{"content":[{"text":"– 2 –","bbox":{"left":0.479216,"top":0.093224,"width":0.04151199999999999,"height":0.015109999999999998}},{"text":"1.","bbox":{"left":0.42,"top":0.139311,"width":0.017075000000000007,"height":0.015094999999999997}},{"text":"Introduction","bbox":{"left":0.459019,"top":0.139311,"width":0.121033,"height":0.015094999999999997}},{"text":"A variety of objects with accretion disks drive supersonic jets, including compact bi- \n naries, AGN nuclei, planetary nebulae, and young stars (Stirling et al. 2001; Schmidt 1997; \n Balick et al. 1998; Reipurth & Bally 2001). Young stars are particularly good places to study \n the dynamics of these outﬂows because the best images easily resolve the widths of jets, and \n because postshock gas cools by radiating emission lines, so images and spectra show where \n the gas is heated. The cavities, shells, and turbulence created by stellar jets as they inject \n kinetic energy and momentum into the surrounding medium are a major way that young \n stars generate feedback to their environments, so by observing how stellar jets interact with \n themselves and their surroundings we can identify the main ﬂuid dynamical processes that \n operate in these systems. Such observations motivate and guide numerical simulations of \n stellar jets, and also provide some insight into how to interpret other astrophysical jets that \n are unresolved spatially.","bbox":{"left":0.117647,"top":0.176103,"width":0.7647250000000001,"height":0.23571599999999998}},{"text":"The most simple and powerful way to study the dynamics within a complex system like \n a stellar jet is simply to watch how it varies with time. With most astrophysical nebulae this \n is not practical because the evolutionary timescales are too long and distances too great for \n multiple epochs of imaging to show substantial motions or structural changes in the emitting \n gas. A notable exception to this is the Crab Nebula, where ‘movies’ at optical and X-ray \n wavelengths revealed a wealth of dynamical information about the jet and its surrounding \n synchrotron nebula (Hester et al. 2002).","bbox":{"left":0.117647,"top":0.426557,"width":0.764785,"height":0.135565}},{"text":"Proper motions of HH objects (the brightest shocks within jets) were ﬁrst observed \n decades ago from ground-based proper motions (Cudworth & Herbig 1979; Herbig & Jones \n 1981, e.g.). These observations show the overall nature of the bipolar ﬂows, but only the \n vastly superior spatial resolution provided by HST makes it possible to clearly resolve cooling \n zones and individual shocks in each region. The nature of the line emission from these objects \n greatly assists in their interpretation. Because shocks in stellar jets typically move into gas \n that is mostly neutral, Balmer line emission occurs primarily in a thin shell excited by \n collisions at the shock front, followed by a cooling zone that is bright in forbidden lines of \n [S II], [O I], and [N II] (Heathcote et al. 1996). Cooling zones are often observed to be oﬀset \n spatially from the shock front, so by combining narrowband Hα and [S II] emission-line \n images we can identify both the location of the shocks and the direction of the ﬂow through \n the shock.","bbox":{"left":0.117647,"top":0.57686,"width":0.764797,"height":0.23571599999999993}},{"text":"Papers based on images from two epochs of HST images exist for the bright, well-resolved \n jets and bow shocks in HH 1 & 2 (Bally et al. 2002), HH 34 (Reipurth et al. 2002), and HH 47 \n (Hartigan et al. 2005). These works focused on measuring precise bulk velocities throughout \n these regions, and the observations were ideal for this purpose because the two epochs in","bbox":{"left":0.117647,"top":0.827315,"width":0.7647249999999999,"height":0.07526099999999991}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}