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CALTECH GE 133 - An Observational Perspective

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Toward Resolving the Outflow Engine: An ObservationalPerspectiveTom RayDublin Institute for Advanced StudiesCatherine DougadosLaboratoire d’Astrophysique de GrenobleFrancesca BacciottiINAF-Osservatorio Astrofisico di ArcetriJochen Eisl¨offelTh¨uringer Landessternwarte TautenburgAntonio ChrysostomouUniversity of HertfordshireJets from young stars represent one of the most striking signposts of star formation. Thephenomenon has been researched for over two decades and there is now general agreement thatsuch jets are generated as a by-product of accretion; most likely by the accretion disk itself. Thusthey mimic what occurs in more exotic objects such as active galactic nuclei and micro-quasars.The precise mechanism for their production however remains a mystery. To a large degree,progress is hampered observationally by the embedded nature of many jet sources as well as alack of spatial resolution: Crude estimates, as well as more sophisticated models, neverthelesssuggest that jets are accelerated and focused on scales of a few AU at most.It is only in the past few years however that we have begun to probe such scales in detailusing classical T Tauri stars as touchstones. Application of adaptive optics, data provided by theHST, use of specialised techniques such as spectro-astrometry, and the development of spectraldiagnostic tools, are beginning to reveal conditions in the jet launch zone. This has helpedenormously to constrain models. Further improvements in the quality of the observational dataare expected when the new generation of interferometers come on-line. Here we review some ofthe most dramatic findings in this area since Protostars and Planets IV including indications forjet rotation, i.e. that they transport angular momentum. We will also show how measurements,such as those of width and the velocity field close to the source, suggest jets are initially launchedas warm magneto-centrifugal disk winds.Finally the power of the spectro-astrometric technique, as a probe of the central engine invery low mass stars and brown dwarfs, is shown by revealing the presence of a collimatedoutflow from a brown dwarf for the first time, copying what occurs on a larger scale in T Tauristars.1. INTRODUCTIONThe phenomenon of jets from young stellar objects(YSOs) has been known for over two decades. While wenow have a reasonably good understanding of how theypropagate and interact with their surroundings on largescales (e.g., see the chapter by Bally, Reipurth and Davis),i.e. hundreds of AU and beyond, how precisely these jetsare generated remains a puzzle. The observed correlationbetween mass outflow and accretion through the star’s disk(e.g. Hartigan et al., 1995; Cabrit et al., 1990) would seemto favour some sort of magnetohydrodynamic (MHD) jetlaunching mechanism but which one is open to question.In particular it is not known whether jets originate from theinterface between the star’s magnetosphere and disk (theso-called X-wind model, see the chapter by Shang et al.;Shang et al., 2002; Shu et al., 2000) or from a wide rangeof disk radii (the disk or D-wind model, see the chapter byPudritz et al.).On the observational front we have begun to probe theregion where the jet is generated and collimated, thus test-ing the various models. Moreover since Protostars andPlanets IV a number of major advances have been madethanks to the availability of high angular resolution imagingand spectroscopy. In particular the use of intermediate dis-1DG Tau[OI]1000 AUFig. 1.— Deconvolved [OI]λ6300 + continuum narrow-band image of the DG Tau jet obtained with the AO system PUEOon the Canada-France-Hawaii Telescope. The spatial resolution achieved was 0.001 = 14 AU at the distance of the TaurusAuriga Cloud. Inset (top-left) is a high contrast image near the source. Adapted from Dougados et al., (2002).persion spectroscopy (long-slit, Fabry-Perot or employingintegral field units) has provided excellent contrast betweenthe line emitting outflow and its continuum-generating par-ent YSO, a pre-requisite to trace outflows right back totheir source (see Section 2). Examples include ground-based telescopes equipped with Adaptive Optics (AO) andthe Space Telescope Imaging Spectrograph (STIS), givingangular resolution down to 0.0005 (see Section 2 & Section3). In addition, as will be explained below, the technique ofspectro-astrometry (see Section 5) is providing insights onscales of a few AU from the source. The new methodolo-gies have brought an impressive wealth of morphologicaland kinematical data on the jet-launching region (≤200 AUfrom the YSO) providing the most stringent constraints todate for the various models. Here we will illustrate theseresults, e.g. measurements of the jet diameter close to thesource, and, where appropriate, comparison with modelpredictions.Jets may be Nature’s way of removing excess angularmomentum from accretion disks, thereby allowing accre-tion to occur. Moreover, they are produced not only byyoung stars but by a plethora of astronomical objects fromnascent brown dwarfs, with masses of around 5.10−2M,to black holes at the centre of AGN, as massive as 5.108M.In between they are generated by X-ray binaries, symbioticsystems, planetary nebulae, and gamma ray burst sources(e.g., Livio, 2004). The range of environments, and the as-tounding 10 orders of magnitude in mass over which the jetmechanism operates, is testimony to its robustness. Thusunderstanding how they are generated is of wide interest tothe Astrophysics Community. Given the quality of data nowcoming on stream, and the prospect of even better angularresolution in the near future, we are hopefully close to un-ravelling the nature of the engine itself. Our suspicion, assupported below, is that this will be done first in the contextof YSO outflows.2. IMAGING STRUCTURES CLOSE TO THE JETBASEYSO jets largely emit in a number of atomic and molec-ular lines (see, for example, Reipurth and Bally, 2001 orEisl¨offel et al., 2000). In the infrared to the ultra-violet,these lines originate in the radiative cooling zones of shockswith typical velocities from a few tens to a few hundredkm s−1. Thus in order to explore the morphology and kine-matics of the jet launching zone, both high spatial resolu-tion narrow-band imaging (on sub-arcsecond scales) andintermediate resolution spectroscopy is required. Leavingaside interferometry of their meagre radio continuum emis-sion (Girart et al., 2002), currently the best


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