Physics
Scientific paper
Jun 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........1o&link_type=abstract
Ph.D. Thesis, Trinity College Dublin, Dublin, Ireland
Physics
2
Star Formation, Hh Objects, Jets, Outflows
Scientific paper
The nature of the accelerated and heated gas along collimated outflows emanating from deeply embedded protostars is investigated. By analysing the shock structures and excitation conditions it is possible to deduce information concerning the flow dynamics and environmental structure governing the morphology of outflows. Narrow-band imaging at near-infrared wavelengths and various spectroscopic techniques are employed in conjunction with bow shock modeling to uncover the underlying gas dynamics and excitation structure.
The L 1634 globule contains two series of aligned molecular shock waves associated with the Herbig-Haro flows HH~240 and HH~241. Near-infrared spectroscopy and narrow-band imaging in the (1,0)~S(1) and (2,1)~S(1) emission lines of molecular hydrogen yield the spatial distributions of both the molecular excitation and velocity, which demonstrate distinct properties for the individual bow shocks. Bow shock models are applied to infer the shock physics, geometry, speed, density and magnetic field properties. The advancing compact bow HH 240C is interpreted as a J-type bow (frozen-in magnetic field) with the flanks in transition to C-type (field diffusion). It is a paraboloidal bow of speed ˜ 42 km s-1 entering a medium of density ˜ 2 × 104 cm-3. The following bow HH 240A can be fit by a C-type model. It has a higher bow speed in spite of a lower excitation, and is propagating through a lower density medium. It is concluded that, while the CO emission originates from cloud gas directly set in motion, the H2 emission is generated from shocks sweeping through an outflow.
The HH 211 outflow is of considerable interest because of its ascribed youth. The outflow is explored through imaging and spectroscopy in the near-infrared. The detection of a near-infrared continuum of unknown origin is confirmed. It is proposed that the continuum is emitted by the driving protostellar source, escapes the core through the jet-excavated cavity, and illuminates the features aligning the outflow. In addition, [Fe II] emission at 1.644 μm has been detected but is restricted to isolated condensations. The ordered structure of the western outflow is modeled as a series of C-type shocks with J-type dissociative apices. Essentially the same conditions are predicted for each bow except for a systematic reduction in speed and density with distance from the driving source. Increased K-band extinctions are found in the bright regions, as high as 2.9 magnitudes, and suggest that the bow shocks become visible where the outflow impacts on dense clumps of cloud material.
Integral field spectroscopy was performed on the highly symmetric HH 212 outflow. Narrow-band images and spectra were simultaneously obtained between 1.5 to 2.5 μm. Images in H2 and [Fe II] transition lines were compared in order to extract the excitation and extinction conditions. Collisional excitation was confirmed as the process leading to the radiation from the inner knots and bows. Lower excitation and extinction are found for the bows which appear to have exited from the dense inner gas. The peak flux positions are compared for all the transition lines detected. For the knots, a trend is found between the measured offsets and the upper level temperatures both along the outflow direction and transverse to the jet axis. An underlying shock structure is implied.
A timescale for the Class O evolutionary stage is suggested which relates the envelope mass to the mass accretion rate as inferred from the outflow luminosity. The deduced timescales are in general agreement with the Class O lifetimes estimated from statistical surveys. It is proposed that in order to investigate the relationship between outflows and protostellar evolution, the individual environmental factors for each outflow need to be examined. Only then can the intrinsic luminosities be revealed and related to the evolution which may be different for each source.
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