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Start Over Author "Lanz, T" Topic accretion (astrophysics)
6 results on '"Lanz, T"'

1. 3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects.

Author

Ibgui, L., Orlando, S., Stehlé, C., Chièze, J.-P., Hubeny, I., Lanz, T., de Sá, L., Matsakos, T., González, M., and Bonito, R.

Subjects
ACCRETION (Astrophysics), RADIATIVE transfer equation, MAGNETOHYDRODYNAMICS, STELLAR chromospheres, THERMODYNAMICS research
Abstract

We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our "gray LTE approach", i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD ( magnetohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared. [ABSTRACT FROM AUTHOR]

Published
2014
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2. 3D numerical modeling of YSO accretion shocks.

Author

Matsakos, T., Chièze, J.-P, Stehlé, C., González, M., Ibgui, L., de Sá, L., Lanz, T., Orlando, S., Bonito, R., Argiroffi, C., Reale, F., and Peres, G.

Subjects
ACCRETION (Astrophysics), RADIATIVE transitions, MAGNETIC fields, ENERGY dissipation, STELLAR chromospheres
Abstract

The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modeling locally the impact of the infalling gas onto the chromosphere. We find that the structure and dynamics of the post-shock region is strongly dependent on the plasma-beta (thermal over magnetic pressure), different values of which may give distinguishable emission signatures, relevant for observations. In particular, a strong magnetic field effectively confines the plasma inside its flux tubes and leads to the formation of quasi-independent fibrils. The fibrils may oscillate out of phase and hence the sum of their contributions in the emission results in a smooth overall profile. On the contrary, a weak magnetic field is not found to have any significant effect on the shocked plasma and the turbulent hot slab that forms is found to retain its periodic signature. [ABSTRACT FROM AUTHOR]

Published
2014
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3. YSO accretion shocks: magnetic, chromospheric or stochastic flow effects can suppress fluctuations of X-ray emission.

Author

Matsakos, T., Chièze, J.-P., Stehlé, C., González, M., Ibgui, L., de Sá, L., Lanz, T., Orlando, S., Bonito, R., Argiroffi, C., Reale, F., and Peres, G.

Subjects
ACCRETION (Astrophysics), COSMIC magnetic fields, RADIATIVE transfer, OSCILLATIONS, SOLAR chromosphere
Abstract

Context. Theoretical arguments and numerical simulations of radiative shocks produced by the impact of the accreting gas onto young stars predict quasi-periodic oscillations in the emitted radiation. However, observational data do not show evidence of such periodicity. Aims. We investigate whether physically plausible perturbations in the accretion column or in the chromosphere could disrupt the shock structure influencing the observability of the oscillatory behavior. Methods. We performed local 2D magneto-hydrodynamical simulations of an accretion shock impacting a chromosphere, taking optically thin radiation losses and thermal conduction into account. We investigated the effects of several perturbation types, such as clumps in the accretion stream or chromospheric fluctuations, and also explored a wide range of plasma-β values. Results. In the case of a weak magnetic field, the post-shock region shows chaotic motion and mixing, smoothing out the perturbations and retaining a global periodic signature. On the other hand, a strong magnetic field confines the plasma in flux tubes, which leads to the formation of fibrils that oscillate independently. Realistic values for the amplitude, length, and time scales of the perturbation are capable of bringing the fibril oscillations out of phase, suppressing the periodicity of the emission. Conclusions. The strength of a locally uniform magnetic field in YSO accretion shocks determines the structure of the post-shock region, namely, whether it will be somewhat homogeneous or if it will split up to form a collection of fibrils. In the second case, the size and shape of the fibrils is found to depend strongly on the plasma-β value but not on the perturbation type. Therefore, the actual value of the protostellar magnetic field is expected to play a critical role in the time dependence of the observable emission. [ABSTRACT FROM AUTHOR]

Published
2013
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4. 3D MODELING OF ACCRETION SHOCKS IN YOUNG STELLAR OBJECTS: SIMULATION OF LABORATORY EXPERIMENTS.

Author

Ibgui, L., González, M., Stehlé, C., Hubeny, I., and Lanz, T.

Subjects
STAR observations, ACCRETION (Astrophysics), HYDRODYNAMICS, RADIATIVE transfer, RADIATION
Abstract

In order to improve the understanding of the physics of ac-cretion shocks around young stellar objects, we have performed a three dimensional simulation of a radiative shock generated in a laser installation. We depict the 3D structure of such a shock. Radiation hydrodynamics is modeled with the HERACLES code; then, radiative transfer post-processing is performed with the IRIS code. [ABSTRACT FROM AUTHOR]

Published
2013
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5. Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Author

Titos Matsakos, Chantal Stehlé, Marco Miceli, Salvatore Orlando, Costanza Argiroffi, J.-P. Chièze, Thierry Lanz, L. Ibgui, Fabio Reale, Rosaria Bonito, L. de Sá, Giovanni Peres, Istituto di Astrofisica Spaziale e Fisica cosmica - Palermo (IASF-Pa), Istituto Nazionale di Astrofisica (INAF), Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique (LERMA), École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université de Cergy Pontoise (UCP), Université Paris-Seine-Université Paris-Seine-Centre National de la Recherche Scientifique (CNRS), Service des Photons, Atomes et Molécules (SPAM), Commissariat à l'énergie atomique et aux énergies alternatives (CEA), Astrophysique Interprétation Modélisation (AIM (UMR7158 / UMR_E_9005 / UM_112)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), Joseph Louis LAGRANGE (LAGRANGE), Université Nice Sophia Antipolis (1965 - 2019) (UNS), COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de la Côte d'Azur, COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Université Côte d'Azur (UCA)-Université Côte d'Azur (UCA)-Centre National de la Recherche Scientifique (CNRS), Orlando, S, Bonito, R, Argiroffi, C, Reale, F, Peres, G, Miceli, M, Matsakos, T, Stehlé, C, Ibgui, L, de Sa, L, Chièze, J, Lanz, T, École normale supérieure - Paris (ENS Paris)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, PSL Research University (PSL)-PSL Research University (PSL)-Université de Cergy Pontoise (UCP), Astrophysique Interprétation Modélisation (AIM (UMR_7158 / UMR_E_9005 / UM_112)), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7), Université Nice Sophia Antipolis (... - 2019) (UNS), Université Côte d'Azur (UCA)-Université Côte d'Azur (UCA)-Observatoire de la Côte d'Azur, Université Côte d'Azur (UCA)-Centre National de la Recherche Scientifique (CNRS), École normale supérieure - Paris (ENS Paris), Université Côte d'Azur (UCA)-Université Nice Sophia Antipolis (... - 2019) (UNS), COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Observatoire de la Côte d'Azur, and Université Côte d'Azur (UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)

Subjects
Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Field strength, X-rays: stars, Astrophysics, stars: pre-main sequence, 01 natural sciences, magnetohydrodynamics (MHD), pre-main sequence, X-rays: stars [accretion, accretion disks, instabilities, magnetohydrodynamics (MHD), shock waves, stars], 010305 fluids & plasmas, Settore FIS/05 - Astronomia E Astrofisica, accretion, 0103 physical sciences, Radiative transfer, Astrophysics::Solar and Stellar Astrophysics, 010303 astronomy & astrophysics, Chromosphere, Solar and Stellar Astrophysics (astro-ph.SR), Astrophysics::Galaxy Astrophysics, Physics, accretion disks, Astronomy and Astrophysics, Plasma, shock waves, Accretion (astrophysics), Magnetic field, T Tauri star, Astrophysics - Solar and Stellar Astrophysics, Space and Planetary Science, instabilities, Physics::Space Physics, Oblique shock, Astrophysics::Earth and Planetary Astrophysics, accretion, accretion disks, instabilities, magnetohydrodynamics (MHD), shock waves, stars: pre-main sequence, X-rays: stars, [PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph]
Abstract

(abridged) AIMS. We investigate the dynamics and stability of post-shock plasma streaming along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. METHODS. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model takes into account the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction. RESULTS. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. For weak magnetic fields, a large component of B may develop perpendicular to the stream at the base of the accretion column, limiting the sinking of the shocked plasma into the chromosphere. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields, the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface. In general, a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the shocked plasma lower than in the case of uniform magnetic field. CONCLUSIONS. The initial strength and configuration of the magnetic field in the impact region of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. The field strength and configuration influence also the energy balance of the shocked plasma, its emission measure at T > 1 MK being lower than expected for a uniform field. The above effects contribute in underestimating the mass accretion rates derived in the X-ray band., 11 pages, 11 Figures; accepted for publication on A&A. Version with full resolution images can be found at http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pdf

Published
2013

6. 3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects

Author

Ivan Hubeny, T. Matsakos, J.-P. Chièze, Rosaria Bonito, Chantal Stehlé, L. Ibgui, T. Lanz, M. González, Salvatore Orlando, L. de Sá, Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique (LERMA (UMR_8112)), Sorbonne Université (SU)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université de Cergy Pontoise (UCP), Université Paris-Seine-Université Paris-Seine-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL), INAF - Osservatorio Astronomico di Palermo (OAPa), Istituto Nazionale di Astrofisica (INAF), Astrophysique Interprétation Modélisation (AIM (UMR_7158 / UMR_E_9005 / UM_112)), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7), Steward Observatory, University of Arizona, Observatoire de la Côte d'Azur, Université Côte d'Azur (UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA), Institut Rayonnement Matière de Saclay (IRAMIS), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris-Saclay, Istituto di Astrofisica Spaziale e Fisica cosmica - Palermo (IASF-Pa), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, PSL Research University (PSL)-PSL Research University (PSL)-Université de Cergy Pontoise (UCP), Université Paris-Seine-Université Paris-Seine-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Université Côte d'Azur (UCA), Ibgui, L, Orlando, S, Stehlé, C, Chièze, JP, Hubeny, I, Lanz, T, de Sá, L, Matsakos, T, González, M, Bonito, R, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Cergy Pontoise (UCP), Astrophysique Interprétation Modélisation (AIM (UMR7158 / UMR_E_9005 / UM_112)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), and COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Université Côte d'Azur (UCA)

Subjects
Physics, Opacity, Radiative cooling, QC1-999, Young stellar object, Astrophysics, [PHYS.ASTR.SR]Physics [physics]/Astrophysics [astro-ph]/Solar and Stellar Astrophysics [astro-ph.SR], Accretion (astrophysics), Radiation flux, N/A, 13. Climate action, Radiative transfer, Astrophysics::Solar and Stellar Astrophysics, Magnetohydrodynamics, Chromosphere, Astrophysics::Galaxy Astrophysics
Abstract

International audience; We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our "gray LTE approach", i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magne-tohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochro-matic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared. a

Published
2013

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