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The Three-Dimensional Dynamic Structure of the Inner Orion The Three-Dimensional Dynamic Structure of the Inner Orion
Nebula Nebula
C. R. O'Dell
Vanderbilt University
W. J. Henney
Universidad Nacional Autónoma de México, Mexico
N. P. Abel
University of Cincinnati
Gary J. Ferland
University of Kentucky
, gary@uky.edu
S. J. Arthur
Universidad Nacional Autónoma de México, Mexico
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O'Dell, C. R.; Henney, W. J.; Abel, N. P.; Ferland, Gary J.; and Arthur, S. J., "The Three-Dimensional Dynamic
Structure of the Inner Orion Nebula" (2009).
Physics and Astronomy Faculty Publications
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The Three-Dimensional Dynamic Structure of the Inner Orion Nebula The Three-Dimensional Dynamic Structure of the Inner Orion Nebula
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http://dx.doi.org/10.1088/0004-6256/137/1/367
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Published in
The Astronomical Journal
, v. 137, no. 1, p. 367-382.
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The Astronomical Journal, 137:367–382, 2009 January doi:10.1088/0004-6256/137/1/367
c
2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE THREE-DIMENSIONAL DYNAMIC STRUCTURE OF THE INNER ORION NEBULA
∗
C. R. O’Dell
1
,W.J.Henney
2
,N.P.Abel
3
, G. J. Ferland
4
, and S. J. Arthur
2
1
Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN 37235, USA; cr.odell@vanderbilt.edu
2
Centro de Radioastronom
´
ıa y Astrof
´
ısica, Universidad Nacional Aut
´
onoma de M
´
exico, Apartado Postal 3-72, 58090 Morelia, Michaoac
´
an, Mexico
3
Department of Mathematics and Physics, College of Applied Science, University of Cincinnati, Cincinnati, OH 45221, USA
4
Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA
Received 2008 August 14; accepted 2008 October 22; published 2008 December 15
ABSTRACT
The three-dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and
newly established data. We find that the widely accepted model of a concave blister of ionized material needs to
be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate
cloud of very optically thick molecules within the body of ionized gas, which is probably the location of the
multiple embedded sources that produce the optical and molecular outflows that define the Orion-S star formation
region. Evidence for this cloud comes from the presence of H
2
CO lines in absorption in the radio continuum and
discrepancies in the extinction derived from radio–optical and optical-only emission. We present an equilibrium
Cloudy model of the Orion-S Cloud, which successfully reproduces many observed properties of this feature,
including the presence of gas-phase H
2
CO in absorption. We also report the discovery of an open-sided shell of
[O
III] surrounding the Trapezium stars, revealed through emission-line ratio images and the onset of radiation
shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a
stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind
from θ
1
Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of
θ
1
Ori C with respect to the Orion molecular cloud, which could mean that this star has only recently begun to
photoionize the Orion Nebula. We show that current observations of the nebula do not rule out such a possibility,
so long as the ionization front has propagated into a pre-existing low-density region. In addition, a young age for
the nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.
Key words: H ii regions – ISM: individual (Orion Nebula, NGC 1976) – stars: formation
1. INTRODUCTION
The brightest portion of NGC 1976, the Orion Nebula, is
commonly called the Huygens region (after Christiaan Huygens,
who published the first drawing of the nebula in 1659) and
its form in three dimensions was the subject of many early
papers (reviewed in O’Dell 2001a). The presently accepted
basic model of a photoionized thin layer of gas flowing off
the side of the Orion molecular cloud (OMC) facing us was
invoked to explain the progressive blueshift of the emission
lines with respect to the OMC (Zuckerman 1973; Ballick et al.
1974). The bright bar along the southeast border of the Huygens
region is a location where the main ionization front (MIF) is
tilted almost along the line of sight to the observer. The other
portions of the MIF have been mapped in three dimensions by
Wen & O’Dell (1995) by a method that assumes all the ionizing
radiation arises from the O7Vp star θ
1
Ori C, and then calculates
the distance to the MIF that would satisfy the known conditions
of gas density and observed Hα surface brightness, this being
a broader application of a method first presented in Baldwin
et al. (1991). Although this three-dimensional (3D) map has
the limitation that it becomes progressively less accurate as
one moves away from the line of sight toward θ
1
OriC,itdid
establish that θ
1
Ori C lies about
5
0.2 pc in front of the MIF,
∗
Based 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 No. NAS 5-26555.
5
Throughout this paper we will adopt a distance of 440 pc to the Orion
Nebula, a value derived (O’Dell & Henney 2008) using the results of what are
currently thought to be the best independent determinations. Earlier papers that
used different assumed distances will have their results scaled to this distance.
confirmed the structure in the bright bar region, and showed
that the nebula was otherwise a concave surface with a large
bump to the southwest of θ
1
Ori C in the Orion-S star formation
region. The surface brightness near θ
1
Ori C can be explained as
emission by a constant density layer of about 0.1 pc thickness
(Pogge et al. 1992). This is only a reference number since the
emissivity must be much higher near the MIF and drops as the
square of the density. In a nebula where there is free expansion
of the photoionized gas and there is a single dominant ionizing
star, a concave shape of the MIF is the natural result, with bumps,
valleys, and ridges reflecting underlying conditions of the host
molecular cloud (MC). The ionization front is expected to be
closer to the ionizing star where the MC density is higher and
further away where the underlying density is lower. The ionized
gas density drops rapidly away from the MIF, but it is not clear
what the density is in the immediate vicinity of θ
1
Ori C since
it is expected that its intense high-velocity wind would create
a hot, low-density cavity around it. This cavity has not been
detected directly except for observations of stand-off shocks in
front of the proplyds closest to θ
1
Ori C (Bally et al. 1998, 2000).
In addition to the rich Orion Nebula Cluster (ONC) centered
on the bright Trapezium stars, there are two star formation
centers imbedded in the OMC. The first is associated with the
deeply imbedded (0.2 pc; Doi et al. 2004) BN–KL infrared (IR)
and radio sources to the northwest of θ
1
Ori C and the second is
in the Orion-S region.
With the discovery of 21 cm absorption lines in the radio
continuum spectrum (van der Werf & Goss 1989)itwas
recognized that there was a Veil of neutral material on the
observer’s side of the nebula, and subsequent absorption line
spectroscopy (Abel et al. 2004b, 2006) established its physical
367
368 O’DELL ET AL. Vol. 137
characteristics and approximate location of about 1 parsec on
the observer’s side (henceforth foreground) of θ
1
Ori C. Recent
detailed reviews have covered the ONC (Muench et al. 2008)
and the nebula plus obscured star formation regions (O’Dell
et al. 2008, and see also O’Dell 2001a, 2001b).
The Huygens region occupies the northeast corner of a much
larger structure called the extended Orion Nebula (EON; G
¨
udel
et al. 2008). It is known that there is a systematic flow of
ionized material into the EON (O’Dell 2001a; Henney et al.
2005), but the lower surface brightness has limited the number
of investigations of this region (Subrahmanyan et al. 2001).
However, the EON is the location of two X-ray bright regions of
hot plasma (G
¨
udel et al. 2008). High-resolution optical (Henney
et al. 2007) and IR (Megeath & Robberto 2006) images of the
EON are now available. Although these images show many
interesting large-scale features, there is neither the detail nor
abundance of stars of the Huygens region.
In this paper, we will present the most relevant information
about the ONC and the Orion Nebula, then integrate this into
a modified picture of the Orion Nebula’s 3D structure and its
history. In Section2we present the most useful information, then
in Section 3 give the results of where this information leads.
2. BACKGROUND INFORMATION
2.1. Emission-Line Images of the Orion Nebula
The Huygens region has been the subject of numerous
imaging studies. Arguably the most useful ground-based study
is that of Pogge et al. (1992), which utilized a Fabry–Perot
system to isolate emission from the Hα,Hβ,[O
III] 5007 Å,
[N
II] 6583 Å and 6548 Å, [SII] 6716 Å and 6731 Å, and [HeI]
6678 Å lines. O’Dell & Wong (1996) presented a mosaic of the
Hubble Space Telescope (HST) Wide Field Planetary Camera 2
(WFPC2) images at the superior resolution of the HST.The
particular advantage of the HST images is that the WFPC2
images allow clear discrimination of not only the isolated [O
III]
5007 Å line, but also both the Hα 6563 Å line and the nearby
[N
II] 6583 Å line in addition to the filters being narrow enough
that they provide a good isolation of the emission lines against
the strong scattered light continuum that primarily arises from
dust grains in the dense region immediately behind the MIF
(O’Dell & Doi 1999). A later HST survey (Henney et al. 2007)
with the Advanced Camera for Surveys (ACS) covered a wider
field of view with pixels one half the angular size of those in the
WFPC2, but the filters used do not allow a clear delineation of
the important emission lines (O’Dell 2004). The Huygens region
has been mapped with the VLA at about 1.
7 resolution (O’Dell
& Yusef-Zadeh 2000) at the extinction free 20.5 cm continuum.
By comparing the surface brightness calibrated (O’Dell & Doi
1999) images and the radio images, it was possible to derive
a map of the optical extinction across the Huygens region and
to generate extinction-corrected versions of the HST WFPC2
emission-line images (O’Dell & Yusef-Zadeh 2000). In this
study, we employ the several forms of the WFPC2 images, all
processed in part using the IRAF package.
6
WeseeinFigure1
that the Orion-S region to the southwest of the Trapezium
resembles the bright bar region in being enhanced in low-
ionization [N
II] emission and being much brighter than adjacent
6
IRAF is distributed by the National Optical Astronomy Observatories,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under cooperative agreement with the National Science
Foundation.
areas. These characteristics are consistent with the brightest part
of the central Huygens region being both closer to θ
1
Ori C
and also being an inclined face of ionized gas. This picture is
consistent with the results of the 3D modeling of Wen & O’Dell
(1995).
2.2. The Orion Nebula’s Veil
Although the optical extinction of the Veil has been rec-
ognized for some time, it has been established only recently
(O’Delletal.1992;O’Dell2002) that most of this extinction
arises from the near side of the ionized zone and not within it.
The detailed analysis of Abel et al. (2005, 2006) established that
the physical conditions in the two 21 cm H
I absorption velocity
components of the Veil are rather different, the energy density in
their component A being dominated by the magnetic field mea-
sured from the Zeeman effect. The elements C, S, Mg, and Si are
ionized in the Veil by the far-ultraviolet (FUV; 5–13.6 eV) radi-
ation that penetrates it, even though the Veil is optically thick to
extreme-ultraviolet (EUV) radiation. There must be a secondary
hydrogen ionization front associated with the Veil and on the
far side (away from the observer and closer to θ
1
Ori C) of the
Veil. Abel et al. (2006) identify emission lines probably arising
from the Veil’s ionization front.
The radial velocities of the various emission and absorption
line components along the line of sight through the Trapezium
are summarized in Table 1.
2.3. H
2
CO is Seen in Absorption in Orion-S
An early study (Johnston et al. 1983)at16
resolution
detected H
2
CO in absorption against the radio continuum with
the location identified as Orion-S, a region with multiple known
molecular emission lines (O’Dell et al. 2008) and well-defined
bipolar molecular outflows in CO (Zapata et al. 2005) and SiO
(Zapata et al. 2006). A higher resolution (5.
1 × 7.
6) study
(Mangum et al. 1993) confirmed the presence of this absorption
feature and the results are shown in Figure 1 and Table 1.In
Figure 1 weseethattheH
2
CO absorption is distinct from the
bipolar outflows, the strongest IR and radio sources, and the
sources of the high-velocity optical outflows (O’Dell & Henney
2008). Mangum et al. (1993) point out that the presence of
H
2
CO in absorption means that the cloud containing it must
lie in front of the ionized gas, a position also taken in more
general form by Johnston et al. (1983) and Wilson et al. (2001).
Since H
2
CO can only exist in a cold dense gas that is optically
thick to FUV (and therefore also EUV) radiation, there will be a
corresponding high extinction at visual wavelengths. Therefore,
we have looked for optical extinction associated with this feature
with the results described in the next section.
2.4. Anomalies in the Extinction in the Orion-S Region
The optical appearance of the Huygens region is strongly
affected by extinction occurring within the Veil. The clearest
example is the Dark Bay feature to the east-northeast of the
Trapezium. This extinction generally decreases away from the
Dark Bay as the line of sight is moved to the southwest.
The extinction has been derived in several fashions. In slit
spectroscopy, the common approach has been to compare the
flux ratios of the strongest Balmer series lines with the ratios
expected from theoretical predictions calculated for the local
conditions (primarily the electron temperature). A good example
of this is the study of Baldwin et al. (1991), who obtained
sample spectra along a well-defined east–west path beginning
No. 1, 2009 THREE-DIMENSIONAL STRUCTURE OF INNER ORION NEBULA 369
Figure 1. This 233
× 219
image is composed of a mosaic of WFPC2 images (O’Dell & Wong 1996) with F502N ([O III]) as blue, F656N (Hα) as green, and F658N
([N
II]) as red. North is up and the labels along the edge depict the right ascension beyond 5:30:00 and the declination south of −5:20:00 (2000). Major outflow features
are labeled in addition to objects discussed in the text. Throughout this paper a position-based designation is used (O’Dell & Wen 1994) except for large individual
features such as Herbig Haro objects. The white dashed ellipse encloses the smaller features collectively discussed in the text as the Orion-S feature. The red lines
and circles represent strong IR H
2
features (Kaify et al. 2000; Stanke et al. 2002) not associated with the BN–KL deeply embedded sources. The strong dark red/blue
contoured lines indicate CO outflow (Zapata et al. 2005) and the pastel-colored pink/light-blue contoured lines indicate SiO outflow (Zapata et al. 2006). The orange
contours are H
2
CO absorption features (Mangum et al. 1993)atthelevelof−80 K km s
−1
(heavier) and −50 K km s
−1
(lighter). The green contours of increasing
thickness depict regions of the difference of extinction (c
DIF
Hβ
as defined in Section 2.4) of 0.1, 0.2, and 0.3. The sharp boundary of the more southerly c
DIF
Hβ
excess
feature is the result of reaching the edge of the WFPC2 field of view. The region most likely to contain the sources of the high-velocity optical outflows isshownasa
dark dashed ellipse (O’Dell & Henney 2008). The open squares indicate the positions of H
2
O maser sources (Gaume et al. 1998). The point sources within the dashed
outline are coded by the shortest wavelength of their detection, with filled white squares indicating the positions of radio-only visible sources (Zapata et al. 2004a,
2004b, 2005), red squares the positions of sources seen only in the mid-IR (Smith et al. 2004; Robberto et al. 2005), and filled orange circles the positions of stars in
the near-IR catalog of Hillenbrand & Carpenter (2000). The white contours show the 350 μm emission in this area (Houde et al. 2004) in units of Jy per 12
beam.
The irregular dashed white line indicates the field where c
Hβ
was determined both by the radio/optical and optical line ratio methods.
about 30
west of θ
1
Ori C and derived the extinction by
comparing Paschen lines with Hγ . The highest spatial resolution
study is that of O’Dell et al. (2003), where calibrated HST
WFPC2 Hα and Hβ images from a single WFPC2 pointing to
the southwest of the Trapezium were compared with theory.
Pixel-by-pixel extinction corrections were obtained as part of a
study of electron-temperature fluctuations. The widest field-of-
view high-resolution determination of the extinction was that of
O’Dell & Yusef-Zadeh (2000) who compared the Hα surface
brightness of a mosaic of Gaussian blurred HST WFPC2 images
with VLA 20 cm images obtained at 1.
7 resolution. In all these
studies similar extinction curves were used (Costero & Peimbert
