Scaling stellar jets to the laboratory: the power of simulations

Advances in laser and Z-pinch technology, coupled with the development of plasma diagnostics and the availability of high-performance computers, have recently stimulated the growth of high-energy density laboratory astrophysics. In particular a number of experiments have been designed to study radiative shocks and jets with the aim of shedding new light on physical processes linked to the ejection and accretion of mass by newly born stars. Although general scaling laws are a powerful tools to link laboratory experiments with astrophysical plasmas, the phenomena modelled are often too complicated for simple scaling to remain relevant. Nevertheless, the experiments can still give important insights into the physics of astrophysical systems and can be used to provide the basic experimental validation of numerical simulations in regimes of interest to astrophysics. We will illustrate the possible links between laboratory experiments, numerical simulations and astrophysics in the context of stellar jets. First we will discuss the propagation of stellar jets in a cross-moving interstellar medium and the scaling to Z-pinch produced jets. Our second example focuses on slab-jets produced at the PALS (Prague Asterix Laser System) laser installation and their practical applications to astrophysics. Finally, we illustrate the limitations of scaling for radiative shocks, which are found at the head of the most rapid stellar jets.Comment: 30 pages, 9 figure

Etude analytique et numérique des flots autour des étoiles jeunes

LORS DU PROCESSUS DE FORMATION STELLAIRE, DE FORMIDABLES EJECTIONS BIPOLAIRES DE MATIERE SONT OBSERVEES, EN PLUS DE L'ACCRETION SUR L'OBJET CENTRAL. DEUX COMPOSANTES CONSTITUENT CES FLOTS BIPOLAIRES : LES JETS (RAPIDES, COLLIMATES ET LEGERS) ET LES FLOTS MOLECULAIRES (PLUS LENTS, LARGES ET DENSES). IL EST SOUVENT ADMIS QUE LE JET EST RESPONSABLE DU FLOT MOLECULAIRE, LORSQUE CE PREMIER ENTRAINE PAR EFFET CHASSE-NEIGE LE MILIEU AMBIANT DANS LEQUEL IL SE PROPAGE. DANS CETTE THESE, NOUS NOUS INTERESSONS PRINCIPALEMENT AUX FLOTS MOLECULAIRES PAR L'ETUDE D'UN MODELE ALTERNATIF ET COMPLEMENTAIRE DE LA VISION STANDARD MENTIONNEE CI-DESSUS. DANS CE MODELE DE >, UNE PARTIE DU GAZ EN CHUTE EN DIRECTION DE L'OBJET CENTRAL EST DEVIEE (TRANSIT) DANS LES DIRECTIONS BIPOLAIRES, EN RAISON DE L'AUGMENTATION DE LA TEMPERATURE ET DE LA DENSITE LORSQUE LA DISTANCE A LA PROTO-ETOILE DIMINUE. NOUS MONTRONS QUE DES SOLUTIONS EXISTENT SANS QU'UN CHAMP MAGNETIQUE SOIT PRESENT, IMPLIQUANT UNE NATURE THERMODYNAMIQUE DU PROCESSUS QUE NOUS DECRIVONS. LES SOLUTIONS MAGNETISEES SONT CEPENDANT EN MEILLEUR ACCORD AVEC LES OBSERVATIONS, CONFIRMANT QUE LE CHAMP MAGNETIQUE EST UN INGRDIENT IMPORTANT DU PROCESSUS DE FORMATION STELLAIRE. GRACE A CE MODELE, NOUS PARVENONS SANS DIFFICULTE A ATTEINDRE LES ENORMES MASSES CHARRIEES DANS LES FLOTS D'ETOILES MASSIVES PUISQUE LE MODELE N'EST PAS LIMITE A CE QU'UN JET PEUT ENTRAINER. LA COMPOSITION DU MILIEU INFLUE EGALEMENT SUR LES SOLUTIONS OBTENUES ET IL EST MONTRE QUE L'ABSENCE DE POUSSIERES DANS LE MILIEU ENTRIANE POTENTIELLEMENT LA FORMATION D'ETOILES PLUS MASSIVES QUE CELLES QUI SE FORMENT DANS L'UNIVERS ACTUEL (OU LA POUSSIERE EST PRESENTE).WHEN A STAR FORMS, NOT ONLY GAS ACCRETES ONTO THE CENTRAL OBJECT BUT POWERFUL BIPOLAR EJECTIONS OF MATERIAL ARE ALSO OBSERVED. THESE BIPOLAR OUTFLOWS ARE GENERALLY SEPERATED INTO TWO COMPONENTS: THE JETS (FAST, HIGHLY COLLIMATED AND LIGHT) AND THE MOLECULAR OUTFLOWS (SLOWER, WIDER AND DENSER). IT IS USELY ACCEPTED THAT THE JETS ARE THE CAUSE OF THE MOLECULAR OUTFLOWS AS THEY ENTRAIN THE GAS OF THE MEDIUM THEY ARE PROPAGATING IN, VIA A SNOW-PLOW EFFECT. IN THIS THESIS, WE ARE PRIMARILY INTERESTED BY MOLECULAR OUTFLOWS AND STUDY AN ALTERNATIVE AND COMPLEMENTARY MODEL OF THE STANDARD JET-DRIVEN MODEL DESCRIBED ABOVE. IN OUR SO-CALLED "TRANSIT MODEL", PART OF THE INFALLING GAZ IS BEING DIVERTED INTO A BIPOLAR OUTFLOW AS THE DENSITY AND TEMPERATURE INCREASE WITH DECREASING DISTANCE TO THE CENTRAL OBJECT. WE SOLVE THE SYSTEM OF EQUATIONS DESCRIBING THE MODEL IN BOTH THE UNMAGNETISED AND MAGNETISED CASE. THE EXISTENCE OF PURELY HYDRODYNAMICAL SOLUTIONS SHOW THAT THE FUNDAMENTAL NATURE OF THE PROCESS DESCRIBED HERE IS THERMODYNAMICAL. HOWEVER, MAGNETISED SOLUTIONS COMPARE BETTER WITH OBSERVATIONS, CONFIRMING THE IMPORTANCE OF THE MAGNETIC FIELD IN THE STAR FORMATION PROCESS. WITH THE TRANSIT MODEL, IT IS POSSIBLE TO REACH WITHOUT DIFFICULTIES THE HUGE MASS OBSERVED IN THE OUTFLOWS OF MASSIVE FORMING STARS : INDEED, IN THE MODEL, THE MASS IN THE MOLECULAR OUTFLOW IS NOT LIMITED TO THE MASS THAT CAN BE ENTRAINED BY AN UNDERLYING JET. WE ALSO STUDIED THE INFLUENCE O F THE COMPOSITION OF THE GAS ON THE SOLUTIONS AND FOUND THAT MORE MASSIVE STARS CAN POTENTIALLY FORM WHEN DUST DOES NOT DOMINATE THE COOLING OF THE GAS, AS IT IS IN THE PRESENT-DAY UNIVERSE.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Etude numérique et expérimentale d'ondes de chocs surcritiques (effets multidimensionnels du rayonnement)

DANS UN CHOC SURCRITIQUE, LE RAYONNEMENT EMIS PAR LE FRONT DE CHOC EST ABSORBE PAR LE MILIEU AMONT, CREANT AINSI UN PRECURSEUR RADIATIF DE TEMPERATURE EGALE A LA TEMPERATURE DU FRONT DE CHOC. NOTRE EQUIPE A EFFECTUE DES EXPERIENCES DE LABORATOIRE AVEC LES LASERS DE PUISSANCE OU NOUS AVONS PU PRODUIRE DANS LE XENON DES CHOCS SUFFISAMMENT RAPIDES POUR ETRE SURCRITIQUES. NOUS AVONS MESURE LA DENSITE ELECTRONIQUE DANS LES PRECURSEURS RADIATIFS AINSI QU'UNE TEMPERATURE EFFECTIVE. LE PRECURSEUR EST MOINS RAPIDE ET PLUS RAIDE DANS LES OBSERVATIONS QUE DANS LES CALCULS NUMERIQUES UNIDIMENSIONNELS D'HYDRODYNAMIQUE RADIATIVE. AFIN DE COMPRENDRE CES DIFFERENCES, NOUS ETUDIONS QUELQUES ASPECTS MULTIDIMENSIONNELS DE TRANSFERT RADIATIF DANS UN CHOC STATIONNAIRE.UN MODELE ANALYTIQUE EN GEOMETRIE CYLINDRIQUE MONTRE QUELLE EST L'INFLUENCE DE L'ETENDUE LATERALE DU CHOC SUR LE CHAMP DE RAYONNEMENT CREE DANS UN MILIEU HOMOGENE, GRIS ET ISOTROPE. IL PERMET DE COMPRENDRE QUE, SUFFISAMMENT LOIN DE LA SOURCE, LE CHAMP DE RAYONNEMENT SUR L'AXE DU CYLINDRE DIMINUE COMME L'ANGLE SOLIDE SOUS LEQUEL ON VOIT LA SOURCE ET EST ATTENUE D'UN FACTEUR EXP(-TAU) EN COMPTANT LA PROFONDEUR OPTIQUE TAU DEPUIS LA SOURCE.ON FAIT ENSUITE UN CALCUL NUMERIQUE DE TRANSFERT RADIATIF AVEC LA METHODE DES CARACTERISTIQUES COURTES DANS UNE STRUCTURE GELEE DE CHOC RADIATIF AFIN D'ETUDIER DANS UN CAS NON GRIS ET EN TROIS DIMENSIONS L'ANISOTROPIE DU CHAMP DE RAYONNEMENT. LES RESULTATS ANALYTIQUES SONT QUALITATIVEMENT CONFIRMES. EN PARTICULIER, ON VOIT COMMENT LES MOMENTS RADIATIFS SONT MODIFIES ENTRE LE CAS INFINI ET LE CAS, SIMILAIRE A CELUI DE LA CONFIGURATION EXPERIMENTALE, OU L'EXTENSION LATERALE DU CHOC EST FINIE.IN SUPERCRITICAL SHOCKS, RADIATION EMITTED BY THE SHOCK FRONT IS ABSORBED UPWIND, THEREFORE CREATINGA RADIATIVE PRECURSOR WHOSE TEMPERATURE IS EQUAL TO THE SHOCK FRONT TEMPERATURE. THE HIGH ENERGYDENSITY LASER LABORATORY EXPERIMENTS WE HAVE PERFORMED ALLOWED US TO GENERATE IN XENON GAS SHOCKSTHAT ARE FAST ENOUGH TO BE SUPERCRITICAL. WE COULD MEASURE ELECTRON DENSITY IN THE RADIATIVE PRECURSOR AND AN EQUIVALENT TEMPERATURE. THE OBSERVED PRECURSOR BEING SLOWER AND STEEPER THAN IN ONE DIMENSIONAL RADIATION HYDRODYNAMICS NUMERICAL COMPUTATIONS, WE STUDIED SOME MULTIDIMENSIONAL ASPECTS OF RADIATIVE TRANSFER IN A STATIONARY SHOCK.A SIMPLE ANALYTICAL MODEL IN CYLINDRICAL GEOMETRY SHOWS THE INFLUENCE OF THE LATERAL EXTENSION OFTHE SHOCK ON THE RADIATION FIELD CREATED IN A HOMOGENEOUS, GREY AND ISOTROPIC MEDIUM. IT ALLOWS USTO UNDERSTAND THAT THE RADIATION FIELD ON THE AXIS OF THE CYLINDER DECREASES LIKE THE SOLID ANGLEUNDER WHICH THE SURFACE OF THE CYLINDER IS SEEN, ATTENUATED BY A FACTOR EXP(-TAU) WHEN COUNTINGTHE OPTICAL DEPTH TAU FROM THIS SURFACE.A NUMERICAL CALCULATION OF RADIATIVE TRANSFER WITH THE SHORT CHARACTERISTICS METHOD IS THENPERFORMED IN THE FROZEN STRUCTURE OF A RADIATIVE SHOCK WITH THE AIM OF STUDYING THE ANISOTROPY OFTHE RADIATION FIELD IN THREE DIMENSIONS IN A NON GREY MEDIUM. ANALYTICAL RESULTS ARE QUALITATIVELYCONFIRMED. IN PARTICULAR, WE SEE THE MODIFICATION OF THE RADIATIVE MOMENTS FROM THE INFINITE CASE TOTHE CASE SIMILAR TO ONE OF THE EXPERIMENTAL CONFIGURATION WHERE THE LATERAL EXTENSION OF THE SHOCKIS FINITE.ORSAY-PARIS 11-BU Sciences (914712101) / SudocMEUDON-Observatoire (920482302) / SudocSudocFranceF

3D modeling of accretion shocks in young stellar objects : Simulation of laboratory experiments

International audienceIn order to improve the understanding of the physics of accretion 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

Multi-dimensional radiative effects in supercritical shock waves

International audienceWe investigate the structure of the radiation field generated in high Mach number shock waves (supercritical shock waves [1]) which have a finite extension R in the direction perpendicular to the shock wave propagation. A simple analytical model allows us to calculate how the radiation field decreases with R for a medium with an homogeneous radiation emission in volume. We then perform a 3D radiative transfer calculation of the radiation field created by a stationary shock wave, and compare the predicted radiative energy density and flux with 1D calculations. We find that the variation of the radiation field with R follows the trend given by the analytical calculation. Finally, we perform radiative-hydrodynamical calculations to investigate further the variation of the radiation field with R and to estimate the decrease of the velocity of the radiative precursor withR

Mass-loss of hot stars studied with spectro-polarimetric interferometry (SPIN)

Astronomical Telescopes and Instrumentation, Polarimetry in Astronomy, Fineschi, S., Proc. SPIE 4843, pp. 484-1491 (2003)International audienc

Radiation Magnetohydrodynamic Models and Spectral Signatures of Plasma Flows Accreting onto Classical T Tauri Stars

International audienc

Radiation Magnetohydrodynamic Models and Spectral Signatures of Plasma Flows Accreting onto Classical T Tauri Stars

International audienc

Radiation Magnetohydrodynamic Models and Spectral Signatures of Plasma Flows Accreting onto Classical T Tauri Stars

International audienc