VELOCITY FIELDS IN H II REGIONS USING HIGH RESOLUTION IMAGING FABRY-PEROT SPECTROMETER

Abstract

ABSTRACT The thesis comprises of two parts: I. Instrumentation II. Observations, results and discussion. An imaging Fabry-Perot spectrometer (IFPS) is designed and constructed for the studies on kinematics of extended astronomical objects (Seema et al., 1992). IFPS comprises of a field aperture, collimating lens and a two dimensional imaging sensor called Imaging Photon Detector (IPD). It is the first time that IPD which uses a resistive anode for position determination is being used in the spectroscopic studies of astronomical objects. Observations were made on Orion and Trifid nebula covering a wide field of view using a 35cm Celestron-14 telescope (f/11 cassegrain) at Gurushikhar, Mt.Abu, India. Orion Nebula: Observations were made in [OIII] 5007A, line with a spectral resolution of 6 km/sec and spatial resolution of 4" covering a field of view of 10.5', to study (i) general velocity flow (ii) high velocity flow and (iii)random motions. Line profiles generated for about 2000 positions showed an asymmetric shape with (a)a narrow component 20 ± 3 km/sec and (b) a broad component 50 ± 3 km/sec. The two components could be interpreted in terms of the interaction of the ionized gas (from the trapezium stars) with the condensations present in the nebula, resulting in the secondary flows. The iso-velocity contour map generated for both the components showed velocity flow in agreement with the champagne flow model (Tenorio-Tagle 1982). A model emission line profile constructed assuming a champagne flow in [OIII] 5007A, line for a position 2' away from theta-1 C Ori showed a reasonably good agreement with the narrow component of the observed profile. Certain high velocity flow (~50 km/s) regions are observed to be superimposed on the main flow of the narrow component. These flows are either radiation pressure driven stellar winds or jets generated during the formation phase of Young stellar objects. The radial velocity was found to be low with no high velocity flow regions in the dark bay indicating that there could be obstructing material to the expanding gas in this region. Velocity profile was generated for the first time across the Orion 'bar' ionization front. The high velocity flows are found only in certain clumps along the bar showing that the bar ionization front is found to be diffuse in [OIII] 5007A, line. The actual velocity obtained by us around the bar ionization front is estimated to be more than 100 km/s, indicating that O++ is produced by shock ionization (at least in part). Turbulence: The structure function B for a region r about 100" around theta-1 C Ori for both the narrow and the broad components showed a correlation of the form B proportional to r. We interpret that this deviation from the standard Kolmogorov model (1941) could be caused due to the presence of compressibility in the fluid (i.e., violation of one of the assumptions in Kolmogorov's model). For regions around theta-2 A Ori, the behaviour of the structure function is found to be irregular. Trifid nebula: Velocity field studies were made for about 48 positions on the nebula in [OIII] 5007A, line covering a field of view of 24'. The isovelocity map in a region 2' around the central hot star HD 164492 showed a picture of symmetric flow in velocities with no density gradients as was observed in the case of Orion nebula. A general red-shift in the radial velocities is observed in the south west direction with a velocity change of 50 km/s. There is a symmetric localized flow with a red-shift velocity of 40 km/s observed about 2.3' southwest of HD 164492 corresponding to a bow shock feature adjoining a condensation.