INTERNAL VELOCITIES IN THE ORION NEBULA: LARGE RADIAL VELOCITY FEATURES 1

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1 The Astronomical Journal, 127: , 2004 June # The American Astronomical Society. All rights reserved. Printed in U.S.A. A INTERNAL VELOCITIES IN THE ORION NEBULA: LARGE RADIAL VELOCITY FEATURES 1 Takao Doi Japan Aerospace Exploration Agency, Jindaiji-Higashi-machi, Chofu-shi, Tokyo , Japan; takao.doi1@jsc.nasa.gov C. R. O Dell Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN and Patrick Hartigan Department of Physics and Astronomy, Rice University, 6100 South Main Street, Houston, TX Received 2003 October 13; accepted 2004 February 17 ABSTRACT A map of high-velocity features in the 3 0 ; 5 0 central region of the Orion Nebula was created from slit spectra with a velocity resolution of 8 km s 1. We identified two new bipolar flows, Herbig-Haro (HH) 725 and HH 726, as well as a highly redshifted flow, HH 512. We also found multiple new high-velocity co-moving features, all lying in the velocity range of 30 to 100 km s 1. The newly discovered Big Arc appears as a slightly blueshifted feature. Unlike the other objects found, the Big Arc is not a shock; rather it is a result of a structural alteration in the nebula nearly extended across the sampled region. Spatial velocities of 19 features belonging to 11 HH objects were obtained by combining these radial velocity data with existing proper motion data. Most HH objects in the H ii region exhibit spatial velocities ranging from 50 to 150 km s 1. By analyzing their threedimensional paths, HH 202 and HH were found to be formed on the curved main ionization front, not on the Veil, as previously proposed. We have been able to locate the source of the HH 201 outflow (possibly the IRc2 feature within the BN/KL region) at 0.21 pc behind the ionization front of the nebula. This is in contrast to the Optical Outflow Source, which gives rise to most of the HH objects in the Orion Nebula and lies only a few hundredths of a parsec behind the ionization front of the nebula. Key words: ISM: Herbig-Haro objects ISM: individual (Orion Nebula) ISM: jets and outflows stars: winds, outflows On-line material: machine-readable table 1. INTRODUCTION The Orion Nebula (M42, NGC 1976) is a thin concave blister of expanding photoionized gas on the facing side of the Orion molecular cloud, OMC-1, which is a massive star formation region (O Dell 2001). As stars form, disk accretion occurs, and some of the infalling material is redirected into collimated bipolar outflows. These bipolar outflows are traced by high-velocity lobes of molecular emission at radio wavelengths (Rodríguez-Franco et al. 1999), by optical emissions of shock-heated Herbig-Haro (HH) objects (Hartigan et al. 1987; O Dell et al. 1997a, hereafter O97a), and by infrared emission of shocked H 2 (Lee & Burton 2000). Numerous high-velocity features have been found and studied in the Orion Nebula. One of the most prominent highvelocity features is the Becklin-Neugebauer/Kleinmann-Low (BN/KL) complex (Allen & Burton 1993), which is located to the northwest of the Trapezium and exhibits numerous H 2 fingers (Taylor et al. 1984; Allen & Burton 1993). HH 201, found by Gull et al. (1973) and Münch & Taylor (1974), is located at the tip of one of the H 2 fingers. Cantó et al. (1980) showed that HH 201 has a blue wing stretched up to about 1 Based in part on observations obtained at the Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation and based on observations with the NASA/ ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS km s 1 in its optical spectrum. Other HH objects associated with the BN/KL complex, such as HH 205, 206, 207, 208, 209, and 210, were identified and studied (Axon & Taylor 1984; Taylor et al. 1986) using spectroscopic observations. The Orion Nebula also contains high-velocity features that have no association with the BN/KL complex. HH 202, which is located to the west of the Trapezium, was first identified as a relatively low blueshifted object by Cantó et al. (1980). The structure and radial velocity have been studied extensively by Meaburn (1986), O Dell et al. (1991), and Clayton & Meaburn (1994). HH 203 and HH 204, located just west of 2 Ori A, were discovered by Münch & Wilson (1962). Spectroscopic studies have been conducted by Taylor & Münch (1978), Cantó et al. (1980), Walsh (1982), and O Dell et al. (1993). The high-resolution radial velocity images by Clayton & Meaburn (1994) and Massey & Meaburn (1995) provided new insight into individual high-velocity features, while lower velocity resolution (50 km s 1 ) studies with Fabry-Perot systems have surveyed the entire Huygens region (the bright central core) of the nebula and have shown that high-velocity features are ubiquitous (O97a; Rosado et al. 2001). The true nature of the HH objects in the Orion Nebula, however, was not revealed until the advent of the highresolution images provided by the Hubble Space Telescope (HST ) because of the objects relatively small-scale size of a few arcseconds. The HST images clearly showed that most HH objects in the Orion Nebula have bow shocks and also revealed that there are numerous knots and filaments inside the bow shocks (O Dell et al. 1997b, hereafter O97b). In

2 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3457 addition, the high spatial resolution images from HST showed numerous compact emission-line objects associated with young stellar objects (O Dell et al. 1993), which were first detected as emission-line sources (Laques & Vidal 1979) and later as radio continuum sources (Churchwell et al. 1987). These objects are now called proplyds (O Dell & Wen 1994), indicating young stellar objects in or near an H ii region. According to Bally et al. (2000, hereafter BOM00), the distance to the Orion Nebula is about 460 pc, which will be used throughout this paper. HST images have been taken over a period of more than 7 yr with a spatial resolution better than 0B1. This has allowed us to measure proper (tangential) motions of a few tens of km s 1 within the Orion Nebula (Doi et al. 2002, hereafter DOH02; O Dell & Doi 2003, hereafter OD03). This new level of accuracy of proper motions supersedes earlier work with ground-based telescopes and HST observations with a shorter time base (Hu 1996; BOM00; Cudworth & Stone 1977; Jones & Walker 1988). We are going to combine the newer proper motion studies of the HST andhigh-resolutionspectroscopy(whichcanprovide radial velocities accurate to a few km s 1 ) to study threedimensional motions of high-velocity features based on tangential and radial velocities of comparable accuracy. In this paper, we present the results of a complete velocity mapping of the high-velocity features in the Huygens region of the Orion Nebula at a new level of accuracy. We conducted our observations using the long-slit echelle spectrograph at Kitt Peak National Observatory (KPNO), which provides improved spatial sampling and high spectral dispersion without order overlap. This allowed us to obtain radial velocities with a much better accuracy than the studies using Fabry-Perot systems (O97a; Rosado et al. 2001). We also conducted the observations covering an area on an order of magnitude larger than those areas covered by the pioneering work of J. Meaburn and his collaborators (Clayton & Meaburn 1994; Massey & Meaburn 1995). We measured three different emission lines: [O iii], H, and[nii]. We describe our observations, data reduction, and characteristics of the radial velocity data in x 2. In x 3, we present the detailed results of the radial velocity measurement. Interesting high radial velocity objects are discussed in x OBSERVATIONS AND DATA REDUCTION 2.1. Observations We observed the Orion Nebula with the echelle spectrograph attached at the f8 Cassegrain focus of the Mayall 4 m telescope at KPNO on 2000 January 21, 23, 24, and October 7, 9, 2001 December 31, and 2002 January 2. The entrance slit width was 130 m or0b8 wide and was oriented north-south, projecting an approximately image onto a 2048 ; 2048 CCD detector with 24 m pixels. Two pixels were binned together along the slit, corresponding to 0B6. We used an echelle grating with a blaze angle of 59: 72 for [O iii], H, and[nii] observations. The velocity scale was 3.6 km s 1 per pixel, and the full width at half maximum (FWHM) intensity of the intrinsically unresolved comparison lines was characteristically 2.1 pixels. Therefore, the velocity resolution was 8 km s 1.An interference filter was used to isolate the [O iii] line in the 45th order, and then another interference filter was used to isolate the H and [N ii] lines simultaneously in the 34th order. A total of 96 slit images obtained at 2 00 intervals in right ascension cover the majority of the Huygens region. The total observed area is from 5 h 35 m 10F4 to5 h 35 m 23F2 in right ascension and from to in declination. The offset guiding available on the Mayall 4 m telescope enabled us to place each slit at a desired position with an accuracy of about Three reference stars, JW352, JW499, and JW698 (Jones & Walker 1988), located across the nebula at almost the same declination, were selected for offsetting the slit. Each night s observations involved obtaining calibration spectra using a Th-Ar lamp at intervals of 1 2 hr in order to monitor drift of the image on the detector. After recording a fiducial star spectrum for use in correcting the tilt of all the spectra, we made two exposures with integration times of 150 s each at each designated slit position under clear conditions. The number of exposures and their duration were increased as dictated by intervening clouds. The typical seeing encountered was about Data Reduction All of the observations were reduced using IRAF 2 and dedicated software tasks. After cosmic-ray cleaning using the gcombine task, we used the IRAF package longslit to calibrate the slit spectra. The tasks in the longslit package also corrected the large-scale distortion of lines along the slit direction as well as line tilting. The calibrated spectra were resampled to 4kms 1 per pixel. The nebular spectra were averaged along the slit to find single pixels, and then the peak velocity was determined. These values were then used to adjust the twodimensional spectra so that they all were aligned in velocity. When compared to the radial velocity map of Wilson et al. (1997), this introduces an uncertainty of 2 kms 1 to the relative velocities within each line s dataset. Reduction of the data was rendered more complex because of the complication of sometimes observing through clouds. This both decreased the signal from the nebula and increased the apparent continuum through the scattering of moonlight. The continuum was identified and subtracted individually from each spectrum. The transparency was corrected by normalizing the emission-line portion of the spectra to the surface brightness calibrated HST images of the nebula (O Dell & Doi 1999). The resulting individual spectra represent information along a north-south sample of the nebula at velocities of 300 km s 1 for H and [N ii]and400 km s 1 for [O iii], although we used data from only 300 km s 1 of the systemic velocity. A characteristic spectrum is shown in Figure 1. The 96 slit spectra were then used to form a velocity cube in [O iii], H,and[Nii]. Each velocity cube is an image of the nebula in a 4 km s 1 range in velocity, extending to 120 km s 1 from the systemic velocity of each line. The heliocentric systemic velocity of these lines is 182 kms 1 (O Dell 2001) Characteristics of Velocity Measurement A typical spectrum has multiple velocity components, as shown in Figure 1. The strongest nebular component is due to emission from the ionized layer of the nebula, where the FWHM is about 15 km s 1 for [O iii], 30 km s 1 for H, and 17 km s 1 for [N ii]. The known and unknown causes for this broadening have recently been summarized and discussed (O Dell 2001; O Dell et al. 2003, hereafter OPP03). The second component is due to scattering of light by dust in the dense 2 IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.

3 3458 DOI, O DELL, & HARTIGAN 5 h 35 m subtracted from the right ascension and added to the declination. Groups of features having common origins or motions indicating physical associations are often given HH numbers (Reipurth 2002). We have identified a number of new features that have never been cataloged before. Those that are not assigned HH numbers are given Doi-O Dell-Hartigan (DOH) numbers in this paper. Fig. 1. Spectrum of a characteristic region in [O iii] k5007 shown in an intensity vs. velocity plot. It consists of three components: the systemic velocity component (main nebula component), the scattered light component, and the blueshifted component. A deblending routine in the IRAF task splot successfully makes a deconvolution into multiple Gaussian components, which fit the original curve as shown with dashed lines. The systemic heliocentric velocity is 18 2kms 1. photon dominated region (PDR). Because of the redshift of the PDR with respect to the emitting regions, this component appears as a doubly redshifted shoulder on the nebular emission line (O Dell et al. 1992; Henney 1998; O Dell 2001). In addition to these inescapable features, if a high-velocity feature is present along that line of sight, its components are added. Spectra were formed using either the original slit spectra or creating spectra from the velocity cubes of isolated samples. The spectra were analyzed using the IRAF task splot and deconvolved into multiple Gaussian components, beginning with the direct and scattered nebular emission, which then allowed recognition of the generally weaker blue and redshifted components. The visibility of a high-velocity component depends critically on its intensity relative to the nebula, which depends on the intrinsic brightness of the feature and its velocity relative to the nebula. Low-redshifted components are harder to identify than low-blueshifted components because of the redshifted nebular scattered light component. We estimate the uncertainty of the radial velocities of the high-velocity components to be 5 kms 1. Unless otherwise noted, we present the radial velocity with respect to the systemic velocity (18 2kms 1 heliocentric) throughout this paper. Spatial velocities must be calculated with respect to the PDR (28 km s 1 heliocentric), which is not moving with respect to OMC-1. In order to do that, a radial velocity with respect to the systemic velocity is converted to a radial velocity with respect to the PDR by adding 10 km s MEASURED RADIAL MOTIONS We present the results of the radial velocity measurements in this section. First, we discuss major features of the Orion Nebula using velocity images and slit spectra near the Trapezium. Second, we focus on each of the blue and redshifted objects. The radial velocity data with respect to the systemic velocity of all the measured features are tabulated in Table 1, in which we use three methods to name the features. The first applies to individual features and is the position-based system developed by O Dell & Wen (1994). In this system (DOH02), the six digit names are the truncated positions (J2000.0) rounded to 0: s 1inrightascensionand1 00 in declination, with 3.1. Velocity Images A particularly useful method of identifying large-scale velocity systems is to study images that are color-coded to simultaneously show different velocity ranges. Figures 2 and 3 show the main velocity features of the Orion Nebula in [O iii] and [N ii] in this fashion. The main nebular component is shown in green in both images. The [O iii] comes from the high-ionization region near 1 Ori C (H þ þ He þ ). On the other hand, the [N ii] emission comes from low-ionization (H þ þ He 0 ) regions of the nebula just ahead of the main ionization front. The thicknesses of the [O iii] and[nii] layers are approximately 6 ; 10 2 and 2 ; 10 3 pc (O Dell 2001), respectively. Because the [N ii] layer is almost 30 times thinner than the [O iii] layer, the nebula appears more crumpled in [N ii] than in [O iii]. [N ii] tends to show the MIF more faithfully. The Bright Bar and east-west Bright Bar are low-ionization features that are dominant in [N ii]. The Bright Bar is an escarpment of the MIF (Becklin et al. 1976; Tielens et al. 1993) moving at the systemic velocity (18 2kms 1 heliocentric). However, the southwest portion of the Bright Bar is redshifted in both [O iii] and [N ii]. In the south part of the Bright Bar, a faint blueshifted region exists only in [N ii]. We can observe various high-velocity blueshifted features. HH 202 is located west of the Trapezium, with proper motion measurements (DOH02; OD03) indicating motion toward the northwest. Only [O iii] shows a jetlike feature extending from HH 202 to near the Dark Arc. On the opposite side of the nebula, HH 203 and HH 204 exist near 2 Ori A and are moving southeast (DOH02). The bow shocks of HH 203 and HH 204 are well defined in [N ii], but the bow shock of HH 203 is not clear in [O iii]. [O iii] shows a jetlike feature extending from HH HH 204 toward the center of the nebula. HH 529 is located to the south of the Trapezium and is composed of various distinctive blueshifted bow shocks, which are moving toward the east (BOM00; DOH02). A newly identified blueshifted feature is the Big Arc, which is an east-west feature to the south of the Trapezium. The Big Arc is clearly seen in [O iii] andh but is very faint in [N ii]. The east portion of the Big Arc starts from the Dark Bay and extends toward the southwest, crossing the jetlike feature connected to HH HH 204. The south portion of the Big Arc extends from the jetlike feature toward the west and crosses over the entire Huygens region. This portion of the feature was first reported in O97a Slit Spectra Features An alternative way of obtaining information about velocity systems is to derive velocities from individual slit spectra and to present the peak velocities on an image of the corresponding part of the nebula. This is particularly useful for crowded regions, such as near the Trapezium, where multiple distinct systems exist that have only small velocity differences. Figure 4 shows an HST mosaic image (O Dell & Wong 1996) in [O iii] of an area near and to the south of the Trapezium combined with velocities measured on individual

4 TABLE 1 Radial Velocities in the Orion Nebula Designation Filter V R a (km s 1 ) FWHM (km s 1 ) Flux b Flux Ratio c Peak d Comments e HH Objects HH H p v HH H p v HH [O iii] p HH [O iii] s HH H p HH [N ii] p HH [N ii] s HH [O iii] p HH [O iii] s HH H p HH [N ii] s HH [N ii] p HH [O iii] p HH [O iii] s HH H p HH [N ii] s HH [N ii] p HH [O iii] t HH [O iii] s HH [O iii] p HH H t HH H p HH H s HH [N ii] t HH [N ii] p HH [N ii] s HH [O iii] t HH [O iii] p HH [O iii] s HH H s HH H p HH [N ii] s HH [N ii] p HH [N ii] t HH [O iii] s HH [O iii] p HH H p HH [N ii] p HH [O iii] p HH [O iii] s HH H p HH [N ii] s HH [N ii] p HH [O iii] s HH [O iii] p HH H p HH H s HH [N ii] t HH [N ii] s HH [N ii] p HH [O iii] s HH [O iii] p HH H s HH H p HH [N ii] s HH [N ii] p HH [N ii] t HH [O iii] p HH [O iii] s HH H p HH [N ii] p HH [O iii] t

5 TABLE 1 Continued Designation Filter V R a (km s 1 ) FWHM (km s 1 ) Flux b Flux Ratio c Peak d Comments e HH [O iii] s HH [O iii] p HH H s HH H p HH [O iii] t HH [O iii] p HH [O iii] s HH H s HH H p HH [O iii] s HH [O iii] p HH H p HH [O iii] s HH [O iii] p HH H p HH [O iii] s HH [O iii] p HH H p HH [O iii] p HH H p i HH [O iii] s HH [O iii] p HH H p HH [O iii] p HH H p HH [O iii] p HH H p HH H p i HH [O iii] p HH H p HH [O iii] p HH H p HH [N ii] p HH [O iii] p HH H p HH [N ii] p HH [O iii] s HH [O iii] p HH H p HH [N ii] s HH [N ii] p HH [O iii] s HH [O iii] p HH H s HH H p HH [N ii] t HH [N ii] p HH [N ii] s HH [O iii] p HH [O iii] p HH [O iii] s HH [O iii] p HH [O iii] s HH [O iii] p HH [O iii] s HH [O iii] p HH [O iii] p HH [O iii] s HH [O iii] t HH [O iii] p HH H p HH [N ii] p HH [N ii] s HH [O iii] p HH [O iii] s HH H p 3460

6 TABLE 1 Continued Designation Filter V R a (km s 1 ) FWHM (km s 1 ) Flux b Flux Ratio c Peak d Comments e HH [N ii] p HH [N ii] s HH H p i HH [N ii] p i HH H p i HH [N ii] p i HH H p v HH [N ii] p v HH H p v HH [N ii] p v HH [O iii] p i HH [O iii] p i HH H p i HH [O iii] p i HH H p HH [N ii] p HH [O iii] p i HH H p i HH [O iii] p i HH [O iii] p i HH [O iii] s HH [O iii] p HH H p i HH [O iii] p HH H p HH [O iii] p HH H p HH [O iii] p HH [O iii] p HH [O iii] s HH H p HH [O iii] p HH H p HH [N ii] p HH [O iii] p HH H p HH [N ii] p HH [O iii] p v HH [O iii] p HH H p HH N ii] p HH [O iii] p HH H p HH [N ii] p HH [O iii] p v HH [O iii] p HH H p HH [N ii] p HH [O iii] p HH H p HH [N ii] p HH [O iii] p HH [O iii] p HH [O iii] p i HH [O iii] p i HH [O iii] p i, LV HH [O iii] p i, LV 2 DOH Objects DOH 1... [O iii] p DOH 1... [O iii] p DOH 1... [O iii] p DOH 2... [O iii] p DOH 2... H p DOH 2... [O iii] p 3461

7 TABLE 1 Continued Designation Filter V R a (km s 1 ) FWHM (km s 1 ) Flux b Flux Ratio c Peak d Comments e DOH 2... [O iii] p DOH 3... [O iii] p DOH 3... [O iii] p DOH 3... H p DOH 3... [N ii] p DOH 3... [O iii] p DOH 3... H p DOH 3... [N ii] p DOH 3... [O iii] p DOH 3... [O iii] p DOH 4... [O iii] p DOH 4... [O iii] p DOH 4... [O iii] p DOH 5... [O iii] p DOH 6... [O iii] p DOH 6... H p DOH 6... [O iii] p DOH 6... H p DOH 6... [O iii] p DOH 6... H p DOH 6... [O iii] p DOH 6... [O iii] p DOH 6... [O iii] p DOH 6... [O iii] p DOH 7... [O iii] p i DOH 7... [O iii] p i DOH 7... [O iii] p i DOH 7... [O iii] p i DOH 8... [O iii] p DOH 8... [O iii] p i DOH 9... [O iii] p i DOH 9... [O iii] s DOH 9... [O iii] p DOH [O iii] p DOH H p DOH [O iii] p DOH [O iii] p DOH H p DOH [O iii] p i DOH [O iii] p DOH [O iii] p DOH [O iii] p i DOH [O iii] p i DOH [O iii] p DOH [O iii] s i DOH [O iii] p i DOH [O iii] p DOH [O iii] p DOH [O iii] p DOH [O iii] p DOH [O iii] p i DOH [O iii] p DOH [O iii] p i DOH [O iii] p i DOH [O iii] p DOH [O iii] p DOH [O iii] s i DOH [O iii] p i DOH [O iii] s i DOH [O iii] p i DOH [O iii] p DOH [O iii] p i DOH [O iii] p i DOH [O iii] p i DOH [O iii] p i

8 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3463 TABLE 1 Continued Designation Filter V R a (km s 1 ) FWHM (km s 1 ) Flux b Flux Ratio c Peak d Comments e DOH [O iii] p i DOH [O iii] p i DOH [O iii] s i DOH [O iii] p i DOH [O iii] p i DOH [O iii] p i DOH [O iii] s i DOH [O iii] p i DOH [O iii] p i DOH [O iii] p i DOH [O iii] p i DOH [O iii] p DOH [O iii] p i DOH [O iii] p DOH [O iii] s i DOH [O iii] p DOH [O iii] p DOH [O iii] p i DOH [O iii] p a Radial velocity with respect to the systemic velocity (182 kms 1 heliocentric). b Observed flux of a Doppler-shifted feature in the same arbitrary units throughout. c Observed flux of a Doppler-shifted feature normalized to the observed flux of the systemic velocity component. d p, s, and t indicate that the radial velocity was measured at the primary, secondary, and tertiary peaks, respectively, of the flux intensity using a Gaussian fitting. e i and v indicate that the radial velocity was measured directly from the slit spectrum visually for the peak intensity (i) and for the maximum velocity (v). [O iii] slit spectra. We grouped features by judging their physical closeness as well as their similar velocities. If features belong to one group, their radial velocities are surrounded by the same shape and color perimeter. Most of these features are visible in [O iii] andh but not in [N ii], indicating that they are features within the H ii region formed by 1 Ori C. In addition, there are numerous redshifted knots around and within the Trapezium in both [O iii] and[nii]. Most of them are proplyds, and some are redshifted bow shocks. Figure 4 shows the most complicated region of the Orion Nebula, containing several HH flows as well as many proplyds and their associated bipolar jets. Radial velocities derived from individual slit spectra are shown superposed on an Orion Nebula [O iii] image taken by the HST Wide Field Planetary Camera 2 (WFPC2). There are four well-known HH objects. The first one is HH 529, which is located to the south of the Trapezium and is composed of a chain of blueshifted bow shocks and knots moving toward the east. The second one is HH 518, which is a series of redshifted bow shocks moving away from proplyd toward the northeast. HH 514 consists of redshifted knots moving toward the north from proplyd HH 523 is a group of blueshifted bow shocks moving away from the Trapezium toward the southeast. To the west of the Trapezium, there are four newly found groups of blueshifted features. They are DOH 1, DOH 2, DOH 3, and DOH 4. Because their flux ratios relative to the nebula are about 1% or less, we cannot identify their counterparts clearly in the HST images due to the fact that the emission-line filter employed passes essentially all velocities and is dominated by the zero-velocity component. Their radial velocities are in a range from 30 to 100 km s 1.HH725is composed of almost linearly aligned blue and redshifted jets centered at proplyd Inside and around the Trapezium, we observed six LVobjects (Laques & Vidal 1979). HH 726 contains blue and redshifted features moving at about 100 km s 1. They are linearly aligned well with LV 2, which also shows a redshifted velocity at about 100 km s 1. On the other hand, LV 5 shows both blue- and redshifted velocities, while LV 4 and LV 6 show only redshifted velocities. DOH 5 is a group of blueshifted features with a radial velocity of about 90 km s 1 and located just north of 1 Ori C. To the northeast of 1 Ori D, we have two new groups of blueshifted features. They are DOH 6 and DOH 10. DOH 6 is the closest to 1 Ori D and has an average radial velocity of about 70 km s 1. It also has a slower group of features on its north side with an average radial velocity of about 60 km s 1. DOH 10 extends further toward the northeast with an average radial velocity of 60 km s 1. Both DOH 6 and DOH 10 are very faint, and there are no counterparts found in the HST images. DOH 7, DOH 8, and DOH 9 are located east of HH 518. DOH 7 is a group of blueshifted features bounded by redshifted features that could either be a part of HH 518 or a part of HH 726. The average radial velocity of DOH 7 is about 50 km s 1. DOH 8 is a group of fast-moving faint features with an average velocity of 70 km s 1.DOH9islocated further southeast of DOH 8. Its average radial velocity is 50 km s HH Objects HH 201 HH 201 is located inside the OMC-1 and is a tip of one of the H 2 fingers expanding from the BN/KL complex (Graham et al. 2003) and just reaching the MIF. It is the superposition of two

9 3464 DOI, O DELL, & HARTIGAN Vol. 127 Fig. 2. Color-coded velocity image of the Orion Nebula in [O iii]. Red is assigned to a radial velocity range from 24 to 64 km s 1, green is from 20 to 20 km s 1, and blue is from 64 to 24 km s 1. Large-scale blueshifted features, as well as numerous blueshifted knots, are seen scattered over the entire Huygens region. Each small numbered frame containing HH objects is shown enlarged in later figures with corresponding numbers. bow shocks (O97b; DOH02). The tip of the primary bow shock is , and its proper motion is 173 km s 1 with position angle P:A: ¼ 314 (DOH02). The tip of the secondary bow shock is , and its proper motion is 47 km s 1 with P:A: ¼ 279 (DOH02). The spectra of these shocks in H are shown in Figure 5a. The maximum radial velocities of and are 260 and 284 km s 1, respectively. These values are also close to their zero-intensity width values (Hartigan et al. 1987), indicating that their shock velocities reach about 300 km s 1 (Hu 1996). Graham et al. (2003) gave 267 km s 1 heliocentric for HH 201 in their [S ii] observation, although they did not resolve and in their spectroscopic observations. The radial velocities of and relative to the PDR are 270 and 294 km s 1, respectively. By combining the tangential (proper motion) velocities with the radial velocities, one can obtain the magnitude and orientation of the velocity vector. The former is expressed in km s 1 with respect to OMC-1 (which has the same velocity as the PDR) and the latter in terms of the orientation angle (OA) between the velocity vector and the observer, with zero being the direction toward the observer. In this case, the spatial velocity and the

10 No. 6, 2004 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3465 Fig. 3. As in Fig. 2, but for [N ii]. Linear features like the Bright Bar and the east-west Bright Bar are clearly seen, indicating that they are low-ionization features. The linear features such as the one from to indicate when a slit observation is missing and the data have been interpolated. The discontinuity in the southwest portion is because the spectra forming this part of the velocity image were less well exposed and more sensitive to the method of correction for scattered moonlight. The labels on the axes are a guide to the position in the position-based designation system used in Orion (O Dell & Wen 1994; DOH02). OA of the primary bow are 321 km s 1 and 33, and those of the secondary bow are 298 km s 1 and 9. The velocity image in [O iii] shows a faint blueshifted broad feature moving at about 50 km s 1, which contains the area that HH 201 occupies. Because the [O iii] emissionofhh201 only comes from the tip of the primary bow, this broad feature is a feature in the H ii region and is discussed in more detail below HH 202 HH 202 is one of the biggest HH objects in the Orion Nebula and is located about west of the Trapezium. Cantó et al. (1980) first identified it as an HH object, while Meaburn (1986), O Dell et al. (1991), and Clayton & Meaburn (1994) measured its radial velocity, and Cudworth & Stone (1977), DOH02, and OD03 measured its proper motion. HH 202 is bright in [O iii], H,and[Nii] and is composed of a main bow shock, HH 202-N, and several bright knots. The HST imaging study of O97b is a useful guide to the appearance of this region. The brightest region is HH 202-S, which moves at 59 km s 1 with P:A: ¼ 329 in the plane of the sky (DOH02). Earlier radial velocity data revealed that HH 202 has two radial velocity components of about 40 and 60 km s 1.In

11 3466 DOI, O DELL, & HARTIGAN Vol. 127 Fig. 4. Radial velocities derived from individual slit spectra shown superposed on an ; portion of an Orion Nebula mosaic [O iii] image (O Dell & Wong 1996). If features belong to one group, their radial velocities are surrounded by the same shape and color perimeter. A vertical line connecting two features indicates that similar features exist along the line. The few redshifted radial velocities are indicated in red. A, B, C, and D indicate 1 Ori A, 1 Ori B, 1 Ori C, and 1 Ori D, respectively. There are numerous rogue features, that is, well-defined spectral features that could not be placed within a grouping. The labels on the axes are a guide to the position in the position-based designation system used in Orion (O Dell & Wen 1994; DOH02). order to explain the existence of these two velocity components, O Dell et al. (1993) proposed a Mach disk model, and Clayton & Meaburn (1994) suggested two overlapping bow shocks. The idea of two bow shocks was also considered by Hartigan (1999) in his He ii line emission study. Recent observations with Fabry-Perot spectrometers showed that HH 202 is connected to a jet that extends about southeast toward an area near the Dark Arc (O97a; Rosado et al. 2001; Takami et al. 2002). With the most recent proper motion data (DOH02; OD03), OD03 proposed that HH 202 originates from the Optical Outflow Source (OOS), which is an embedded source lying beyond the MIF and is the source of several other high-velocity outflows. Because there were no strong IR or radio sources observed in this area (BOM00), it is not clear if the OOS is a single object or consists of multiple objects. The jet is bright in [O iii], faint in H, and not visible in [N ii]. The high spatial resolution of the present study allows us for the first time to obtain the radial velocities of the jet as well as those of individual features inside HH 202 as showninfigures6and7. HH 202 is composed of four bright knots in [O iii]. They are , , , and , and all of them are located within the main bow of HH 202. For the first time, we have identified three different radial velocity components in this area. Their averaged velocities are 55 km s 1 (fast), 33 km s 1 (moderate), and 15 km s 1 (slow). We could not resolve the slow radial velocity component in H because of the large FWHM of the systemic velocity component. Feature is located on the west wing of the main bow and is bright in [O iii], but faint in [N ii]. The bright knot in [O iii] is located just southwest of The knot is further inside of the main bow and bright in [O iii] and H. Knot has sometimes been called HH 202-S. The features , , and are located just north of the HH 202 main bow and have only moderate and slow velocity components. The feature is elongated in the east-west direction and is connected to , although also has only moderate and slow velocity components. The HH 202 jet is only visible in [O iii] andh. Thejet forms almost a straight line with P:A: ¼ 316. It is connected to HH 202 at and , both of which have only moderate and slow velocity components. On the other hand, and each have three velocity components. A radial velocity of faster than 60 km s 1 was recorded in both and in [O iii] andh. However, the flux of the fast velocity component is very small compared to the other two velocity components. The features ,

12 No. 6, 2004 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3467 Fig. 5. Slit spectra showing high radial velocity flows of four objects with P:A: ¼ 0.(a) HH 201. Two slits were combined to show the characteristics of both and (b) Proplyd , where the maximum velocity of the redshifted jet reaches about 196 km s 1.(c) Proplyd , which has both blueshifted and redshifted jets. (d) LV2,whichhas a very faint blueshifted trail of a possible counterjet , and form the initial visible section of the jet. They also have two velocity components. The slower velocity component is about 10 km s 1, but the faster velocity component increases from 32 to 52 km s 1 in [O iii] as the jet advances to the northwest. The radial velocity of the faster component of HH 202-S is 57 km s 1, which belongs to the fast component (average velocity 55 km s 1 ) of the shocks. The radial velocity relative to the PDR is 67 km s 1. Combining this result with the proper motion data gives a spatial velocity of 89 km s 1 with OA ¼ HH HH 204 HH 203 and HH 204 are large and bright blueshifted objects located west of 2 Ori A. Since their discovery (Münch & Wilson 1962), many studies have been carried out to obtain radial velocities (Taylor & Münch 1978; Cantó et al. 1980; O Dell et al. 1993), proper motions (Cudworth & Stone 1977; BOM00; DOH02), and spectrophotometric data (Walsh 1982). The heliocentric radial velocities are 48:0 1:4 kms 1 for Fig. 6. Velocity image of HH 202 and its associated jet in [O iii]. North is up. The radial velocity range is from 40 to 80 km s 1. Each rectangular enclosure shows an area in which a one-dimensional spectrum was created from the velocity cubes to conduct a Gaussian deconvolution analysis. Each label next to the rectangular enclosure is composed of the position-based identifier on the top and radial velocity on the bottom. If there is more than one radial velocity component of one location, the radial velocity that has the highest flux is put in bold. Four bright regions are located inside the main bow. The feature is sometimes called HH 202-S. A very faint jet extends toward the southeast from HH 202. HH 203 and 24:5 3:3 kms 1 for HH 204 (O Dell et al. 1993). The proper motions are km s 1 with P:A: ¼ for HH 203 and km s 1 with P:A: ¼ for HH 204 (DOH02). As a result of using Fabry-Perot spectrometers, recent observations also indicate that HH 203 is connected to a high radial velocity jet that extends about northwest into the heart of the Orion Nebula (O97a; Rosado et al. 2001; Takami et al. 2002) as shown in Figures 8 and 9. In the present study, the radial velocities are 64, 63, and 65 km s 1 for HH 203 and 34, 38, and 35 km s 1 for HH 204 in [O iii], H, and[nii], respectively. The present radial velocity values are in good agreement with the results of O Dell et al. (1993) for HH 203 and slightly different for HH 204. As pointed out in O97a and Takami et al. (2002), the high radial velocity jet is highly ionized, being seen in [O iii], H, and He i m but not in [N ii]. Takami et al. argued that the main reason for the high visibility in the He i m

13 3468 DOI, O DELL, & HARTIGAN Vol. 127 Fig. 9. As in Fig. 6, but for HH 203 and its associated jet in H. The bright H emission shows the locations of bow shocks in HH 203 and HH 204. Fig. 7. As in Fig. 6, but for H. The radial velocity range is from 40 to 80 km s 1. The feature , which is also called HH 202-S, is a dominant feature in H. Fig. 8. As in Fig. 6, but for HH 203 and its associated jet in [O iii]. The [O iii] emission along the jet shows that the jet is in the H ii region and feeds into HH 203 directly. emission line is the high density of the jet, which increases the collisional excitation. However, it is more probable that the Doppler shift causes the He i emission line to fall off the absorption core of the nearly static ambient nebular He i. At low velocities, the nebula is very optically thick to the He i m emission (Vaughan 1968), but the highly blueshifted jet emission readily escapes. As shown in Figures 8 and 9, the [O iii] andh intensities vary along the jet and are clumpy. The westernmost feature in the jet is , and of all the jet components, it has the slowest radial velocity of 49 km s 1 in [O iii]. It leads to , which consists of three distinctive knots, with a radial velocity of 58 km s 1 in [O iii]. The radial velocity of the jet increases to 65 km s 1 at This section is the fastest component of the jet in both [O iii] andh. Thejet looks like it bends almost 60 to the south to form , which is the brightest feature in [O iii]. From to , which is the tip of HH 203, the jet extends almost straight with P:A: ¼ 120, while keeping a radial velocity of about 65 km s 1. It is interesting to note that the [O iii] intensity suddenly drops in and Instead, the H intensity strengthens, and the [N ii] emission suddenly appears in , , and , which form HH 203. The features and show almost a linear morphology in H and [N ii]; is a bow shock in H and [N ii]. These HH 203 features have a fairly consistent radial velocity of about 65 km s 1. Because this high radial velocity jet clearly feeds HH 203, we will call it the HH 203 jet hereafter. There is also evidence of a high-ionization flow feeding HH 204, as illustrated in Figure 10. On examination of the velocity images from about 24 to 36 km s 1 (which are well below the HH 203 shock and jet velocity of about 65 km s 1 ), one sees a bifurcated feature that collapses to a single feature as one proceeds to the southeast. This single feature then feeds exactly into the HH 204 shock. Extraction of quantitative data on this feature is difficult because the low relative blueshift leaves this component at a

14 No. 6, 2004 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3469 Fig. 11. H velocity image of the redshifted flow originating from proplyd North is up. The velocity range is from 60 to 80 km s 1,with the measured velocities labeled. The arrow indicates the proper motion vector of (DOH02). Fig. 10. Velocity image for HH 204 and its associated jet in [O iii]. The radial velocity range is from 24 to 36 km s 1. North is up. The labeled radial velocities were determined from spectra formed from the [O iii] velocity cube. The rectangular enclosures in bold are located at the same places as in Figs. 8 and 9; other rectangular enclosures are selected to isolate features associated with the HH 204 jet, which seems to feed directly into HH 204. point where the velocity profile of the emission from the core is still about 20% of the line core, and the additional component cannot be clearly seen except in the asymmetry it produces in the line profile. In a case like this, in which one is trying to extract information from a weak line distorting the emission from the MIF, the situation is further complicated by local changes in the velocity of the MIF. When searching velocity images created under the assumption that the MIF emission all occurs at 0 km s 1, a local blueshift of the MIF will put more energy into the blueshifted images, giving a false appearance of a blueshifted component. We have tested this effect by measuring the line core velocities in the regions indicating an HH 204 jet, finding a local velocity shift in the MIF of 4 kms 1. However, the intensity of the blueward wing of the line core, even after correction for this MIF shifting, is twice that of the MIF emission and is quite believable. Figure 10 shows the velocities indicated by fitting a Gaussian line to the blue shoulder of the MIF emission. We see that the radial velocities of the features associated with the HH 204 jet are always significantly slower than those of both HH 203 ( 65 km s 1 )and HH 204 ( 34 km s 1 ), although the jet velocities do become more negative as HH 204 is approached, even having similar velocity values in a second component as it is reached. The averaged radial velocities of HH 203 and HH 204 are 74 and 46 km s 1 relative to the PDR, respectively. Hence, the spatial velocities of HH 203 and HH 204 are 104 km s 1 with OA ¼ 45 and 103 km s 1 with OA ¼ 63, respectively Redshifted Flow from (HH 512) A monopolar microjet (HH 512) coming from the proplyd was noted by BOM00. The proplyd shows a cusp without a tail (O Dell & Wong 1996). The microjet is visible in H, [Nii], and [S ii], but not in [O iii] inthehst images, extending about 1B7 withp:a: ¼ 60 in H. Figure 5b shows that the microjet is highly redshifted. The maximum intensity of the radial velocity occurs at around 116 km s 1, and the emission reaches about 196 km s 1. Figure 11 shows that the faint redshifted jet extends further toward the northeast than is indicated by the HST images. Three individual knots ( , , and ) are visible along the jet. Their radial velocities are 76, 68, and 64 km s 1, respectively, which establishes a pattern of decreasing velocity with increasing distance from their proplyd source. Because this redshifted jet must be moving toward the MIF, it is possible that the last components of the jet are approaching a higher density region, entrain more material, and are decelerating. The knots and are not visible in the HST images, but is. DOH02 determined the proper motion of to be km s 1 with P:A: ¼ 53 7 in H. This gives a dynamical age for of approximately 620 yr. The spatial motion of is 141 km s 1 with OA ¼ 113. If the proper motion of also applies to the 116 km s 1 microjet emanating from , its spatial motion would be 168 km s 1 with OA ¼ HH 514 BOM00 observed a microjet emerging from proplyd at P:A: ¼ 357 and a compact pair of bow shocks about 3B4 and3b8 downstream from the proplyd along the axis of the jet. DOH02 measured the proper motion of the pair of bow shocks and gave 37 km s 1 with P:A: ¼ 8.BOM00also measured the radial velocities of the bow shocks with the HIRES spectrograph on the Keck I telescope at Mauna Kea and gave a redshifted radial velocity of 135 6kms 1 with respect to the systemic velocity. The present radial velocity measurement of the bows gives 136 km s 1, which is in good agreement with BOM00. Combining the proper motion data and the radial velocity, we have a spatial velocity of 131 km s 1 with OA ¼ 164. BOM00 also reported the existence of a counterjet with a radial velocity of 222 5kms 1 at proplyd , and the velocity gradually decreases to 168 km s 1 (heliocentric) at about to the south of the proplyd. We observed this very faint and narrow high-velocity emission with a radial

15 3470 DOI, O DELL, & HARTIGAN Vol. 127 velocity of approximately 156 km s 1 at 4 00,andwealso detected a fairly strong feature with a radial velocity of 40 km s 1 at 2 00 south of the proplyd HH 516 HH 516 ( ) is a partial bow shock located with P:A: ¼ 202 from 1 Ori C. BOM00 and OD03 gave an averaged proper motion of 26 km s 1 and P:A: ¼ 90.The position of the maximum radial velocity of 91 km s 1 in H was found to correspond to the tip of the bow shock in the present study. The spectra of [N ii] andh are almost identical, and the radial velocity decreases gradually toward the wing of the bow shock. However, the [O iii] spectrum only shows emission from the tip of the bow shock. These emission characteristics match a bow shock model (Hartigan et al. 1987) in which a shock moves toward the observer with a small orientation angle and a shock velocity of about 100 km s 1. With the proper motion data, HH 516 was computed to move at 106 km s 1 with OA ¼ 14 in three-dimensional space HH 518 HH 518 consists of three redshifted bow shocks moving toward the northeast in the area south and southeast of the Trapezium as shown in Figure 4. Because they do not show any [S ii] emission (DOH02), the bow shocks are all in the H ii region. O97a first observed these redshifted shocks. The welldefined bow, , has an average radial velocity of 84 km s 1. The second fainter bow, , has a much slower radial velocity of 56 km s 1. The third bow, , which has a bright leading knot with a stretched wing, shows two different radial velocities of 98 and 68 km s 1.Thisthird bow is in a very crowded region where we also identified blueshifted features called DOH 7. Further northeast from , we observed several faint redshifted features with radial velocities of about 100 km s 1. They could be an extension of HH 518 or a redshifted jet from LV 2. There are no clear bow shocks visible in the HST images. OD03 gave averaged proper motions of 19 km s 1 with P:A: ¼ 39 for and 21 km s 1 with P:A: ¼ 22 for DOH02 gave a proper motion of 42 km s 1 with P:A: ¼ 57 for Combining these proper motion data with the radial velocities with respect to the PDR, we have spatial velocities of 76 km s 1 with OA ¼ 166,51km s 1 with 156, and 96 km s 1 with 154 for , , and , respectively. The fact that all the orientation angles are within 12 suggests that these bow shocks comprise one redshifted flow. Extending the symmetry axis and the proper motion vectors of these bow shocks, we find a possible origin for these shocks. Proplyd lies near this axis and shows a continuous stellar spectrum as well as a redshifted broad feature. The maximum radial velocity of the redshifted broad feature is about 36 km s HH 523 HH 523 consists of numerous bow shocks to the east of the Trapezium as shown in Figure 4. These bow shocks are visible in H and [O iii] but not in [S ii], indicating that they formed in the H ii region (DOH02). A velocity image in [O iii] shows four bright blueshifted knots, , , , and The knot is close to the bow shock, whose proper motion is 71 km s 1 with P:A: ¼ 106 (DOH02), and shows two different radial velocities of 63 km s 1 and 43 km s 1. Combining the proper motion and the larger radial velocity indicates that the shock is moving at 102 km s 1 with OA ¼ 44 in three-dimensional space HH 529 After the blueshifted features in HH 529 were first observed by Lee (1969), several authors have detected this object (Castañeda 1988; Massey & Meaburn 1995) in spectroscopic observations. However, the true nature of this object was not clear until O97a conducted a survey offast-moving objects over the entire Huygens region by using Fabry-Perot spectroscopy, and BOM00 studied HH 529 by using high-resolution spectroscopy and HST imaging. HH 529 contains a series of eastward oriented bow shocks and knots, which extend approximately from a location south of the Trapezium. Previous proper motion studies (BOM00; DOH02) show that almost all of the components are moving east. OD03 proposed that HH 529 also originates from the OOS as does HH 269, which is a group of bows moving west. BOM00 also measured radial velocities of several components of HH 529. HH 529 is visible in [O iii]andh but not in [N ii]. It is also visible in He i m (Takami et al. 2002). Thus, HH 529 is located in the H ii region. In this study, we have measured the radial velocities of most of the components of HH 529. Features and show very distinctive peaks at about 62 km s 1 in [O iii], H, and[nii]. BOM00 gave 63 2kms 1, which is in very good agreement with ours. The HST image in [O iii] shows that these features are part of a linear filament and are moving at 85 km s 1 with P:A: ¼ 102 (BOM00; DOH02; OD03); therefore, their spatial motion is 113 km s 1 with OA ¼ 49. Feature is the tip of a bow with a stretched wing, and the average radial velocity in all three lines is 61 km s 1. BOM00 gave 65 2kms 1. The proper motion of the knot behind the bow is 131 km s 1 with P:A: ¼ 99 (BOM00; DOH02); hence, it is moving at 149 km s 1 with OA ¼ 62. Feature is a well-defined bow shock preceding The average radial velocity in all three lines is 45 km s 1.Theproper motion of the bow is 80 km s 1 with P:A: ¼ 107 (BOM00; DOH02), giving a spatial velocity of 97 km s 1 with OA ¼ 56. The leading bow shock of HH 529 contains , , and This is the largest bow shock in HH 529. The average radial velocity is 44 km s 1,andthe proper motion is 54 km s 1 with P:A: ¼ 100 (DOH02), giving aspatialvelocityof76kms 1 with OA ¼ 45. The feature is a faint bow moving at 67 km s 1 with P:A: ¼ 126 (DOH02; OD03) and a maximum radial velocity of 28 km s 1, giving a spatial velocity of 77 km s 1 with OA ¼ 60. We see three different spatial velocity groups in HH 529, although all the measured features are moving in a similar range of orientation angle from 45 to 62. The fastest feature is the bow tip , which is moving at 149 km s 1.The second group contains , , and , which are moving at about 100 km s 1. The two leading bow shocks, and , form the slowest group, which is moving at about 75 km s HH 626 HH 626 is an incomplete double ellipse centered on the proplyd (OD03). The radial velocity of this ellipse is 34 km s 1 in [O iii]. The proplyd spectrum shows a faint stellar continuum with a broad blueshifted component of 32 km s 1. The proper motions of the northwest ( ) and southeast ( ) components are 42 km s 1 with P:A: ¼ 229

16 No. 6, 2004 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3471 and 32 km s 1 with P:A: ¼ 219, respectively, in [O iii] (OD03). The radial velocity of the incomplete ellipse with respect to the PDR is 44 km s 1. Similar values for both the proper motion and the radial velocity agree with the hypothesis that the incomplete ellipse is the result of shocks formed from the interaction of ionized gas with either a general wind or a photoevaporative flow from the proplyd (OD03); therefore, we may be seeing an expanding spherical shock front moving away from the central proplyd. The radial velocity with respect to the PDR of the proplyd of 42 km s 1 mayalsoindicatean initial expanding shell of high-velocity gas New HH Objects East-West Bipolar Flow from Proplyd (HH 725) Proplyd is located about toward P:A: ¼ 224 from 1 Ori C and has a round head and a tail (O Dell & Wen 1994; O Dell 1998). No microjets coming from this proplyd have been observed in the HST images (BOM00); however, the current [O iii] observations show that it has bipolar jets, as seen in Figure 5c. The redshifted jet is also visible in H, but the blueshifted jet is not. The radial velocities at the peak intensity are 88 km s 1 for the blueshifted jet and 104 km s 1 for the redshifted jet. Figure 4 shows that the adjacent slit spectra record the extensions of both the blueshifted and redshifted flows almost in an east-west direction on both sides of the proplyd. The averaged radial velocity and the position angle of the blueshifted flow are 92 km s 1 and 270, with the values for the redshifted flow being 68 km s 1 and 90. The blueshifted flow keeps its radial velocity almost constant at about 92 km s 1 and extends for about The redshifted flow decelerates from 104to64kms 1 and extends for about Both their positional alignment with the proplyd and the similar velocities of the flows and the jets of the proplyd suggest that these flows (designated as HH 725) originate from proplyd LV 2 Bipolar Flow (HH 726) Meaburn (1988) first pointed out that there is a highvelocity outflow from LV 2 in his [O iii] spectra. Meaburn et al. (1993) conducted a more detailed study on several LV objects with a long-slit echelle spectrograph and found a redshifted velocity spike in LV 2 at about 120 km s 1 with respect to the systemic velocity. They argued that it could be an outflow from a monopolar jet. Subsequent spectroscopy (Massey & Meaburn 1995) revealed that the jet extends about 2 00 toward the southeast. The jet is also visible in the HST images (BOM00). The jet position angles derived from the HST and the ground-based data are very similar at P:A: 120 (Henny et al. 2002). Figure 5d shows the redshifted microjet of LV 2 with an intensity peak at 105 km s 1. We also see a possible counterjet that was first seen by Henny et al. (2002). There is a subtle intensity peak of the counterjet at about 116 km s 1,andthe emission can be traced to about 140 km s 1. Figure 4 shows that the newly found HH 726 flow is aligned with LV 2. The blueshifted flow is located to the northwest and the redshifted flow to the southeast, being visible in [O iii] andh. The average radial velocity and position angle of the blueshifted flow are 104 km s 1 and 310 7, while those of the redshifted flow are 99 km s 1 and The velocities and the position angles of Fig. 12. [O iii] velocity image for DOH 10, 11, and 12 showing the radial velocity range from 40 to 80 km s 1. North is up. HH 726 are very close to those of the microjets in LV 2, strongly suggesting that HH 726 is the result of a bipolar flow from that object DOH Objects Most of the DOH objects are very faint filamentary features located near the center of the Orion Nebula. This is the first time that a detailed radial velocity measurement of these objects has been conducted. They are only visible in [O iii] and H, which suggests that they are in the H ii region. All of the objects are blueshifted, and their radial velocities vary in a wide range from 30 to 100 km s 1. Figure 4 shows many of the DOH objects, which surround the Trapezium. DOH 1 is located toward the west, and the radial velocity ranges from 30 to 70 km s 1. DOH 2, which overlaps the northeast portion of the Dark Arc, has a radial velocity range from 40 to 60 km s 1. The northern half of DOH 2 may contribute to the initial part of the HH 202 jet. DOH 3 is just northwest of HH 725 and has a radial velocity range from 30 to 70 km s 1. Although DOH 3 is the brightest of the DOH objects, there is no corresponding feature visible in the HST images. DOH 4 is between DOH 3 and the Trapezium. The average radial velocity is about 35 km s 1. DOH 5 is located within the Trapezium, has an average radial velocity of about 90 km s 1, and extends in a north-south direction. This object is not a part of HH 726, although it cuts through the middle of the blueshifted flow of HH 726. DOH 6 is located northeast of 1 Ori D. The radial velocity ranges from 60 to 90 km s 1. DOH 7 is in the middle of the redshifted flow of HH 518. The radial velocity varies from 30 to 70 km s 1. DOH 8 is located southwest of HH 523. It has the fastest radial velocity component measured, reaching about 100 km s 1. DOH 9 is located south of HH 523. The radial velocity ranges from 40 to 80 km s 1. DOH 10 is located east of DOH 6. The radial velocity range is from 30 to 70 km s 1, which is a smaller velocity than that of DOH6. Figure 12 shows some of the DOH objects located north of the Trapezium. DOH 11 extends in a north-south direction, with a radial velocity varying from 30 to 60 km s 1. DOH 12 is located east of DOH 11 and has an average radial velocity of about 50 km s 1. It extends in a southeastnorthwest direction and has numerous knots as well as a complicated filamentary structure. It connects to DOH 13 at its northwest end.

17 3472 DOI, O DELL, & HARTIGAN Vol. 127 Fig. 13. As in Fig. 12, but for DOH 13. HH 201 is not visible in this velocity range in [O iii]. Figure 13 shows DOH 13, which resembles a double arc. The outside arc consists of , , , and , and the inside arc consists of , , , , , and Both arcs have radial velocity ranges from 30 to 60 km s 1. Figure 14 shows the DOH objects near the eastern end of the Big Arc. DOH 14 is a linear feature extending in a northeastsouthwest direction. The radial velocity varies from 30 to 90 km s 1. DOH 15 has two branches, which meet at Both branches also extend in a northeast-southwest direction. The north branch consists of , , , and , and the south branch consists of , , , , and The radial velocities of both branches range from 40 to 80 km s 1. Fig. 14. As in Fig. 12, but for DOH 14 and 15. Fig. 15. Velocity image of the Big Arc. The radial velocity range imaged is from 16 to 36 km s 1. The numbers underneath each position-based identifier show the local shift of the MIF velocity peak with respect to the average along the full slit. If there are two velocity peaks, two values corresponding to the shifts of the two peaks are shown. The Big Arc south section tends to have bigger blueshifted peaks, whereas the Big Arc east section tends to have two peaks Big Arc The Big Arc, a prominent blueshifted feature in the Orion Nebula, is easily visible and is labeled in Figure 2. It was first seen in the low-spectral resolution study of O97a, but in this study, we see that the previously known east-west feature to the south of the Trapezium actually curves northward at the east end. It is composed of the east section, which extends from the intersection with the HH 204 jet toward the northeast by about 77 00, and a south section, which extends in the eastwest direction by about It is one of the biggest structures in the Orion Nebula. In no section does the Big Arc appear as a distinct velocityshifted feature; rather, it is an apparent enhancement in the blueward side of the shoulder of the emission from the main emitting layer. This means that it could simply be a weak component of low velocity shift, or it could be a result of blueshifting of the MIF. We have established that the Big Arc is the result of highly localized velocity shifting of the MIF along the Big Arc, rather than being a separate physical entity such as a shock. We did this by using the same approach that was presented in x In that case, we saw that the HH 204 jet features were real, even after consideration of shifting emission into the blueward velocity windows by small blueward shifts in the MIF. In the case of the Big Arc features, the MIF blueshifting is larger, and the apparent blueshifted features should be attributed to this fact. A ratio of images of the nebula at the systemic velocity of 0 km s 1 and at 8kms 1 shows the Big Arc in the same fashion as the color velocity images sampling the shoulders of the MIF emission core. In Figure 15, we show the velocity of the primary emission line in [O iii] coming from the MIF, as measured along the Big Arc. The HH 203 and HH 204 jets begin to be visible at the Big Arc, a feature discussed in x To the

18 No. 6, 2004 LARGE RADIAL VELOCITY FEATURES IN ORION NEBULA 3473 Fig. 16. Color-coded velocity image of blueshifted objects in the Orion Nebula in [O iii]. Red is assigned to a radial velocity range from 24 to 40 km s 1, green from 44 to 60 km s 1, and blue from 64 to 80 km s 1. The positions of BN, IRc2, and the OOS are also indicated by filled white circles. The dominant blueshifted features are HH 202 and HH HH 204 with their associated jets. The Big Arc extends from east to west to the south of the Trapezium. DOH objects are seen scattered in the northern half of the Huygens region. The labels on the axes are a guide to the position in the position-based designation system usedin Orion (O Dell & Wen 1994; DOH02). northeast from this intersection, the MIF emission actually splits into two components of similar intensity. In the east section, DOH 14 and DOH 15 are co-located in the Big Arc and aligned well in the direction of the Big Arc, but their well-defined separate velocities indicate that they are separate entities with no direct relation to the Big Arc. 4. DISCUSSION There are many interesting fast-moving objects in the Orion Nebula, the most interesting of which are blueshifted. These are summarized in Figure 16, which shows our full field in the radial velocity range from 24 to 80 km s 1. The two prominent objects are HH 202, with its associated jet, and HH HH 204, with the associated HH 203 jet. The jets driving all these HH objects originate in or near the OOS. There are similarities and differences between the HH objects and their jets, with the HH 203 jet being much faster than the HH 202 jet. HH 202 shows a very complicated velocity structure, indicating that it is not a simple bow shock. In Figure 16, the white region in HH 202 has the largest line width, hence, the