C. R. O'Dell
Department of Space Physics and Astronomy, M.S.108, Rice University, P.O. Box 1892, Houston, TX 77251 USA
The Orion constellation contains many regions rich in interstellar gas and dust and sites of recent star formation, but no region has received the amount of attention of the Orion Nebula (M42, NGC 1976) and its associated Trapezium Cluster. This attention is well deserved because NGC 1976 is the closest bright HII region, is located at a relatively high galactic latitude, suffers little extinction besides that of its associated components, and contains what is certainly the densest and what may be one of the youngest sets of coeval stars.
The basic elements of the structure of NGC 1976 are well understood. Most of the emission comes from a thin layer of emitting gas on the facing side of the giant molecular cloud OMC-1 (Zuckerman 1973). Wen & O'Dell (1995) have been able to build a three-dimensional model of this main emitting surface by use of the basic considerations of the photoionization equilibrium of a thin layer of gas. The emitting layer is thin with respect to the dimension of the nebula, with the scale height for the density decrease away from the main ionization front corresponding to only an angle of 10 whereas the scale of the brightest part of the nebula is about 200. The presence of an associated foreground layer of neutral Hydrogen was revealed in the 21-cm absorption line study of van der Werf & Goss (1989). The most obvious component of this ``lid'' is the very high column density region to the east of the Trapezium stars which causes the feature commonly called the ``Dark Bay'' (O'Dell et al. 1992). it is also seen in the absorption lines of low ionization potential energy ions which appear in the spectra of the Trapezium stars and the nearby associated O9 star Ori (O'Dell et al. 1993a). The distance between the main ionization front and the overlying lid is about 250 i.e., about the same dimension as the apparent size of the nebula. Almost all of the radiation we see from NGC 1976 arises from photoionization caused by Ori because of its much greater ionizing continuum luminosity that results from its temperature being higher than nearby stars.
The physics of the nebula is relatively straightforward since absorption by the Lyman continuum dominates the transfer of radiation. Material near the ionization front lying on OMC-1 is photoionized and heated, resulting in a large pressure difference with the open cavity near the Trapezium stars which in turn leads to a flow of material from OMC-1 though the ionized emitting layer and eventually flowing out of the opening in the southwest portion of the lid. In the approximation of a single star of ionizing photon luminosity Q photon illuminating a thin ionized layer, the apparent surface brightness of the nebula in the H line in photons will be
where the terms have the meaning adopted by Osterbrock (1989) and r is the distance between the star and the ionization front. A more accurate formulation of the problem would require correction for differences in the columns doing the absorbing and that of the observer. This relation explains the general drop of surface brightness as one looks away from Ori with the obvious irregularities being caused by roughness in the height of the ionization front.
The eponymous Trapezium Cluster is dominated in visual light images by the four bright O stars projected near the center of the nebula. The brightness of the nebula makes the nearby associated stars difficult to see in the visual, but examination of this region in the infrared (Herbig 1982, McCaughrean & Stauffer 1994) indicates that this is an extremely rich cluster, containing about 700 members (mostly faint, cool, low mass stars) most of which are still in contraction towards the main sequence and having an age of about years (Prosser et al. 1994). The cluster itself appears about the same dimension as the nebula, so that one would expect a few of the outlying stellar members to be found both within the overlying lid and imbedded in the near parts of OMC-1. There is no significant motion of this cluster with respect to OMC-1 since the former has a radial velocity of 272.5 (Marschall & Mathieu, private communication 1995) and the molecules in the latter 261 (Goudis 1982, Table 3.3. VIII).
Given this good basic knowledge, it was only natural that this region would become the primary program in the author's deferred Guarantee Time Observer program (GTO). The first of the observations were made as part of the Early Release Observations program executed immediately after the installation of WFPC2 in December 1993. The rest were made between November 1994 and March 1995. Four of the 15 fields that were imaged were done as part of a General Observer program of John Bally. Figure 1 shows an outline of the fields imaged superimposed on a star chart prepared by Strand (1958). The four Bally fields were all close together and only an outline of that composite is shown.
Figure: The fifteen fields imaged with WFPC2 are shown superimposed on the atlas of Strand (1958). Four fields that were close together near the center are shown as one highly irregular field.
For surveying the nebula in emission lines, we observed with the f502n filter in order to isolate the [OIII] line at 5007Å with the f656n filter to isolate the H line at 6563Å and with the f658n filter to isolate the [NII] line at 6583Å. For surveying the stellar content, we used the f547m filter, which simulates the UBV system V filter and is free of the effects of strong nebular emission lines. Multiple exposures were taken of each field in order to allow removal of cosmic ray hits. After standard processing and bringing the four CCD images together into a four component mosaic, we performed astrometric solutions for each pointing, which allow determination of coordinates of objects to about 0.1. We then combined the 15 fields into a composite mosaic by rotation to a common position angle and fitting the images of stars in overlapping fields. The result is shown in Figure 2 where we have used the ionization energy coding of f502n is blue, f656n is green, and f658n is red. The color balance was chosen to give an appearance close to that seen in a large optical telescope.
In the mosaic we see a wealth of interesting large objects, some of which are composed of structure near the resolution limit of the WFPC2 and have escaped previous detection, while others were already known but show new and interesting detail.
The region to the south of the Trapezium shows many high ionization shock fronts at a variety of scales. These shocks were originally detected in the high resolution spectroscopy studies of Lee (1969) and Castañeda (1988) who could isolate the highly blue shifted [OIII] components (-25 to -104 ) but could not see any corresponding features in ground-based images. The radial velocity of the main ionization front is about 17 (O'Dell et al. 1993a). We now see that these were samples of a clustering of shocks close to the center of brightness of the nebula. They probably occur from the interaction of flows away from the young stars in that region with the ambient nebular gas. The high ionization of these shocks is probably due to radiative preionization by Ori.
Figure: The combined WFPC2 image of M42 is shown with the color coding described in the text and the color balance set to resemble the visual appearance. A logarithmic scale of intensity was employed.
The large features southwest of Ori have been known for some time (Münch & Wilson 1962) and are designated as HH 203 and HH 204 (the more southerly object). The usual interpretation is that they are both Herbig Haro objects (Walsh 1982) although an interpretation based on their being interactions of a stellar wind with ambient nebular gas has been made (Taylor & Münch 1978). Both objects are very low ionization and HH 203 has a radial velocity of -48 while that of HH 204 is -25 . The proper motion measured by Cudworth & Stone (1977) indicates tangential velocities of 61 k for HH 204 if the distance to the nebula is 440 pc (Warren & Hesser 1977). The WFPC2 images show that the northeast boundary of HH 203 can be traced nearly back to Ori, actually stopping at one of the young stellar objects discussed in the next section (O'Dell & Wen 1994). This may be a chance superposition of images in a crowed field or it may indicate that the northwest boundary is actually a jet with the bright end (HH 203) as the shock at its end. If this evidence is ignored, then HH 203 can be interpreted as a shock formed by a jet moving into a region with a significant density gradient as has been modeled by Henney (1995). The latter interpretation seems more likely as the central feature of HH 203 that points out to the northwest from the bright tip can be seen in extension for about 90 and is known to posses a high and nearly constant radial velocity at the same values as HH 203 (Hu 1996). HH 204 is more clearly symmetric in form, the brightest portion probably being the mach disk of a shocked jet.
Allen & Burton (1993) first demonstrated that most of the Herbig Haro objects lying to the northwest of the Trapezium (HH 201, 205--210) show a symmetry indicating a common point of origin near the position of the strong infrared source IRc2. They interpreted these objects as being multiple bullets expelled from a common origin, as indicated by the proper motions of a few of them (Jones & Walker 1985). Allen & Burton used infrared images in and [FeII] which delineate spokes trailing behind the optical objects best seen in [OI] (Axford & Taylor 1984). Our WFPC2 images confirm the conclusions of Allen & Burton about the forms of the objects and indicate high proper motions in several additional HH objects (Hu 1996) and that their is a common age of about 700 years if they are indeed the products of ejections. Since simultaneous multiple ejections from a single source would require introduction of a new physical mechanism alternative interpretations should be sought. One such mechanism has been proposed by Stone et al. (1995), who argue that such features would be produced from Rayleigh-Taylor instabilities at the interfaces of shocks formed by an initial slow stellar wind from IRc2 being overtaken by a faster later wind from the same source. A central source of a shock which breaks down is very attractive, but a double wind mechanism would produce instabilities in all directions. Since the region around IRc2 is heavily obscured, one would expect to no see optical counterparts of many of the infrared sources. This is not the case as most of the infrared sources have optical counterparts (O'Dell 1995). A mechanism which could explain both the symmetry and the commonality of optical and infrared sources is that the instabilities arise from a wind driven shock moving into a zone of diminishing density. In this case, such an interpretation would require that the near side of OMC-1 must be facing the observer, which is indeed the case.
The object designated as HH 202 is a U-shaped complex lying to the west-northwest of the Trapezium and with the open ends pointed to the southeast, i.e., towards the brightest parts of the nebula. The object has been studied several times since its discovery by Cantó et al. (1980). Its nature is not clear. Our images show that the two bright knots are really quite distinct, although lying exactly on the same curved edge that resembles a shock. The most extensive ground-based spectroscopic studies indicate that it is kinematically very complex, appearing to contain both an expanding shell or shock and also a high velocity jet type feature that goes out to -67 km (O'Dell et al. 1991), although the spatial resolution of that study was so low that the two bright components were unresolved. The most likely interpretation of these images and the spectroscopy is that this is a shock at the end of a jet arising from a source to its southeast and that the two bright spots are mach disks formed at dense regions in the ambient nebular material.
A detailed analysis of the region around HH 202 has been made by Xihai Hu (1996). He confirmed the presence of a high ionization large high velocity feature to the immediate south of HH 202 (Meaburn 1986) and has also found a smaller similar feature to the west-northwest of the bright portions of HH 202. Evidently this region is a complex of multiple shocked and accelerated material.
An irregular elliptical shell has been to the west-southwest of the Trapezium has recently been studied by Walter et al. (1995). This object is of major axes 41x23 and centered at 5:35:09.16-5:23:44 (E2000) and elongated almost exactly east-west. Spectra have been obtained of the brightest knots in it and they are shown to be of very low ionization, dominated by [NII], [SII], and H. The radial velocity is about -17 km , which means it is blueshifted about -34 km with respect to the main ionization front. Large relative motions are also indicated by its proper motion, which has been studied by Feibelman (1976) and more accurately by Cudworth & Stone (1977). The latter determined that the object is expanding in the plane of the sky at 2.9 0.2 which corresponds to 60 km . There is no obvious driving source for this shell. It has been given, for convenience, the designation HH 269, although it may be quite different from other objects of this class.
The mystery deepens to open further examination of the WFPC2 mosaic. We see a similar form but slightly smaller emission line shell centered about 69 east of HH 269 and a fainter similar ellipse to its west. A more diffuse and larger ellipse is centered about 64 to the southeast from the center of HH 269. In line with the three bright east-west objects, and centered about 110 east of the middle of HH 269, is yet another ellipse of similar size, but in this case it is dark, indicating a high density un-ionized shell or cylinder. I can think of only one mechanism which would produce four shells in a row. They could be produced by a single jet which has passed completely through four clouds, forming a cylinder of compressed material in each. If this is indeed the case, the clouds are probably in the lid that lies across the front of the nebula, where the work of van der Werf & Goss (1989) demonstrate that there are at least three obvious velocity systems.
The Trapezium Cluster is quite young and extremely dense. In the usual ground-based image of M42, the impression one receives is largely determined from the few very bright O type stars; however, a more realistic view comes from infrared images since most of the stars are cool. The first HST images, made before the refurbishment mission, indicated that the stars were not simple systems but that many of them had circumstellar material. This circumstellar material is rendered visible either through photoionization from the outside or through being seen in projection. I have coined the term ``proplyds'' to indicate young stars with circumstellar material that is rendered visible by being in or near an HII region (O'Dell et al. 1993b). The proplyds can either be bright or dark and their appearance will depend upon the viewing angle. In most cases, the central star is seen, sometimes only in the infrared. In only one case is a star not seen at all, which probably means that that object has a very dense disk of material which is viewed almost exactly edge on.
A detailed summary of the stellar and nearly stellar objects found from examination of these images is given in a paper now in press (O'Dell & Wong 1996). Suffice it to say that we measured 489 such objects of V=22 or brighter and found that 145 were resolved, most of them showing a central star. The proplyds which are inside the cavity between the main ionization front and the lid are illuminated on the side facing Ori with a brightness that varies according to equation 1 (O'Dell & Wen 1994), which means that those closest to the photoionizing star are brightest and easiest to detect. In fact, almost all of the low mass stars at small projected distances from Ori show that they have circumstellar material. A more detailed discussion is found in the paper by John Bally in the proceedings of this conference.
Figure: Four of the dark proplyds are shown, this subset illustrates well the range of forms and relative brightnesses of the central stars. The color coding is the same as in Figure 2. Each field is 4.1 square, corresponding to 1800 AU.
The stars of the Trapezium cluster that are shielded from photoionizing radiation from Ori present a very different appearance. This condition would apply for stars within the overlying lid or on the observer's side of this lid. In this case the circumstellar material is seen not through photoionization of the gaseous component, rather, by extinction of the background image of the Orion Nebula by the dusty circumstellar cloud. Seven such objects are now known and they present an excellent opportunity for detailed study of the circumstellar material surrounding these young low mass stars. These objects are discussed intensively in a paper by McCaughrean & O'Dell (1996) and in a poster paper presented at this conference.
The variety of forms seen in these dark proplyds is consistent with a model of them all being stars with circumstellar disks, rather than shells or spherically symmetric distributions, since they vary from being almost circular in appearance to the extreme case of an object almost dumbbell in form. This last object resembles extremely well the theoretically expected shape for a circumstellar disk. In that object, the central star is not seen directly because of the nearly planar viewing angle but can be detected through light scattered from the back inside face of the dumbbell of gas and dust.
Figure: The unusual dark proplyd 114-426 is shown. The area is the same as in Figure 3. The left hand image employs the same color coding as as Figures 2 and 3 while the right hand image was made from the f547m continuum image. The field is the same as Figure 3.
The extinction can be used to determine the amount of material in the dark proplyds. These objects have a line of sight optical depth that increases from nearly zero at the detection limit to a large value in the middle. The point spread function of even the PC portion of WFPC2 is finite and fills in background light into the middle of these very dark centers. One can use the extinction to determine the column density of material (O'Dell & Wen 1994) and when this is done for the seven objects now known, one obtains lower limits to the circumstellar masses from --g (i.e., up to ) and these masses are probably much lower than the actual values.
Dust is certainly obvious in many of the bright proplyds also with a large fraction of them showing obscuration of the nebula near the center of the proplyd even while the outer part of the gaseous component is being photoionized.
I am grateful to the pre-launch GTO's for their generous sharing of precious HST observing time. John Bally of the University of Colorado has shared his WFPC2 images in the central portion of NGC 1976. Mark J. McCaughrean of the Max Planck Institute for Astronomy in Heidelberg has led the way in the analysis of the dark disk proplyds. I acknowledge with gratitude the continued good support of the Space Telescope Science Institute science support team, the Goddard Space Flight Center HST offices, and financial support through NASA grant NAG 5-1626.
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