INTERNAL VELOCITIES IN THE ORION NEBULA - IOPscience

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The proper motions of Herbig-Haro (HH) objects in the Orion Nebula were measured with a Ж10 ... images of the Orion Nebula gave the first indications of fea-.
The Astronomical Journal, 124:445–463, 2002 July # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.

INTERNAL VELOCITIES IN THE ORION NEBULA: LARGE PROPER-MOTION FEATURES1 Takao Doi NASA Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058-3696; [email protected]

C. R. O’Dell Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN 37235

and Patrick Hartigan Department of Physics and Astronomy, Rice University, 6100 South Main Street, Houston, TX 77005-1892 Received 2002 March 7; accepted 2002 April 1

ABSTRACT The proper motions of Herbig-Haro (HH) objects in the Orion Nebula were measured with a 10 km s1 accuracy using the Hubble Space Telescope Wide Field Planetary Camera 2 images in [S ii], [N ii], H, and [O iii], taken 4–6 yr apart. Seven HH flows in the outer region as well as seven HH objects in the inner region of the BN-KL complex were identified. The H2 finger system was confirmed to be created by an explosive event that took place approximately 1000 yr ago. We found a new HH flow toward the northwest originating from the vicinity of OMC-1S, which may be a part of the low-velocity bipolar flows centered at FIR 4. The proper motion of HH 202 was also measured with high accuracy for the first time. The proper-motion vectors of HH 202 and HH 203/204 are aligned well with their projected symmetric axis, which may indicate that they emanated from the same source. The proper-motion measurements in various emission lines provide generally the same results in the wide range of velocity from 20 to 400 km s1, as expected for shocks in a steady state. Key words: Herbig-Haro objects — ISM: individual (Orion Nebula) — ISM: jets and outflows — stars: winds, outflows Perot systems that cover the complete Huygens region (the bright central core) of the nebula (O’Dell et al. 1997a, hereafter O97a; Rosado et al. 2001), and these have shown that extreme radial velocity features are ubiquitous. Detection of changes of positions of features also offers the potential for discovery of moving objects. Variations in the recorded structure of the Orion Nebula produced a wealth of papers and discussion in the 19th century before the initiation of astronomical photography. As the time base of photographs increased, motions began to appear in the nebula (Cudworth & Stone 1977; Jones & Walker 1985, hereafter JW85). Because shocks radiate in spatially compact cooling zones, it is relatively easy to separate the shocks’ emissions from the nebula’s background with images in narrowband filters. Such filters have now been commonly available for over 40 yr for use with groundbased telescopes, but seeing limits the spatial resolution of these images to around 100 . According to Bally, O’Dell, & McCaughrean (2000, hereafter BOM), the distance to the Orion Nebula is about 460 pc, which will be used throughout this paper, so 1>0 corresponds to a distance of 460 AU. The movement of 1>0 over a time base of 20 yr corresponds to a tangential velocity (often called proper motion in this paper) of 109 km s1 . Although one can measure the position of symmetric objects to a fraction of their apparent size, it is truly rare that photographic images have recorded the Orion Nebula at 100 resolution. The proof of this lies in the fact that multiple objects of several arcseconds size have been found on HST images that are not seen on published images for which superb seeing is claimed. Of course, some remarkable narrow field-of-view (FOV) ground-based images are now being made using seeing compensating techniques (McCullough et al. 1995; Simon, Close, & Beck 1999), but their long

1. INTRODUCTION

The Orion Nebula (NGC 1976, M42) is composed of a slowly expanding thin zone of photoionized gas on the facing side of the host Orion Molecular Cloud upon which many high-velocity features are superposed (O’Dell 2001). In the foreground, there is a veil of neutral material through which the Orion Nebula and the core of the Orion Nebula Cluster are seen. Although narrow-bandpass filter images do show a host of fine-scale structure within the nebula itself as well as the jets, outflows, and shocks that arise from the young stars in the eponymous cluster, it is in images resolved in radial velocity that one begins to appreciate the amount of material that is moving with speeds above the sonic speed of the nebula (about 17 km s1 ). Ground-based images of the Orion Nebula gave the first indications of features now known to be of high tangential velocity, but it was the order-of-magnitude improvement of resolution made possible by the Hubble Space Telescope (HST) that allowed our best views of this region (O’Dell, Wen, & Hu 1993; O’Dell & Wen 1994; O’Dell & Wong 1996; Bally et al. 1998). Individual slit spectra began to identify regions of extreme radial velocity (Canto´ et al. 1980; Castan˜eda 1988), but high-velocity resolution images by J. Meaburn and his collaborators (Clayton & Meaburn 1994; Massey & Meaburn 1995) started to allow clear delineation of unquestionably extreme radial velocity objects and structures. Lower velocity resolution studies have been made with Fabry-

1 Based in part 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 5-26555.

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446

DOI, O’DELL, & HARTIGAN

time-based counterparts do not exist. Hence, ground-based studies detect only tangential motions of a significant fraction of 100 km s1 . The advent of a longer time base for HST images allows our best opportunity to measure proper motions in the Orion Nebula. The optical resolution of HST is about 0>08, and the time base for observations with the spherical aberration–free Wide Field Planetary Camera 2 (WFPC2; Holtzman et al. 1995) now exceeds 7 yr, so one can hope to measure tangential velocities of a few tens of km s1 . In this paper, we present the results of the first program dedicated to measuring proper motions in the Orion Nebula with the WFPC2. The complete range of ionization states observed with our filter set and the longer time base of the observations allow a significant improvement in the accuracy of proper motion reported in BOM, where the coverage of location, filters, and time base had important limitations. We describe our observations in x 2 and present the propermotion results from them in x 3. A brief discussion of the results is presented in x 4. A more complete discussion of motions within the Orion Nebula will be made when the present authors complete a high-resolution map of the inner Orion Nebula, which will provide a radial-velocity data set.

2. OBSERVATIONS AND DATA REDUCTION

Our observations were made with the WFPC2 aboard the HST as part of program GO8121. The first (2000 March 30) was to the southeast (GO8121NW) of the Trapezium and the second (2000 September 13) to the northwest (GO8121SE), shown in Figure 1. The filters were selected to isolate emission lines that trace different zones of photoionization and of shocked material, these lines being F502N ([O ˚ ), F656N (H 6563 A ˚ ), F658N ([N ii] 6583 A ˚ ), iii] 5007 A ˚ and F673N ([S ii] 6717+6731 A). The total exposure time was 780 s for F502N and F656N and 1800 s for F658N and F673N. We can measure the proper motions by comparing these images with similar earlier WFPC2 images. The northwest FOV is essentially identical to that of program GO976NW (O’Dell et al. 1997b, hereafter O97b), which included our F673N filter (4.95 yr difference), but the overlapping filters and FOVs for the other observations came from a combination of programs: GO5976S (4.38 yr difference), GO5085S1 (5.65 yr difference for northwest and 5.19 yr for southeast), GO5085S3 (5.84 yr difference), GO5085S4 (5.84 yr difference), GO5085S10 (5.84 yr difference), GO5469LV3 (5.49 yr difference), GO5469HST10 (5.49 yr difference for northwest and 5.03 yr difference for southeast), and ERO5193 (6.71 yr difference for northwest and 6.25 yr difference for southeast). Table 1 presents the time difference of two epoch images used for the proper-motion measurements for various emission lines. Figure 1 shows the FOVs of the first- and the second-epoch images. These observations were reduced using IRAF,2 Space Telescope Science Institute STSDAS, and dedicated software tasks. After cosmic-ray cleaning using the gcombine task, the CCD individual images were combined using the 2 IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science foundation.

Vol. 124

wmosaic task, which placed the individual images into the correct relative position and also corrected them for field distortions. Since the wmosaic task assumes that the CCDs have not drifted in position with respect to one another (which is generally not the case), one must align individual CCDs (GO5976 and GO8121) or fractions of them (GO8121 plus all other programs) prior to looking for changes indicating proper motions or changes of structure. This was done using stars in common to the two epochs of data and involved two to 10 Orion Nebula Cluster stars. The typical alignment error was less than one-tenth of a pixel (0>01). Features to be measured for proper motion were identified primarily by examination of the aligned images through a blinking procedure. This procedure appears to have a threshold of detection of a fraction of 1 pixel. A few additional features that are identified with objects of known high radial velocity (Canto´ et al. 1980; Axon & Taylor 1984; Hu 1996, hereafter H96; BOM)3 were also measured. The proper motions were determined using a code developed by Hartigan et al. (2001), which is based on a leastsquares algorithm described by Currie et al. (1996). The code follows the emission within a small rectangular FOV centered on a feature of interest. The square of the difference of the intensity between the two images is summed over the FOV. The images are shifted relative to one another and the difference is recalculated, with the proper motion being indicated by where the sum of the differences of the images becomes a minimum. The typical measurement error was 0.2 pixels (corresponding to 8–10 km s1 ). The task was made more difficult when the feature changed in appearance between the two epochs. When the velocity measurement was affected by morphology changes between the two epochs, the measurement data were discarded. However, it should be noted that a morphology change less than a few pixels is difficult to detect with visual inspection and that it is still possible for a small-scale morphology change (1) to affect the proper-motion measurement by up to 40–50 km s1 . The range of the measured velocities varies from about 20 to 400 km s1 . Since the measurement error is essentially independent of the velocity, the fractional error of the velocity is less for features with larger velocities. The uncertainty of the direction of the motion is about 16 for a motion of 50 km s1 , about 8 for 100 km s1 , and about 3 for 300 km s1 . 3. MEASURED PROPER MOTIONS

The northwest region is already known to contain multiple high proper motion features (Jones & Walker 1985; H96; BOM; Lee & Burton 2000, hereafter LB00), and the region has been shown to have multiple high radial velocity features (O97a) and features whose forms suggest that they are shocks (O97b). The dominant source for those shocks is the Becklin-Neugebauer Kleinmann-Low (BN-KL) complex (Genzel & Stutzki 1989; Allen & Burton 1993; Menten & Reid 1995; McCaughrean & Mac Low 1997). On the other hand, the majority of the features in the southeast section show medium to low proper motions. Most of them appear to originate from the vicinity of OMC-1S, which is a strong far-infrared source observed to have various out3 All radial velocities in this paper are heliocentric. To obtain radial velocities with respect to the local standard of rest, subtract 18.1 km s1 .

No. 1, 2002

LARGE PROPER-MOTION FEATURES IN THE ORION NEBULA

447

Fig. 1.—HST mosaic image from O’Dell & Wong (1996) is shown with the perimeter of the fields imaged with the WFPC2. The image fields of the first epoch are shown with thin solid lines. The image fields of the second epoch are shown with thick lines. Each small frame containing HH objects will be shown enlarged in later figures corresponding to the numbers of the frames. The regions of the BN-KL complex and OMC-1S are also indicated.

flows (Ziurys, Wilson, & Mauersberger 1990; SchmidBurgk et al. 1990; Rodrı´guez-Franco, Martı´n-Pintado, & Wilson 1999). The measured proper motions are presented in Table 2. In order to present the results in a meaningful way, we have

assigned the features we measured to four groups, each having a common physical property or region of origin. They are (1) features associated with BN-KL; (2) features associated with OMC-1S; (3) HH 202, HH 203, and HH 204; and (4) bipolar objects.

448

DOI, O’DELL, & HARTIGAN TABLE 1 HST Image Data Second Epoch GO8121NW

First Epoch

Dt (yr)

GO5976N ............... GO5976S ................ GO5085S1 .............. GO5085S3 .............. GO5085S4 .............. GO5085S10 ............ GO5469LV3 ........... GO5469HST10 ....... ERO5193 ................

4.95 ... 5.65 5.84 5.84 5.84 5.49 5.49 6.71

GO8121SE

Filter

Dt (yr)

Filter

... [N ii], H, [O iii] [N ii], H, [O iii] [N ii], H, [O iii] [N ii], H, [O iii] [N ii], H, [O iii] [S ii], [N ii], H, [O iii] [N ii], H, [O iii]

... 4.38 5.19 ... ... ... 5.02 5.03 6.25

... [S ii] [N ii], H, [O iii] ... ... ... [N ii], H, [O iii] [S ii], [N ii], H, [O iii] [N ii], H, [O iii]

[S ii]

All of the features in the first group originate from the BN-KL complex in the northwest region. The features in the second group originate from the vicinity of OMC-1S and are in the southeast region. The third group contains large shocks on both sides of the center of the nebula and has a common projected symmetric axis. The fourth group contains proplyds with their associated shocks. We use two methods to name the features in Table 2. The first applies to individual features and is the position-based system introduced by O’Dell & Wen (1994). In this system, the six-digit names are the truncated positions (J2000.0) rounded off to 0 91 in right ascension and 100 in declination, with 5h35m subtracted from the right ascension and 5 200 added to the declination. For example, the object 112-152 lies within the 0 91  100 box whose northwest corner is at 5h35m119 2, 5 210 52>0. If two or more adjacent objects have the same identifier, we add a letter to distinguish them. The letters are assigned according to the right ascension value, with the lower value receiving an earlier letter in the alphabet. In the case of identical right ascension, the object closer to the north receives an earlier letter. Groups of features having common origins or motions indicating physical associations are often given HH numbers.4 3.1. Features Associated with BN-KL Figures 2–7 contain features belonging to BN-KL. We may further classify this region into the outer region, shown in Figures 2–6, and the inner region, shown in Figure 7, around the core of the BN-KL complex. The outer region is marked by multiple fingers of moving material. The main body of these features is seen only in H2, with the tips seen in the infrared [Fe ii] (Allen & Burton 1993) and in the optical lines (O97b), the latter observation indicating that the tips lie at or near the main ionization front of the nebula, since placement within the underlying photon-dominated region or farther would produce high optical wavelength extinction. The previous HST investigation (O97b) showed that there are eight fingers. Seven of those fingers are analyzed in the present study: HH 210 at a clock position of 11:00, HH 601 at 12:00, HH 602 at 12:30, HH 205/206/207 at 1:00, HH 603 at 1:30, HH 604 at 2:00, and HH 201. 4 B. Reipurth 2002, A General Catalog of HH Objects, http:// casa.colorado.edu/hhcat.

Figure 2 contains HH 210 and HH 601. HH 210 is the 11:00 finger and has a pronounced bow shape containing numerous knots and complicated filaments in [S ii]. We measured the five fastest moving features; 155-039 is located at the tip of the bow shock moving at 248 km s1 . However, 154-040b, located just behind 155-039, is moving at 389 km s1 , which is much faster than 155-039; 154-040b will overtake 155-039 in about 10 yr. The fastest moving feature is 154-040a, which is located farther behind the bow tip and is moving at 425 km s1 . In the first-epoch image, 154-040a and 154-041 are seen as one feature, which is the brightest feature in [O iii] (O97b). However, in the second-epoch image, 154-040a has moved away, leaving 154-041 behind. In [O iii], 154-040a is the brightest feature and has a small bow attached to it. All five features are moving almost in parallel. The average tangential velocity hVi is 285 km s1 , and the average position angle hP.A.i is l5 . JW85 measured the proper motion of HH 210 between 1963 and 1983, using plates of infrared emulsion with an RG-8 filter taken with the Lick 120 inch reflector. They found the measured tangential velocity was 215 km s1 and P:A: ¼ 16 . Their P.A. is in good agreement with ours. Their velocity is smaller than ours, probably because of the fact that the groundbased observation was not capable of resolving the inside of HH 210. Features 140-029, 139-034, and 140-036 form the 12:00 finger and are designated as HH 601. They are bright only in [S ii] and do not show clear bow structures. Their average tangential velocity is 216 km s1 and hP.A.i = 341 . Figure 3 contains both the 12:30 and 1:00 fingers. Four measured features, 129-035, 130-035, 131-036, and 130-041, form the front part of the 12:30 finger. They are called HH 602. The average tangential velocity is 144 km s1 and hP.A.i = 359 . The features 133-053 and 133-055 also form a part of the 12:30 finger. The average tangential velocity is 129 km s1 and hP.A.i = 346 . All of the features in the 12:30 finger are visible only in [S ii]. The 1:00 finger consists of HH 205, HH 206, and HH 207. HH 205 and HH 207 have well-defined bow structures. All of them are blueshifted (Axon & Taylor 1984). HH 205 has bright knots on the north wing of the bow in both [S ii] and H. The tip is designated as 116-023, which is also visible in [O iii] and is moving at 320 km s1 in H. BOM found that its proper motion was 346 km s1 in H, but JW85 and H96 gave much smaller velocities of 236 and 253 km s1, respec-

TABLE 2 Proper Motions in the Orion Nebula

Designation

Filter

VT (km s1)

P.A. (deg)

Figure

Epoch 2/1

Commentsa

BN-KL Region 154-040a HH 210 ....... 154-040b HH 210 ....... 154-041 HH 210 ......... 155-039 HH 210 ......... 155-040 HH 210 ......... 139-034 HH 601 ......... 140-029 HH 601 ......... 140-036 HH 601 ......... 129-035 HH 602 ......... 130-035 HH 602 ......... 130-041 HH 602 ......... 131-036 HH 602 ......... 133-053 HH 602 ......... 133-055 HH 602 ......... 116-023 HH 205 ......... 116-023 HH 205 ......... 116-023 HH 205 ......... 117-024 HH 205 ......... 119-040 HH 206 ......... 120-033a HH 206 ....... 120-033a HH 206 ....... 120-033b HH 206 ....... 120-033b HH 206 ....... 121-039 HH 206 ......... 122-038 HH 206 ......... 122-038 HH 206 ......... 122-038 HH 206 ......... 122-038 HH 206 ......... 123-040 HH 206 ......... 124-052 HH 207 ......... 125-050 HH 207 ......... 125-050 HH 207 ......... 125-051 HH 207 ......... 115-107 HH 603 ......... 116-112 HH 603 ......... 115-101 ...................... 115-102 ...................... 115-136 HH 604 ......... 116-137 HH 604 ......... 116-138 HH 604 ......... 117-137 HH 604 ......... 119-141 HH 604 ......... 120-144 HH 604 ......... 125-147 HH 604 ......... 126-146 HH 604 ......... 112-152 HH 201 ......... 112-152 HH 201 ......... 112-152 HH 201 ......... 112-152 HH 201 ......... 113-155a HH 201 ....... 113-155a HH 201 ....... 113-155b HH 201 ....... 115-154 HH 201 ......... 115-154 HH 201 ......... 115-155 HH 201 ......... 115-155 HH 201 ......... 136-224 HH 208 ......... 137-149a HH 209 ....... 137-149b HH 209 ....... 137-149c HH 209 ....... 126-214 ...................... 129-158 ......................

[S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [N ii] H [O iii] [S ii] [S ii] [S ii] [N ii] [S ii] [N ii] [S ii] [S ii] [N ii] H [O iii] [S ii] [S ii] [N ii] H [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [N ii] H [O iii] [S ii] H [S ii] [S ii] H [S ii] [N ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii]

425 389 211 248 153 193 243 212 217 145 106 106 98 159 344 320 326 281 137 293 284 175 185 335 252 230 255 245 196 83 291 203 167 177 177 75 70 88 88 70 83 92 83 83 145 176 149 175 171 56 40 53 44 55 45 34 0 55 76 63 65 84

7 17 14 15 21 346 335 342 0 10 339 10 342 349 337 335 334 332 332 345 347 347 349 338 344 340 334 1 342 315 329 350 342 322 329 232 225 315 315 315 333 323 315 297 327 315 312 313 315 264 293 273 315 295 304 328 ... 333 325 349 273 354

2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7

8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5085S4 8121NW/5085S4 8121NW/5085s4 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5085S4 8121NW/5976N 8121NW/5085S4 8121NW/5976N 8121NW/5976N 8121NW/5085S4 8121NW/5085S4 8121NW/5085S4 8121NW/5976N 8121NW/5976N 8121NW/5085S4 8121NW/5085S4 8121NW/5085S4 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5085S4 8121NW/5085S4 8121NW/5085S4 8121NW/5976N 8121NW/5085S4 8121NW/5976N 8121NW/5076N 8121NW/5085S4 8121NW/5976N 8121NW/5085S4 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N

n, h, o n, h, o n, h, o n, h, o n, h, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O s

n, h, o N, H, O h, O h, O N, H, O

n, H, O N, H, O s, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O n, h, O N, H, O n, h, O

n, O n, h, O n, O h, O N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O

TABLE 2—Continued

Designation

Filter

VT (km s1)

P.A. (deg)

Figure

129-210 ...................... 129-216 ...................... 135-233 ...................... 152-228 ...................... 152-229 ...................... 159-231 ......................

[S ii] [S ii] [S ii] [S ii] [S ii] [S ii]

44 49 35 36 50 63

309 288 225 59 90 98

7 7 7 7 7 7

Epoch 2/1

Commentsa

8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N 8121NW/5976N

N, H, O N, H, O N, H, O N, H, O N, H, O N, H, O

8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 10 10 10 10 10 10 11 11 11 11 11 11 10 11 11 11

8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/5469HST10 8121SE/ERO5193

s, n, O s, n, O o

12 12 12 12 12

8121NW/5976N 8121NW/5469LV3 8121NW/5469LV3 8121NW/5976N 8121NW/5976N

OMC-1S Region 149-352 HH 529 ......... 150-353 HH 529 ......... 151-353 HH 529 ......... 151-353 HH 529 ......... 151-353 HH 529 ......... 155-355 HH 529 ......... 155-355 HH 529 ......... 160-352 HH 529 ......... 160-353 HH 529 ......... 164-406 HH 529 ......... 164-406 HH 529 ......... 165-405 HH 529 ......... 165-405 HH 529 ......... 169-357 HH 529 ......... 169-358 HH 529 ......... 169-400 HH 529 ......... 152-359 HH 606 ......... 155-404 HH 606 ......... 157-355 ...................... 162-341 HH 605 ......... 165-333 HH 605 ......... 172-326 HH 605 ......... 172-326 HH 605 ......... 182-327 HH 523 ......... 182-327 HH 523 ......... 182-327 HH 523 ......... 182-330 HH 523 ......... 182-330 HH 523 ......... 182-330 HH 523 ......... 183-329 HH 523 ......... 183-329 HH 523 ......... 183-329 HH 523 ......... 166-433 HH 528 ......... 166-440 HH 528 ......... 166-440 HH 528 ......... 167-440 HH 528 ......... 170-448 HH 528 ......... 174-453 HH 528 ......... 180-509 HH 528 ......... 181-512 HH 528 ......... 182-505 HH 528 ......... 182-513 HH 528 ......... 183-509 HH 528 ......... 185-509 HH 528 ......... 178-453 ...................... 178-514 ...................... 178-514 ...................... 193-456 ...................... 219-433 ......................

H H [S ii] [N ii] H [N ii] H H H [S ii] [N ii] [S ii] H H H H [S ii] [S ii] H H H H [O iii] [N ii] H [O iii] [N ii] H [O iii] [N ii] H [O iii] [S ii] [S ii] H [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] [S ii] H [S ii] [N ii]

51 105 77 74 67 147 98 78 97 40 51 64 77 61 48 52 30 26 52 25 44 43 40 85 86 86 61 55 51 78 71 65 35 27 61 22 18 27 15 16 25 33 31 18 70 130 130 26 39

73 105 100 108 98 93 102 109 116 123 163 123 105 98 143 119 143 143 233 72 43 53 60 113 106 98 98 104 112 109 105 106 129 125 143 143 139 162 197 219 207 176 237 204 57 51 53 143 116

S, o S, N, o S, N, o h, O n, O S, n, o S, n, o S, n, o n, h, O n, h, O S, n, o S, N, o S, N, o S, n S

S

S

n, h, O n, O n, h, O n, h, O N, H, O n, h, O n, h, O n, h, O n, h, O n, h, O n, h, O N, H, O n, O n, h, O S, H, O

HH 202-203-204 115-255 HH 202 ......... 115-255 HH 202 ......... 115-255 HH 202 ......... 116-255 HH 202 ......... 116-256 HH 202 .........

[S ii] [N ii] H [S ii] [S ii]

72 67 106 44 40

329 312 328 321 328

o

n, h, o n, h, o

LARGE PROPER-MOTION FEATURES IN THE ORION NEBULA

451

TABLE 2—Continued

Designation

Filter

VT (km s1)

P.A. (deg)

Figure

Epoch 2/1

118-248 HH 202 ......... 118-256a HH 202 ....... 118-256a HH 202 ....... 118-256a HH 202 ....... 118-256b HH 202 ....... 118-256b HH 202 ....... 222-502 HH 203 ......... 222-502 HH 203 ......... 222-502 HH 203 ......... 222-503a HH 203 ....... 222-503a HH 203 ....... 222-503a HH 203 ....... 222-503b HH 203 ....... 222-503b HH 203 ....... 222-503b HH 203 ....... 222-505 HH 203 ......... 222-505 HH 203 ......... 222-505 HH 203 ......... 223-459 HH 203 ......... 223-502 HH 203 ......... 225-517 HH 204 ......... 225-517 HH 204 ......... 225-517 HH 204 ......... 225-519 HH 204 ......... 225-519 HH 204 ......... 225-519 HH 204 ......... 227-519 HH 204 ......... 227-519 HH 204 ......... 227-519 HH 204 ......... 228-518 HH 204 ......... 228-518 HH 204 ......... 228-518 HH 204 ......... 228-519 HH 204 ......... 228-519 HH 204 ......... 228-519 HH 204 .........

[S ii] [S ii] [N ii] H [S ii] H [S ii] [N ii] H [S ii] [N ii] H [S ii] [N ii] H [S ii] [N ii] H [S ii] [S ii] [S ii] [N ii] H [S ii] [N ii] H [S ii] [N ii] H [S ii] [N ii] H [S ii] [N ii] H

44 45 59 65 42 48 71 90 98 91 104 108 54 70 54 51 70 79 28 58 85 87 98 109 115 109 91 87 83 94 74 75 104 75 91

340 326 343 326 333 339 130 153 147 102 143 141 121 143 121 154 149 156 100 119 136 143 143 143 148 143 131 159 131 125 124 132 126 121 143

12 12 12 12 12 12 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13

8121NW/5976N 8121NW/5976N 8121NW/5469LV3 8121NW/5469LV3 8121NW/5976N 8121NW/5976N 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/5976S 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193 8121SE/5976S 8121SE/ERO5193 8121SE/ERO5193

2 7 7 7 7 7 7 7 7

8121NW/5976N 8121NW/5976N 8121NW/5469LV3 8121NW/5469LV3 8121NW/5469LV3 8121NW/5976N 8121NW/5469LV3 8121NW/5469LV3 8121NW/5469LV3 8121NW/5085S4 8121NW/5085S4 8121NW/5085S4 8121NW/5976N 8121SE/5469HST10

Commentsa n, h, o O

n, O O

O

O

O

n, h, O n, h, O O

O

O

O

O

Bipolar Objects 132-046 HH 607 ......... 161-236a HH 513 ....... 161-236a HH 513 ....... 161-236a HH 513 ....... 161-236a HH 513 ....... 161-236b HH 513 ....... 161-236b HH 513 ....... 161-236b HH 513 ....... 161-236b HH 513 ....... 114-132 HH 608 ......... 114-132 HH 608 ......... 114-132 HH 608 ......... 133-129 HH 608 ......... 169-333 HH 514 .........

[S ii] [S ii] [N ii] H [O iii] [S ii] [N ii] H [O iii] [N ii] H [O iii] [S ii] H

79 59 55 56 56 103 94 137 140 99 90 100 83 37

199 267 270 270 270 271 267 272 267 281 273 256 77 8

9

N, H, O

S

N, H, O S, n, o

a S, N, H, and O mean that the feature is not visible in [S ii], [N ii], H, and [O iii], respectively; while s, n, h, and o mean that the proper motion of the feature is not measurable because of no matching images in both epochs or too much morphological change in [S ii], [N ii], H, and [O iii], respectively.

tively. The P.A.s of all the previous observations agree very well with the present hP.A.i = 334 . The average tangential velocity of HH 205 is 318 km s1. HH 206 contains six scattered knots with very faint bows attached to them. The bows are visible only in [S ii]. The shocks are moving almost in parallel. The leading feature is 120-033a, which is moving at 293 km s1. The average tangential velocity is 235 km s1 and hP.A.i = 344 . BOM

found that the tangential velocity of 120-033a was 300 km s1 and P:A: ¼ 330 . H96 gave 251 km s1 and 333 for the average proper motion. In HH 207, feature 125-051 is a knot located just behind the faint tip of the bow 125-050. The brightness of the knot decreased considerably between the first and the second epoch in [S ii]. However, the overall bow shape was preserved. The average tangential velocity and P.A. are 186 km

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Fig. 2.—Second-epoch HST image in [S ii] is shown including the 11:00 finger HH 210 and 12:00 finger HH 601. HH 210 shows a typical bow shock structure containing numerous knots and filaments. HH 601 is composed of faint filaments and knots. The rest of the proper-motion data shown in this figure were measured in [S ii]. The length of each arrow indicates the velocity as well as the distance traveled in 50 yr.

s1 and 334 . In HH 207, 125-050 is the only feature also visible in [N ii] and H, and its tangential velocity was measured in the H image. Figure 4 shows the 1:30 finger, designated as HH 603, where 115-107 is the leading knot and 116-112 is a fainter knot with an associated bow structure. These knots are only visible in [S ii]. The average tangential velocity and P.A. are 177 km s1 and 326 . The features 115-101 and 115-102 do not belong to the BN-KL complex, since their direction of

motion is almost perpendicular to that of HH 603 and their velocities are much slower than those of HH 603. Feature 115-102 forms the south wing of the bow. These features also are only visible in [S ii]. Their average tangential velocity and P.A. are 73 km s1 and 229 . Figure 5 contains the 2:00 finger, which has three distinct groups. They are designated as HH 604. The leading group has 115-136, a small bow, followed by 116-137, 116-138, and 117-137, which together form a faint bow. The second

Fig. 3.—HH 602 forms the 12:30 finger. HH 205, HH 206, and HH 207 form the 1:00 finger. HH 205 and HH 206 contain high-velocity shocks, whose velocities exceed 300 km s1 . HH 607 contains the bipolar object 132-042 and its associated bow shock 132-046. ‘‘ (H) ’’ located after the name of a feature indicates that the proper-motion data were measured in H. The rest of the data were measured in [S ii]. The length of each arrow indicates the velocity as well as the distance traveled in 50 yr.

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Fig. 4.—HH 603 forms the 1:30 finger. Features 115-102 and 115-101 form a bright bow shock, which is not a part of the BN-KL objects. The length of each arrow indicates the velocity as well as the distance traveled in 50 yr.

group contains 119-141, which is a knot behind the tip of the third bow, and 120-144, which is the fourth bow. The third group has the fifth bow, which contains the brightest knot 126-146, and the sixth bow, 125-147. All of the bow shocks are visible only in [S ii] except 126-146 and 120-144, which are weakly visible in [N ii] and H. All of the features in the groups were preserved well between the first and second epochs. The average tangential velocities and P.A.s are 82 km s1 and 320 for the first group, 88 km s1 and 319 for the second group, and 114 km s1 and 312 for the third group. The average tangential velocity of the 2:00 finger is 95 km s1 , which is much smaller than those of the other fingers. Figure 6 has HH 201, which is the brightest HH object in the northwest region in [S ii]. HH 201 contains numerous knots and filaments. As O97b suggested, it can be seen clearly that HH 201 is the superposition of two bow shocks. The tip of the primary bow shock is 112-152, moving at 176 km s1 with a P.A. of 315 . The tip of the secondary bow shock is 113-155a, moving at 56 km s1 with a P.A. of 264 . In the primary bow shock, the rest of the knots inside are

Fig. 5.—HH 604 forms the 2:00 finger consisting of three distinct groups. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

moving at about 50 km s1 , which is much slower than the tip. BOM found a tangential velocity of 179 km s1 and a P.A. of 320 for the tip of the primary bow shock. JW85 and H96 reported 167 and 170 km s1 for the tangential velocity and 304 and 324 for the P.A., respectively. The central region of the BN-KL complex is shown in Figure 7. There are quite a few features moving radially from the central core of BN-KL. This is the first time that one can measure proper motions of objects closer to the BN-KL core with high accuracy. HH 208 is located approximately 700 to the east of BN and does not show any measurable proper motion. It has a bright core surrounded by an almost spherical fainter shell in [S ii]. This core appears to have changed in structure in a disorganized fashion that would correspond to motions of about 50 km s1 . Since HH 208 is a blueshifted object (Axon & Taylor 1984; O97b), the present results suggest that HH 208 is moving almost directly toward us. This agrees well with the fact that HH 208 shows a deficit of [O iii] surface brightness in the nebular background (O97b). This deficit indicates that the dust con-

Fig. 6.—HH 201 is composed of two superposed bow shocks. The leading tip of the primary bow shock is 112-152 moving at 176 km s1 with P:A: ¼ 314 . The leading tip of the secondary bow is 113-155a moving at 56 km s1 with P:A: ¼ 279 . The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

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Fig. 7.—Positions of infrared sources IRc2, IRc9, and BN are shown with solid circles. The dashed circle shows a 1000 or 0.02 pc diameter region, from which most of the inner HH objects emanate. This area was found by extending each velocity arrow of the inner HH objects in its reverse direction, taking into account the proper-motion measurement error. HH 208, which is located near the IRc2-BN complex, does not show any measurable proper motion. HH 513 contains the bipolar object 164-235 with its associated bow shocks 161-236a and 161-236b. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

centration in the shock blocks out the background nebula emission, thus the shock is located in front of the nebula. HH 209 is located approximately at 1100 north of IRc9. It contains three bright knots in [S ii]. Their average tangential velocity and P.A. are 65 km s1 and 336 . If we extend the velocity vector of HH 209 backward, it goes through IRc9 and near BN and IRc2. Feature 129-210 is a bright knot with a small faint bow moving at 44 km s1 with P:A: ¼ 309 . Feature 126-214 is a knot in an irregular filament. Feature 129-216 is a knot leading a faint bow. Features 129-210, 129-216, and 129-214 are moving away from the BN-KL core toward the east. Feature 135-233 is a flatshaped bow also moving away from BN and at 35 km s1 with P:A: ¼ 225 . Features 152-228 and 152-229 are a part of a bow-shaped filament moving away from the vicinity of IRc2 and BN. Feature 159-231 is the tip of a small bow moving at 63 km s1 with P:A: ¼ 98 . Finally, 129-158 is a knot moving at 84 km s1 . This is the only object that does not seem to originate from either IRc2 or BN. Instead, 129158 seems to originate from a star, 135-219, which has a weak extension in H in the direction toward 129-158, as pointed out in O97b. All of the objects in this region are visible only in [S ii], which indicates lower excitation energy and, consequently, lower shock speed compared to the HH objects in the outer region. 3.2. Features Associated with OMC-1S Figure 8 shows proper motions of three groups of features moving toward the east, northeast, and southeast from the vicinity of OMC-1S. The east flow, designated as HH 529, was first pointed out by BOM, and most of its features

are bright in H and faint in [S ii]. The northeast flow, designated as HH 605 and extending into Figure 9, is a part of a newly identified HH flow passing the region east of Trapezium and is bright in H. On the other hand, the flow toward the southeast, HH 606, is bright in [S ii] and almost invisible in H. HH 529 is composed of a series of bow shocks and knots. The leading bow contains a bright knot 169-357 and a secondary knot 169-358. The left wing of the bow is 169-400. The bow is brighter in H than in [S ii]. The average tangential velocity is 54 km s1 and hP.A.i = 120 . BOM found a proper motion of 82 km s1 and P.A. of 100 for 169-357. The second bow is located at the south end of the leading bow and is faint. It contains two knots, 165-405 and 164406. The average tangential velocity is 58 km s1 and hP.A.i = 129 . The third bow contains 160-352 in its left wing and a knot 160-353 behind the bow. The average tangential velocity is 88 km s1 and hP.A.i = 113 . The fourth bow contains 155-355, a bright knot behind the bow. These four bows are moving almost parallel. BOM measured the proper motions of 160-353 and 155-355, finding 72 km s1 and 100 for 160-353 and 139 km s1 and 100 for 155-355. The radial velocity of 155-355 was measured to be 93  2 km s1 relative to the nebula (BOM). Hence, the spatial velocity of 155-355 is given to be 154 km s1 with an orientation angle (90 if the bow shock moves in the plane of the sky) of 53 . The fifth group in HH 529 contains 151-353, 150-353, and 149-352. They are connected to each other with linear filaments. Features 151-353 and 150-353 are moving parallel with other HH 529 objects. However, 149-352 is moving more toward the northeast than the other two features,

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Fig. 8.—HH 529 and HH 605 are bright in H. HH 606 is only visible in [S ii]. HH 529 is composed of five distinct bow shocks. HH 605 shows only one bow shock containing 162-341 in this figure and extends further to the northeast, as shown in Fig. 9. HH 606 consists of two faint knots. ‘‘ [S ii] ’’ located after the name of a feature indicates that the proper-motion data were measured in [S ii]. The rest of the data were measured in H. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

which may have caused a kink of the filament connecting 150-353 and 149-352. The average proper motion for 150353 and 151-353 is 81 km s1 and 103 . BOM gave hVi = 85 km s1 and hP.A.i = 100 . The radial velocity of the filaments was also measured by BOM to be 71  2 km s1 relative to the nebula. The orientation angle of the filament is then given to be 49 . Hence, 151-353 and 150-353 are moving in the wake of the bow shock that contains 155-355. The second flow newly designated as HH 606 moves toward the southeast from the vicinity of OMC-1S and contains 155-404 and 152-359. They are faint knots and visible in [S ii], [N ii], and H. The average tangential velocity is 28 km s1 and hP.A.i = 143 . The flow seems to extend toward the southeast as HH 528, shown in Figure 10. The third flow HH 605 moving toward the northeast contains a knot 162-341, which is a part of a well-defined bow in H. The flow continues into Figure 9. A knot 157-355 is moving at 52 km s1 with P:A: ¼ 233 . It is bright in H but invisible in [S ii]. Since it is moving toward the southeast of OMC-1S, it is not a part of any flows mentioned above, and the origin of the knot is not clear.

Figure 9 shows the leading portion of the northeast flow called HH 605. Feature 165-333 is part of a flat-tipped bow, and 172-326 is the leading knot of the stretched wing of a bow. The average tangential velocity of HH 605 is 38 km s1 and hP.A.i = 57 . HH 523 is an irregularly shaped filament, which is well defined in [N ii], H, and [O iii] but invisible in [S ii]. It contains three knots: 182-327, 182-330, and 183329. They are moving in parallel with an average tangential velocity of 71 km s1 and hP.A.i = 106 . BOM measured the proper motion of 183-327 as V ¼ 96 km s1 and P:A: ¼ 100 . HH 523 originates from the direction of Trapezium, and its possible origin is proplyd 175-324, which is geometrically well aligned with HH 523. Figure 10 shows the main body of the southeast flow called HH 528. It is a group of knots and filaments moving almost in parallel. They are bright in [S ii], faint in [N ii] and H, and not visible in [O iii]. The average tangential velocity is 32 km s1 and hP.A.i = 140 . Compared to the east flow, the southeast flow is slower and does not show any clear bow shocks. BOM measured the proper motion of 167-440, finding V ¼ 39 km s1 and P:A: ¼ 110 . The faint filament

Fig. 9.—HH 605 consists of two bow shocks. HH 523 is a thin irregularly shaped filament. HH 514 is a bipolar object with its associated shock 169-333. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

Fig. 10.—HH 528 is composed of numerous filaments and knots moving toward the southeast. Their average velocity is 32 km s1 . Feature 178-453 does not belong to HH 528. The length of each arrow indicates the velocity as well as the distance traveled in 200 yr.

LARGE PROPER-MOTION FEATURES IN THE ORION NEBULA

Fig. 11.—Oval-shaped object with complex filaments and knots is a part of HH 528. This is located just on the Orion Bar. Feature 178-514 does not belong to HH 528. Both 178-453 in Fig. 10 and 178-514 originate from an unknown source located farther southwest in the Orion Nebula. The length of each arrow indicates the velocity as well as the distance traveled in 200 yr.

containing 174-453 is leading the southeast flow to an ovalshaped large feature, the terminus of HH 528, which is near the Orion Bar in Figure 11. This oval-shaped large feature, which is very well defined in [S ii], contains numerous knots and filaments. The proper-motion measurement reveals two different groups. The first group, which consists of 181-512, 182-505, 183509, and 185-509, has hVi = 23 km s1 and hP.A.i = 217 . The second group, consisting of 182-513 and 180-509, has hVi = 24 km s1 and hP.A.i = 187 , which is closer to that of the southeast flow. The filamentary structure is still clear in [N ii] but becomes less clear in H and [O iii]. Feature 178-514 is a knot moving at almost an opposite direction to HH 528, while 178-453, in Figure 10, is a faint knot moving almost parallel with 178-514. These last two objects do not belong to HH 528 and must have originated from further southwest in the Orion nebula. Feature 193-456 is a part of a small filamentary structure located in the northeast of HH 528. The motion is similar to that of the southeast flow, which suggests that 193-456 could be a part of the same flow.

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(Mu¨nch & Wilson 1962; Walsh 1982) as HH 203 and HH 204. This is not an example of simple large-scale bipolar flow because all of these shocks are blueshifted (O97a). Since all three features are brightest in low-ionization [S ii] emission, a feature absent in the central shocks formed within photoionized gas, then the interpretation (O97b) is that these are shocks being formed in the veil of neutral material that lies in the foreground of the nebula (van der Werf & Goss 1989). In the case of HH 203, the asymmetry of the shock is argued to be the result of a driving jet of material impinging at an angle on a stationary slab (Henney 1995). The fact that the envelope of the shock in HH 203 is filled with [O iii] emission leads to the conclusion that the geometry is such that photoionizing radiation from h1 Ori C illuminates the inside of the shock from behind (O97b). This feature is shared by HH 202 (O97b), probably indicating a similar geometry. Figure 12 shows the flow within HH 202. Although the object has the overall form of a shock, it has two bright regions. HH 202-N includes the 118-248 feature, which is of low ionization, whereas the HH 202-S group includes the other features in Figure 12, which show a wider range of ionizations, including 116-255 and 116-256 being bright in [O iii]. The only previous attempt to measure proper motions in this region is that of Cudworth & Stone (1977). From their approximate position and description (‘‘ the western edge of a loop of nebulosity near P1783 ’’), we conclude that they measured the group of bright features called HH 202-S here. They measured a westward motion only, at 33  11 km s1 . No motion in declination was seen, although they remarked that ‘‘ the feature appears to have lengthened in this coordinate [declination] over the epoch difference of our plates [75 yrs] ’’. Object 118-248 is moving at 44 km s1 toward P:A: ¼ 340 , while the average proper motion of the HH 202-S group is hVi = 59 km s1 and P:A: ¼ 329 . The former P.A. is oblique to the HH 202–HH 203–HH 204 line of symmetry, while that of the HH 202-S group is similar.

3.3. Features Associated with HH 202, HH 203, and HH 204 Extending across the bright center of the Orion Nebula is an aligned group of shocks sharing a common axis of symmetry with P:A: ¼ 131 311 . The northwest portion (Canto´ et al. 1980; O’Dell, Wen, & Hester 1991; O97b) is now designated as HH 202, and the southeast portion

Fig. 12.—HH 202 consists of HH 202-N and HH 202-S. HH 202-N is a bow-shaped structure containing 118-248. HH 202-S contains numerous knots including 116-255 and 116-256, which are bright in [O iii]. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

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Fig. 13.—Both HH 203 and HH 204 are located close to the bright star h2 Ori A. HH 203 looks like a bow shock truncated in half along the symmetrical axis. HH 204 resembles the terminal structure of HH 528. The length of each arrow indicates the velocity as well as the distance traveled in 100 yr.

Figure 13 shows the motion in HH 203 and HH 204, which are close to the bright star h2 Ori A. HH 203 shows the steady progression of direction of motions dropping off from the apex of the shock that is expected for shock models. The average proper motion of the six features closest to the apex is 73 km s1 and 134 . HH 204 is quite different from HH 203 in its appearance in that it is a host of irregularly oriented fine features and in this manner resembles the terminal structure of HH 528. The group motion of the five features measured is hVi = 92 km s1 and hP.A.i = 137 . The P.A.s of the motion of HH 203 and HH 204 agree well with the HH 202–HH 203–HH 204 projected line of symmetry. H96 estimated that the tangential velocity of HH 203 was less than 90 km s1 . BOM reported that HH 203 and HH 204 were both moving at P:A: ¼ 140 and that hVi = 74 km s1 for HH 203 and hVi = 59 km s1 for HH 204. Their values for HH 203 are in good agreement with the present values. However, they somehow gave a lower tangential velocity for HH 204 than we did. If the radial velocities of HH 203 and HH 204 with respect to the host molecular cloud are 74 and 50 km s1 (O’Dell et al. 1993), then these shocks are moving at spatial velocities and angles with respect to the line of sight of 104 km s1 and 45 and 105 km s1 and 61 , respectively. Therefore, they are moving along almost a similar path in space. This result strongly indicates that they are related to each other and were ejected from their origin at nearly the same time, since their spatial velocities and positions are almost identical. 3.4. Bipolar Objects We have identified four bipolar objects, two of which are newly found. The first object is HH 607, which consists of proplyd 132-042 and its associated bow shock 132-046, shown in Figure 3. Feature 132-042 has a round faint shell surrounding a star in the middle with faint microjets extending toward P:A: ¼ 180 , called the south jet, and P:A: ¼ 0 , called the north jet. The south jet is 100 long in [S ii] and is also seen in [N ii]. The north jet is 0>2 long and is much fainter than the south jet. The bow shock 132-046 is geometrically aligned to the axis of symmetry of the microjets. The proper motion of the bow is V ¼ 79 km s1 and P:A: ¼ 199 . Hence, the dynamical age of the bow is estimated to be 122 yr. The shock associated with the north jet

is not clearly seen, although a possible candidate is 131-036. However, 131-036 is not on the axis of symmetry of the system and may belong to HH 602. The second object HH 608 consists of 123-131 and its associated bipolar shocks 114-132 and 133-129. Feature 123-131 is a proplyd first reported by O’Dell & Wong (1996) and is located at the geometric center between 114-132 and 133-129. Features 114-132 and 133-129 look like the tips of bow shocks moving in opposite directions. The P.A.s of the motions of 114-132 and 133-129 are 270 and 77 , respectively. Feature 133-129 is visible only in [S ii]. On the other hand, 114-132 is visible in [N ii], H, and [O iii]. The estimated ejection times of both objects are approximately 370 yr ago for 114-132 and 330 yr ago for 133-129. The third object is HH 513 with its associated shocks of 161-236a and 161-236b, shown in Figure 7. BOM reported the object as bipolar jets with a bow-tie structure. We measured only the south part of the jets, because the other half lies outside of our GO8121 images. Knots 161-236a and 161-236b are moving at 88 km s1 toward P:A: ¼ 269 . The fourth object is HH 514, located near Trapezium, as shown in Figure 9. This object was identified as a bow shock ejected from the star 170-337, which has a short microjet and an extension further out (BOM). The bow moves away from the star at 37 km s1 and P:A: ¼ 8 . 4. DISCUSSION

There are two major flows of HH objects in the Orion Nebula. We see this clearly in Figure 14, which is a summary of all of the HH objects we have identified in the present study. The first flow originates from the IRc2-BN complex in the northwest region. The other originates from the vicinity of OMC-1S in the southeast region. We discuss each flow in more detail in this section. We also discuss the motions of the related HH 202–HH 203–HH 204 objects. We conclude with a comparison of velocities in different ions of shocks with multiple stages of ionization. 4.1. BN-KL Region All of the HH objects in this group move away from a common origin in the IRc2-BN complex. The outer region contains HH 210, HH 601, HH 602, HH 205, HH 206, HH

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Fig. 14.—HST mosaic image from O’Dell & Wong (1996) is shown with all of the proper-motion data in the present study. Each arrow represents the average velocity of the features belonging to the designated HH objects, and its length indicates the velocity as well as the distance traveled in 300 yr. It is clearly seen that there are two major HH outflows in the Orion Nebula. The H2 finger system is radiating from an area indicated by the dashed circle in the IRc2-BN complex in the northwest region. The other group of flows is radiating from the vicinity of OMC-1S in the southeast region.

207, HH 603, HH 604, and HH 201. Most of them show a bow shock structure in [S ii]. The proper-motion direction of each object is well aligned with its symmetrical axis. The tangential velocity of each object is around 200–400 km s1 . The tips of the bow shocks are seen as knots in [N ii], H, and [O iii]; hence, the fastest moving portion of a shock is the tip of the bow. However, the fact that the [O iii] emission requires only a shock with a speed of about 90 km s1 and that the [O iii] emission is seen only at the tip of the bow, not in the entire bow shock whose proper motion exceeds 100 km s1 , indicates that those HH objects are moving in the wake of other shocks moving ahead of them. Most of the outer HH objects are also bright in [Fe ii] emission (Allen &

Burton 1993). On the other hand, the main body of these features is seen only in H2 (LB00). This indicates that the tips of the fingers lie at or near the main ionization front of the nebula, since placement within the underlying photondominated region or farther away would produce high optical wavelength extinction. The inner region contains HH 208, HH 209, 126-214, 129-210, 129-216, 135-233, 152-228, 152-229, and 159-231. They are only seen in [S ii]. Their average proper motion is around 50 km s1 . Stolovy et al. (1998) and Schultz et al. (1999) examined the inner finger system around the IRc2BN complex. The inner finger system contains numerous bullets, like the outer finger system. However, the bullets in

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the inner finger system emit only in H2, not in [Fe ii]. The lower excitation energy indicates that the bullets are interacting at lower speeds with the medium. Figure 7 clearly shows that the HH objects are radiating from the vicinity of the IRc2-BN complex. We can assume that they are the tips of the bullets in the inner finger system, although we cannot identify the corresponding IR objects except for HH 209. The lack of recognition of optical counterparts indicates that the inner fingers suffer from higher extinction, which means that they are probably more deeply imbedded in the molecular cloud than the outer fingers. The mechanism of forming the H2 finger system is not well understood. Allen & Burton (1993) proposed one or multiple explosive events in which supersonic flying ejecta from the IRc2-BN complex formed the finger system. Stone, Xu, & Mundy (1995) proposed in situ Rayleigh-Taylor instabilities in time-variable winds or in a slow-moving wind being overtaken by a faster moving wind. H96 analyzed HH 201, HH 205, HH 206, and HH 210 and concluded that they originated from the same region at the same time, about 1000 yr ago. LB00 showed that the more distant [Fe ii] bullets are, in general, moving faster and concluded that the finger system is an explosive event. However, they failed to determine the time of the explosive event as a result of the large uncertainties in their proper-motion speed data. Figure 15 shows the proper motion of the tips of the bow shocks for both the inner and outer regions against the projected distance from the IRc2. It clearly shows that HH objects in the finger systems increase their proper-motion speed almost linearly with their projected distance, confirming that the finger systems in both the inner and outer regions were possibly created by an explosive event that took place approximately 1010  140 yr ago. Many authors have tried to locate the origin of the bullets by analyzing the proper motions of HH objects in the outer

Fig. 15.—Tip velocities of all of the bow shocks that radiate from the IRc2-BN complex are shown against their projected distances from IRc2. The proper-motion velocity increases linearly with the distance from IRc2. The solid line, which is the best fit for the data, represents a time of flight of 1010 yr. The top and bottom dashed lines correspond to 870 and 1150 yr, respectively.

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region (Allen & Burton 1993; O97b; LB00). They came to the conclusion that the bullets originated from the IRc2-BN complex but failed to pinpoint the origin because of the fact that the HH objects in the outer region are too far away from the origin and that the accuracy of their propermotion data was not good enough for the purpose. It is still true for our data of the outer region. However, using the proper-motion data in the inner region, we can locate the origin of the bullets more accurately. All of the moving HH objects in the inner region show that they have a common origin in a 1000 circle, as seen with dashed lines in Figures 7 and 14. The circle corresponds to approximately 0.02 pc in diameter and includes both BN and IRc2 near its boundary. BN is reported to be an early B-type star embedded in the Orion Molecular Cloud (Scoville et al. 1983). WynnWilliams et al. (1984) argue that the source associated with IRc2 is 10 times more luminous than BN. IRc2 is a group of four far-infrared sources (Dougados et al. 1993). There is a radio source designated ‘‘ I ’’ less than 100 to the south of IRc2 (Churchwell et al. 1987; Menten & Reid 1995) on which water masers are centered (Gaume et al. 1998). The IR2/I complex was observed to have bipolar outflows (Morino et al. 1998; Rodrı´guez-Franco et al. 1999). Hence, IRc2 or the IRc2/I complex are good candidates as the driving source of the bullets. It should be noted that the circle in Figure 7 is offset from IRc2 toward the northwest, which corresponds to the region of the blue and red bullets observed with the CO emission by Rodrı´guez-Franco et al. (1999). 4.2. OMC-1S Region We have three major flows of HH objects in this region. HH 529 consists of bow shocks and filaments moving toward the east. The average tangential velocity is 74 km s1 with hP.A.i = 111 . HH 528 consists of knots and a complex lattice of filaments moving at hVi = 32 km s1 and hP.A.i = 140 . They are lower excitation features. The third flow is HH 605 moving toward the northeast. The average tangential velocity is 38 km s1 with hP.A.i = 57 . Both HH 529 and HH 605 are well defined in [N ii] and H and not visible in [S ii]. This indicates that the shocks of HH 529 and HH 605 form in the H ii region, where the ambient medium is already ionized by ultraviolet light from h1 Ori C. On the other hand, the low ionization feature of HH 528 indicates that these shocks form behind the main ionization front or in the veil where the light from h1 Ori C cannot reach them. Two different outflows were measured by radio observation in this area. The low radial velocity bipolar outflows exit near FIR 4 (Ziurys, Wilson, & Mauersberger 1990; Schmid-Burgk et al. 1990). The blueshifted portion oriented along P:A: ¼ 31 and the redshifted portion along P:A: ¼ 211 are almost centered around FIR 4 (SchmidBurgk et al. 1990). The measured radial velocity is less than 30 km s1 . High-velocity bipolar outflows were also found in the vicinity of the low-velocity outflows (Rodrı´guezFranco et al. 1999). The orientation of the high-velocity outflows is almost perpendicular to that of the low-velocity ones. The P.A. of the redshifted component is P:A: ¼ 131 and that of the blueshifted component is P:A: ¼ 311 . The measured radial velocity reaches 110 km s1 . The source of the high-velocity flows is not yet clear, although there are three infrared sources embedded together within a few arcseconds of space (Gaume et al. 1998) near the geometrical center of the bipolar flows, designated as ‘‘ B ’’ in Figure 14.

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BOM findings that the possible source of HH 529 is inside the incomplete dark oval in the OMC-1S were confirmed in the present study. The infrared sources, designated as ‘‘ B ’’ in Figure 14, are a little to the south of the axis of HH 529. Since HH 529 is a blueshifted outflow, it is not a part of the high-velocity bipolar outflows found by Rodrı´guez-Franco et al. (1999). Also, HH 529 is not a part of the low-velocity bipolar outflows found by Schmid-Burgk et al. (1990) because FIR 4 is located further away from the axis of HH 529. However, HH 605 could be a part of the low-velocity bipolar outflows, since FIR 4 is close to the axis of HH 605. The radial-velocity study of HH 605 will clarify the above possibility. If we extend the axis of high-velocity bipolar outflows toward P:A: ¼ 131 , it goes through the middle of HH 528, which could mean that HH 528 might be a part of the bipolar flows. If so, we are seeing the low radial velocity component of the bipolar flows in HH 528, hence HH 528 is moving almost parallel to the plane of the sky. This supports BOM’s hypothesis that HH 528 runs along the interface between the H ii region and the background molecular cloud and rams the Orion Bar to produce the flattened head of the flow at its southeastern terminus. This also means that HH 528 is not an extension of HH 606. 4.3. HH 202–HH 203–HH 204 HH 202, HH 203, and HH 204 show some of the same characteristics, as pointed out in x 3.3. They have similar forms, low ionization at the tips, and a diffused and filled emission in [O iii], all of this arguing that they are formed in the foreground veil of neutral material. Moreover, their geometric axes point toward one another (P:A: ¼ 131 311 ). However, there are no obvious candidate sources along this common axis as it passes to the northeast of the OMC-1S infrared sources. Their common axis is very similar to that of the high-velocity molecular outflow found by Rodrı´guezFranco et al. (1999), which comes from OMC-1S. If there is a connection between these two flows (HH 202–HH 203– HH 204 and the OMC-1S high-velocity molecules), then one must explain the displacement of 0.06 pc. This could be a lateral flow of the veil material of 16 km s1 , with the approximate age of 4000 yr identified from the magnitude of the proper motions. BOM reported that the bow shocks belonging to HH 269 were moving rapidly westward away from OMC-1S and probably arose from that region. Those shocks have common features with HH 202–HH 203–HH 204, both in form, in having extended enclosed [O iii] emission, and in being blueshifted. In addition, there is a further similarity in that HH 269 has a counterpart moving in the opposite direction (HH 529) that is also blueshifted, although in this case the shocks constituting HH 529 are clearly formed in photoionized gas. The curious preponderance of blueshifted radial velocities may be the result of a multilobed flow, such as one sees around BN-KL, with a strong observational bias toward seeing optically the blueshifted flows. This could be due to the facts that the tangential or redshifted flows from the embedded source will remain behind the ionization front of the nebula and that the high extinction within the dense underlying shocked zone will extinguish the optical emission of any associated shocks. This would mean that selectively only the blueshifted optical components will have been observed in these optical-wavelength HST observations,

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although the opposite alignment of the flows is not an inherent feature of this explanation. 4.4. Proper-Motion Measurement in Different Emission Lines HST high-resolution images made it possible for the present proper-motion study to measure individual features such as knots and filaments inside a bow shock with various emission lines. Table 2 shows more proper-motion data for [S ii] than for the other emission lines because in [S ii] the features are more visible and the proper-motion measurement is less affected by morphological changes. The H images show more photometric changes between two epochs than those in [S ii]. As Hartigan et al. (2000) suggested, the enhanced variability at H may arise because most of that emission comes from collisional excitation in the immediate vicinity of the shock front. Hence, any sudden changes in preshock density of the medium would change the H intensity and sometimes result in morphology changes. On the other hand, the emission of [S ii] comes from the cooling zone following the collisional excitation zone, thus it would average the variability over a 10–100 yr cooling time. Even though [N ii] emission should come from the cooling zone region close to that of [S ii], the propermotion measurement in [N ii] is not as extensive as in [S ii], as noted above. Figure 16 shows how emission lines affect proper-motion measurements. Each data point represents proper-motion measurement in [S ii] as well as in other emission lines. The dashed lines show the error of the present measurement corresponding to 10 km s1 . The empty circles with error bars are data from LB00 for the [Fe ii] emission. Since their observations were carried out over a 4 yr time difference on the ground, their velocity measurement error is much larger than that of the present study. As most of the data points are between the dashed lines or close to them, the proper motion in various emission lines appears to be the same over the wide range of velocity. The [Fe ii] observation is also consistent with ours because most of the error bars fall between the dashed lines. A shock wave consists of a shock front followed by a collisional excitation zone and cooling zone in which [O iii], [N ii], and [S ii] emission layers form. A high-speed shock wave accelerates ambient particles to only 34 of the shockfront velocity. Hence, the above emission layers remain attached to the shock front instead of to the particles passing through the shock wave. The fact that all of the propermotion measurements in the various emission lines provide generally the same results in the wide range of velocity indicates that the pattern of the layers inside the shock wave remains the same between any two epochs of observation. Therefore, most of these shocks do not deviate enough from a steady-state to produce significant differential motions in the cooling zone of the various emission lines. 5. CONCLUSIONS

In the present study, we successfully measured the proper motions of HH objects in the Orion Nebula with a 10 km s1 accuracy using HST WFPC2 images in [S ii], [N ii], H, and [O iii], taken 4–6 yr apart. We identified all of the HH flows in the northwest and southeast regions of the Orion Nebula and found two new bipolar objects. The main results of the present study are as follows:

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Fig. 16.—All of the proper-motion data measured in [N ii], H, and [O iii] are plotted against the proper-motion data in [S ii]. The open circles with error bars are the proper-motion data in [Fe ii] from LB00. The dashed lines show the measurement error of 10 km s1 in the present study.

1. We measured the proper motions of seven major HH flows in the outer region. We also measured the proper motions of seven HH objects in the inner region of the H2 finger system for the first time. We confirmed that the finger system was created by an explosive event that took place approximately 1000 yr ago. The origin of the system, defined by the proper motion of the inner system, is close to the IRc2-BN complex. 2. We found a new HH flow toward the northeast; hence, there are three HH flows originating from OMC-1S in the southwest region of the Orion Nebula. The newly found HH flow HH 605 may be a part of the low-velocity bipolar flows centered at FIR 4 found by Schmid-Burgk et al. (1990). HH 528 can be a part of the bipolar flow centered near the infrared source B found by Rodrı´guez-Franco et al. (1999). 3. The proper motion of HH 202 was measured with high accuracy for the first time. Although HH 202 and HH 203/ 204 are blueshifted, the fact that the proper-motion vectors

are aligned well with their common projected symmetric axis may indicate that they emanated from the same unknown source. 4. The proper-motion measurements in [S ii], [N ii], H, and [O iii] provide generally the same results in the wide range of the proper-motion velocities from 20 to 400 km s1 . This agreement will arise in any shocks where the cooling zones do not deviate significantly from those in a steady state.

We are grateful to Bo Reipurth for cataloging eight new HH flows found in the present study. The work of C. R. O. and P. H. on this program was supported in part by grant GO8121 from the Space Telescope Science Institute and the Alexander von Humboldt Foundation of Germany. T. D.’s work on this program was supported by the National Space Development Agency of Japan.

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