Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne
C. R. O'Dell
Department of Space Physics and Astronomy, M.S.108, Rice University,
P.O. Box 1892, Houston, TX 77251 USA
Abstract:
The Orion Nebula and its associated cluster has been surveyed with
the Hubble Space Telescope's WFPC2 in the strongest emission lines
of HI, [NII], and [OIII] in addition to the f547m filter, which
mimics the UBV systems V filter. Images from 15 different
positions are combined into a continuous mosaic presenting an
unequaled view of this region. Several large scale structures are
revealed for the first time, some of which are shocks and Herbig-Haro
objects, while others seem to fit into neither classification. We
have carefully examined 489 stellar or nearly stellar images down to
V=22 and find that 145 of these sources show imaging evidence for
circumstellar material. Of these 145 non-stellar objects, all but
seven are circumstellar clouds of gas and dust photoionized on the
outside by emission from 
Ori and the remaining seven
objects are not photoionized but appear in silhouette against the
bright background of the nebula. The distribution of forms indicates
that the circumstellar material is a disk. Extinction measurements
are used to establish lower limits to the masses of these disks,
which are as large as 
.
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
27
2.5
(Marschall & Mathieu, private communication
1995) and the molecules in the latter 26
1
(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.
Acknowledgments:
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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