The launch and collimation ….

Stellar jets transport significant amounts of energy and momentum away from the powering source important role in its evolution.

The jet production mechanism works on very small spatial scales the central jet engine is often heavily embedded

Many open questions remain to be resolved, eg, the nature of the accretion/ejection relationship the jet generation mechanims whether the process is similar on all masses and length scales

Models of jet generationModels of jet generation

It is widely accepted that to make a fast collimated jet requiured: accretion disk

large-scale magnetic field.

This is because neither gas pressure nor radiation pressure are enough tocollimate large momentum flux.

MAGNETO-CENTRIFUGAL MODELS:

During the accretion process, magneto-centrifugal forces are responsible for launch acceleration of the stellar jet collimation

Observational challengesObservational challenges

Main observational difficulties in testing models lie: YSO are heavily embedded Infall and outflow kinematics are complex and confused close to the source Spatial and temporal scales ar relatively small.

Ex: considering the length scales involved for Taurus (d~140 pc):

Jet acceleration and collimation zone ~ 1-40 AU above the plane of disk 0.1” res.

Jet engine operates on sclales of < 5 AU 0.01” res.

Observational difficulties persit even the jet has travelled far from the source: emission lines mark the location of shock fronts and post-shock cooling zone (length scales ~ tens AU) Jet widths are typically ~ 15 AU

Hence, resolving the jet internal structure, excitation and kinematics is heavily dependent on high spatial resolution data

The region of the jet launchThe region of the jet launch

Outflows are ultimately powered by the release of Eg liberated by accretion onto the YSO ( only ~ 10% ; the rest is radiated).

It is likely that the acceleration of an outflow and its collimation occur at different location and involving different processes:

Most outflow are probably launched at radii of at most a few AU , while Most jets have beams ~50-500 AU wide at the point where

they first become visible Indication that collimation occurs at larger distances from

the source than the launch region.

Collimation Collimation l>>wl>>w

Need to determine the jet width as close as possible to the powering sourceNeed to determine the jet width as close as possible to the powering source Ideally: at a few RIdeally: at a few R* * 10 101212cm cm 0.5mas for a d~140pc (Taurus) 0.5mas for a d~140pc (Taurus)

““Problems”:Problems”: ++ObservationObservation limited by the spatial resolution.limited by the spatial resolution. ++IntrinsecIntrinsec: * Powering source, deeply embedded Powering source, deeply embedded high optical/ir high optical/ir

extinction extinction jet are observed beginning from few arcsec jet are observed beginning from few arcsec from the powering source.from the powering source. *Light reflected by the disk produces a bright background (Light reflected by the disk produces a bright background ( low low contrast) contrast) that makes difficult to measure the that makes difficult to measure the

jet width.jet width.

0.5-10” (70-1500AU)

Case A : jet collimation “far”

from the source

Case B: jet collimation “close”

to the source Mundt & Raga:i= mean jet aperture inside the box

a=mean jet aperture outside the box

Measured in 15 jets:i 0 a 3º

i>3a

Seems to favour Case A

“Forbidden observing box”

MECHANISMS OF COLLIMATION OF THE JETMECHANISMS OF COLLIMATION OF THE JET

Currently, there are three main magnetohydrodynamic (MHD) models, which differ mainly in the origin of magnetic forces which drive the jet:1.- The stellar wind 2.- The X-wind3.- The Disk-wind

The Stellar wind modelStellar wind model

The jet launching point is the stellar surface

PROBLEMS: Difficulties in achieving sufficient angular momentum extraction to slow the stellar rotation to the observed via stellar wind alone.

The X-Wind modelX-Wind model

The magnetic X-point (point where the stellar magnetosphere intersects the disk) is the point of origin of a magneto-centrifugally driven wind, fueled bymatter injected onto the open field lines.

The magnetic forces on the open field lines, at scales of ~ 0.03 AU from thesource, are responsible for collimating the wind into a jet

X-WINDX-WIND

The Disk-wind modelDisk-wind model

Centrifugally driven winds are launched from a magnetised disk surface launch occurs not only close to the source, but also up to a few AU along the disk (~0.03 to 5 AU)

DISK-WINDDISK-WIND

Jets structure……..

Knots: steady crossing-shockssteady crossing-shocks (model from Raga, Cantó)

A jet which initially has a pressure higher than the environment, expands freely until its pressure falls below the pressure of the ambient

It is then recollimated by a curved “incident shock”

Where the incident shock converges on the jet axis, it sets up a “reflectedshock”

Since it is now again overpressured compared to its environment, it will Expand and the whole pattern is repeated.

Advantadges:Advantadges:

Makes possible to extensively explore parameter space.Predict the jet appearance in various emission lines and calculate theoretical long-slit spectra.

Problems:Problems:

Not very realistic, eg, If the observed knots are identified with the predicted crossing-shock cells, the observed length-to-width ratio is an order of magnitude smaller than the calculated (~1.4 x Mj; Mj~20; Mj, Mach number).

Observed proper motion cannot be reproduced in these stationary models.

** Jets: knots/interknots ** Jets: knots/interknots discontinuous intensity discontinuous intensityKnotsKnotscrossing shockscrossing shocks

Differences between pDifferences between pj j y py pee give rise to a pair of shock waves give rise to a pair of shock waves

(incident/reflected(incident/reflected))

Problems:Problems: stationary knotsstationary knots

M=sin-1(aT/Vj)

M= Mach angleaT=sound velocityVj=jet velocity

SHOCK MODELS IN HHsSHOCK MODELS IN HHs

HHs: Observational signature of the shock produced by thecollision of two fluids with their velocities ranging 10 -- 300 km/s

Models: plausible scenarios

1.- Internal shocks within a moving fluid (jet or wind)2.-Entrainment of dense gas by jet or wind.3.- Bow-shock produced when the wind is intersected by an obstacle.4.- A dense knot (“bullet”) goes through an obstacle.

(Models 2 and 4 are similar, except in the relationshipbetween the densities of the high-velocity and stationarygas).

IWS

Internal shocks in a moving fluid

Schematic diagram showing a two-shock working surface formed by the interaction of a flow of high velocity V2 with a previously ejected flow of slower velocity V1. The working surface moves with an intermediate velocity Vws.

Velocity variationsVelocity variations

Variabilities in the ejection velocity results in the generation of internal “working surfaces” as the high velocity sections of the flow catches up the slow ones previously ejected..

v V2

Shock

V1

Vws

Shock

The working surface of a jetThe working surface of a jet

Two fluid colliding supersonically: two shocks are generated:1.- a shock in which material from the environment is accelerated

bow-shockbow-shock2.- 2.- a shock in which the jet material is decelerated jet shockjet shock

The whole double shock structure is the working surfaceworking surface

The shocks are separated by a contact discontinuity

““working surfaceworking surface”

Structure of the working surfaceStructure of the working surface

Incident jet gas (1, v1) encounters gas (2,v2 < v1). Impact occurs at the Mach disk, also known at the jet shock. Gas traversing this front creates the curved bow-shock on the right. In between the two fronts, material spills out laterally, forming the cocoon

The widest bow-shocks observedhave a clumpy morphology and display short-temporal fluctuations: The inter-shock material is not an homogeneous fluid: two flows withdistinct properties enter and areunlikely to mix completely. The weakerShock, at lower temperature, cools more rapidly, forms a dense shell. Breaksas fresh material joins it.

Models: density maps of the working surface after 800 yr (a) and 1200 yr (b)

Spatial non-coincidence of the emission from different lines