Browsing by Author "Hu, Xihai"

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    High velocity flows, shocks in the star forming H II region: The Orion Nebula
    (1996) Hu, Xihai; O'Dell, C. Robert
    We have studied the high velocity flows and related shock phenomena in the Orion Nebula. Hubble Space Telescope WF/PC and WFPC2 images have been used in morphological studies and proper motion studies of the objects of interest. The high spectral resolution echelle slit spectra have been obtained for the central Orion Nebula in order to study both high velocity flows associated with compact objects and large scale high velocity systems. A group of Herbig-Haro objects located to the north of the Trapezium cluster have been studied in detail. Their morphology, proper motions and motions along the line of sight have been investigated. A bow shock model has been applied to predict the emission line profiles of bow shocks and the resultant line profiles were compared with the observed spectra. The comparison was successful except there is a discrepancy for one of the objects. The possible origin and evolution of this group of objects have been discussed. Two Herbig-Haro objects(HH203/HH204) in the vicinity of $\theta\sp2$A have also been studied both through the HST images and slit spectra. The possible nature of HH203 was suggested by comparing the emission line image with bow shock model predicted emission in inhomogeneous density environment. The spectra of Knot C has been presented and the nature of this object was discussed. We also reported the discovery of a Herbig-Haro like object-HH269, which is located to the southwest of the Trapezium cluster. A slit spectra mapping of the vicinity of the Trapezium cluster and HH202 have been done. The high velocity flows associated with several "proplyds" have been studied. A 1$\sp\prime$ diameter "receding disk" near the Trapezium cluster was found in (OIII) line spectra. A couple of complicated extended high velocity systems were found near HH202. The spectra of this region were presented.
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    Study of inner region of the Orion nebula using the Hubble Space Telescope
    (1994) Hu, Xihai; O'Dell, C. Robert
    Narrow band filter images of the inner Orion nebula were taken by the the Hubble Space Telescope WF/PC in 1990 and 1991. Two data sets were reduced and astrometric solutions were obtained for the two fields with good accuracy. The three major emission line filters for the 1991 data were calibrated into absolute energy units. The continuum contribution was removed from the images, and the $\rm H\alpha$ contribution was removed from the (NII) images. Calibration constants were found to differ from predicted prelaunch values, and possible explanations were investigated. VLA and HST images of two compact sources near the center of the Orion Nebula were compared. The VLA and HST images are similar when the two are scaled to the same units. VLA images show a higher concentration of surface brightness, reflecting its better resolution; whereas, the HST images, due to greater sensitivity of the HST, showed extended cometary form. The positions of three Herbig-Haro objects on HST images were compared with ground based observations at various wavelengths. The basic agreement between these positional observations can be used to analyze the structure of these sources, because of the different excitation mechanism for each source.