Deflagration to detonation transition by amplification of acoustic waves in type Ia supernovae

We study a new mechanism for deflagration to detonation transition in thermonuclear supernovae (SNe Ia), based on the formation of shocks by amplification of sound waves in the steep density gradients of white dwarfs envelopes. Given a large enough jump in density a small pressure and velocity perturbation, produced by the turbulent deflagration, turns into a shock down of the gradient, where it will dissipate and heat up the media. With the right frequency and amplitude the heating can be enough to initiate a detonation, which can propagate backward and up the density gradient. We studied planar and spherical geometry. In the planar case we made a parametric study of the frequency and amplitude. We found it possible to obtain a detonation for perturbations down to Mach number M=0.003. In the spherical case, geometrical damping makes it harder to initiate a detonation, but considering a small He atmosphere (<0.01 Msol) makes it possible again to obtain a detonation down to small perturbation (M=0.002). In the context of thermonuclear supernovae, this could be a mean to turn a turbulent flame producing sound waves to a detonation.Comment: accepted for publication in Astronomy and Astrophysic

Hydrodynamic modeling of accretion shocks on a star with radiative transport and a chromospheric model

International audienceAccretion flows on the surface of a star is modeled using a high resolution hydrodynamic 1D ALE code (ASTROLABE) coupled to radiative transfer and line cooling, along with a model for the acoustic heating of the chromospheric plasma

Hydrodynamic modeling of accretion shocks on a star with radiative transport and a chromospheric model

International audienceThe aim of the project (ANR STARSHOCK) is to understand the dynamics and the radiative properties of accretion columns, linking the circumstellar disk to the surface photosphere of Young Stellar Objects. The hydrodynamics is computed first, using a high resolution hydrodynamic 1D ALE code (AS-TROLABE) coupled to radiative transfer and line cooling, along with a model for the acoustic heating of the chromospheric plasma. Spectra are then post-processed with a 1D radiative transfer code (SYNSPEC), using DFE solver and an extended atomic database covering a wavelength range from X rays to visible

Accretion shock stability on a dynamically heated YSO atmosphere with radiative transfer

Theory and simulations predict Quasi-Periodic Oscillations of shocks which develop in magnetically driven accretion funnels connecting the stellar disc to the photosphere of Young Stellar Objects (YSO). X-ray observations however do not show evidence of the expected periodicity. We examine here, in a first attempt, the influence of radiative transfer on the evolution of material impinging on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code ASTROLABE. The mechanical shock heating mechanism of the chromosphere only slightly perturbs the flow. We also show that, since the impacting flow, and especially the part which penetrates into the chromosphere, is not treated as a purely radiating transparent medium, a sufficiently efficient coupling between gas and radiation may affect or even suppress the oscillations of the shocked column. This study shows the importance of the description of the radiation effects in the hydrodynamics and of the accuracy of the opacities for an adequate modeling

New insight on accretion shocks onto young stellar objects - Chromospheric feedback and radiation transfer

Context. Material accreted onto classical T Tauri stars is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match observations. They all rely on a static model for the chromosphere and on the assumption that radiation and matter are decoupled. Aims. We explore the effects of a shock-heated chromosphere and of the coupling between radiation and hydrodynamics on the structure and dynamics of the accretion flow. Methods. We simulated accretion columns that fall onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the first two moment equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ion densities from collisional-radiative model. Results. The chromospheric acoustic heating affects the duration of the cycle and the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and of the chromosphere: the whole column is pushed up by the inflating chromosphere over several times the steady chromosphere thickness. These last two conclusions are in agreement with the computed monochromatic intensity. Acoustic heating and radiation coupling affect the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on which model is considered.French ANR StarShock projectFrench National Research Agency (ANR) [ANR-08-BLAN-0263-07, ANR-11-IDEX-0004-02]; French ANR LabEx Plas@Par projectFrench National Research Agency (ANR) [ANR-08-BLAN-0263-07, ANR-11-IDEX-0004-02]; PICS [6838]; Programme National de Physique Stellaire of CNRS/INSU; Observatoire de ParisOpen access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Accretion shock stability on a dynamically heated YSO atmosphere with radiative transfer

International audienceTheory and simulations predict Quasi-Periodic Oscillations of shocks which develop in magnetically driven accretion funnels connecting the stellar disc to the pho-tosphere of Young Stellar Objects (YSO). X-ray observations however do not show evidence of the expected periodicity. We examine here, in a first attempt, the influence of radiative transfer on the evolution of material impinging on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code ASTROLABE. The mechanical shock heating mechanism of the chromosphere only slightly perturbs the flow. We also show that, since the impacting flow, and especially the part which penetrates into the chromosphere, is not treated as a purely radiating transparent medium, a sufficiently efficient coupling between gas and radiation may affect or even suppress the oscillations of the shocked column. This study shows the importance of the description of the radiation effects in the hydro-dynamics and of the accuracy of the opacities for an adequate modeling

Modelling the Accretion on Young Stars, Recent Results and Perspectives

International audienc

New probing techniques of radiative shocks

International audienceRadiative shock waves propagating in xenon at a low pressure have been produced using 60 joules of iodine laser (&#955; = 1.315 &#956;m) at PALS center. The shocks have been probed by XUV imaging using a Zn X-raylaser (&#955; = 21 nm) generated with a 20-ns delay after the shock creating pulse. Auxiliary high-speed silicon diodes allowed performing space- and time-resolved measurement of plasma self-emission in the visible and XUV. The results show the generation of a shock wave propagating at 60 km/s preceded by a radiative precursor. This demonstrates the feasibility of radiative shock generation using high power infrared lasers and the use of XRL backlighting as a suitable diagnostic for shock imaging