This document summarizes simulations of jets in star forming regions. It discusses simulations of jets from binary sources, and how jets can be collimated by their ambient environment. The simulations show that a toroidal magnetic field helps collimate interacting binary jets. When jets propagate through an evacuated ambient medium, they become accelerated and more collimated. Intensity maps of these simulations can help interpret observations of outflows from young stellar objects.

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Simulations of Jets in Star Forming Regions
Gareth Murphy

2. 08/10/10 Laboratoire d'AstrOphysique de Grenoble 2/35
Collaborators
• F. Bacciotti
• L. Drury
• T. Lery
• C. Combet
• R. Curran
• S. O’Sullivan
• D. Spicer
• T. Ray
• E.T. Whelan

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Outline
1. Introduction
2. Numerical Code: Atlas
3. Simulations of Jets from Binary Sources
4. Jets in Evacuated Ambient Media
5. Conclusions

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1. Introduction

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Low Mass Star Formation
• Stars form inside collapsing
molecular cores
• Centrifugal forces flatten the
envelope in thin accretion
disks
• Orbiting matter decays toward
the central object - increasing
tangential velocity thanks to
the conservation of angular
momentum
• Charged particles drag frozen
magnetic field lines towards
the centre
• Approximately 10% of the
material is ejected from the
disk and travels along open B
field lines in bipolar direction.
Class 0
Class I/II Class III

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Jets and Magnetic field
• Wind velocity is ~ escape velocity close to the central object
• Magnetic field is necessary for collimation and to accelerate wind to the high
velocities associated with protostellar jets.
• Mechanism may be either a disk wind (Blandford & Payne 82, Ferreira 97) or
an X-Wind (Shu 94) or both

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Binary Jets
• Large numbers of young binaries exist in
Taurus, Ophichus clouds.
• Binary TTS frequency is 60% in Taurus-
Auriga (Ghez et al 93)
• Most forming stars have bipolar outflows
• Can a young binary produce a binary
outflow?
• Only ~10 candidate binary jets out of ~ 800
HH objects
• Is there a BINARY JET DESERT?
• What affects direction of jets?
• What is the effect of the circumbinary disk (if
any)?
• Do the jets interact - collide - merge - can
interaction be observed?
• What sort of jets interact? What is the role of
the magnetic field?

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Why Simulate Jets?
• Jets are important tracers of star
formation process.
• Forbidden line emission from jets is
easier to observe than the central object.
• Jet probes the central launching engine.
• We can try to explain morphology and
emission produced by nonlinear physical
processes.
• We can try to explain and predict
observations.
• We can test theory, produce synthetic
observations (emission maps etc) which
can be useful for observers.
• Simulations provide a laboratory in which
we can include effects of ambient
environment, magnetic fields, radiative
cooling, turbulence.
• Jets from low-mass YSOs have long
lifetimes ~10 kyr

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3. Numerical Code: Atlas

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Numerical Simulations
Fluid equations
+ Maxwell’s Laws
+ Assumptions
= Ideal MHD Equations
• Time-integrate the partial differential
equations
• Solve for B, E, u and rho
numerically.
∂ ρ
∂t
∇⋅ρu=0
∂ ρu
∂t
∇⋅ρuup IBB=0
∂B
∂t
∇⋅uB−Bu=0
∂ E
∂t
∇⋅[ Ep∗u−u.B B]−Lcooling=0
E=
1
2
ρu2

p
γ−1

1
2
B2
p∗¿ p
1
2
B2

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AMR MHD Code: Atlas
• Written by D.S. Spicer, Stephen O’Sullivan - NASA GSFC
• 3D Parallel AMR MHD Godunov code
• Roe-Balsara Riemann solver
• Corner transport upwind method – unsplit scheme so can use the
• Staggered Mesh (Constrained Transport scheme) to maintain div B=0
• Piecewise parabolic method for reconstruction of the fluxes
• Paramesh v3.3 - Parallel Structured AMR with face-centred fields
• Optically thin radiative cooling

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Adaptive Mesh Refinement
• The grid adapts
itself to the physical
changes within,
tracking the
features of the
result as the
computation
progresses
• Richardson
Truncation Error
Estimation

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Numerical Test Problems
Hydro Shock – Mach Reflection MHD Orszag-Tang Vortex
MHD
Blast
Wave

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2. Simulations of Jets from Binary
Sources

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Binary Protostars Make Binary Jets

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Binary Protostars Don’t Make Binary Jets
• DG Tau (Coffey et al 2005),
• T Tau (triple) (Schwartz 1975)
• L1642-2 (Reipurth et al 1990),
• Z Cma (Millan-Gabet & Monnier 2002),
• Sz 68 (Reipurth & Zinnecker 1993),
• SR 24 (Struve and Rudkjobing 1949),
• XZ Tau (Haas, Leinert, Zinnecker 1990)
• CoKu Tau/1 (Movsesyan 1989)
• DO Tau (Tessier et al 1994)
There are plenty of multiple sources which only drive one bipolar jet

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Jet Binaries, HH1, HH111, HH154

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Jets from L1551
IRS 5
• L1551 IRS 5 system
• Observations of binary disks and
bipolar binary jets from L1551 IRS 5
• Unlikely to be a cavity in the jet –
velocity difference too great + bipolar
+binary source all point towards 2
jets.
• 2 jets - fast and slow jet
• 1 bow shock visible
• Wiggling along course of jets – South
jet appears to turn in direction of
north jet

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Two Jets From LDN1551 IRS 5
(Fridlund and Liseau 98)

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Initial Conditions
• Assumed distance of 140 parsecs
• Density=500 /cm^3, temperature=1000 K
• Velocities 300km/s (North Jet) and 100km/s (South Jet)
• Sinusoidal varying injection velocity (Raga et al. 1990), assuming an
amplitude of 30% in the velocity with a period of 8 years for each jet.
• The ambient medium is modelled with a uniform density (5000 /cm^3)
• Stagger launching of the two jets - faster northern jet is launched 150
years after the slower southern jet.
• The velocity profile is a positive cosine - with its maximum at centre and
its minimum (zero) at r_jet.
• Profile chosen based on the observations of Bacciotti et al. (2000),
where the highest velocities are thought to be located in the centre of
the jet.

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Orbiting Jets
Density contours 3d hydro orbiting jet simulation. Orbital Period=250
years

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Parallel Interacting Jets
3D midplane slice isopycnics for parallel
hydrodynamic interacting jets

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Magnetic Field Configuration
• Polarimetry measurements of Scarrott (1988) (optical) , Lucas and Roche
(1997) (infra-red) and Curran (2006) (submm) all point towards a toroidal
magnetic field around the cloud.
• We use a 0.01 mG toroidal field in the ambient medium.

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Magnetically Confined Interaction
3D midplane density
slice

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Interacting Binary Jets
• Jet Interaction – kink in structure caused by bow shock impinging on
neighbour’s beam
• Ambient toroidal field – enhances interaction – collimates the jets
• Orbital motion has negligible effect (at this stage)
• X-rays from source probably just scattered out of protostars
• Murphy et al (PPV, 2005)

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4. Jets collimated by environment

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Motivation: Circulation Model
• Combet et al (2006)
• Solutions to the model present both
density and velocity gradients.
• The model also implies a prehistory
of jets and outflows.
• The model applies to both low and
massive star formation.
• Large-scale model far from launch
region
• We launch a jet into the medium
and see does the medium affect the
morphology and the dynamics of the
jet

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Jets (Re)Collimated by Environment
• Prehistory of outflows – a previous outflow creates a channel or
cavity in the molecular cloud
• The cavity fills in on the order of 10^6 years (Quillen et al. 2005)
• The jet on entering a cavity is accelerated, and recollimated.
Direction may also be affected.
• Frank & Mellema (1996,1999) – 2.5D HD and MHD study of
WAW entering a cavity for PNe
• Opher et al (2004) - 3D MHD studies
• Our study - 2D and 3D HD and MHD collimated jet enters a
cavity
• Note: Model requires a coarse mesh at a high level of refinement
so models take longer to run.

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2D Calculations –Effect of density
gradient
Density gradient
Constant density Medium
•2 main
effects
•Collimation
•Increased
structure in
the cocoon

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3D jet in evac ambient medium

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3D calculations - Intensity Maps
Evacuated Cavity Medium
Jet is accelerated and
collimated
Constant Density Medium

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5. Conclusions

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Conclusions: Binary Jets
• We show that the toroidal ambient magnetic field
helps to collimate and refocus the two jets, and
increases the interaction between the two jets
• The density ratio between the jet and ambient
medium controls the interaction, underdense jets
have larger cocoons and are much more likely to
interfere than more ballistic jets
• Bow shock temperatures are high enough to produce
some weak X-rays, but the interacting region does
not produce X-rays by thermal means (implies either
scattering or some non-thermal process).
• SMBBH?

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Super Massive Binary Black Hole Merger

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Conclusions: Evacuated Ambient Media
• The circulation model suggests the presence of an
evacuated cavity around the axis of symmetry
• We modelled this evacuated cavity and showed the
collimation and acceleration of the jet.
• Not a perfect circulation model – but have some
results to look for in observations

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Merci beaucoup

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Intensity Maps
• Intensity Maps
• Emissivity = (transition probability) * hf *N^2 * x_e * (abundance S/H) *
(ionic abundance Si/Hi) * (level pop)
• Bacciotti & Eisloffel (1999)