sky, the proper motions translate accurately to space velocities, which range from 220 to 330 km s with a 1

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1 The Astrophysical Journal, 559:L157 L161, 2001 October The American Astronomical Society. All rights reserved. Printed in U.S.A. PROPER MOTIONS OF THE HH 111 JET OBSERVED WITH THE HUBBLE SPACE TELESCOPE 1 Patrick Hartigan, 2 Jon A. Morse, 3 Bo Reipurth, 3 Steve Heathcote, 4 and John Bally 3 Received 2001 July 25; accepted 2001 August 23; published 2001 September 10 ABSTRACT New Ha and [S ii] images of the HH 111 jet taken with the Hubble Space Telescope reveal marked proper motions and morphological changes when compared with similar images obtained 4 years earlier. Knots in the jet, which are dominated by emission from nested bow shocks, generally move ballistically, with no evidence for turbulent motions even in regions where the emission has a complex morphology. These bow shocks sometimes overtake one another; the new images show this occurred in knot L about 80 years ago. Photometric variability, clearly visible for the first time at subarcsecond scales, can confuse ground-based measurements that require many years between epochs to detect reliable proper motions. With the exception of the bow shock L, whose wings expand laterally, the jet moves mainly along its long axis. Because HH 111 lies nearly in the plane of the sky, the proper motions translate accurately to space velocities, which range from 220 to 330 km s with a typical uncertainty of 5 km s. The fastest knots are associated with object E at the base of the visible jet, where a cooling layer is in the process of forming behind one of the shocks. Velocity differences between adjacent knots within the optically bright part of the jet are typically 40 km s, in line with predictions of nonmagnetic shock models based on emission-line fluxes. This agreement limits the component of the magnetic field perpendicular to the axis of the jet to be mg. Subject headings: ISM: Herbig-Haro objects ISM: jets and outflows ISM: kinematics and dynamics shock waves 1. INTRODUCTION Herbig-Haro (HH) objects are regions of shocked gas within protostellar outflows (see Reipurth & Heathcote 1997 for a review). Proper motions and radial velocities of HH objects are highly supersonic (Herbig & Jones 1981; Schwartz 1975), and many are bow shocks, as determined by their large internal motions (Schwartz 1981; Hartmann & Raymond 1984), line profiles (Hartigan, Raymond, & Hartmann 1987), positionvelocity diagrams (Raga & Böhm 1986), and direct imaging (Reipurth et al. 1997). Velocity variations most likely cause the multiple bow shocks observed in many jets (Reipurth 1989). At the base of stellar jets, there is always a young star whose luminosity is dominated by an active accretion disk. Through a process not well understood, the system collimates a portion of the material accreting onto the star into a jet (Königl & Pudritz 2000). Stellar jets can extend for parsecs and often exhibit changes in the ejection angle at the source that may be caused by precession (e.g., Bally & Devine 1994; Gomez, Kenyon, & Whitney 1997). The observed location of shocks in jets and the emissionline ratios from the gas that cools behind these shocks greatly constrain numerical simulations of the dynamics. Numerical simulations show that shocks may arise as jets interact with the surrounding medium (Norman et al. 1982), with previously shocked gas (Cioffi & Blondin 1992), and when fast material overtakes slower material in an unsteady flow (e.g., Stone & Norman 1993; de Gouveia dal Pino & Benz 1994). Each of 1 Based on observations made 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 Department of Physics and Astronomy, Rice University, 6100 South Main, Houston, TX Center for Astrophysics and Space Astronomy, Campus Box 389, University of Colorado, Boulder, CO Cerro Tololo Inter-American Observatory, NOAO, Casilla 603, La Serena, Chile. L157 these cases manifests a distinctive morphology and time evolution for the system. Sharp Ha features form as neutral gas becomes collisionally excited when it enters the front (Chevalier & Raymond 1978; Hartigan et al. 2000); these features mark the location of shocks observationally in much the same way as (unobservable) pressure contours do in numerical simulations. Hence, there is a direct way to connect observations and models of stellar jets. Forbidden lines such as [S ii] radiate behind the shocks as the gas cools and give additional information about density, temperature, and shock velocity. Proper-motion measurements of jets also help to decipher the physical conditions there. Because magnetic fields play a role in collimating jets in most models (e.g., Shu et al. 2000), it is important to measure field strengths in jets. Unfortunately, fields in jets are very difficult to measure directly. Models of the emission-line ratios within HH objects with weak fields typically suggest shock velocities of km s (10% of the flow speed), consistent with the notion that HH objects move within the wakes of previous ejections. However, higher shock velocities can also produce the observed emission-line ratios if one includes a larger magnetic field in the models (Hartigan, Morse, & Raymond 1994). The handful of fields measured so far (Morse et al. 1992, 1993, 1994) using the ratio of the observed electron density in the cooling zone to that expected from models with no fields indicate strengths of 30 mg, which is insufficient to collimate the jets. However, these fields refer to the material in front of large bow shocks in the flow 0.1 pc from the driving source and not necessarily to conditions within the jet near the star. When corrected for projection effects, which are smallest if the flow lies nearly in the plane of the sky, the difference between proper motions of adjacent knots equals the shock velocity when the knots collide. Hence, proper-motion measurements break the theoretical degeneracy between field strength and shock velocity that occurs in emission-line studies. Ground-based measurements of proper motions, while useful, cannot provide a direct connection to numerical simulations

2 L158 PROPER MOTIONS OF HH 111 JET Vol. 559 because photometric variations in the jet knots begin to become important when the time between epochs is a few years. In this time interval, jet knots typically move at most 1 on the sky. Hence, it is difficult from the ground to distinguish true motion from photometric variability, especially when these shocks have significant internal structure on subarcsecond scales. The HH 111 jet in Orion is an excellent place to study internal motions in jets because it is bright, lies nearly in the plane of the sky, and has high proper motions (Reipurth 1989; Reipurth, Raga, & Heathcote 1992). In a previous paper (Reipurth et al. 1997, hereafter Paper I), we identified the shocks within HH 111 from deep narrowband Hubble Space Telescope (HST) images and discussed their cooling zones. Recent NICMOS images trace the HH 111 jet nearly all the way to the driving source and demonstrate that the jet widens steadily as it moves outward (Reipurth et al. 2000). In this Letter, we report proper-motion measurements within the HH 111 jet by comparing the images in Paper I with a new epoch of Ha and [S ii] images taken with HST 4 years later. The new observations allow us to follow the motion of individual knots, identify those that vary rapidly, and quantify internal motions within the jet accurately for the first time. 2. OBSERVATIONS AND DATA REDUCTION We imaged the HH 111 jet through the F656N and the F673N filters with the Wide Field Planetary Camera 2 (WFPC2) on HST on 1998 November 7. Roughly 4 years passed between the new observations and those presented in Paper I. Total exposure times for the Ha and [S ii] images were 5100 and 5200 s, respectively. We reduced the new images and aligned them with the previous epoch using the same procedures described in Paper I and in Reipurth et al. (2001). The rms uncertainty in the global alignment fits are equivalent to a few kilometers per second or 1% of the typical measured motions. Measuring proper motions in nebulous objects that vary photometrically requires some care. Although a cross-correlation between segments of two images gives an estimate of the proper motion (Reipurth, Raga, & Heathcote 1996), multiplicative cross-correlation peaks of diffuse features produce a rather broad maximum in our data and do not yield results as accurate as those based on a least-squares technique (Currie et al. 1996; Morse et al. 2001). For each knot or bow shock, we first examined the images to identify features whose emission-line fluxes did not vary strongly between the epochs. The square of the difference between the counts in the two epochs summed over the box that defines each object determines how well the features at the two epochs line up. Shifting the alignment by integer pixels over some buffer size and recalculating the sum generate a correlation image whose minimum corresponds to the proper motion of the object. Centering tasks within IRAF then pinpoint the shift to within a fraction of a pixel. Statistical errors with this procedure are typically only 0.1 pixel, or about 1% of the measured motions, similar to the precision of the image alignment. Of more concern with all estimates of proper motions are systematic errors. To quantify these, we explored a range of buffer sizes for each measurement and also varied the size and shape of the rectangle that defines the object. The resulting range of proper motions gives a systematic error, which is typically 2% 3%. The procedure also works for large objects, in which it measures an average proper motion of the system. We tested the routine on a variety of nebulous objects by shifting the images and recovering the offsets with the code. 3. PROPER MOTIONS AND VARIABILITY WITHIN HH 111 Proper motions within HH 111 appear in Figure 1 and Table 1. The observed velocities in the plane of the sky listed beside the boxes that outline the objects are in kilometers per second, assuming a distance to HH 111 of 460 pc. With the exception of knot L, which shows some lateral expansion, the motion of the gas within HH 111 lies along the main axis of the flow (Table 1). The proper motion of the outer bow shock V, combined with its radial velocity of 56 km s (Reipurth 1989), implies that the orientation angle of the jet to the plane of the sky is 11, nearly identical to the value obtained by Reipurth et al. (1992) from ground-based proper motions. Flow times from the source, assuming ballistic motions ( dv/dt p 0), range from about 160 yr for knot E to over 1100 yr for knot V (Table 1). When corrected for the viewing angle of 11, the observed proper motions give the true velocities of the line-emitting material within the jet. The range of these velocities equals the velocities of the shocks that will form there. Figure 2 shows how the space velocities in the jet vary with distance from the source. The entire range is only 100 km s, so it is not surprising that [O iii] emission, which occurs only when shock velocities exceed 90 km s (Hartigan et al. 1987), exists only very weakly in one location (knot L) in HH 111 (Morse et al. 1993; Noriega-Crespo, Garnavich, & Raga 1993). A typical velocity variation in the bright portion of the jet between knots EandLis 40 km s. This value agrees well with the shock velocities determined from emission-line ratios in weakly magnetized shocks with preshock densities of cm (Hartigan et al. 1994). In fact, the magnetic fields that lie parallel to the shocks (i.e., perpendicular to the jet) must be less than about 1 mg to account for the good agreement between the observations and theory. Fields larger than this value require shock velocities in excess of 60 km s to account for the emission-line ratios. This upper limit can probably be improved upon by modeling line ratios, fluxes, and geometries of specific knots in the jet. Several features within the HH 111 jet exhibit significant morphological changes in the 4 years that passed between epochs. In the most severe cases, these changes made reliable proper-motion measurements impossible. Brightness changes affect the Ha image more than the [S ii] image, probably because Ha responds immediately to changes in the preshock density while [S ii] averages this variability over the 30 yr cooling time. However, [S ii] also changes morphology and flux in some of the knots. Morphological changes are most easy to observe by constructing a movie, in which intermediate frames are created by shifting the first epoch image forward in time, shifting the second epoch backward in time, and performing a weighted average of the shifted images. This procedure yields a series of interpolated images, which is of great use as a visual aid to follow changes in the flow. 5 The most notable features in the HH 111 movies include: 1. The majority of the features retain their shapes between the epochs. There are no indications of rapid turbulent motions in the knots or along the wings of the bow shocks. 2. Knots E 1 and E 2 clearly move much faster than their surroundings and appear to outline the wings of what should become a new, prominent bow shock in the coming decades. Knot E 1 has brightened in [S ii], perhaps owing to the onset 5 See hartigan/movies.html.

3 No. 2, 2001 HARTIGAN ET AL. L159 Fig. 1. Motions in the HH 111 jet calculated from [S ii] and Ha HST images separated by 4 years. Velocities in the plane of the sky in kilometers per second appear alongside the box that defines the object in each case. The boxes were chosen to follow distinctive features in the images and may differ in size and shape between the two filters for the same knot. Motions perpendicular to the long axis of the jet are small, except for knot L 3 (see also Table 1). Internal velocities between adjacent knots are typically km s (Fig. 2). The arrows show the distance traveled in 20 years by each object. of a cooling zone behind this shock. This dynamic region bears watching in future epochs. 3. Proper motions of the bow shock defined by L 1,L 3, and L 4 show that it overran the smaller knot L 2 80 years ago and is in the process of separating from this knot. The bow wing L 3 has a clear component to its motion away from the axis of the jet. 4. The [S ii] emission in knot V 3, which lies behind the large bow shock V 2, split into two pieces between the epochs. This region behind the bow shock V is where one might expect to see a Mach disk, which is highly time-variable in many numerical simulations (Blondin, Fryxell, & Königl 1990). Morse et al. (1993) argued that the bridge of [S ii] emission that connects the bow shock V all the way back to knot T some 10 upstream marks the location where material in the jet decelerates. However, Mach disks need not be present in episodic flows, and there are other explanations for knot V 3. This knot may lie along the wings of the larger bow shock and only appears in projection near the axis of the flow. Alternatively, knot V 3 may represent a slower portion of the flow that has been shocked recently by the large bow. These last two explanations would explain why the proper motion of knot V 3 is less than that of V 2. Future images of this region will help to clarify these scenarios.

4 L160 PROPER MOTIONS OF HH 111 JET Vol. 559 TABLE 1 Proper Motions in the HH 111 Jet Object Filter DY DX Velocity a Age b E 3... [S ii] (1.5) 3.1 (1.3) 295 (4) 170 E 2... Ha (1.5) 0.3 (1.5) 321 (5) 159 [S ii] (1.4) 0.7 (1.3) 327 (4) 157 E 1... Ha (2.1) 0.1 (1.5) 327 (5) 157 F 4... Ha (1.4) 9.4 (1.5) 297 (4) 187 F 2... [S ii] (1.1).4 (1.3) 298 (4) 189 F 1... Ha (1.1).7 (1.1) 235 (4) 246 [S ii] (1.1).6 (1.3) 250 (4) 231 G 1... [S ii] (1.1).8 (1.3) 286 (4) 230 H... Ha (1.1) 3.0 (1.3) 257 (4) 276 [S ii] (1.1) 0.7 (1.1) 265 (4) 265 I 3... Ha (1.4) 2.7 (1.7) 280 (5) 271 I 2... [S ii] (1.5) 9.6 (1.3) 244 (4) 299 I 1... Ha (1.3).1 (1.1) 230 (4) 334 [S ii] (1.1) 7.0 (1.1) 264 (4) 289 J... Ha (1.4) 0.9 (1.3) 271 (4) 302 [S ii] (1.1) 3.4 (1.1) 265 (4) 308 K 1... Ha (1.1) 7.0 (1.1) 301 (4) 299 [S ii] (1.3) 7.1 (1.4) 284 (4) 315 L 4... Ha (1.6) 0.9 (1.4) 264 (4) 356 L 3... Ha (1.3) 36.4 (1.4) 271 (4) 352 L 2... Ha (1.3) 3.5 (1.4) 244 (4) 393 [S ii] (1.3) 8.2 (1.4) 222 (4) 431 L 1... Ha (1.4).4 (1.3) 278 (4) 353 V 3... Ha (1.4) 2.5 (1.3) 229 (4) 1408 V 2... Ha (1.1) 5.8 (1.1) 293 (4) 1110 [S ii] (1.1) 1.0 (1.1) 281 (4) 1154 Note. DY and DX are proper motions in milliarcseconds per year (and with a 1 j error) along and perpendicular to the axis of the jet, respectively. Positive values of DX indicate motions to the right (roughly south) in Fig. 1, while positive values of DY denote motion along the main axis of the jet away from the source. The position angle of the jet on the sky is a Tangential velocity in kilometers per second, assuming a distance of 460 pc. b Time in years for the object to move from the source to its present location at its current velocity. 4. CONCLUSIONS New proper-motion measurements of HH 111 based on two sets of HST WFPC2 Ha and [S ii] images separated by 4 years provide great insight into the dynamics of this collimated stellar jet. Interactions between multiple bow shocks that move between 220 and 320 km s dominate the dynamics in HH 111. The range of internal motions observed, 40 km s, agrees with those predicted from radiative shock models with weak magnetic fields. Motions are generally ballistic and lie along Fig. 2. Space velocities in the HH 111 jet plotted against distance from the source for Ha (squares) and [S ii] (asterisks) for each object in Table 1. The jet is nearly in the plane of the sky, so projection effects are small; an orientation angle of 79 to the line of sight was used to correct the observed proper motions and projected distances to the space velocities and true distances shown in the figure. Motions are directed to the right (away from the source). When the velocity decreases with distance by more than the sound speed ( 10 km s ), shocks will form as the fast material overtakes the slow gas; alternatively, an increasing velocity with distance generates a rarefaction wave (e.g., Hartigan & Raymond 1993). A key result from this Letter is that the uncertainties in the motions ( 4 5 km s ) are significantly smaller than the observed variations in the space velocities ( km s ), which allows us to estimate a typical shock velocity in the jet accurately for the first time. the main axis of the jet. Significant photometric variability occurs between the epochs, which makes ground-based propermotion measurements of these systems problematic. Objects of note include a cooling zone that is currently forming behind the shock in knot E 1, a variable shock structure within knot V, and the aftermath of merging bow shocks in knot L. 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