CRASH: a Radiative Transfer Scheme

We present a largely improved version of CRASH, a 3-D radiative transfer code that treats the effects of ionizing radiation propagating through a given inhomogeneous H/He cosmological density field, on the physical conditions of the gas. The code, based on a Monte Carlo technique, self-consistently calculates the time evolution of gas temperature and ionization fractions due to an arbitrary number of point/extended sources and/or diffuse background radiation with given spectra. In addition, the effects of diffuse ionizing radiation following recombinations of ionized atoms have been included. After a complete description of the numerical scheme, to demonstrate the performances, accuracy, convergency and robustness of the code we present four different test cases designed to investigate specific aspects of radiative transfer: (i) pure hydrogen isothermal Stromgren sphere; (ii) realistic Stromgren spheres; (iii) multiple overlapping point sources, and (iv) shadowing of background radiation by an intervening optically thick layer. When possible, detailed quantitative comparison of the results against either analytical solutions or 1-D standard photoionization codes has been made showing a good level of agreement. For more complicated tests the code yields physically plausible results, which could be eventually checked only by comparison with other similar codes. Finally, we briefly discuss future possible developments and cosmological applications of the code.Comment: 17 pages, 14 figures, accepted for pubblication in MNRAS, high res figures available at http://www.arcetri.astro.it/science/cosmology/IGM/radtrans.htm

CRASH3: cosmological radiative transfer through metals

Here we introduce CRASH3, the latest release of the 3D radiative transfer code CRASH. In its current implementation CRASH3 integrates into the reference algorithm the code Cloudy to evaluate the ionisation states of metals, self-consistently with the radiative transfer through H and He. The feedback of the heavy elements on the calculation of the gas temperature is also taken into account, making of CRASH3 the first 3D code for cosmological applications which treats self-consistently the radiative transfer through an inhomogeneous distribution of metal enriched gas with an arbitrary number of point sources and/or a background radiation. The code has been tested in idealized configurations, as well as in a more realistic case of multiple sources embedded in a polluted cosmic web. Through these validation tests the new method has been proven to be numerically stable and convergent. We have studied the dependence of the results on a number of physical quantities such as the source characteristics (spectral range and shape, intensity), the metal composition, the gas number density and metallicity.Comment: accepted for publication in MNRA

Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator

The aim of this work is to model the jets produced by conical wire arrays on the MAGPIE generator, and to design and test new setups to strengthen the link between laboratory and astrophysical jets. We performed the modelling with direct three-dimensional magneto-hydro-dynamic numerical simulations using the code GORGON. We applied our code to the typical MAGPIE setup and we successfully reproduced the experiments. We found that a minimum resolution of approximately 100 is required to retrieve the unstable character of the jet. We investigated the effect of changing the number of wires and found that arrays with less wires produce more unstable jets, and that this effect has magnetic origin. Finally, we studied the behaviour of the conical array together with a conical shield on top of it to reduce the presence of unwanted low density plasma flows. The resulting jet is shorter and less dense.Comment: Accepted for publication in Astrophysics & Space Science. HEDLA 2010 conference procedings. Final pubblication will be available on Springe

Inhomogeneous Reionization Regulated by Radiative and Stellar Feedbacks

We study the inhomogeneous reionization in a critical density CDM universe due to stellar sources, including Population III objects. The spatial distribution of the sources is obtained from high resolution numerical N-body simulations. We calculate the source properties taking into account a self-consistent treatment of both radiative (ie ionizing and H2 -photodissociating photons) and stellar (ie SN explosions) feedbacks regulated by massive stars. This allows us to describe the topology of the ionized and dissociated regions at various cosmic epochs and derive the evolution of H, He, and H2 filling factors, soft UV background, cosmic star formation rate and the final fate of ionizing objects. The main results are: (i) galaxies reionize the IGM by z~10 (with some uncertainty related to the gas clumping factor), whereas H2 is completely dissociated already by z~25; (ii) reionization is mostly due to the relatively massive objects which collapse via H line cooling, while objects whose formation relies on H2 cooling alone are insufficient to this aim; (iii) the diffuse soft UV background is the major source of radiative feedback effects for z<15; at higher z direct flux from neighboring objects dominates; (iv) the match of the calculated cosmic star formation history with the one observed at lower redshifts suggests that the conversion efficiency of baryons into stars is ~1%; (v) we find that a very large population of dark objects which failed to form stars is present by z~8. We discuss and compare our results with similar previous studies.Comment: 34 pages, emulateapj.sty, LaTeX, 13 figures. MNRAS, submitte

Reionization History from Coupled CMB/21cm Line Data

We study CMB secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code CRASH. The reionization history is consistent with the WMAP Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for l > 4000 and reaches a maximum amplitude of l(l+1)C_l/2\pi ~ 1.6 x 10^{-13} on arcmin scale, i.e. l as large as several thousands. We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21cm line emission fluctuations obtained from the same simulations. The two signals are highly anti-correlated on angular scales corresponding to the typical size of HII regions (including overlapping) at the 21cm map redshift. We show how the CMB/21cm cross-correlation can be used to: (a) study the nature of the reionization sources, (b) reconstruct the cosmic reionization history, (c) infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities.Comment: 7 pages, 5 figures, MNRAS in pres

Detecting Unresolved Binaries in TESS Data with Speckle Imaging

The Transiting Exoplanet Survey Satellite (TESS) is conducting a two-year wide-field survey searching for transiting exoplanets around nearby bright stars that will be ideal for follow-up characterization. To facilitate studies of planet compositions and atmospheric properties, accurate and precise planetary radii need to be derived from the transit light curves. Since 40 - 50% of exoplanet host stars are in multiple star systems, however, the observed transit depth may be diluted by the flux of a companion star, causing the radius of the planet to be underestimated. High angular resolution imaging can detect companion stars that are not resolved in the TESS Input Catalog, or by seeing-limited photometry, to validate exoplanet candidates and derive accurate planetary radii. We examine the population of stellar companions that will be detectable around TESS planet candidate host stars, and those that will remain undetected, by applying the detection limits of speckle imaging to the simulated host star populations of Sullivan et al. (2015) and Barclay et al. (2018). By detecting companions with contrasts of delta m < 7 - 9 and separations of ~0.02 - 1.2'', speckle imaging can detect companion stars as faint as early M stars around A - F stars and stars as faint as mid-M around G - M stars, as well as up to 99% of the expected binary star distribution for systems located within a few hundred parsecs.Comment: Accepted for publication in The Astronomical Journal; 16 pages, 8 figures, 2 table

On the Formation of Cathedral Chapters and Cathedral Culture : Lund, Denmark, and Scandinavia, c. 1060–1225

The cathedral was one of the most important institutions in medieval Europe. The local as well as ecclesiastical elite gathered around it and its bishop; the liturgy was celebrated day and night, year after year; the cathedral served as educational institution of the clergy. The cathedral chapter, i.e. the organized clergy that served at the cathedral, played a decisive part in all this.The overarching purpose of this doctoral thesis has been to re-interpret essential aspects of the formation and functions of cathedral chapters in Scandinavia in the period c. 1060–1225. This is achieved through a number of articles on the cathedral chapter at St Lawrence’s in Lund, and, to a lesser extent, other cathedral chapters in medieval Denmark and Scandinavia. After a general introduction on the subject, starting with clarifications of those concepts that have been essential to this study and my understanding and application of them, i.e. ‘formation’, ‘functions’, ‘cathedral chapter’, and ‘cathedral culture’, I recapitulate what has been achieved, misinterpreted, or even neglected by previous scholarship by focusing on the formation and functions of cathedral chapters in a Scandinavian setting. In this section, I also present my own hypotheses and methodological approaches and define the chronological and geographical delimitations of this work. Finally, I introduce the sources of this work by categorizing them. In the second major part of the introduction, I present the context, hypotheses, methods, and results of the articles; I also present and discuss whether there has been any subsequent research or similar approaches on the various topics dealt with.In my articles, I consider four general topics: first, the textual transmission of normative text of the cathedral chapter in Lund and especially the customary, the Consuetudines Lundenses from c. 1120 (Articles 1 and 4); second, the concepts of ‘law & learning’ are examined in the context of the cathedral chapter at St Lawrence’s in Lund in the first quarter of the twelfth century (Article 2). Third, by focusing on the cult of saints in Scandinavia c. 1000–c. 1200 in relation to the emerging capitular institutions, I both introduce a new concept, ‘cathedral culture’, and demonstrate the importance of the local cathedral clergy and chapter are in the introduction and maintenance of both foreign and local saints’ cults (Article 3). Finally, I examine the various legal developments within the process of appointing bishops that were practiced in Latin Christendom until the mid-1220s (Article 5), arriving at the conclusion that this process and the formation of cathedral chapters were reciprocal and resulted in a more legally regulated electoral procedure.The major results of this work are the following. First, by a redefinition of the concept of ‘cathedral chapter’, meaning that any clerical organization occupied with the liturgical service at the cathedral church of the diocese could be designated as such rather than define this institution from those legal or administrative functions that were added later, and by demonstrating that the liturgical function was the primary function of the capitular institution throughout the Middle Ages, I contribute to a new, more organic view of this ecclesiastical institution. Second, by using a more flexible terminology, I demonstrate that a focus on the process of formation rather than the search for a fixed date of foundation is rewarding when dealing with the capitular institutions in this period, which are characterized by institutional, legal, and ideological transition. Finally, I introduce a new conceptual framework, ‘cathedral culture’, which not only facilitates our understanding of the activities undertaken and performed by the chapters and their members in the cathedral and its surrounding milieu, i.e. as agents of ecclesiastical tradition, but also this concept helps us to see the emergence of a specific culture out of a monastic, contemplative culture and closely related to a nascent scholastic, erudite culture