Browsing by Author "O'Dell, C. Robert"
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Item A survey of stars and compact sources in the Orion Nebula(1996) Wong, Shui Kwan; O'Dell, C. RobertFour monochromatic (H$\alpha$, (N II), (O III) and 5500 A continuum) mosaics of the inner region of the Orion Nebula have been made by using data obtained with the Wide Field Planetary Camera 2 of the Hubble Space Telescope. These mosaics have a common area of $7\sp\prime52\sp{\prime\prime} \times 9\sp\prime25\sp{\prime\prime}$ which is the largest area of the Orion Nebula covered by the Hubble Space Telescope images in a single study. Making use of these Wide Field Planetary Camera 2 data, a survey of compact sources and stars down to V $\approx$ 22 has been done. A total of 489 sources that are not Herbig-Haro objects have been found. 145 of them can be classified as protoplanetary disks (proplyds). The proplyds have been classified into eight different forms. The forms which show strong evidence of interaction with a stellar force are concentrated near $\Theta\sp1$C Ori. Those forms that do not show such evidence are more widely spread.Item Cometary knots in the Helix Nebula(1996) Handron, Kerry Dorinda Patrick; O'Dell, C. RobertThe central star of the Helix Nebula, the closest planetary nebula, is ringed by hundreds of Cometary Knots. Presented here are images taken with WFPC2 of a field in the northern portion of the bright ring and some physical parameters and models derived from the data. The field of view contains more than thirty well formed Cometary Knots and reveals that the entire bright ring is composed of knot-like structures. The data is in three narrow emission line filters, H$\alpha$, (OIII) and (NII). Optical thickness to ionizing photons for the knots is established, physical properties such as density in the ionized portions of the knots and dust mass are determined and the beginning of a model to explain the detailed structure set forth.Item High velocity flows, shocks in the star forming H II region: The Orion Nebula(1996) Hu, Xihai; O'Dell, C. RobertWe 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.Item Spectrophotometry of Bowen resonance fluorescence lines in three planetary nebulae(1992) Miller, Christopher O.; O'Dell, C. RobertThis thesis studies the Bowen resonance fluorescence mechanism of OIII in three planetary nebulae: NGC 6210, 7027, and 7662. It utilizes the best spectra to date of this phenomena in the wavelength range of 3100 A-3850 A. The data are presented in the form of flux ratios relative to the strong 3444.1 A line and compared to expectations based on current theory. Harrington's theoretical predictions which take into account the additional enhancement of some of the Bowen resonance fluorescence lines due to pumping of next higher energy level, OIII 2p3d $\sp3$P$\sbsp{1}{0}$, show agreement with NGC 7027 and 7662; however, predictions by Neufeld agree poorly for all three planetary nebulae studied. Enhancement by charge exchange reactions is quite evident in NGC 6210. Finally, the HeII 3203 A line is used to determine efficiencies of 0.57 and 0.48 for NGC 7027 and 7662 respectively, only slightly higher than Kallman and McCray's prediction of 0.42; while NGC 6210 yielded an impossible value of 1.37.Item Study of inner region of the Orion nebula using the Hubble Space Telescope(1994) Hu, Xihai; O'Dell, C. RobertNarrow 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.Item Study of the Orion Bright Bar using the Hubble Space Telescope and a recalibration of three HST filters(1995) Hasan, Abbas Syed; O'Dell, C. RobertUsing Images of the Orion Nebula from the Hubble Space Telescope and calibrated ground based images, we were able to recalibrate three narrow band HST filters, namely F656N, F658N and F502N. The results show that the first two filter profiles had kept the same shape as that published in the Wide Field Planetary Camera 1 (WFPC1) handbook but had decreased in overall transparency by about 40% each. Results for the F502N filter indicate that the profile had remained unchanged but had shifted towards the lower wavelength side by 2.9A. Using the new profiles, we investigated the 'wrong-sided' brightness phenomenon displayed in the SW portion of the Orion Bright Bar. It was concluded that the anomaly is not an artifact of filter contamination but is real and the result of a geometric effect. In short, it is due to a 'bump' in the nebula.Item The structure of the central Orion Nebula(1994) Wen, Zheng; O'Dell, C. RobertWe have studied Hubble Space Telescope images of the Orion Nebula. Many fine scale features of the nebula and compact objects embedded in the nebula were found. Some of the compact objects were resolved or partially resolved, and exhibit comet or elongated shapes. We propose that those compact objects associated with infrared sources are protoplanetary systems. Many high ionization, high velocity bow shock structures were found in the region where the protoplanetary disk systems are concentrated. High resolution spectra of (S III) $\lambda$ 6312 A line were obtained to study the kinematic properties of the S$\sp{++}$ ions. Four velocity systems were found. Three of them were explained. The first two arise in the main and the foreground layer of the nebula, respectively. The third one results from the reflection of the first system by dust particles inside or around the nebula. An east-west velocity gradient was found for the main emission. We also studied the turbulent motion of the S$\sp{++}$ ions. The results do not agree with predictions of the available turbulence model of H II regions. High resolution (N II) $\lambda\lambda$ 6548, 6583 A, (S II) $\lambda\lambda$ 6716, 6731 A, H$\alpha$, He $\lambda$ 5876 A, and (O I) $\lambda$ 6300 A, and (S III) $\lambda$ 6312 A spectra of the nebula were obtained to study the characteristic motions of their corresponding ions. The results are consistent with those of previous detailed studies of O$\sp+$, O$\sp{++}$, and S$\sp{++}$ ions, indicating the existence of an emission from the foreground lid. Combined with results of previous emission and absorption line studies, we conclude that the line-of-sight structure of the nebula can be approximated as two emitting layers. One litter is the main body of the nebula lying at the near side of the molecular cloud OMC-1, the other is the layer at the far side of the foreground neutral lid. The gas lying in between these two layers is very tenuous. We introduced a method for modeling the geometry of the nebula. The method utilizes a density map and an H I recombination emission map of the nebula. It was applied to the main emitting layer of the nebula, which was the first attempt to quantitatively model the geometry of a nebula. The results indicate that the method is quite successful. The model geometry shows some of the features expected and improves our understanding of the prominent features observed in the nebula.