JETS FROM YOUNG STARS: RADIATIVE MHD SIMULATIONS

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1 JETS FROM YOUNG STARS: RADIATIVE MHD SIMULATIONS O. TEŞILEANU 1,2, A. MIGNONE 1, S. MASSAGLIA 1 1 Dipartimento di Fisica Generale, Università degli Studi di Torino, via P. Giuria 1, Turin, Italy, tesileanu@ph.unito.it 2 Research Centre for Atomic Physics and Astrophysics, University of Bucharest, P.O. Box MG-6, RO Bucharest-Mãgurele, Romania Received October 10, 2008 With the recent improvements in available observational data, simulating the radiative processes in YSO jets will provide a valuable tool for model discrimination. The present work describes the various strategies for the implementation of radiative cooling losses in a time-dependent manner to MHD simulation codes, with an emphasis on the code we use, PLUTO. Post-processing routines for the realistic computation of emission lines are now available. Preliminary results of the 1D modelling of the HH30 jet with synthetic emission line ratios computations is presented. 1. INTRODUCTION Jets are widespread phenomena in the Universe, existing in a wide range of objects and temporal and spatial scales. Collimated, supersonic outflows of matter can be found in the most powerful form originating from Active Galactic Nuclei but also, in the other end of the scales range, in our Galaxy in regions of star formation, originating from young stellar objects. Between these two extreme cases, jets were discovered to be associated to neutron stars, massive X-ray binary systems (for example SS433), symbiotic stars, and galactic stellar mass black holes (microquasars). Because of the resemblances of the morphologies, similar physical mechanisms are believed to be at work at all scales. Some of the young stellar objects (YSOs) in our Galaxy are sources of jets, at relatively small scales they extend on ranges of ~ 0.01 to few parsecs, and have core velocies of the order of few hundreds kilometers per second (at a sound speed of ~ 10 kms 1 ). The rapid evolution of the observational capabilities and the relatively small distance to these sources made them the ideal candidates for comparison with theory and for discrimination between theoretical models of jet formation and propagation. Paper presented at the National Conference of Physics, September, 2008, Bucharest Mãgurele, Romania. Rom. Journ. Phys., Vol. 54, Nos. 7 8, P , Bucharest, 2009

2 772 O. Tesileanu, A. Mignone, S. Massaglia 2 Initially only the regions of high and peculiar emissions from the YSO jets have been discovered by Herbig [1] and Haro [2]. We present in Fig. 1 an image of the first two HH objects discovered, imaged at high resolution by the Hubble Space Telescope. Fig. 1 Hubble Space Telescope images of HH1 and HH2. The present work will focus on the numerical study of Herbig-Haro jets, including the radiative cooling processes due to collisionally-excited line radiation. Departing from magnetohydrodynamic (MHD) simulations, synthetic observations (emission line ratios) are obtained and compared with astronomical observations. 2. RADIATIVE COOLING Radiative cooling becomes important for the dynamical evolution of the system whenever the cooling timescale becomes comparable to or lower than the dynamical timescale. This is the case with schocked astrophysical flows like those encountered in YSO jets. The total energy E is evolved according to the standard MHD equations: E Ept uu BBSE (1) t where S E is a radiative loss term, and pt pb 2 2 denotes the total pressure (thermal + magnetic) of the fluid. For a detailed description of the way this system is solved by the MHD code PLUTO, we refer to [3].

3 3 Jets from young stars: radiative MHD simulations 773 The radiative loss term may be computed in various ways, depending on the accuracy needed and available computational power. These approaches will be described in this section TABULATED COOLING The tabulated cooling module provides a way to solve the internal energy equation dp ( 1) 2 n ( T) with n dt m m when the cooling/heating function (T) is not known analytically but rather is available as a table sampled at discrete points, i.e., j ( Tj). In the equation above, is the density, n the particle number density, is the ratio of specific heats and m p and m e are the proton and electron masses, respectively. p e (2) 2.2. MINEQ COOLING The Multi-Ion Non-Equilibrium cooling is a newly developed cooling module for the PLUTO MHD code that integrates a complex ionization network of 29 ion species: H I, H II, He I, He II, C I to V, N I to V, O I to V, Ne I to V, and S I to V, and the collisionally excited line emission for these ion species in the approximation of a 5-level atom. For each ion, we solve the additional equation ( Xi ) Xiu Si (3) t coupled to the original system of conservation laws. In Eq. (3), the first index () corresponds to the element, while the second index (i) corresponds to the ionization stage. Specifically, Xi Ni N is the ion number fraction, N i is the number density of the i-th ion of element, and N is the element number density. The source term S i accounts for ionization and recombination. The total line emission from these species enters in the source term S E in Eq. (1) and should give a good approximation of radiative cooling for the above conditions ([4]). For a detailed description of this cooling function, testing and sample applications to astrophysics, we refer to [5].

4 774 O. Tesileanu, A. Mignone, S. Massaglia SIMPLIFIED NON-EQUILIBRIUM COOLING The SNEq cooling consists of the introduction of one supplementary variable to the MHD system, the fraction of neutrals fn fhi / fh, representing the fraction of neutral hydrogen in the plasma. The fraction of neutrals obeys the following non-homogeneous advection equation: fn v fn ne[ cr ci fn cr] (4) t where v is the velocity, n e the electron number density, and c i and c r the ionization and recombination coefficients of hydrogen. This is coupled to the energy equation. S E is computed as the sum 16 different line emissions from from some of the most common elements, k = Ly, H, HeI ( Å), CI ( Å), CII (156 m), CII (2325 Å), NI (5200 Å), NII ( Å), OI (63 m), OI ( Å), OII ( 3727 Å), MgII ( 2800 Å), SiII (35 m), SII ( Å), FeII (25 m), FeII (1.6 m). 3. EMISSION LINE RATIOS With the detailed non-equilibrium ionization balance computed in MINEq cooling, one is able to accurately compute the emission line intensities and emission line ratios. These can be further on directly compared to observations. The observational data for HH30 comes from [6], the cited paper being also an analysis of the physical quantities following the procedure in [7]. We instead computed the emission line ratios departing from the 1D MHD data (physical parameters). HH30 is an ideal candidate for this type of studies as it lies almost in the plane of the sky and has a quasilinear, very collimated shape. A key (reasonable) assumption is that all the emission from the jet comes from post-shock regions in the flow, resulting from instabilities or variations in the jet injection speed. An initial perturbation (the setup in [8]) in velocity evolves in a shock propagating through the jet medium. We follow its evolution in time, compute the line emissions and then integrate over the distance corresponding to the resolution of observational data. The basic parameters for the simulations are the jet density, perturbation amplitude in velocity, transversal magnetic field. An investigation of the parameter space is underway, but a preliminary acceptable result (given the simplicity of the model) was obtained for the parameters mentioned in Fig. 2. The variation in the emission along the jet cannot be captured with this simple method as it probably comes from secondary or interacting shocks. Further studies will include the extension to 2D.

5 5 Jets from young stars: radiative MHD simulations 775 Fig. 2 Simulation of line ratios between the doublets of NII ( Å), OI ( Å), and SII ( Å) (lines: [SII]/[OI] solid, [OI]/[NII] dashed). Comparison with HH30 data (circles). 4. CONCLUSIONS Including radiative losses in the MHD simulations of YSO jets demonstrated to be essential for reliable predictions in terms of jet propagation and morphology. The detailed cooling and non-equilibrium ionization computation is desirable if predictions are to be made in terms of line emissions. Preliminary results from 1D and 2D simulations are encouraging, and further efforts will be made in order to obtain predictions for more (and more complex) HH objects. Acknowledgements. The present work has been supported by the European Union (contract MRTN-CT ) within the Marie Curie RTN JETSET. REFERENCES 1. G. H. Herbig, ApJ, 113, 697 (1951). 2. G. Haro, Astron. J., 55, 72 (1950). 3. A. Mignone, S. Massaglia, G. Bodo et al., ApJS, 170, 228 (2007). 4. A. C. Raga, G. Mellema, P. Lundqvist, ApJS, 109, 517 (1997). 5. O. Teºileanu, A. Mignone, S. Massaglia, A&A, 488, 429 (2008). 6. F. Bacciotti, J. Eislöffel, T. P. Ray, A&A, 350, 917 (1999). 7. F. Bacciotti, J. Eislöffel, A&A, 342, 717 (1999). 8. S. Massaglia, A. Mignone, G. Bodo, A&A, 442, 549 (2005).