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 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 to collimate large momentum flux. MAGNETO-CENTRIFUGAL MODELS: During the accretion process, magneto-centrifugal forces are responsible for  launch  acceleration of the stellar jet  collimation

Observational 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 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 ~ 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  l>>w Need to determine the jet width as close as possible to the powering source Ideally: at a few R *  cm  0.5mas for a d~140pc (Taurus) “Problems”: +Observationlimited by the spatial resolution. + Observation : limited by the spatial resolution. + Intrinsec + Intrinsec: Powering source, deeply embedded high optical/ir extinction  jet are observed beginning from few arcsec * Powering source, deeply embedded  high optical/ir extinction  jet are observed beginning from few arcsec from the powering source. from the powering source. Light reflected by the disk produces a bright background (  low contrast) that makes difficult to measure the jet width. *Light reflected by the disk produces a bright background (  low contrast) that makes difficult to measure the jet width.

0.5-10” ( AU) 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 >3  a Seems to favour Case A “Forbidden observing box”

MECHANISMS 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-wind 3.- The Disk-wind

Stellar wind model The Stellar 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.

X-Wind model The X-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 by matter injected onto the open field lines. The magnetic forces on the open field lines, at scales of ~ 0.03 AU from the source, are responsible for collimating the wind into a jet

X-WIND

Disk-wind model The Disk-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-WIND

X-wind vs D-wind X-wind X-wind - from a single annulus; interaction region between the inner disk edge and protostellar magnetosphere Disk-wind: Disk-wind: from a spread of disk radii 0.1AU 5-10 AU

Jets structure ……..

steady crossing-shocks Knots: steady 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 “reflected shock” Since it is now again overpressured compared to its environment, it will Expand and the whole pattern is repeated.

Advantadges:  Makes possible to extensively explore parameter space.  Predict the jet appearance in various emission lines and calculate theoretical long-slit spectra. 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  discontinuous intensity Knots  crossing shocks Differences between p j y p e give rise to a pair of shock waves (incident/reflected ) Problems:stationary knots Problems: stationary knots  M =sin -1 (a T /V j )  M= Mach angle a T =sound velocity Vj=jet velocity

SHOCK MODELS IN HHs HHs: Observational signature of the shock produced by the collision of two fluids with their velocities ranging 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 relationship between the densities of the high-velocity and stationary gas).

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 V 2 with a previously ejected flow of slower velocity V 1. The working surface moves with an intermediate velocity V ws. Velocity 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 V2V2 Shock V1V1 V ws Shock

The 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-shock 2.- jet shock 2.- a shock in which the jet material is decelerated  jet shock working surface The whole double shock structure is the working surface The shocks are separated by a contact discontinuity

“working surface “working surface”

Structure of the working surface Incident jet gas (  1, v 1 ) encounters gas (  2, v 2 < v 1 ). 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 observed have a clumpy morphology and display short-temporal fluctuations: The inter-shock material is not an homogeneous fluid: two flows with distinct properties enter and are unlikely to mix completely. The weaker Shock, at lower temperature, cools more rapidly, forms a dense shell. Breaks as 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