RUI: Optical and Infrared Observations of Herbig-Haro Objects

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9417209 Schwartz A comprehensive program of optical and infrared observations of the shock wave structure of Herbig-Haro objects (stars) will be carried out. A basic aim of the study is to understand the dynamics of high-velocity jets from young stellar objects and the manner in which the high velocity material transfers momentum and energy to the ambient molecular medium. The fundamental hypothesis is that molecular hydrogen emission is produced by low-velocity shocks via entrainment by high-velocity jet material and/or in the extended wings of the bow shocks in the working surface of a jet penetrating into the molecular medium. In both cases, molecular hydrogen is accelerated to intermediate velocities between those found for optical emission lines in the high-temperature Herbig-Haro shocks and in the more widespread carbon monoxide (C0) outflows in which there is momentum coupling. A major unresolved issue concerns the relation of the optical shocks, which involve shock velocities greater than about 50 km per second, and the shocks which produce molecular hydrogen emission which would require shock velocities less than about 30 km per second. In many cases, the optical shocks appear to be coincident spatially with the molecular shocks, yet it is unlikely that both the optical and the molecular hydrogen emission arise from a common postshock gas. High spatial resolution images of selected Herbig-Haro objects will be obtained in area of the spectrum of the optical emission lines of Hydrogen alpha and SII and in the infrared lines of molecular hydrogen and FeII . The FeII /(molecular hydrogen) line intensity ratio can be used to infer the depletion of gas phase heavy elements onto grains, and thus to determine whether the gas being shocked is jet gas or ambient medium gas. The SII /(Hydrogen alpha) ratio is a measure of excitation of the gas, and in general the emission from molecular hydrogen is expected to be strongest in or near structures with the largest SII /(Hydrogen alpha) intensity ratio. For two optically obscured Herbig-Haro outflows, the 1.64 micron wavelength line of FeII will be used to trace the "optical" component of the shocks. The acquisition of high resolution spectral data in both the optical and infrared portions of the spectrum will be used to examine the radial velocities and velocity dispersions of the postshock gas. The data should resolve the question of whether the low-excitation optical emission arises from the same postshock gas as the emission from molecular hydrogen. The spatially-resolved spectra will also permit a discrimination between magnetic precursor and bow shock models. In the larger context of star formation, the interpretations of the observations to be carried out will lend considerable insight into the question of the origin of bipolar molecular outflows from young stellar objects. Recent theoretical models suggest that high-velocity jets provide the driving power for the more widespread low-velocity molecular outflows seen in C0 emission. The study of low-excitation Herbig-Haro objects and emission from molecular hydrogen found at the boundaries of the high and low-velocity flows can be used to estimate the efficiency with which energy and momentum are transferred to the ambient gas.