Deciphering the 3-D Orion Nebula-IV: The HH~269 flow emerges from the Orion-S Embedded Molecular Cloud
DDraft version February 2, 2021
Preprint typeset using L A TEX style AASTeX6 v. 1.0
DECIPHERING THE 3-D ORION NEBULA-IV: THE HH 269 FLOW EMERGES FROM THE ORION-SEMBEDDED MOLECULAR CLOUD
C. R. O’Dell
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235-1807
N. P. Abel
MCGP Department, University of Cincinnati, Clermont College, Batavia, OH, 45103
G. J. Ferland
Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506
ABSTRACTWe have extended the membership and determined the 3-D structure of the large (0.19 pc) HH 269sequence of shocks in the Orion Nebula. All of the components lie along a track that is highly tiltedto the plane-of-the-sky and emerge from within the Orion-S embedded molecular cloud. Their sourceis probably either the highly obscured mm 9 source associated with a high N2H+ density core (morelikely) or the more distant star COUP 632 (less likely). The former must be located in the PhotonDominated Region (PDR) underlying the ionized surface of the Orion South Cloud, while the latterwould be embedded within the cloud. The flows seem to be episodic, with intervals of 1900 to 2600years or 700 to 2600 years if COUP 632 is the source.
Keywords:
ISM:bubbles-ISM:HII regions-ISM: individual (Orion Nebula, NGC 1976)-ISM:lines andbands-ISM:Photon-Dominated-Region(PDR)-ISM:HHobjects INTRODUCTIONThis paper is the fourth in a series reporting on prop-erties of features within the Orion Nebula. PaperI(O’Dell et al. 2020a) dealt with large-scale features, es-pecially the foreground layer of ionized material, theplacement of the embedded Orion-S Molecular Cloud(henceforth the Orion-S Cloud), and the more distantoverlying ionized layer designated as the nearer ionizedlayer (the NIL). Paper II (O’Dell et al. 2021a) identifieda new major feature of the nebula lying ESE–WNWacross nebula and with the Orion-S Cloud on its north-ern boundary. These first two papers used spectra av-eraged over blocks of 10 (cid:48)(cid:48) × (cid:48)(cid:48) (velocity resolution of 10km s − ). Paper III (O’Dell et al. 2021b) used severalfull spatial resolution (about 2 (cid:48)(cid:48) ) sequences of spectra,denoted therein as Profiles, to determine the propertiesof the brightest part of the Orion Nebula that occurs atthe NE boundary of the Orion-S Cloud. In that studya central 30 (cid:48)(cid:48) diameter region designated as the Cross-ing was spectroscopically very complex and informed thespatial structure of the photoionized layer of the Orion-SCloud that faces the observer. These three papers give summaries of our preceding knowledge of the structureof the Huygens Region (the bright central portion of theOrion Nebula) and the progressive improvement of thisknowledge.From a star formation point of view, the arguablymost interesting region identified in Papers I–III is theOrion-S Cloud. These earlier papers established thatthis feature is a host for young stars, many of whichhave collimated outflows. The study of these outflowsrequires using images and spectra in a different manner.The most important procedural differences are that inthe present study we draw on time-lapse images madewith the Hubble Space Telescope to identify high veloc-ity flows associated with HH 269 and that we employhigh spatial resolution radial velocities of these featuresto determine their 3-D locations and motions. This thenallows us to precisely constrain the location of the sourcedriving these outflows.The nomenclature for velocity and groupings of datain the present study are the same as in Paper III. Ra-dial velocities are given in km s − in the Heliocentric a r X i v : . [ a s t r o - ph . GA ] J a n system . 1.1. Outline of this paper
The observational material used here is described inSection 2. The importance of the motions in the Cross-ing is explained in Section 3, while the new properties ofthe HH 269 flow are described in Section 4. The 3-D po-sitions and motions of the HH 269 Groups are presentedin Section 4.2 and their origin in Section 5. A nearbyun-aligned series of shocks is described in Section 6. Theepisodic nature of the HH 269 flow is discussed in Sec-tion 7. The results of this paper are related to the earlierpapers in this series are presented in Section 8. OBSERVATIONSThe spectroscopic data we use in this study are thesame as in Paper III of this series, and we have madeeven more extensive use of archival Hubble Space Tele-scope (HST) images. Again, we draw on the high-spectral-resolution ”Spectroscopic Atlas of Orion Spec-tra” of Garc´ıa-D´ıaz et al. (2008), compiled from a seriesof north-south slit spectra at intervals of 2 (cid:48)(cid:48) and a ve-locity resolution of 10 km s − . THE IMPORTANT ROLE OF THE ORION-SCROSSINGThe Orion-S Crossing (henceforth, the Crossing) is a30 (cid:48)(cid:48) diameter region centered at 5:35:13.95 -5:23:49.2 InPapers II and III we established that this is the region oforigin of changes in the foreground Nearer Ionized Layer(NIL) and the ionized surface of the Orion-S embeddedmolecular cloud. It is a high (closer to the observer)region on the NE edge of the Orion-S Cloud.Within the Crossing we define as the Core the regionwhere the three Profiles discussed in PaperIII show thatthe signals from the V long , [O III] ([O III ] 500.7 nm emis-sion), and V short , [O III] emission abruptly become com-parable. The former isa longer wavelength componentof the deconvolved line profile that is usually identifiedwith the main ionization front (MIF) while the latter isa shorter wavelength component usually identified withforeground lower velocity material. This common regionis shown in red outline in Figure 1. The Core agrees inposition with the areas in Paper III where we found thatthe signal ratios (S short , [O III] /S long , [O III] ) are anoma-lously strong. usually from the MIF.Within the Core we found blue-shifted componentsnot found in nearby spectra. Their velocities are givenin Table 1 along with velocities from HH 269 featuresidentified in Figure 2. Since the spectra were relativelywide, the location of these components are most likely Conversion to LSR is done by subtracting 18.1 km s − to be in the central region covered by the three pro-files. This region was included in the tangential velocitystudy of O’Dell et al. (2015), which found multiple hightangential features. The outer panels of Figure 1 showtwo tangential velocity motion images (first epoch imageover second epoch image). The light-line ellipse enclosesthe region where moving objects were found (seven in[N II ] and two in [O III ]).Three compact sources in the Crossing and lying nearthe axis of HH 269 are shown in all panels of Figure 1:COUP 632 Getman et al. (2005) (5:35:14.40 -5:23:50.9),DRS 1186 DaRio et al. (2009) (5:35:14.29 -5:23:53.1),and mm 9 Eisner & Carpenter (2006) (5:35:13.72 -5:23:50.6).Using both the average radial and tangential velocitieswe calculated the spatial motions presented in Table 2.The high velocity features arising from the Crossing aremoving at about 65 km s − at an angle Θ (cid:39) ◦ from theplane-of-the-sky (toward the observer) toward PositionAngle (PA) (cid:39) ◦ . PROPERTIES OF THE HH 269 SERIES OFSHOCKSThe series of features associated with HH 269 areamong the largest structures within the Huygens Re-gion. Their two brightest components were first studiedin detail by Walter et al. (1995) and subsequently des-ignated as HH 269-East and HH 269-West (Bally et al.2000) (henceforth, East and West in this study). The H study of Stanke et al. (2002) covered the full ExtendedOrion Nebula (EON) and O’Dell et al. (2015) assignedStanke’s more distant feature 2-4 as part of HH 269 onthe basis of its orientation along the axis of the East-West features. O’Dell et al. (2015) added multiple smallmoving features to the west of the West component.4.1. Two new components of HH 269
We now add two groupings of shocks (the Middle-Group and the Crossing-Group) to extend the series ofcomponents to the east (the latter addition was sug-gested in O’Dell et al. (2015)), as shown in Figure 2. Amajor feature within the Crossing is the low ionization”squiggly” feature called the West-Jet in O’Dell et al.(2015). In Paper III we established that the West-Jetname is not a good description because only the westend of it has a detectable motion, while a [N II ] brightfeature at 5:35:13.90 -5:23:52 is the highly tilted side of astationary escarpment, facing north. The latter featureis called the Ledge in Paper III and all of the West-Jet isnow called the Extended-Ledge, which is a mix of mov-ing and stationary features. All of these lie along an axisof PA = 275 ◦ . The observed and derived characteristicsof the components are presented in Tables 1, 2, and 3. [N II] Motions [O III] Motions Figure 1 . These three 30 (cid:48)(cid:48) × (cid:48)(cid:48) panels are centered on the Crossing (the large black circle). The center panel is made fromHST camera WFC3 images with a scale of 0 . (cid:48)(cid:48)
04 pixels (HST program GO 12543, (O’Dell et al. 2015)), The color coding isRed-[N II ], Green-H α and Blue-[O III ]. The other panels depict motions, the left in [N II ] and the right in [O III ]. They are theratio of early over late images, so motion is indicated by the dark edge outside of a bright edge as described in O’Dell et al.(2015). The red line gives the boundaries of the Core, where the V low , [OIII] and V long , [O III] signals are comparable. The longthin black lines are artifacts at the edges of the earlier images. The three small filled circles are at the positions of the compactsources within the Crossing that lie near the central axis of the HH 269 flow. The white ellipse encloses the features for whichtangential motions could be determined (the Crossing-Group). The black arrows indicate the average tangential velocities forthe group (from Table 1 for both [N II ] (V tan , [N II] = 25 ±
12 km s − , PA = 270 ± ◦ ) and [O III ] (V tan , [O III] = 32 ±
12 km s − ,PA = 276 ± ◦ ). The irregular white lines designate the central contours of the N H + Cores 2 (south) and 3 (north) from Teng& Hirano (2020), Section 5.1.1. The square indicates the position from which the HH 1132 east-jet emerges, as discussed inSection 5.1.
Table 1 . Tangential and Radial Velocities of HH 269 Components a Group V tan , [N II] PA [N II] V [N II] V tan , [O III] PA [O III] V [O III] Crossing-Group 25 ±
12 270 ±
10 -11 ± ±
12 276 ±
35 -30 ± b ±
11 273 ±
16 -9 ± ±
19 262 ±
14 - 40 ± c ± ±
13 -13 ± ±
16 270 ±
11 -23 ± ±
19 288 ± ± a Position Angles (PA) in ◦ are from O’Dell et al. (2015) except where measured in this study and tangential velocities arecalculated using an assumed distance of 388 pc (Kounkel et al. 2017). The positions of non-Crossing Groups are indicated inFigure 2 and are given in Table 3. b V [N II] is from Large Sample 80,-30 in Paper II. c V [N II] is from Walter et al. (1995). V tan (motion in the plane-of-the-sky) and V r (ra-dial velocity) have been determined for the sequence ofHH 269 components Crossing-Group—Middle-Group—East—West (Table 1). This allows the calculation of thespatial velocity (V − D ) and the angle (Θ) with respectto the plane-of-the-sky with the results shown in Table 2.Having the separations in the plane-of-the-sky (Table 3)and the velocity vectors, one can calculate the relativepositions along the line-of-sight. Using these results wehave calculated the positions of each component withrespect to the Crossing-Group (Table 4), where we seethat the HH 269 sequence is highly tilted with respectto the plane-of-the-sky.We conclude that the Crossing is where the axis of the HH 269 sequence emerges from behind the surface of theOrion-S Cloud. The source of the components must beat or east of the Crossing-Group. ORIGIN OF THE HH 269 COMPONENTSTwo basic approaches are used to determine the lo-cation of the source of the HH 269 sequence, proximityto the axis of the HH 269 components, and locationssuggested by V − D . We will see that these suggest twolikely sources.5.1. Origin from positions and directions
The dashed black line with PA = 275 ◦ in Figure 2shows that the axis of the HH 269 components pass nearthree compact sources (from the west mm 9, DRS 1186,and COUP 632) within the Dark Arc (the dark feature Figure 2 . This 143.2 (cid:48)(cid:48) × (cid:48)(cid:48) F658N [N II ] motions image centered at 5:35:10.0 -5:23:50 shows the East and West componentsof HH 269 designated in Bally et al. (2000), the Westernmost components designated in O’Dell et al. (2015), the Middle-Groupcomponents studied in O’Dell et al. (2015) that we now designate as part of the HH 269 system, and the Crossing-Groupcomponent highlighted in this study. The dashed black line indicates the common axis of the components at PA = 275 ◦ andthe dashed white lines ± ◦ uncertainty in that value. The H knot 2-4 (Stanke et al. 2002) lies 91 (cid:48)(cid:48) at PA = 277 ◦ from the mostwesterly Westernmost marked component. The positions of COUP 632, DRS 1186, and mm 9 are shown as open circles andthe filled white square indicates the position from which the east-moving HH 1132 east-jet emerges, as discussed in Section 5.1.The large circle on the east end indicates the Crossing from Figure 1. Table 2 . HH 269 Components 3-D Velocities and AnglesDesignation Line V − D a Θ a Crossing-Group [N II ] 69 ±
12 57 ± ◦ ” [O III ] 65 ±
12 61 ± ◦ Middle-Group [N II ] 46 ±
12 49 ± ◦ ” [O III ] 76 ±
20 62 ± ◦ East [N II ] 58 ±
10 44 ± ◦ West [N II ] 88 ±
15 34 ± ◦ ” [O III ] 68 ±
20 40 ± ◦ Average [N II ] & [O III ] 66 ±
14 48 ± ◦ a V − D is the spatial velocity of the Group in km s − , Θ isthe angle of the velocity vector out of the plane-of-the-skyand toward the observer. The assumed radial velocity of theOrion-S Cloud was 27 km s − . The average was calculatedgiving triple weight to the much more numerous [N II ] mo-tions. Table 3 . Positions a of HH 269 ComponentsDesignation RA DECCrossing-Group 5:35:13.71 -5:23:52Middle-Group 5:35:11.08 -5:23:49East 5:35:09.65 -5:23:47West 5:35:07.77 -5:23:46Westernmost 5:35:07.12 -5:23:44Westernmost 5:35:06.98 -5:23:43Westernmost 5:35:06.46 -5:23:43Westernmost 5:35:05.75 -5:23:41Stanke 2-4 5:34:59.8 -5:23:30 a along the top of the Crossing). A plausible uncertaintyof the PA of ± ◦ would correspond to an uncertaintyin Declination of ± . (cid:48)(cid:48) Table 4 . 3-D Positions of HH 269 ComponentsComponent ∆Tangential a Adopted Θ ∆D a Crossing-Group 0.0 59 ◦ ◦ -0.11East 0.105 44 ◦ -0.17West 0.157 — -0.22 a Distances are in pc relative to the Crossing-Group in theplane-of-the-sky (∆Tangential) and along the line of sight(∆D, where negative numbers are toward the observer). an alignment of the HH 269 axis with any of the threecompact sources. At a greater distance the alignmentpasses close to AC Ori.5.1.1. mm 9 and an associated N H + clump mm 9 is located in the NW portion of the ellipticalregion defining the shocks associated with the Crossing-Group. It is located slightly offset within the Crossing-Group but along the axis of the HH 269 flow if that axishas the allowable error of 1 ◦ . It was measured at 3 mmas a > σ detection by Eisner & Carpenter (2006), whonote that it has no near infrared counterpart.In addition, mm 9 lies within Core 2 found by Teng& Hirano (2020) in their 5 (cid:48)(cid:48) resolution map in N H + emission in the J = 3–2 line along a north-south featurewithin the background Orion Molecular Cloud. Two ofthe Teng & Hirano (2020) cores are shown in Figure 1.Core 2’s heliocentric velocity is about 24.6 km s − . Morerecently Hacar et al. (2020) mapped the same regionwith 10 (cid:48)(cid:48) resolution in the N H + J = 7–6 transition, anemission-line selectively coming from very high densitygas n(H ) > cm − . In that study they establisheda strong correlation of high density N H + knots andvery young stars, which argues that this region containsa potential source of the material causing the HH 269shocks. Although Teng & Hirano (2020) argued that theCore 2 lies in the background Orion Molecular Cloud,Hacar et al. (2020) place it within the Orion-S Cloud.5.1.2. DRS 1186
DRS 1186 lies closest to the HH 269 axis of 275 ◦ . Lit-tle is known about it, although the discovery paper (Za-pata et al. 2004) shows the source to have a micro-jetof about 0 . (cid:48)(cid:48) tan , [N II] = 8 km s − but is atthe limit of detectability. The shock’s position gives nosupport for association of DRS 1186 with HH 269.5.1.3. COUP 632
COUP 632 lies on the northern boundary of the axisof HH 269. Appendix A9 of Rivilla et al. (2013) sum-marizes well the characteristics of COUP 632, althoughthey incorrectly assign it as the source of HH 529 O’Dellet al. (2015). It is seen in X-rays, through the infrared,and in short wavelength radio radiation.COUP 632 was identified in O’Dell et al. (2015) as thesource of the HH 1132 east-jet that emerges with PA= 107 ◦ from within the Orion-S Cloud at the positionshown with a square in Figures 1 and 2. The emergenceposition is quite obvious in time sequence F656N H α images (O’Dell et al. 2015) rendered as a movie. Theaxis of this east-jet points exactly at COUP 632 with anopening-COUP 632 separation of 6 . (cid:48)(cid:48) ◦ . A bipolar system would require the west-jet tobe pointed toward PA = 287 ◦ . A compounding problemto association of the HH 1132 east-jet and the HH 269shocks is that both are blue-shifted. The east-movingHH 1132 moves at a spatial velocity of 116 km s − withΘ = 32 ◦ (O’Dell et al. 2015) while the Crossing-Groupshocks have PA = 275 ◦ , Θ=59 ± ◦ , and space velocity67 ±
12 km s − .A strong selection effect exists for finding blue-shiftedcomponents in the Huygens Region because the red-shifted component would be headed toward the MIFand disappearance behind the PDR. Likewise, a red-shifted jet originating behind the PDR will not emergeinto the low extinction ionized gas. A summary of out-flows O’Dell et al. (2015) finds 27 mono-polar flows (allblue-shifted), eight bipolar flows (none with a well estab-lished red-shift), and seven multi-polar flows (all blue-shifted). Therefore, there is plenty of evidence for non-bipolar flows in the Huygens Region and COUP 632 maybe such a source.If COUP 632 is the source, then it lies within the Orion-S Cloud. The separation of the Crossing-Groupof moving features from COUP 632 is 10 . (cid:48)(cid:48) ◦ ,puts COUP 632 (0.031 pc) beyond the plane containingthe Crossing-Group shocks. The separation in the plane-of-the-sky of the Crossing-Group shocks and the SE-NWTransition established in Paper II is 23 (cid:48)(cid:48) (0.0427 pc).This means that COUP 632 is still within the Cloud. Ifit is also the source of the HH 1132 east-jet, then thisinterpretation is fully consistent with the break-out ofthat feature. TWO SERIES OF SHOCKS WITHIN THECROSSING THAT ARE NOT RELATED TOHH 269There are large-scale shocks of note falling within theCrossing that have no relation to HH 269. In the tan-gential velocities study of O’Dell et al. (2015) usingHST WFC3 and WFPC2 images, a series of concen-tric strongly curved arcs was found in [O
III ] motionsimages in the Crossing, as shown in Figure 11 of thatpaper. These lie between mm 9 and the Dark Arc andwere attributed to outflow from an undetected source afew arcseconds north of mm 9. We show in Figure 3 thatthese features are also seen in the superior WFC3 [O
III ]images adjusted in brightness and contrast to best dis-play this area. We now see that they are not a circularmotion away from an empty position designated as theBlank-West in O’Dell et al. (2015). Instead, they are thelead features in a series of bow-shock shapes driven bya more distant source to the SSW. At 7 . (cid:48)(cid:48) MOST OF THE HH 269 COMPONENTS AREFORMED FROM EPISODIC OUTFLOWSThe series of well-defined bow-shocks that we see inthe Middle-Group, East, and West components arguefor the driving jet being episodic because each periodof flow would produce a single shock or a set of tightlygrouped shocks. The V tan and separations of the fourmain components indicate the time intervals betweenoutflows: Crossing-Group and Middle-Group, 1900 yrs;Middle-Group and East, 2500 yrs; and East and West,2600 yrs. These time intervals are similar to those foundin other outflows in the Huygens Region (O’Dell & Hen-ney 2008).The HH 269 Crossing-Group component lacks thewell-defined bow-shocks seen in the Middle-Group, East,and West components. This is consistent with this com-ponent not representing a single outflow, rather, that it
F502N [O III]
Figure 3 . The same Field-of-View as Figure 1 except nowshowing the filter WFC3 F502N [O
III ] image. The displayis adjusted to best show the faint arcs north of source mm9 but inside the Dark Arc. Their motions were reported inO’Dell et al. (2015). is where a submerged outflow passes through the nearside of the Orion-S Cloud. DISCUSSION AND CONCLUSIONSIn Paper III we established that the Crossing is cen-tered on a local rise in the nearer side of the Orion-S Cloud, which then flattens to the SW. Now we seethat the Crossing-Group, the first visual components ofHH 269, appears there. This suggests that a changein the local topography has determined where the flowproducing the Crossing-Group breaks out.This conclusion is reinforced by the fact that the E-W line along which the HH 269 features appear is alsowhere the [O
III ] emission changes from domination by acomponent clearly near the MIF to lower velocity com-ponents associated with the NIL, a behavior that ex-tends to the SW. • Taken together, these points mean that the struc-ture of the nearer ionized layer of the Orion-S Cloudinfluences where one can see the results of the HH 269flow, and this flow does not influence the structure. • Two young stars are candidate sources of the colli-mated outflow that drives HH 269-the mm 9 source asso-ciated with N H + Clump 2 and the nearby COUP 632. • At 10 . (cid:48)(cid:48) ◦ or slightly less (O’Dell et al.2015). The reciprocal of its motion is 288 ◦ , produc-ing a poor match to HH 269’s PA = 275 ± ◦ . Moretroubling is that both HH 1132 and HH 269 are clearlyblue-shifted (V r , [N II] =-40 km s − (O’Dell et al. 2015)and -14 ± − , respectively). However, multi-polaroutflows in the Huygens have been found (O’Dell et al.2015). If this is the source, it lies in the western portionof the Orion-S Cloud. • mm 9 is located within the high-density N H + Clump 2, which is a likely source of new star formation,and must lie close to the surface of the ionized layer ofthe Orion-S Cloud that faces the observer. Given itshigh extinction, it is likely to be within or beyond theunderlying PDR. • The spatial separations and velocities of the com-ponents of HH 269 argue that these are the result ofintermittent jet activity at intervals of about 1900 to2600 years. ACKNOWLEDGEMENTSThe observational data were obtained from observa-tions with the NASA/ESA Hubble Space Telescope, ob-tained at the Space Telescope Science Institute (GO12543), which is operated by the Association of Uni-versities for Research in Astronomy, Inc., under NASAContract No. NAS 5-26555; the Kitt Peak NationalObservatory and the Cerro Tololo Interamerican Obser-vatory operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agree-ment with the National Science Foundation; and the SanPedro M´artir Observatory operated by the UniversidadNacional Aut´onoma de M´exico. We have made exten-sive use of the SIMBAD data base, operated at CDS,Strasbourg, France and its mirror site at Harvard Uni-versity, and to NASA’s Astrophysics Data System Bib-liographic Services. GJF acknowledges support by NSF(1816537, 1910687), NASA (ATP 17-ATP17-0141), andSTScI (HST-AR- 15018).REFERENCES