster
Il profilo filosofico e culturale di un metafisico neoclassico. A proposito di "Il filosofo e la fede. Il cristianesimo ‘moderno’ di Gustavo Bontadini" di Leonardo Messinese
No full textThis paper examines Leonardo Messinese’s volume, Il filosofo e la fede. Il cristianesimo ‘moderno’ di Gustavo Bon-tadini. From the book emerges a philosophical portrait of the Milanese thinker useful to understand the fun-damental points of his theoretical proposal: neoclassical metaphysics, brought out by the Author in relation to the dispute with Emanuele Severino. Some lesser-known cultural, civil and political aspects of Bontadini’s intellectual biography are also exposed. The intent of the volume is to spread a greater knowledge of the fig-ure of Bontadini within Italian philosophical culture, particularly that of Catholic inspiration
Astrophysical implications of the proton-proton cross section updates
No full textThe p(p, e(+) v(e))H-2 reaction rate is an essential ingredient for theoretical computations of stellar models. In the past several values of the corresponding S-factor have been made available by different authors. Prompted by a recent evaluation of S(E), we analysed the effect of the adoption of different proton-proton reaction rates on stellar models, focusing, in particular, on the age of mid and old stellar clusters (1-12Gyr) and on standard solar model predictions. By comparing different widely adopted p(p, e(+) v(e))H-2 reaction rates, we found a maximum difference in the temperature regimes typical of main sequence hydrogen-burning stars (5 x 10(6)-3 x 10(7) K) of about 3%. Such a variation translates into a change of cluster age determination lower than 1%. A slightly larger effect is observed in the predicted solar neutrino fluxes with a maximum difference, in the worst case, of about 8%. Finally we also notice that the uncertainty evaluation of the present proton-proton rate is at the level of few parts per thousand, thus the p(p, e(+) v(e))H-2 reaction rate does not constitute anymore a significant uncertainty source in stellar models
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