Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines

Context. The accretion process in Classical T Tauri Stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims. We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams.We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods. We model the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We explore different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) line profiles. Results. The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of C IV doublet are characterized by two main components: one narrow and redshifted to speed $\approx$ 50 km s$^{-1}$ and the other broader and consisting of subcomponents with redshift to speed in the range 200 $\approx$ 400 km s$^{-1}$. The profiles of OVIII lines appear more symmetric than C IV and are redshifted to speed $\approx$ 150 km s$^{-1}$. Conclusions. Our model predicts profiles of C IV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation.Comment: 11 pages, 10 figure

Coronal fuzziness modelled with pulse-heated multistranded loop systems

Coronal active regions are observed to get fuzzier and fuzzier (i.e. more and more confused and uniform) in harder and harder energy bands or lines. We explain this evidence as due to the fine multi-temperature structure of coronal loops. To this end, we model bundles of loops made of thin strands, each heated by short and intense heat pulses. For simplicity, we assume that the heat pulses are all equal and triggered only once in each strand at a random time. The pulse intensity and cadence are selected so as to have steady active region loops ($\sim 3$ MK), on the average. We compute the evolution of the confined heated plasma with a hydrodynamic loop model. We then compute the emission along each strand in several spectral lines, from cool ($\leq 1$ MK), to warm ($2-3$ MK) lines, detectable with Hinode/EIS, to hot X-ray lines. The strands are then put side-by-side to construct an active region loop bundle. We find that in the warm lines ($2-3$ MK) the loop emission fills all the available image surface. Therefore the emission appears quite uniform and it is difficult to resolve the single loops, while in the cool lines the loops are considerably more contrasted and the region is less fuzzy. The main reasons for this effect are that, during their evolution, i.e. pulse heating and slow cooling, each strand spends a relatively long time at temperatures around $2-3$ MK, and that it has a high emission measure during that phase, so the whole region appears more uniform or smudged. We make the prediction that the fuzziness should be reduced in the hot UV and X-ray lines.Comment: 27 pages, 14 figure

The Sun as an X-Ray Star. IV. The Contribution of Different Regions of the Corona to Its X-Ray Spectrum

We study X-ray-synthesized spectra of solar regions as templates to interpret analogous stellar spectra. We define three classes of coronal structures of different brightness, low (background quiet corona), medium (active regions), and high (active region cores), and determine their contribution to the solar X-ray emission measure versus temperature, EM(T), luminosity, and spectrum. This study defines the extent of the solar analogy quantitatively and accurately. To this end, we have selected a large sample of full-disk Yohkoh soft X-ray telescope observations taken between the maximum and the minimum of solar cycle 22, obtaining the contribution of each class to the whole Sun's EM(T). From the EM(T) distributions, we synthesize the X-ray spectra of the Sun and of the single classes of solar coronal regions as they would be collected with the ROSAT Position Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging Spectrometer. We find that the Sun during the cycle fits well in the stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio (HR) and surface X-ray flux, FPSPC, both increase going from the background corona to the active regions and the cores of the active regions, and range between the values of low and intermediate activity stars. We suggest that the coronae of these stars may be explained as the effect of structures similar to those present on the Sun and that the various levels of X-ray luminosity, HR, and FPSPC are achieved by changing the surface coverage of the different classes of coronal regions

A Interpretação de Platão Inaugurada pela Escola de Tübingen e por mim Apresentada em Sentido Epistemológico como “Paradigma Hermenêutico” Alternativo Àquele Dominante

Este texto versa, em primeiro lugar, sobre a interpretação de Platão inaugurada pela Escola de Tübingen, explicando como essa escola, com fino trato histórico, leva em conta a especificidade da revolução cultural em curso na época de Platão. Na sequência, explicita as razões que motivaram o autor do texto que ora se apresenta a que fizesse em anos idos uma reapresentação da nova interpretação de Platão. Não fica de fora um importante juízo de Hans Krämer sobre essa reapresentação. Finalmente, comenta as publicações dos anos oitenta em diante sobre Platão e a história do platonismo, e sobre as contribuições de teóricos recentes, em defesa do novo paradigma de leitura das obras de Platão.Este texto versa, em primeiro lugar, sobre a interpretação de Platão inaugurada pela Escola de Tübingen, explicando como essa escola, com fino trato histórico, leva em conta a especificidade da revolução cultural em curso na época de Platão. Na sequência, explicita as razões que motivaram o autor do texto que ora se apresenta a que fizesse em anos idos uma reapresentação da nova interpretação de Platão. Não fica de fora um importante juízo de Hans Krämer sobre essa reapresentação. Finalmente, comenta as publicações dos anos oitenta em diante sobre Platão e a história do platonismo, e sobre as contribuições de teóricos recentes, em defesa do novo paradigma de leitura das obras de Platão.This text is, first, on the interpretation of Plato inaugurated by Tübingen’s School of interpretation of Plato, explaining how this school, with accurate historical trait, takes into account the specificity of cultural revolution in progress at the time of Plato. In sequence, it explains the reasons that motivated the author of this paper, some years ago, to make a replay of the new interpretation of Plato. This paper brings forth an important opinion of Hans Krämer about this replay. Finally, comments are made on publications from the eighties until now on Plato and the history of Platonism, and on recent theoretical contributions in defense of the new paradigm of reading the works of Plato.Este texto versa, em primeiro lugar, sobre a interpretação de Platão inaugurada pela Escola de Tübingen, explicando como essa escola, com fino trato histórico, leva em conta a especificidade da revolução cultural em curso na época de Platão. Na sequência, explicita as razões que motivaram o autor do texto que ora se apresenta a que fizesse em anos idos uma reapresentação da nova interpretação de Platão. Não fica de fora um importante juízo de Hans Krämer sobre essa reapresentação. Finalmente, comenta as publicações dos anos oitenta em diante sobre Platão e a história do platonismo, e sobre as contribuições de teóricos recentes, em defesa do novo paradigma de leitura das obras de Platão.Este texto versa, em primeiro lugar, sobre a interpretação de Platão inaugurada pela Escola de Tübingen, explicando como essa escola, com fino trato histórico, leva em conta a especificidade da revolução cultural em curso na época de Platão. Na sequência, explicita as razões que motivaram o autor do texto que ora se apresenta a que fizesse em anos idos uma reapresentação da nova interpretação de Platão. Não fica de fora um importante juízo de Hans Krämer sobre essa reapresentação. Finalmente, comenta as publicações dos anos oitenta em diante sobre Platão e a história do platonismo, e sobre as contribuições de teóricos recentes, em defesa do novo paradigma de leitura das obras de Platão

A interpretação de Platão inaugurada pela escola de Tübingen e por mim apresentada em sentido epistemológico como "paradigma hermenêutico" alternativo àquele dominante

This text is, first, on the interpretation of Plato inaugurated by Tubingen's School of interpretation of Plato, explaining how this school, with accurate historical trait, takes into account the specificity of cultural revolution in progress at the time of Plato. In sequence, it explains the reasons that motivated the author of this paper, some years ago, to make a replay of the new interpretation of Plato. This paper brings forth an important opinion of Hans Kramer about this replay. Finally, comments are made on publications from the eighties until now on Plato and the history of Platonism, and on recent theoretical contributions in defense of the new paradigm of reading the works of Plato

Large Scale Properties of Coronal Heating along the Solar Cycle

We discuss various studies of the global properties of coronal heating. Some of them find power laws tying the X-ray luminosity with the magnetic flux of individual structures, of the whole Sun, and of active solar-type stars. Others are based on methods to model the Sun as an X-ray star. We also briefly discuss solar-like active stars and how the Sun fits in the whole scenario. We use a new model, including all flares, of the Sun as an X-ray star to describe the evolution of the corona along the solar cycle and the implications on the heating of closed coronal structures. We point out that, as activity increases, more heating is released into the confined coronal plasma and such a heating has to be, on average, more intense in order to explain the widespread evidence of a temperature increase with activity. By the same token, nanoflare heating (if existent) has to increase and decrease along the cycle differently from flares

Modeling an X-ray flare on Proxima Centauri: Evidence of two flaring loop components and of two heating mechanisms at work

We model in detail a flare observed on Proxima Centauri with the EPIC-PN on board XMM-Newton at high statistics and high time resolution and coverage. Time-dependent hydrodynamic loop modeling is used to describe the rise and peak of the light curve, and a large fraction of the decay, including its change of slope and a secondary maximum, over more than 2 h. The light curve, the emission measure and the temperature derived from the data allow us to constrain the loop morphology and the heating function and to show that this flare can be described with two components: a major one triggered by an intense heat pulse injected in a single flaring loop with half-length ≈1.0 × 1010 cm, the other one by less intense heat pulses released about 1/2 h after the first one in related loop systems, probably arcades, with the same half-length. The heat functions of the two loop systems appear very similar: an intense pulse located at the loop footpoints followed by a low gradual decay distributed in the coronal part of the loop. The latter result and the similarity to at least one solar event (the Bastille Day flare in 2000) indicate that this pattern may be common to solar and stellar flares. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member states and the USA (NASA)

A multidisciplinary diagnostic approach preliminary to the restoration of the country church "San Maurizio" located in Ittiri (SS), Italy

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