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CALTECH GE 133 - Observations of Jets and Outflows from Young Stars

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Observations of Jets and Outflows from Young StarsJohn BallyUniversity of Colorado at BoulderBo ReipurthUniversity of HawaiiChristopher J. DavisJoint Astronomy CentreThis review concentrates on observations of outflows from young stars during the last 6years. Recent developments include detections of an increasing number of Herbig-Haro flowsat X-rays and UV wavelengths, high resolution studies of irradiated jets with HST, wide-fieldimaging of parsec-scale outflows with ground-based CCDs and near-IR imagers, completesurveys of visual and near-IR emission from shocks in the vicinity of entire molecular cloudswith wide-field imagers, far infrared studies with ISO and the Spitzer Space Telescope, andhigh angular sub-mm, mm, and cm wavelength aperture synthesis array data-cubes showingboth the spatial and velocity structure of jets and outflows.1. INTRODUCTIONOutflows are one of the manifestations of the birth of ayoung star that are easiest to observe. More than a half cen-tury ago, Herbig (1950, 1951) and Haro (1952, 1953) foundthe first examples of the peculiar nebulae which have cometo be known as Herbig-Haro (HH) objects. Located in ornear dark clouds in regions suspected of having undergonerecent star formation, HH objects were first thought to beyoung stars or their associated reflection nebulae. However,by the 1970s, their spectra were interpreted as indicativeof mostly low-excitation shock waves. By the 1980s someof the growing list of HH objects were found to trace highlycollimated jets powered by young stars (Dopita et al., 1982;Mundt and Fried, 1983; Reipurth et al., 1986), associatedwith high proper-motion bipolar outflows (Herbig & Jones,1981).During the 1960s and 70s, additional manifestations ofoutflow activity were discovered. These include P-Cygniprofiles and other spectroscopic indicators of powerful stel-lar winds emerging from young stars, high-velocity OH andH2O masers, bipolar molecular outflows (e.g., Snell et al.,1980), as well as shock-excited near-IR emission lines ofspecies such as [FeII] and H2. By the 1980s, a number ofyoung stellar objects (YSOs) were found to produce radiocontinuum jets visible at centimeter wavelengths. Althoughit was not at first apparent, by the early 1990s, observa-tions and theoretical considerations made it clear that mostmanifestations of outflow activity were produced by shockspowered by forming stars.The advent of large-format CCDs and the launch of theHubble Space Telescope (HST) during the 1990s usheredin new developments. Wide-field visual wavelength sur-veys revealed that outflows can attain parsec-scale dimen-sions with lengths exceeding 10 pc, and that they blow-outof their molecular clouds (Bally and Devine, 1994; Reipurthet al., 1997; Eisl¨offel and Mundt, 1997). Narrow band fil-ter imaging revealed a new population of protostellar jets inHII regions which are rendered visible by the UV radiationfields of nearby massive stars (Reipurth et al., 1998a; Cer-nicharo et al., 1998; Bally and Reipurth, 2001). HST hasresolved the sub-arcsecond scale structure and cooling lay-ers of dozens of HH objects. HST has enabled the measure-ment of proper motions on images taken less than one yearapart - a time-scale shorter than the typical cooling time.1.1. The Importance of OutflowsAlthough no theory of star formation anticipated jets andoutflows, it is now clear that the production of these flowsis a fundamental aspect of star formation. Collimated out-flows occur in most astrophysical systems in which accre-tion, rotation, and magnetic fields interact. Due to theirproximity, large numbers (nearly a thousand masers, HHobjects, and molecular outflows are now known; Wu et al.2004), and the diversity of available tracers, protostellaroutflows make ideal laboratories for the investigations ofthe physics, chemistry, acceleration, collimation, propaga-tion, and impacts of these systems. The results of thesestudies should be relevant to many other classes of astro-physical outflow.Outflows provide a fossil record of the mass loss, andtherefore, the mass-accretion histories of forming stars.Outflow symmetries provide clues about the dynamical en-vironment of the engine; S-and Z-shaped symmetries indi-cate that the outflow axis has changed over time, perhapsdue to precession induced by a companion, or interactions1with sibling stars in a cluster. C-shaped bends indicate mo-tion of surrounding gas (side-winds), or the motion of theoutflow source itself. Outflows have a profound impact ontheir surroundings. Jets and winds create cavities in theapparent cloud, inject energy and momentum into the sur-rounding clouds which in the absence of massive stars maydominate the generation of turbulence and cloud motions.The terminal shocks in outflows dissociate molecules, sput-ter grains, and can re-set the chemical evolution of clouds toan initial state. Shocks also drive chemistry, thereby alter-ing the chemical composition of the impacted media. Out-flows may play a fundamental role in sculpting and disrupt-ing their parent clouds. They may play a role in determiningfinal stellar masses and the shape of the Initial Mass Func-tion. Outflows may also carry away some of the angularmomentum of matter accreting onto the forming star.This review will concentrate on developments since thelast Protostars and Planets conference held in 1998. Duringthe last 7 years, observational and computational capabil-ities have increased greatly. Major new developments in-clude the detection of X-rays from an increasing numberof protostellar outflows, and the measurement of the emis-sion and absorption spectra of some HH objects in the UV(Section 2). At visual and near-IR wavelengths, the formatsof detectors has continued to increase with pixel countsdoubling approximately every two years. This has led tothe first complete imaging surveys of entire giant molec-ular clouds with 4 meter-class telescopes in narrow-bandfilters sensitive to shock tracers. 8-meter class telescopes,adaptive optics, a new generation of high-resolution andmulti-object spectrographs, and the Advanced Camera forSurveys on HST have had major impacts (Section 3). TheSpitzer Space Telescope is surveying the Galactic plane andmany of the nearby clouds, and IRAC and MIPS imagesare providing stunning images of the mid-IR emission fromjets, outflows, and the properties of their sources. Ther-mal imaging with Gemini has traced entrained warm dustin jets. Upgrades at the VLA, and a new generation of


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