14 research outputs found
YSO jets: MHD simulations with radiative cooling
The new High Power Computing facilities available for the scientific community allowed the use of increasingly complex codes for the numerical simulation of physical processes. Recent magnetohydrodynamic (MHD) simulations of
astrophysical jets could finally include non-ideal effects, such as the radiative cooling we will discuss in this work in the context of protostellar jets. This makes the simulations more reliable and, with the recent improvements in available observational data, will provide a valuable tool for model discrimination. From 2D adaptivemesh
refinement (AMR) simulations, synthetic surface brightness maps for the line emissions are computed, to be compared with observations
Numerical simulations of radiative magnetized Herbig-Haro jets: the influence of pre-ionization from X-rays on emission lines
We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a
time-dependent injection velocity by numerical simulations with the PLUTO code.
Using a comprehensive set of parameters, we explore different jet
configurations in the attempt to construct models that can be directly compared
to observational data of microjets. In particular, we focus our attention on
the emitting properties of traveling knots and construct, at the same time,
accurate line intensity ratios and surface brightness maps. Direct comparison
of the resulting brightness and line intensity ratios distributions with
observational data of microjets shows that a closer match can be obtained only
when the jet material is pre-ionized to some degree. A very likely source for a
pre-ionized medium is photoionization by X-ray flux coming from the central
object.Comment: Accepted for publication in Ap
Simulating radiative astrophysical flows with the PLUTO code: a non-equilibrium, multi-species cooling function
Context. Time-dependent cooling processes are of paramount importance in the evolution of astrophysical gaseous nebulae and, in particular, when radiative shocks are present. Given the recent improvements in resolution of the observational data, simulating these processes in a more realistic manner in magnetohydrodynamic (MHD) codes will provide a unique tool for model discrimination.
Aims. The present work introduces a necessary set of tools that can be used to model radiative astrophysical flows in the optically-thin plasma limit.
We aim to provide reliable and accurate predictions of emission line ratios
and radiative cooling losses in astrophysical simulations of shocked flows.
Moreover, we discuss numerical implementation aspects to ease future improvements
and implementation in other MHD numerical codes.
Methods. The most important source of radiative cooling for our plasma conditions comes from the
collisionally-excited line radiation.
We evolve a chemical network, including 29 ion species, to compute the ionization balance
in non-equilibrium conditions. The numerical methods are implemented in the PLUTO code for astrophysical fluid dynamics and particular attention has been devoted to resolve accuracy and efficiency issues arising from cooling timescales considerably shorter than the dynamical ones.
Results. After a series of validations and tests, typical astrophysical setups are simulated in 1D and 2D,
employing both the present cooling model and a simplified one. The influence of the cooling model
on structure morphologies can become important, especially for emission line diagnostic purposes.
Conclusions. The tests make us confident that the use of the presented detailed radiative cooling treatment
will allow more accurate predictions in terms of emission line intensities and shock dynamics
in various astrophysical setups
Time-dependent MHD shocks and line intensity ratios in the HH 30 jet: a focus on cooling function and numerical resolution
Context. The coupling between time-dependent, multidimensional MHD numerical
codes and radiative line emission is of utmost importance in the
studies of the interplay between dynamical and radiative processes in many
astrophysical environments, with particular interest for problems involving radiative
shocks.
There is a widespread consensus that line emitting knots observed in Herbig-Haro jets
can be interpreted as radiative shocks. Velocity perturbations at the jet base
steepen into shocks to emit the observed spectra.
To derive the observable characteristics of the emitted spectra, such as line intensity ratios,
one has to study physical processes that involve the solution of the MHD equations coupled
with radiative cooling in non-equilibrium conditions.
Aims.
In this paper we address two different aspects relevant to
the time-dependent calculations of the line intensity ratios of forbidden transitions, resulting from the
excitation by planar, time-dependent radiative shocks traveling in a stratified medium.
The first one concerns the impact of the radiation and ionization processes included in the cooling model,
and the second one the effects of the numerical grid resolution.
Methods.
Dealing with both dynamical and radiative processes in the same numerical
scheme means to treat phenomena characterized by different time and length
scales. This may be especially arduous and computationally
expensive when discontinuities are involved, such as in the case of shocks.
Adaptive mesh refinement (AMR) methods have been introduced in order to alleviate
these difficulties. In this paper we apply the AMR methodology to the treatment of radiating shocks
and show how this method is able to vastly reduce the integration time.
Results.
The technique is applied to the knots of the HH 30 jet to obtain the observed line intensity ratios and
derive the physical parameters, such as density, temperature and ionization fraction. We consider
the impact of two different cooling functions and different grid resolutions on the results.
Conclusions.
We conclude that the use of different cooling routines has effects on
results whose weight depends upon the line ratio considered. Moreover, we
find the minimum numerical resolution of the simulation grid behind the shock to achieve convergence
in the results. This is crucial for the forthcoming 2D calculations of radiative shocks
Young stellar object jet models: From theory to synthetic observations
Context. Astronomical observations, analytical solutions, and numerical simulations have provided the building blocks to formulate the current theory of young stellar object jets. Although each approach has made great progress independently, it is only during the past decade that significant efforts have been made to bring the separate pieces together. Aims. Building on previous work that combined analytical solutions and numerical simulations, we apply a sophisticated cooling function to incorporate optically thin energy losses in the dynamics. On one hand, this allows a self-consistent treatment of the jet evolution, and on the other hand, it provides the necessary data to generate synthetic emission maps. Methods. Firstly, analytical disk and stellar outflow solutions are properly combined to initialize numerical two-component jet models inside the computational box. Secondly, magneto-hydrodynamical simulations are performed in 2.5D, correctly following the ionization and recombination of a maximum of 29 ions. Finally, the outputs are post-processed to produce artificial observational data. Results. The values for the density, temperature, and velocity that the simulations provide along the axis are within the typical range of protostellar outflows. Moreover, the synthetic emission maps of the doublets [O i], [N ii], and [S ii] outline a well-collimated and knot-structured jet, which is surrounded by a less dense and slower wind that is not observable in these lines. The jet is found to have a small opening angle and a radius that is also comparable to observations. Conclusions. The first two-component jet simulations, based on analytical models, that include ionization and optically thin radiation losses demonstrate promising results for modeling specific young stellar object outflows. The generation of synthetic emission maps provides the link to observations, as well as the necessary feedback for further improvement of the available models. © 2014 ESO