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arXiv:astro�ph/0604328
v1
14
Apr
2006
Astronomy & Astrophysics manuscript no. 5068 c
# ESO 2006
April 17, 2006
Outflows from the high�mass protostars NGC 7538 IRS1/2
observed with bispectrum speckle interferometry
Signatures of flow precession
S. Kraus 1 , Y. Balega 2 , M. Elitzur 3 , K.�H. Hofmann 1 , M. Meyer 4 , Th. Preibisch 1 , A. Rosen 1 , D. Schertl 1 ,
G. Weigelt 1 , and E. T. Young 4
1 Max Planck Institut f�ur Radioastronomie, Auf dem H�ugel 69, 53121 Bonn, Germany
2 Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Zelenchuk region, Karachai�Cherkesia, 357147, Russia
3 Department of Physics & Astronomy, University of Kentucky, Lexington, KY 40506, USA
4 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received February 22, 2006; accepted April 10, 2006
ABSTRACT
Context. NGC 7538 IRS1 is a high�mass (30 M # ) protostar with a CO outflow, an associated ultracompact H II region, and a linear methanol
maser structure, which might trace a Keplerian�rotating circumstellar disk. The directions of the various associated axes are misaligned with
each other.
Aims. We investigate the near�infrared morphology of the source to clarify the relations among the various axes.
Methods. K'�band bispectrum speckle interferometry was performed at two 6�meter�class telescopes---the BTA 6m telescope and the 6.5m
MMT. Complementary IRAC images from the Spitzer Space Telescope Archive were used to relate the structures detected with the outflow at
larger scales.
Results. High�dynamic range images show fan�shaped outflow structure in which we detect 18 stars and several blobs of diffuse emission. We
interpret the misalignment of various outflow axes in the context of a disk precession model, including numerical hydrodynamic simulations
of the molecular emission. The precession period is # 280 years and its half�opening angle is # 40 # . A possible triggering mechanism is non�
coplanar tidal interaction of an (undiscovered) close companion with the circumbinary protostellar disk. Our observations resolve the nearby
massive protostar NGC 7538 IRS2 as a close binary with separation of 195 mas. We find indications for shock interaction between the outflow
activities in IRS1 and IRS2. Finally, we find prominent sites of star formation at the interface between two bubble�like structures in NGC 7538,
suggestive of a triggered star formation scenario.
Conclusions. Indications of outflow precession have been discovered to date in a number of massive protostars, all with large precession
angles (# 20--45 # ). This might explain the difference between the outflow widths in low� and high�mass stars and add support to a common
collimation mechanism.
Key words. stars: formation -- stars: individual: NGC 7538 IRS1, NGC 7538 IRS2 -- techniques: bispectrum speckle interferometry, interfer�
ometric
1. Introduction
Protostellar disks and outflows are essential constituents of
the star formation process. For high�mass protostellar objects
(HMPOs), direct evidence for the presence of compact cir�
cumstellar disks is still rare, whereas outflows seem to be
omnipresent in the high�mass star forming regions. Outflows
remove not only angular momentum from the infalling mat�
ter, but also help to overcome the radiation pressure limit to
protostellar accretion, by carving out optically thin cavities
along which the radiation pressure can escape (Krumholz et al.
2005).
Send offprint requests to: skraus@mpifr�bonn.mpg.de
How outflows are collimated is a matter of ongoing de�
bate and may depend on the stellar mass of the outflow�
driving source. One of the arguments in support of this con�
clusion is that outflows from high�mass stars appear less col�
limated than the outflows and jets from their low�mass coun�
terparts (Wu et al. 2004). Therefore, it has been suggested that
outflows from HMPOs might be driven by strong stellar winds,
lacking a recollimation mechanism. Since HMPOs typically
form in dense clusters, another possibility is confusion by the
presence of multiple collimated outflows.
However, since there is evidence that the binary frequency
is significantly higher for high�mass than for low�mass stars
(e.g., Preibisch et al. 1999), another possibility is that out�

2 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
flows from HMPOs simply appear wider, assuming they un�
dertake precession. A few cases where outflow precession
have been proposed for HMPO outflows (e.g. Shepherd et al.
2000; Weigelt et al. 2002, 2005) show precession angles of
# 20 to 45 # ; considerably wider than the jet precession an�
gles of typically just a few degrees observed towards low�
mass stars (Terquem et al. 1999). This is in agreement with the
general picture that high�mass stars form at high stellar den�
sity sites and therefore experience strong tidal interaction from
close companions and stellar encounters.
The detection of precessing jet�driven outflows from
HMPOs adds support to the hypothesis of a common forma�
tion mechanism for outflows from low to high�mass stars.
Furthermore, jet precession carries information about the
accretion properties of the driving source and, simultaneously,
about the kinematics and stellar population within its closest
vicinity, yielding a unique insight into the crowded places
where high�mass star formation occurs.
In this paper, we report another potential case of outflow
precession concerning the outflow from the high�mass (30 M # ,
Pestalozzi et al. 2004) protostellar object NGC 7538 IRS1.
We obtained bispectrum speckle interferometry of IRS1
and IRS2, which provides us with the spatial resolution to
study the inner parts of the outflow, detecting filigreed fine
structure within the flow. Information about even smaller
scales is provided by the intriguing methanol maser feature,
which was detected at the position of this infrared source
and which was modeled successfully as a protostellar disk
in Keplerian rotation (Pestalozzi et al. 2004). To search for
outflow tracers on larger scales, we also present archival
Spitzer/IRAC images. In addition, this allows us to relate the
sources studied with bispectrum speckle interferometry with
the overall star forming region and we find new hints for
triggered star formation in this region.
1.1. Previous studies of NGC 7538
The NGC 7538 molecular cloud is located in the Cas
OB2 association in the Perseus spiral arm at a distance of
# 2.8 kpc (Blitz et al. 1982). Several authors noted that
NGC 7538 might present a case of triggered or induced star
formation since it shows ongoing star formation at various
evolutionary stages, apparently arranged in a northwest (most
developed) to southeast (youngest evolutionary stage) gradi�
ent (McCaughrean et al. 1991).
At optical wavelengths, the appearance of the region is
dominated by diffuse H II emission, which extends several ar�
cminutes from the southeast to the northwest (Lynds & O'Neil
1986). In 1974, Wynn�Williams, Becklin, & Neugebauer de�
tected eleven infrared sources (IRS1�11) in the NGC 7538 re�
gion, wherein IRS1--3 are located on the southeast�corner of
the fan�shaped H II emission in a small cluster of OB�stars.
IRS1 is the brightest NIR source within this cluster and is em�
bedded within an ultracompact (UC) H II region whose size
was estimated to be # 0. ## 4 (n e # 10 5 cm -3 , measured in
5 and 15 GHz CO continuum, Campbell 1984). The spectral
type was estimated to be O7 (Akabane & Kuno 2005), which
implies a luminosity # 9.6 � 10 4 L # . VLA observations with
a resolution down to 0. ## 1 (=180 AU) also revealed a double�
peaked structure of ionized gas within the UC core (peaks sep�
arated by # 0. ## 2), which was interpreted as a disk collimating
a north�south�oriented outflow (Campbell 1984; Gaume et al.
1995). This interpretation is also supported by the detec�
tion of elongation of the dust�emitting region at mid�infrared
(MIR) wavelengths (5 �m: Hackwell et al. 1982; 11.7 �m and
18.3 �m: De Buizer & Minier 2005) and imaging studies per�
formed in the sub�millimeter continuum (350 �m, 450 �m,
800 �m, 850 �m, 1.3 mm: Sandell & Sievers 2004, show�
ing an elliptical source with a size of # 11. ##
6 � 7. ##
6 along
PA 1 # -80 #
) and CO line emission (Scoville et al. 1986, show�
ing a disk�like structure extending # 22 ##
in the east�west direc�
tion). Also, polarization measurements of the infrared emission
around IRS1 can be construed in favor of the disk interpreta�
tion (Dyck & Capps 1978; Tamura et al. 1991). Kawabe et al.
(1992) carried out interferometric CS (J=2 # 1) observations
and found a ring�like structure, which they interpret as a nearly
face�on protostellar disk of dense molecular gas.
Further evidence for outflow activity was found
by Gaume et al. (1995), who measured the profile of the
H66# recombination line and derived high velocities of
250 km s -1 , indicating a strong stellar outflow from IRS1.
CO (J=1 # 0) spectral line mapping showed a bipolar
flow (Fischer et al. 1985). The mass outflow rate �
M outflow from
IRS1 was estimated to be # 5.4 � 10 -3 M # yr -1 (Davis et al.
1998). Interferometric observations by Scoville et al. (1986,
beam size 7 ## ) show that the blue and red�shifted lobes are
separated by 28 ## with a position angle of -45 # , and IRS1 is
located on this axis just between the lobes of this high�velocity
(-76 to -37 km s -1 ) CO outflow. In comparing the data ob�
tained with various beam sizes (Campbell 1984; Kameya et al.
1989), these seem to indicate a change in the position angle
of the flow direction at different spatial scales, ranging from
PA # 0 # at 0. ## 3, PA # -25 # at 2 ## , PA # -35 # at 7 ## , to PA -40 #
at 16 ## .
Within the immediate (# 0. ## 5) vicinity of IRS1, a
large variety of masers has been discovered, including
OH (Dickel et al. 1982), H 2 CO (formaldehyde, Rots et al.
1981; Hoffman et al. 2003), NH 3 (ammonia, Madden et al.
1986), CH 3 OH (methanol, five features A, B, C, D, E
were detected at 6.7 and 12.2 GHz: Menten et al. 1986;
Minier et al. 1998, 2000), 15 NH 3 (Johnston et al. 1989), and
H 2 O (Kameya et al. 1990). Some of the masers show only
vague signs for a systematic alignment within linear ( 15 NH 3 ,
PA # -60 # ) or ring�like structures (H 2 O, methanol�maser fea�
ture E). However, the methanol�maser feature A represents one
of the most convincing cases of systematic alignment, in both
linear spatial arrangement (PA # -62 # ) and well�defined veloc�
ity gradient, observed to date in any maser source. The qualita�
tive interpretation of this structure as an edge�on circumstellar
disk (Minier et al. 1998) was later confirmed by the detailed
1 Following the convention, we measure the position angle (PA)
from north to east.

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 3
modeling of Pestalozzi et al. (2004), which showed that the
alignment in the position--line�of�sight (LOS) velocity diagram
of maser feature A can be modeled accurately assuming a pro�
tostellar disk with Keplerian rotation.
Aiming for a more complete picture, several authors (e.g.
Minier et al. 1998; De Buizer & Minier 2005) also tried to in�
corporate the presence of methanol maser features B, C, D,
and E in the circumstellar disk model for feature A and inter�
preted them as part of an outflow which is oriented perpedicu�
lar to feature A. Since these maser features are southwards of
the putative circumstellar disk, it remains unclear why they ap�
pear blue�shifted with respect to feature A (Minier et al. 1998),
whereas the southern lobe of the CO�outflow is red�shifted.
Besides the circumstellar disk interpretation for the origin
of the maser feature Amentioned above, an alternative scenario
was proposed by De Buizer & Minier (2005), who suggested
that feature A might trace the walls of an outflow cavity.
The region was also intensively observed in the infrared.
Survey images of the infrared continuum emission were pre�
sented by Campbell & Persson (1988, H, K) and Ojha et al.
(2004, J, H, K s ) and showed diffuse emission, which ex�
tends from the IRS1--3 cluster in a fan�shaped structure to�
wards the northeast and north, approximately tracing the op�
tical H II region. The northeast border of this NIR emitting re�
gion also appears very pronounced in the continuum�subtracted
H 2 2.122 �m maps by Davis et al. (1998), possibly tracing
the illuminated surfaces of nearby molecular clouds or the in�
ner walls of a vast outflow cavity. Furthermore, Davis et al.
(1998) discovered two bowshock�shaped structures, centered
roughly on the IRS1--3 cluster and orientated again along the
northwest--southeast direction (PA # -30 # ) in H 2 2.122 �m.
With imaging at arcsecond resolution and the use of sev�
eral spectral filters (J, H, K, [Fe II] 1.65 �m, Br# 2.165 �m,
H 2 2.122 �m, and 3.29 �m), Bloomer et al. (1998) attempted
to identify the source and mechanism of the outflow. Based
on a cometary�shaped morphology in the [Fe II] line images
and shell�like rings observed in the J, H, and K�bands, these
authors propose a stellar wind bowshock model in which the
motion of IRS2 relative to the molecular cloud produces the
diffuse NIR emission within the vicinity of the IRS1--3 cluster.
The first K�band speckle images, taken with the 3.5 m�
telescope on Calar Alto were presented by Alvarez et al. (2004)
and showed substructure in the vicinity of IRS1; namely, two
strong blobs (A, PA # -45 # ; B, PA # -70 # ), a diffuse emission
feature (C, PA # 0 # ) as well as several faint point�like sources
(a� f ).
2. Observations
2.1. Bispectrum speckle interferometry
The first set of observations was performed on 2002�09�24
using the 6.0 m BTA (Big Telescope Alt�azimuthal) tele�
scope of the Special Astrophysical Observatory located on
Mt. Pastukhov in Russia. Additional data were gathered 2004�
12�20 with the MMT (Multiple Mirror Telescope) on Mt.
Hopkins in Arizona, which harbors a 6.5 m primary mirror.
As detector, we used at both telescopes one 512�512 pixel
quadrant of the Rockwell HAWAII array in our speckle cam�
era. All observations were carried out using a K'�band fil�
ter centered on the wavelength 2.12 �m with a bandwidth
of 0.21 �m. During the BTA observation run, we recorded
420 speckle interferograms on NGC 7538 IRS1 and 400 in�
terferograms on the unresolved star BSD 19�901 in order to
compensate for the atmospheric speckle transfer function. The
speckle interferograms of both objects were taken with an ex�
posure time of 360 ms per frame. For the MMT observa�
tions, the star 2MASS 23134580+6124049 was used for the
calibration and 120 (200) frames were recorded on the target
(calibrator) with an 800 ms exposure time. The modulus of
the Fourier transform of the object (visibility) was obtained
with the speckle interferometry method (Labeyrie 1970). For
image reconstruction we used the bispectrum speckle inter�
ferometry method (Weigelt 1977, Weigelt & Wirnitzer 1983,
Lohmann et al. 1983, Hofmann & Weigelt 1986). With pixel
sizes of 27.0 mas (BTA) and 28.7 mas (MMT) on the sky, the
reconstructed images possess fields of views of 13. ## 8 (BTA)
and 13. ## 1 (MMT), respectively.
We found that the BTA data allows the highest spatial res�
olution (and is therefore perfectly suited for the identification
of point�sources within the field), whereas the image recon�
structed from the MMT data allows a high dynamic range in
the diffuse emission. Therefore, we show the diffuse emission
within an image of moderate resolution (reconstructed from
MMT data, see Figure 1a) and perform point�source identifi�
cations within the higher resolution image reconstructed from
BTA data (Figure 1b). In order to distinguish point�sources and
diffuse structures reliably, we reconstructed images of various
resolutions (146 mas, 97 mas, 72 mas) and carefully exam�
ined changes in the peak brightness of the detected features.
Whereas for point�sources the peak brightness increases sys�
tematically, it stays constant or decreases for diffuse structures.
To perform an absolute calibration of the astrometry in our
images, we measured the position of IRS1 and IRS2 in the Two
Micron All Sky Survey (2MASS) K s Atlas images and use the
determined absolute positions as reference for our astrometry.
We estimate that the accuracy reached in the relative astrometry
is # 0. ## 1. The absolute calibration introduces further errors (#
0. ## 2).
2.2. Spitzer/IRAC Archive data
In order to relate our high�resolution images with the mor�
phology of the NGC 7538 molecular cloud at large scales,
we examined archival 3.6, 4.5, 5.8, and 8.0 �m images (PI:
G. G. Fazio), taken with the Infrared Array Camera (IRAC,
Fazio et al. 2004) on the Spitzer Space Telescope. The four
bands are recorded simultaneously using two InSb (3.6 �m,
4.5 �m) and two Si:As (5.8 �m, 8.0 �m) detectors. The central
wavelengths and bandwidths of the IRAC bands (Hora et al.
2004) are 3.56 �m (## = 0.75 �m), 4.52 �m (## = 1.01 �m),
5.73 �m (## = 1.42 �m), and 7.91 �m (## = 2.93 �m).
Each image consists of 256 � 256 pixels, corresponding to
a # 5 # � 5 # field�of�view on the sky. The data used include
48 Spitzer pointings taken on 2003 December 23 in the High

4 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Fig. 1. Bispectrum speckle images
(K # �band) reconstructed from data
taken with a) the 6.5m MMT and
b) the 6m BTA telescope. To show
the weak emission features, the in�
tensity of IRS1 was clipped to
2% of the total flux. Within the
high�resolution image (b), speckle�
noise artifacts appear around IRS1
(marked with a circle). These weak
features represent small distortions
of the point�spread�function (PSF)
on the 1%�level and do not influ�
ence the reliability of the identifi�
cation of point sources within the
image. The absolute coordinates of
IRS1 are # = 23 h 13 m 45. s 35 and
# = +61 #
28 # 10. ## 84 (J2000, de�
termined from 2MASS, accuracy #
0. ## 5).

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 5
Dynamic Range (HDR) mode. In HDR mode, for each point�
ing, images are taken with two exposure times (0.6 s and 12 s)
in order to record both bright and faint structures. However,
the two brightest sources, IRS1 and IRS9, are saturated even
within the 0.6 s exposure.
We used the mopex software (2005�09�05 version), released
by the Spitzer Science Center (SSC), to process both the long
and short exposure images. Beside the basic calibration steps
applied by the Basic Calibrated Data (BCD) pipeline (S11.0.2),
we performed Radhit detection, artifact masking, and pointing
refinement. Finally we generated a mosaic in which the satu�
rated pixels of the long exposure image were replaced by the
corresponding pixels of the 0.6 s exposure. The optical design
of IRAC induces a shift of # 6. # 8 between the 3.6/5.8 �m and
4.5/8.0 �m pointings, leaving an overlap of 5. # 1 between all four
bands.
In Figure 2, color composites of the 3.6/4.5/5.8 �m and
4.5/5.8/8.0 �m band images are shown.
The diffuse emission in three of the four IRAC bands
is dominated by Polycyclic Aromatic Hydrocarbons (PAHs,
Churchwell et al. 2004), which trace the border of regions
excited by the UV photons from HMPOs particularly well.
Contributions are also expected from several vibrational lev�
els of H 2 (Smith & Rosen 2005b), atomic lines, CO vibrational
bands, and thermal dust grain emission.
3. Results
3.1. Bispectrum speckle interferometry: Small�scale
structures around IRS1/2
3.1.1. IRS1 Airy disk elongation and diffuse
emission
In our speckle images, the Airy disk of IRS1 itself appears
asymmetric, being more extended towards the northwest di�
rection (PA # -70 # , see Figure 1b and inset in the lower left
of Figure 3). In the same direction (PA # -60 # ), we find two
strong blobs (A, B + B # ) of diffuse emission at separations of
#1 ## and 2 ## . These blobs and additional diffuse emission seem
to form a conical (fan�shaped) region with a 90 # opening angle
extending from IRS1 towards the northwest.
A careful examination of the power spectrum of IRS1 has
shown that the detected asymmetry of IRS1 is not caused by a
companion, but seems to represent diffuse emission. Therefore,
we can rule out a close binary system of similar�brightness
components down to the diffraction limit of # 70 mas. For the
case of a binary system with components of significantly dif�
ferent brightnesses, we can put upper limits on the brightness
of the hypothetical companion as a function of the projected
binary separation (see Figure 4).
The PA of the elongation of the Airy disk is similar to the
PA of K' blobs A, B, and B # . Another strong feature (C) can
be seen towards PA# 0 # . The blobs seem to be connected by
a bridge of diffuse emission extending from feature B to C.
Overall, the diffuse emission seems to form a fan�shaped re�
gion which is extending from IRS1 towards the northwest with
an opening angle of nearly 90 # . We identified some further fea�
Fig. 4. By measuring the speckle noise around the PSF of IRS1,
we can rule out binarity of IRS1 on a 3#�level as a function of
apparent separation and intensity ratio.
tures and list their position angles and separations in Table 1.
The directions, which were reported for various outflow trac�
ers, are also listed in this table and illustrated in Figure 5.
Our features A, B + B # , and C appear to coincide with the
features A, B, and C identified by Alvarez et al. (2004). A com�
parison suggests that features A + B + B # , C, D correspond to
the peaks 1W, 1N, and 1NW in Tamura et al. (1991).
3.1.2. Binarity of NGC 7538 IRS2
IRS2 appears resolved as a close binary system. Using an im�
age reconstructed from BTA data with a spatial resolution of
80 mas (see inset in the upper�left corner of Figure 3), we de�
termined the separation to be 195 mas and found a PA of -123 #
for the 1. m 9 fainter companion (2002�09�24). We designate the
brighter component in the K'�band as IRS2a and the fainter as
IRS2b.
3.1.3. Detection of fainter cluster members
Besides IRS1 and IRS2a/b, we were able to identify 18 ad�
ditional fainter point�like sources (a�r) within the BTA image,
whose positions and K'�band magnitudes are listed in Table 2.
In order to test whether these sources are physically re�
lated to NGC 7538, one can compare the stellar number density
for the brightness range 11. m 0 to 12. m 0 in our speckle image,
N Speckle = 18/128 arcsec 2 # 2.1 � 10 6 /deg 2 , with the num�
ber expected from the cumulative K�band luminosity function
(KLF) of the NGC 7538 field 2 N field # 1.8�10 3 /deg 2 . Although
these number densities were obtained with different spatial res�
olution, the clear over�density of stars in our speckle image is
significant and we conclude that most of the detected stars are
likely members of the NGC 7538 star forming region. When
2 The K�band luminosity function by Balog et al. (2004) for the
whole NGC 7538 region, corrected with the on�cluster KLF, and cu�
mulated for the magnitude range 11. m 0 to 12. m 0 was used.

6 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Table 1. NGC 7538 IRS1 outflow directions reported for various tracers.
Tracer Structure Beam Size Scale PA of Dynamical Ref. Comments
[ ## ] [ ## ] outflow Age f (Velocities in [km s -1 ])
direction [ # ] [10 3 yrs]
Methanol masers 0.03 c +19 a,d -- [8] see Fig. 3
1.0 cm cont. inner core # 0. ## 5 0.13 0.4 +0 a < 0.03 [9] t e = 0.15 yrs e
1.3 cm cont. inner core # 0. ## 4 0.11 0.4 +0 a < 0.02 [5]
6.0 cm cont. inner core # 0. ## 75 0.35 0.4 +0 < 0.04 [1]
1.0 cm cont. outer core # 0. ## 5 0.13 1.0 �25 a > 0.03 [9] t e = 0.3 yrs e
1.3 cm cont. outer core # 0. ## 4 0.11 1.0 �25 a > 0.02 [5]
6.0 cm cont. outer core # 0. ## 75 0.35 1.0 �15 ... �20 > 0.04 [1]
6.0 cm cont. outer core # 0. ## 5 0.4 1.0 �25 a > 0.03 [9]
MIR 11.7�m IRS1 Elongation 0.43 3 �45 0.16 [10] see Fig. 6k
MIR 18.3�m IRS1 Elongation 0.54 4 �45 0.16 [10] see Fig. 6l
NIR K'�band IRS1 Elongation 0.3 0.6 �78 0.03 -- see Fig. 3
NIR K'�band feature A 0.3 1.6 �65 0.08 -- see Fig. 3
NIR K'�band feature F 0.3 2.1 �33 0.11 -- see Fig. 3
NIR K'�band feature B 0.3 3.0 �39 0.16 -- see Fig. 3
NIR K'�band feature B' 0.3 3.3 �57 0.18 -- see Fig. 3
NIR K'�band feature C 0.3 4.8 +6 0.25 -- see Fig. 3
NIR K'�band feature E 0.3 6.2 �20 0.33 -- see Fig. 3
NIR K'�band feature D 0.3 7.4 +10 0.39 -- see Fig. 3
NIR K'�band eastern wall 0.3 -- +25 -- -- see Fig. 3
NIR K'�band western wall 0.3 -- �65 -- -- see Fig. 3
[Fe II] 1.65 �m 1 15 a N�S a 0.8 [7] around IRS2; see Fig. 6i
H 2 northern bow 30 a N�S 1.5 [6]
H 2 1 27 a �25 a 1.4 [7] shell�like structure; see Fig. 6i
H 2 southern bow 45 a 155 a 2.3 [6] see Fig. 6h
IRAC bands southern bow 1 40 145 2.2 -- see Fig. 6a...e
CO low velocities 7 5 a,b E�W 0.9 [3] -11 < �
V b < -6; 2 < �
V r < 9; see Fig. 6g
CO high velocities 7 15 b �35 15 [3] -17 < �
V b < -11; 9 < �
V r < 15; see Fig. 6g
CO 34 18 b �50 a 15 [2] -24 < �
V b < -8; 9 < �
V r < 22
CO 16 13 a,b �40 14 [4] -14 < �
V b < -9; 11 < �
V r < 16; see Fig. 6 f
CO 45 12 a,b �50 a 10 [3] -24 < �
V b < -8; 9 < �
V r < 22
Note -- �
V r and �
V b are measured relative to the velocity of methanol maser feature A ( �
V = V - 56.25 km s -1 )
a Estimated from figures presented within the reference paper; therefore, with limited accuracy.
b The half�separation between the red� and blue�shifted CO lobe is given.
c For VLBI observations, we give the estimated error on the absolute position of the measured maser spots.
d The expected outflow direction is given; i.e., perpendicular to the measured orientation of maser feature A.
e Electron recombination time given in the reference paper.
Assuming an outflow velocity of 250 km s -1 , which was measured by Gaume et al. (1995) within the H66# recombination line. For the
CO emission, we also use the measured CO outflow velocity and provide the corresponding dynamical age in brackets. Since all velocities are
measured along LOS, this timescale can only provide upper limits.
References: [1] Campbell (1984); [2] Fischer et al. (1985); [3] Scoville et al. (1986); [4] Kameya et al. (1989); [5] Gaume et al. (1995);
[6] Davis et al. (1998); [7] Bloomer et al. (1998); [8] Minier et al. (2000); [9] Franco�Hern�andez & Rodr��guez (2004);
[10] De Buizer & Minier (2005)
using the KLF for the IRS 1�3 region instead of the whole
NGC 7538 field, the stellar over�density in our speckle image
becomes even more evident (N IRS1-3 cluster # 1.4 � 10 3 /deg 2 ).
Since these stars are about 5 to 6 magnitudes fainter than IRS1,
they are likely to be part of the associated intermediate mass
stellar population.
The arrangement of the stars within the fan�shaped nebula
does not appear to be random, but follows the S�structure of the
diffuse blobs (see Figure 3). Most remarkable, more than half
of the stars seem to be aligned in a chain reaching from feature
B to C (PA # 45 # ). Within the diffuse blobs close to IRS1 (A,
B, B #
), no stars were found, whereas embedded in blob C, three
stars could be detected.
3.2. Spitzer/IRAC: Morphology at large spatial
scales
Imaging of NGC 7538 at optical wavelengths showed that
diffuse emission can mainly be found in the vicinity of
IRS5 (Lynds & O'Neil 1986). At near�infrared (NIR) wave�
lengths (Ojha et al. 2004), a diffuse structure can be found ex�

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 7
Table 2. Point sources identified in our speckle images. For details about IRS1 and the binary system IRS2a/b, we refer to the
text. We identify components a to f with the stars already discovered in the image by Alvarez et al. (2004).
Name RA (J2000) a DEC (J2000) a K' Magnitude b Comment
a 23 h 13 m 45. s 33 61 # 28 # 21. ## 02 10. m 94
b 44. s 81 10. ## 63 11. m 29
c 44. s 95 12. ## 33 11. m 46
d 45. s 11 09. ## 69 11. m 73
e 45. s 37 15. ## 12 11. m 77 embedded in feature C
f 45. s 34 15. ## 93 11. m 73 embedded in feature C
g 45. s 36 14. ## 80 11. m 73 embedded in feature C
h 45. s 31 14. ## 72 11. m 86
i 45. s 29 13. ## 74 11. m 71
j 45. s 27 13. ## 25 11. m 73
k 45. s 25 13. ## 05 11. m 81
l 45. s 24 13. ## 28 11. m 83
m 45. s 22 12. ## 71 11. m 71
n 45. s 36 12. ## 99 11. m 75
o 45. s 50 10. ## 50 11. m 60
p 45. s 29 18. ## 42 11. m 73
q 45. s 16 19. ## 69 11. m 77
r 45. s 33 08. ## 48 11. m 71
a For the astrometry, the relative errors are of the order of 0. ## 1. The absolute calibration using the reference position of IRS1 in 2MASS
introduces further errors (0. ## 2).
b The photometry was done relative to IRS1 with an uncertainty of 0. m 3. For the conversion to absolute photometry, we assumed a IRS1
magnitude of 8. m 9 (Ojha et al. 2004).
tending from the IRS1�3 cluster towards the northwest with the
strongest emission around IRS5.
The Spitzer/IRAC images reveal a more complex, bubble�
like structure (see Figure 2b, c), whose western border is
formed by a pronounced ridge�like filament connecting IRS1�
3 with IRS4 and reaching up to IRS5 (see Figure 2a). At the
western border of the bubble a wide conical structure is lo�
cated, with a vertex on 2MASS 23135808+6130484. Another
conical structure can be detected close to the northern bor�
der of the bubble. Several other outflow structures can be
found in the IRAC image; most noteworthy, the unidirec�
tional reflection nebula around 2MASS 23144651+6129397,
2MASS 23131691+6129076, and 2MASS 23130929+6128184
(see Figure 2a). The sources 2MASS 23131660+6128017 and
2MASS 23133184+6125161 appear to be embedded in a shell�
like cloud.
Besides the position of the strongest near�infrared sources,
Figure 2 shows also the position of the submillimeter
(450 �m, 850 �m) clumps reported by Reid & Wilson (2005).
These clumps trace the filaments and knots of the bubble,
which can be seen in the IRAC images, very well. Besides
this, the submillimeter clumps suggest another bubble�like
structure to the southwest of IRS4 (see also the images
in Reid & Wilson 2005). This bubble seems to be invisible
at near� and mid�infrared wavelengths, although several NIR
sources are located on its border (2MASS 23130929+6128184,
2MASS 23133184+6125161).
As already pointed out by Reid & Wilson (2005), it is in�
teresting to compare the position of the detected H 2 O masers
with the position of the centers of high�mass star formation in
the region and to find agreement in many cases (IRS1--3, IRS9,
NGC 7538S). However, as can be seen in Figure 2, for four
locations of H 2 O masers, no MIR counterpart can be found in
the IRAC images (the detection limits for point sources in the
four IRAC bands are roughly 3.6, 5.3, 31, and 34 �Jy for the
IRAC bands at 3.6, 4.5, 5.8, and 8 �m assuming medium sky
background).
4. Discussion
4.1. Nature of the observed K'�band emission
In the wavelength range of the K'�band filter (# 0 = 2.12 �m,
## = 0.21 �m), we record not only continuum radiation
(e.g. scattered light, thermal dust emission, stellar contin�
uum emission), but also line emission (e.g. H 2 ). However,
both Bloomer et al. (1998, see Figure 6) and Davis et al.
(1998) did not detect significant amounts of H 2 emis�
sion around IRS1. Furthermore, deep optical imaging by
Elsaesser et al. (1982) and Campbell & Persson (1988) reveal a
weak optical source offset 2. ## 2 north of the radio source IRS1.
The latter authors argue that the strong extinction (A V = 13)
derived for IRS1 makes it highly unlikely that this optical emis�
sion is connected to IRS1 itself but that it most likely represents
scattered light. The measured offset suggests that the faint op�
tical source should be associated with blobs A and B in our im�
ages, making scattering the most likely radiation mechanism
for the detected K'�band emission. This conclusion is also sup�
ported by polarization measurements (Dyck & Lonsdale 1979)
which show a strong polarization of the 2 �m emission, trac�
ing either scattered light or light transmitted through aligned

8 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
grains. Henceforth we presume continuum to be the most im�
portant contributor to the detected emission.
4.2. Methanol maser feature A: Protostellar disk or
outflow?
We note that the 2MASS position of IRS1
(# = 23 h 13 m 45. s 35, # = +61 #
28 #
10. ##
8, J2000) and the po�
sition of the methanol maser feature A (# = 23 h 13 m 45. s 364,
# = +61 # 28 # 10. ## 55, J2000) reported by Minier et al. (2000)
coincide within the errors 3 . Therefore, the methanol masers
and the outflow driving source are likely causually connected,
however a random coincidental alignment cannot be ruled out.
Since methanol masers can trace both protostellar disks
and outflows, it is not a priori clear how the linear alignment
of the methanol maser feature A and the observed veloc�
ity gradient should be interpreted. For IRS1, both claims
have been made (Pestalozzi et al. 2004; De Buizer & Minier
2005). However, detailed modeling has provided strong
quantitative support for the disk interpretation but is still
missing for the outflow interpretation. Furthermore, a study
by Pestallozi et al. (in prep.) suggests that simple outflow
geometries cannot explain the observed properties of feature A.
A major difference between these two scenarios is the
orientation of the disk associated with the outflow driving
source: Whereas in the disk scenario the methanol masers
are lined up within the disk plane (PA # -62 # ), the outflow
scenario suggests an orientation of the disk plane perpendicular
to the maser alignment (PA # +28 # ). The observed asym�
metry in our NIR speckle images, as well as the elongation
of the emission observed in the 11.7 and 18.3 �m images
by De Buizer & Minier (2005), can be explained within both
scenarios:
Scenario A: If maser feature A traces an outflow cavity, the
detected asymmetry might simply reflect the innermost walls
of this cavity (oriented northwest), whereas the southeastern
cavity of a presumably bipolar outflow might be hidden due to
inclination effects.
Scenario B: Alternatively, if the masers trace an edge�on
circumstellar disk, the asymmetry of the infrared emission
could trace the western wall of an outflow cavity with a wide
half�opening angle. The asymmetry cannot be attributed to the
disk itself because the detection of stellar radiation scattered
off the disk surface at such a large distance is highly unlikely.
For completeness, we also mention the interpretation
by Kameya et al. (1989), who attributed the change between
the direction observed in the UCH II region (PA 0 # ) and the
high�velocity CO flow (PA -60 # ) to flow deflection, either by
large�scale magnetic fields or due to density gradients.
3 The astrometric accuracy of the 2MASS
catalogue was reported to be # 0. ## 15 (see
http://ipac.caltech.edu/2mass/releases/allsky/doc/expl�sup.html).
We proceed now to discuss both scenarios within an
outflow�cavity model (Sec. 4.3) and a precessing jet model
(Sec. 4.4), incorporating the large amounts of evidence col�
lected by various authors over the last three decades.
4.3. Scenario A: Outflow cavity model
Since the intensity of the diffuse emission in our images seems
to decrease with distance from IRS1 and the vertex of the fan�
shaped region appears centered on IRS1, we cannot support
the hypothesis by Bloomer et al. (1998), who identified IRS2
as the likely source of the diffuse NIR emission. Instead, the
observed fan�shaped region can be interpreted as a cavity that
was formed by outflow activity from IRS1. Because the walls
of the fan�shaped structure are well�defined, we can measure
the opening angle of the proposed outflow cavity from the east�
ern wall (PA 25 # ) to blob A (PA -65 # ), obtaining a wide total
opening angle of 90 # .
The unidirectional asymmetry of IRS1 in the NIR and MIR
images (see Figure 6) is naturally explained in this context as
scattered light from the inner (< 1500 AU) walls of the cavity.
This scenario is also consistent with the southeast--northwest
orientation of the CO outflow, aligned roughly parallel to the
methanol masers (PA # -62 # ). Blobs A, B, B # are located
within the same direction and might resemble either clumps in
the cavity or recent ejecta from the outflow. The various blobs
might also indicate the presence of several outflows.
In order to resolve the misalignment of the radio�continuum
core with respect to the other outflow tracers, it was proposed
that the radio�continuum emission might arise from a photoe�
vaporated disk wind (Lugo et al. 2004).
However, as noted above, the methanol maser feature A
lacks a quantitative modeling up to now.
4.4. Scenario B: Precessing jet model
4.4.1. Constraints from the methanol maser disk
The circumstellar�disk modeling presented by Pestalozzi et al.
(2004) reproduces the observational data for maser feature A in
minute detail. Assuming a central mass of 30M # and Keplerian
rotation, this model confines the inner (r i # 290 AU) and
outer (r o # 750 AU) radii of the disk (these radii scale as
(M/30M # ) -1/3
with the central mass M). The model does not
set strict constraints on the inclination and orientation of the
disk on the sky.
An uncertainty in the disk inclination arises from the as�
sumption that methanol is formed within a surface layer of the
disk from photoevaporation of H 2 O. The midplane of the disk
might therefore be inclined within certain limits. A distinct in�
clination is suggested by the fact that the NIR/MIR continuum
emission, as well as the H 2 shock tracer line emission, appears
more pronounced towards the north than towards to south (see
Figures 1 and 6h). An inclination of the northern outflow to�
wards us is also indicated by CO outflow observations (e.g.
Kameya et al. 1989), which show the blueshifted CO lobe of
IRS1 towards the northwest (see Figures 6 f & 6g).

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 9
The disk orientation on the sky can only be constrained by
the maser observations with a limited accuracy since the masers
only trace a narrow latitudinal arc of the disk, missing potential
disk warping. Nevertheless, it is still reasonable to identify the
disk orientation with the linearly�aligned feature A.
4.4.2. Indications for disk and jet precession
Assuming that the alignment of the masers is representative of
the orientation of the disk midplane (i.e. assuming disk warp�
ing is negligible 4 ), it is evident that the direction perpendic�
ular to the disk plane (PA +19 # , the expected outflow direc�
tion) is significantly misaligned from the axes of the bipolar
CO outflow and the NIR fan�shaped region (PA # -20 # ; illus�
trated within Figure 3). Also, the observed bending (Campbell
1984; Gaume et al. 1995) in the radio continuum could indi�
cate a change in the outflow direction. Whereas the inner core
(# 0. ## 5) is orientated along PA # 0 # , the outer core (0. ## 5 --
1. ##
0) bends slightly towards the west (PA # -25 #
). This might
indicate that the outflow changed its direction by this amount
within the times needed by the jet to propagate the appropriate
projected distances (# 25 and # 50 years).
The bending detectable in the UC H II region on scales of
# 1. ## 0 seems to continue at larger scales within the morphology
observed in our speckle images, suggesting an S�shaped fine�
structure of the diffuse emission extending from IRS1 initially
towards the northwest and further out towards north. The blobs
A, B, and B # observed close to IRS1 (PA # -60 # ) might rep�
resent the most recent ejecta, whereas the weak features which
appear further away in our images (C, D) might trace earlier
epochs of the history of the outflow.
Based on these indications, we suggest a disk and jet pre�
cession model. The fan�shaped diffuse emission in which the
S�structure is embedded can be explained as scattered light
from the walls of an outflow cavity, which was cleared by the
proposed wandering jet.
The western wall of this wide, carved�out outflow cavity
might appear within our NIR and the MIR images as an elon�
gation of IRS1. Since this elongation extends mainly towards
the northwest, there must be an additional reason why the west�
ern wall of this cavity appears more prominent than the east�
ern wall. A possible explanation might be shock excitation of
the western wall, which would cool through emission in shock
tracer lines like H 2 , which is contributing to the recorded K'�
band.
Assuming the precession period derived in Sec. 4.4.3, the
outflow (which currently points towards PA # 19 # ) would have
excited the western wall of this cavity # 140 years ago, which
corresponds roughly to the H 2 radiative cooling time.
The arrangement of the fainter cluster members embedded
within the diffuse emission can be understood in this context,
too: Taking into account that IRS1 is still deeply embedded in
its natal circumstellar cloud, the jet would have cleared the en�
4 Interestingly, the converse assumption (that disk warping is non�
negligible) implies disk precession as well, as the jet would be
launched perpendicular to the warped surface of the inner part of the
disk.
velope along its wandering path. The decreasing column den�
sity results in lower extinction along the jet's path, revealing
the fainter stars which likely formed in the vicinity of IRS1.
The fainter stars might therefore be detectable only in those
regions where the precessing jet reduces the extinction suffi�
ciently. Within the blobs closest to IRS1, stars may be unde�
tectable because of either inclination effects or confusion with
the significantly higher surface brightness of blobs IRS1 A, B,
and B # (limiting the sensitivity to detect point sources), or be�
cause of the high density of the outflowing material itself, pro�
viding intrinsic extinction.
The outflow tracers observed at rather large scales (CO,
H 2 , see Figure 6) are oriented roughly in the same direc�
tion as the NIR fan�shaped structure. The CO channel maps
by Scoville et al. (1986, Figure 6g) suggest a change in the
orientation of the CO outflow lobes for low and high veloci�
ties. Whereas the low velocity CO outflow is oriented along the
east�west direction, the high velocity lobes are oriented along
PA # -35 # . As CO traces material swept�up by the outflow and
has a relatively long cooling time (of the order of 10 4 yrs), the
different orientations observed at low� and high velocities are
more difficult to interpret.
Finally, we speculate that the precession model might also
explain why the velocities of the methanol maser features B, C,
D, and E are in the same range as the velocities of the CO out�
flow (De Buizer & Minier 2005), but show opposite signs for
the LOS velocity with respect to feature A (maser features B,
C, D, and E are blue�shifted, whereas the southern CO lobe is
red�shifted). Assuming precession, the CO outflow would trace
the average outflow direction around the precession axis (with
the southern axis oriented away from the observer), whereas the
methanol masers might trace clumps very close to the source,
which were excited more recently when the southern part of the
outflow was pointing towards the observer 5 .
In general, precession can explain the change in the flow
orientation, but potential alternative explanations include den�
sity gradients in the surroundings of IRS1, the presence of mul�
tiple outflows, and flow deflection.
4.4.3. Analytic precession model
In order to get a rough estimate for the precession parameters,
we employ a simple analytical model with constant radial out�
flow speed v. On the radial motion we superpose a precession
with period P prec , leading to the wave number
k =
2#
vP prec
; (1)
by the time that ejected material travels a distance r from the
source, the direction of the jet axis changes by the angle kr.
To describe the jet propagation in three dimensions we in�
troduce a Cartesian coordinate system centered on IRS1 whose
5 This is consistent with the precession parameters determined in
Sect. 4.4.3, where we find that the half�opening angle of the precession
# is larger than the inclination of the precession axis with respect to
the plane of the sky .

10 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
z�axis is along the line of sight (see Figure 7, top). The preces�
sion axis is in the y - z plane inclined by angle to the plane of
the sky 6 , and the jet axis makes an angle # with it. For counter�
clockwise 7 precession, the coordinates of material at distance r
from the origin are
#
# # # # # # # # #
x
y
z
#
# # # # # # # # #
= r �
#
# # # # # # # # #
sin # cos #
cos # cos + sin # sin # sin
- cos # sin + sin # sin # cos
#
# # # # # # # # #
(2)
where # = # 0 + kr is the jet's azimuthal angle from the x�
axis. The initial phase # 0 can be taken as 0 # since the direction
perpendicular to the methanol feature A seems to coincide with
the eastern wall of the outflow cavity. The PA # of the y�axis
can be set from the average angle of the fan�shaped region in
the speckle images as # = -30 # . Using v = 250 km s -1 , as
measured by Gaume et al. (1995) within the H66# recombina�
tion line, leaves as free parameters P prec , #, , and the sense
of rotation. Trying to fit the orientation of the maser disk, the
orientation of the UC H II region, and the position of the NIR
blobs with these parameters simultaneously, we find reason�
able agreement with a precession period P prec = 280�10 yrs, a
precession angle # = 40 #
�3 #
, a counter�clockwise sense of ro�
tation, and small inclination = 0 #
� 10 # . At larger inclination
angles (# 10 # ) loops start to appear, significantly degrading the
agreement. In Figure 7, we show the projected trajectory of the
proposed wandering jet with the thick line, whereas the thin
lines give the path obtained with a variation of �5 # in # and ,
resembling the finite width of the flow.
The analytic model presented in this section might suffice
in order to get a rough estimate of the precession parameters,
although it does not take into account the interaction of the flow
with the ambient medium nor the excitation and cooling of the
ambient material.
These parameters can be used to predict how the orienta�
tion of the methanol maser disk changes with time. Using the
PA at the phase # 0 = 0 # as reference, one expects that the PA
changes only marginally (less than 1 # ) within 10 yrs. A much
more significant change of 10 # (20 # ) would be expected after
36 yrs (50 yrs), which would be detectable with future VLBI
observations.
4.4.4. Numerical molecular hydrodynamic
simulations
A large number of studies about the structure and evolu�
tion of precessing protostellar jets can be found in litera�
ture (e.g. Raga & Biro 1993; V�olker et al. 1999; Raga et al.
2004; Rosen & Smith 2004; Smith & Rosen 2005a), although
most of these studies focus on jets from low�mass stars with
rather narrow precession angles. As the number of simula�
tions carried out for wide precession angles is much more
limited (e.g. Cliffe et al. 1996), we performed a new hydrody�
namic simulation. Besides the general morphology, we aim for
6 Positive values of indicate an inclination out of the plane of the
sky towards the observer.
7 For the sense of rotation, we follow the convention that counter�
clockwise rotation (as measured from the source along the precession
axis) corresponds to a positive sign of the phase #.
comparing the position of the newly discovered fainter stars
with the column density variations caused by a precessing jet,
which was beyond the scope of earlier studies.
We use the version of the ZEUS�3D code as modified
by Smith & Rosen (2003), which includes some molecular
cooling and chemistry, as well as the ability to follow the
molecular (H 2 ) fraction. The large precession envisioned for
the flow associated with NGC 7538 IRS1 requires that the sim�
ulation be performed on a very wide computational grid. Due to
computational limits, we were restrained to use for this simula�
tion a 3D Cartesian grid of 275 zones in each direction, where
each zone spans 2 � 10 14 cm in each direction. This grid bal�
ances the desire for some spatial resolution of the flow with
the ability to simulate a sufficiently large part of the observed
flow associated with NGC 7538 IRS1. Still, the total grid size
(# 0.018 pc) is smaller than the projected distance between IRS
1 and K'�band feature C.
Owing to the rather small physical size of the grid, we have
chosen a nominal speed of 150 km s -1 , reduced from the in�
ferred value of 250 km s -1 for this source. The flow is pre�
cessed with a nearly 30 # precession angle, with the amplitude
of the radial components of the velocity 0.55 of that of the axial
component. The precession has a period of 120 years, which
leads to 1.25 cycles during a grid crossing time. The flow is
also pulsed, with a 30% amplitude and a 30 year period. This
short period assists in the reproduction of the multiple knots
of K'�band emission near NGC 7538 IRS1. The jet flow also
is sheared at the inlet, with the velocity at the jet radius 0.7
that of the jet center. We have chosen a jet number density of
10 5 hydrogenic nuclei cm -3 , while the ambient density is 10 4 .
The simulated jet radius is 4.0 � 10 15 cm (20 zones). Thus, the
time�averaged mass flux is 2.6 � 10 -6 M # yr -1
, which is three
orders of magnitude lower than the value as determined for the
CO outflow (Davis et al. 1998). Similar calculations of the mo�
mentum flux and kinetic energy flux, or mechanical luminosity,
yield values of 3.8 � 10 -4 km s -1 M # yr -1 and 4.7 L # , respec�
tively.
After the convolution with a PSF resembling the resolution
obtained in real observations the H 2 emission in our simula�
tion shows a morphology which is similar to the one seen in
the K'�band speckle image. In particular, the simulations might
also explain features D and E as associated with the proposed
precessing jet (compare Figure 8a with Figure 3). The simula�
tions also show that the CO emission, which can be expected
for a precessing jet at larger distances from the driving source,
appears very smooth, which is also in accordance with the CO
observations made for NGC 7538 IRS1.
We note that the fainter stars e to n reported in Sec. 3.1.3
are located in the region where the column density in our pre�
cessing jet simulation appears particularly low (see Figure 8c,
left column), supporting the scenario proposed in Sec. 4.4.2.
4.4.5. Possible precession mechanisms
Several mechanisms have been proposed which can cause
jet bending or jet precession, although most of them were
established for low� and intermediate stars and can cause

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 11
precession angles of only a few degrees (Fendt & Zinnecker
1998; Eisl�offel & Mundt 1997). For the case of high�mass
stars and larger precession angles (# # 40 # ), Shepherd et al.
(2000) summarized the three most promising concepts that
could induce precession into circumstellar disks. We discuss
how well these mechanisms can explain the observations of
IRS1. For all cases, it is assumed that the outflow is launched
close to the center of the disk and that a precession of the
inner parts of the disk will translate into a precession of the
collimated flow (Bate et al. 2000).
1. Radiative�induced warping: Armitage & Pringle (1997)
suggested that geometrically thin, optically thick accretion
disks can become unstable to warping if the incident radiation
from the stellar source is strong enough. As this warping
instability is expected to occur only at disk radii larger than a
critical radius R crit , we can estimate whether radiative�induced
disk warping is expected at the inner part of the IRS1 disk.
Using a stellar mass of M # 30 M # (Pestalozzi et al. 2004),
a mass accretion rate of the order of the mass outflow rate
�
M acc # �
M outflow # 5.4�10 -3 M # yr -1 (Davis et al. 1998), and a
luminosity L # 9.6 � 10 4 L # (Akabane & Kuno 2005), we use
equation 5 by Armitage & Pringle (1997) and the assumptions
listed in Shepherd et al. (2000) and obtain a critical radius
R crit # 200 pc. Since this is far beyond the inner edge of the
disk where the jet collimation is expected to happen, it is
very unlikely that the radiation emitted by the star or due to
accretion causes any noticeable warping within the disk.
2. Anisotropic accretion events: The impact/merging of
(low mass) condensations can change the orientation of the
disk angular momentum vector. In such a dramatic event,
angular momentum can be transferred from the impactor
onto the accretion disk, potentially resulting in a net torque
in the rotation of the disk. To estimate the precession angle,
which could result from anisotropic accretion, very detailed
assumptions about the disk, the impacting condensation, and
their kinematics must be made. Since no data is available to
estimate these quantities, we refer to the example computed
by Shepherd et al. (2000) and note that in extreme cases, such
an accretion event could cause a sufficiently large precession
angle in the case of NGC 7538 IRS1 as well. However, in this
scenario one would expect rather sudden changes in the jet
direction rather than a smooth precession.
3. Tidal interactions with a companion: Warping and pre�
cession of the disk could be caused by tidal interactions with
one or more companions on non�coplanar orbits. We assume
the simplest case of a binary: with stellar masses M p (primary)
and M s (secondary), an orbit with inclination i with respect to
the disk plane, and a semimajor axis a. The mass ratio shall
be denoted q = M p /M s and will be assumed as unity. Our
observations place an upper limit on the separation of such a
companion (see Figure 4). Two cases can be considered:
3a) circumprimary disk (a > r o ): Because tidal torques
would truncate the disk at about 0.3 times the binary separa�
tion (Lubow & Ogilvie 2000), we obtain a lower limit for the
binary separation (for a circular orbit), namely, a > 2 500 AU.
However, a binary with such a large separation would be not
suited to explain the observations since the orbital period
would be > 2 � 10 4 yrs (M p + M s = 30 M # ), implying a disk
precession rate of > 4 � 10 5 yrs (Bate et al. 2000). Assuming
an extreme eccentricity might yield a short precession period
of the order of 10 2 yrs but implies strong, periodic interactions
between the companion and the disk during each perihelion
passage. As this would quickly distort and truncate the disk, we
see this assumption contradicts the methanol maser structure,
which suggests a smooth extension of the methanol layer from
# 290 AU to # 750 AU.
3b) circumbinary disk (a < r i ): Smoothed particle hy�
drodynamic simulations by Larwood & Papaloizou (1997)
showed that a binary on a non�coplanar orbit with large
inclination i could cause strong quasi�rigid body precession
of the circumbinary disk (for q > 10) and strong warping,
especially on the inner edge of the disk (q # 1). The same
authors report that the disk precession frequency # prec should
be lower than the orbital frequency of the binary # binary .
To make an order�of�magnitude estimation for the orbital
period that would be expected for this hypothetical IRS1
binary system, one can assume # prec # # binary /20 (Bate et al.
2000) to obtain P binary # 14 yrs for the binary period, cor�
responding to a separation of a binary # 19 AU (# 7 mas).
This binary separation then puts a lower limit on the radius
of the inner edge of the circumstellar disk. As this scenario
can trigger the fast disk precession without truncating the
extended disk structure traced by the methanol masers, we
consider a circumbinary disk as the most plausible explanation.
4.5. The IRS2 companion and flow interaction with
the IRS2 UC H II region
The spectral type of IRS2 was estimated to be
O4.5 (Akabane & Kuno 2005), corresponding to a lumi�
nosity of # 6.4 � 10 5 L # . Using the measured K # �band flux
ratio, one can make rough estimates for the spectral type of the
two components reported in Sec. 3.1.2. By assuming the total
luminosity is attributed only to the two components, we obtain
a spectral type of O5 for IRS2a and O9 for IRS2b (using the
OB star luminosities from Vacca et al. 1996).
Within our images, the wide�opening angle outflow cone
from IRS1 seems to extend well out to IRS2. This offers an
explanation for the shock tracer line emitting region that was
imaged around IRS2 (Bloomer et al. 1998, see Figure 6i). The
bowshock�like morphology of the [Fe II] and H 2 emission sug�
gests that the shock is excited from the south (which is roughly
the direction towards IRS1). In the direction opposite IRS1,
the [Fe II] and H 2 emission even shows a cavity�like structure,
which also appears in the 6 cm�radio continuum. Bloomer et al.
(1998) suggested a stellar wind bowshock scenario, in which
IRS2 moves with a speed of #10 km s -1 towards the south�
west through the ambient molecular cloud. We note that the
morphology could also be explained by interaction between the
IRS1 outflow and IRS2 outflows. Based on its young age, IRS2

12 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
might also launch a powerful wind itself, causing the distinct
shock zone which appears within the shock tracer emission (see
Figure 6i) and which is also detectable in our K'�band image
(arc�like morphology between features G and H).
4.6. Outflow structures from IRS1 at larger spatial
scales
Figure 6a to d shows mosaics of the vicinity of IRS1 in the
four IRAC bands. Although IRS1 and IRS2 appear saturated in
these images (shown in logarithmic scaling) and banding (verti�
cal and horizontal stripes produced by IRS1 and IRS9) appears
especially in the 5.8 and 8.0 �m images, structures potentially
related to IRS1 can be observed. About 40 ## towards the south�
east of IRS1, a bowshock structure can be seen, which is also
present in the H 2 image by Davis et al. (1998). This bowshock
points in the same direction as the redshifted lobe of the CO
outflow (see Figure 6f ) and just opposite to the outflow direc�
tion identified in our speckle image at small scales. Thus, it is
possible that this bow traces the southeastern part of the IRS1
outflow. In our speckle image, the inner part of this southeast�
ern outflow is not visible; likely a result of strong intervening
extinction.
Furthermore, it is interesting to note that the ``ridge'' con�
necting the IRS1--3 cluster with IRS4 and IRS5 follows the
western wall of the outflow direction identified in our speckle
image (see top of Figure 6). It is possible that the total extent
of the IRS1 outflow also reaches much further northwest than
the structure seen in the speckle image, contributing to the ex�
citation of the western part of the bubble seen in the IRAC
bands and the shocks in the H 2 image by Davis et al. (1998,
see Figure 6h).
5. Evidence for triggered star formation in the
NGC 7538 star forming region
It has been proposed by many authors that star formation
seems to propagate southeastwards throughout the NGC 7538
complex (Werner et al. 1979; McCaughrean et al. 1991;
Ojha et al. 2004). This is indicated by the spatial arrangement
of the members of this star forming region, which also seems
to agree with the expected evolutionary sequence: starting
about 3 # northwest of IRS1, O stars located in the H II region
represent the most developed evolutionary state, followed by
the IRS1--3 cluster and their associated UCH II regions, with
the compact reflection nebula around IRS9 representing the
youngest member of this star formation site. In agreement
with this picture, Balog et al. (2004) measured the reddening
of stars throughout NGC 7538 and found a gradient in red�
dening with the most heavily reddened sources in the southeast.
The presented IRAC images can also be interpreted in
support of this scenario since the ''ridge''�like feature connect�
ing IRS1�3, IRS4, IRS5 seems to trace the interface between
the northeastern bubble (visible at NIR/MIR wavelengths)
and the submillimeter bubble, which appears in the 450 �m
and 850 �m�maps by Reid & Wilson (2005, see Figure 2).
This suggests that in NGC 7538, star formation was triggered
by the compression of gas just at the interface layer of these
expanding bubbles, sequentially initiating the formation of the
observed chain of infrared sources.
Ojha et al. (2004) suggested that IRS6, the most luminous
source in the NGC 7538 region, might be the main excit�
ing source responsible for the optical H II region. Inspecting
the IRAC color composites, this scenario is supported by the
morphology of the bright, curved structure west of IRS6. In
the IRAC 8 �m band (red in Figure 2c), this structure ap�
pears particularly prominent. As it is known that emission
in this IRAC band is often associated with PAHs, this sug�
gests that this region is illuminated by strong UV radia�
tion from IRS6. Other features, such as the conical structure
around 2MASS 23135808+6130484and the structure northeast
of IRS7, also show a symmetry towards IRS6.
6. Summary and Conclusions
Bispectrum speckle interferometry and archival Spitzer/IRAC
imaging of the massive protostars NGC 7538 IRS1/2 and their
vicinity are presented. We summarize our results as follows:
1. The clumpy, fan�shaped structure seen in our speckle im�
ages most likely traces recent outflow activity from IRS1,
consistent with the direction of the blue�shifted lobe of the
known CO outflow. A bowshock structure noticeable in
the IRAC images # 40 ## southeast of IRS1 suggests that
the total extent of the outflow might be several parsecs.
The outflow might have also contributed to shaping and
exciting the bubble�like structure, which is prominent in
all four IRAC bands (although contributions from several
other sources, especially IRS6, are also evident).
2. A companion around the high�mass star NGC 7538 IRS2
was discovered. Furthermore, we see indications for inter�
actions between the IRS1 flow and outflows or stellar winds
from IRS2 (nebulosity surrounding IRS2).
3. A jet precession model seems suitable to describe the fea�
tures observed within our NIR images, simultaneously ex�
plaining the misalignment between the putative methanol
maser disk, the UCH II region, and the outflow tracers de�
tected at larger scales (CO). A simple analytic precession
model was used to extract order�of�magnitude estimates for
the precession parameters. Using these we estimate tidal in�
teraction of a close binary system with a circumbinary disk
as the most plausible gyroscopic mechanism, which is trig�
gering the precession.
4. The presented molecular hydrodynamic simulations can re�
produce some of the fine�structure observed in our NIR
images and indicate that the arrangement of the detected
fainter stars might be explained as a column�density effect,
caused by the proposed precessing jet.
5. The prominent sites of ongoing high�mass star formation
in NGC 7538 seem to be located just at the interface be�
tween two bubble�like structures --- one is visible in the
presented IRAC images, the other traced by submillimeter
observations. The gas compression caused by the expan�

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 13
sion of these bubbles might have triggered star formation
in this region.
While it is well established that the outflows of HMPOs
generally appear less collimated than those of their low�mass
counterparts, the recent discovery of evidence for outflow
precession for an increasing number of massive YSOs might
indicate a common launching mechanism for all outflow
driving sources of all stellar masses. The observed widening in
HMPO outflows might be due to selection effects (Shepherd
2005) and/or precession of a collimated jet. The large preces�
sion angles reported for IRAS 20126+4104 (Shepherd et al.
2000), S140 IRS1 (Weigelt et al. 2002), IRAS
23151+5912 (Weigelt et al. 2005), and now NGC 7538 IRS1
(this paper) might point towards a rather dramatic precession
mechanism, maybe the presence of very close, high�mass
companions on non�coplanar orbits.
We strongly encourage further observations of IRS1, es�
pecially to detect potential companions either by near�infrared
long�baseline interferometry or radial velocity measurements.
Acknowledgements. We thank all the participants of the NGC 7538
collaboration for very fruitful discussions that contributed to the
achievement of this paper. The collaboration consists of Roy Booth,
John Conway, James De Buizer, Moshe Elitzur, Stefan Kraus, Vincent
Minier, Michele Pestalozzi, and Gerd Weigelt.
We also acknowledge the BTA and MMT staff for their support of this
run, and D. Apai and I. Pascucci for assistance during the MMT ob�
servations.
SK was supported for this research through a fellowship from the
International Max Planck Research School (IMPRS) for Radio and
Infrared Astronomy at the University of Bonn.
The numerical hydrodynamic simulations were executed on the
Armagh SGI Origin 2000 computer (FORGE), acquired through the
Particle Physics and Astronomy Research Council (PPARC) JREI ini�
tiative with SGI participation.
This work is based in part on archival data obtained with the Spitzer
Space Telescope, which is operated by the Jet Propulsion Laboratory,
California Institute of Technology under a contract with NASA.
This publication makes use of data products from the Two Micron All
Sky Survey, which is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute
of Technology, funded by the National Aeronautics and Space
Administration and the National Science Foundation.
References
Akabane, K. & Kuno, N. 2005, A&A, 431, 183
Alvarez, C., Hoare, M., Glindemann, A., & Richichi, A. 2004,
A&A, 427, 505
Armitage, P. J. & Pringle, J. E. 1997, ApJ, 488, L47+
Balog, Z., Kenyon, S. J., Lada, E. A., et al. 2004, AJ, 128, 2942
Bate, M. R., Bonnell, I. A., Clarke, C. J., et al. 2000, MNRAS,
317, 773
Blitz, L., Fich, M., & Stark, A. A. 1982, ApJS, 49, 183
Bloomer, J. D., Watson, D. M., Pipher, J. L., et al. 1998, ApJ,
506, 727
Campbell, B. 1984, ApJ, 282, L27
Campbell, B. & Persson, S. E. 1988, AJ, 95, 1185
Churchwell, E., Whitney, B. A., Babler, B. L., et al. 2004,
ApJS, 154, 322
Cliffe, J. A., Frank, A., & Jones, T. W. 1996, MNRAS, 282,
1114
Davis, C. J., Moriarty�Schieven, G., Eisl�offel, J., Hoare, M. G.,
& Ray, T. P. 1998, AJ, 115, 1118
De Buizer, J. M. & Minier, V. 2005, ApJ, 628, L151
Dickel, H. R., Rots, A. H., Goss, W. M., & Forster, J. R. 1982,
MNRAS, 198, 265
Dyck, H. M. & Capps, R. W. 1978, ApJ, 220, L49
Dyck, H. M. & Lonsdale, C. J. 1979, AJ, 84, 1339
Eisl�offel, J. & Mundt, R. 1997, AJ, 114, 280
Elsaesser, H., Birkle, K., Eiroa, C., & Lenzen, R. 1982, A&A,
108, 274
Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154,
10
Fendt, C. & Zinnecker, H. 1998, A&A, 334, 750
Fischer, J., Sanders, D. B., Simon, M., & Solomon, P. M. 1985,
ApJ, 293, 508
Franco�Hern�andez, R. & Rodr��guez, L. F. 2004, ApJ, 604,
L105
Gaume, R. A., Goss, W. M., Dickel, H. R., Wilson, T. L., &
Johnston, K. J. 1995, ApJ, 438, 776
Hackwell, J. A., Grasdalen, G. L., & Gehrz, R. D. 1982, ApJ,
252, 250
Hoffman, I. M., Goss, W. M., Palmer, P., & Richards, A. M. S.
2003, ApJ, 598, 1061
Hofmann, K.�H. & Weigelt, G. 1986, A&A, 167, L15+
Hora, J. L., Fazio, G. G., Allen, L. E., et al. 2004, in Microwave
and Terahertz Photonics. Edited by Stohr, Andreas; Jager,
Dieter; Iezekiel, Stavros. Proceedings of the SPIE, Volume
5487, pp. 77�92 (2004)., ed. J. C. Mather, 77--92
Hutawarakorn, B. & Cohen, R. J. 2003, MNRAS, 345, 175
Johnston, K. J., Stolovy, S. R., Wilson, T. L., Henkel, C., &
Mauersberger, R. 1989, ApJ, 343, L41
Kameya, O., Hasegawa, T. I., Hirano, N., Takakubo, K., &
Seki, M. 1989, ApJ, 339, 222
Kameya, O., Morita, K.�I., Kawabe, R., & Ishiguro, M. 1990,
ApJ, 355, 562
Kawabe, R., Suzuki, M., Hirano, N., et al. 1992, PASJ, 44, 435
Krumholz, M. R., McKee, C. F., & Klein, R. I. 2005, ApJ, 618,
L33
Labeyrie, A. 1970, A&A, 6, 85
Larwood, J. D. & Papaloizou, J. C. B. 1997, MNRAS, 285, 288
Lohmann, A. W., Weigelt, G., & Wirnitzer, B. 1983,
Appl. Opt., 22, 4028
Lubow, S. H. & Ogilvie, G. I. 2000, ApJ, 538, 326
Lugo, J., Lizano, S., & Garay, G. 2004, ApJ, 614, 807
Lynds, B. T. & O'Neil, E. J. 1986, ApJ, 306, 532
Madden, S. C., Irvine, W. M., Matthews, H. E., Brown, R. D.,
& Godfrey, P. D. 1986, ApJ, 300, L79
McCaughrean, M., Rayner, J., & Zinnecker, H. 1991, Memorie
della Societa Astronomica Italiana, 62, 715
Menten, K. M., Walmsley, C. M., Henkel, C., et al. 1986, A&A,
169, 271
Minier, V., Booth, R. S., & Conway, J. E. 1998, A&A, 336, L5
Minier, V., Booth, R. S., & Conway, J. E. 2000, A&A, 362,
1093
Ojha, D. K., Tamura, M., Nakajima, Y., et al. 2004, ApJ, 616,
1042

14 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Pestalozzi, M. R., Elitzur, M., Conway, J. E., & Booth, R. S.
2004, ApJ, 603, L113
Preibisch, T., Balega, Y., Hofmann, K.�H., Weigelt, G., &
Zinnecker, H. 1999, New Astronomy, 4, 531
Raga, A. C. & Biro, S. 1993, MNRAS, 264, 758
Raga, A. C., Riera, A., Masciadri, E., et al. 2004, AJ, 127, 1081
Reid, M. A. & Wilson, C. D. 2005, ApJ, 625, 891
Rosen, A. & Smith, M. D. 2004, MNRAS, 347, 1097
Rots, A. H., Dickel, H. R., Forster, J. R., & Goss, W. M. 1981,
ApJ, 245, L15
Sandell, G. & Sievers, A. 2004, ApJ, 600, 269
Scoville, N. Z., Sargent, A. I., Sanders, D. B., et al. 1986, ApJ,
303, 416
Shepherd, D. 2005, in IAU Symposium, 237--246
Shepherd, D. S., Yu, K. C., Bally, J., & Testi, L. 2000, ApJ,
535, 833
Smith, M. D. & Rosen, A. 2003, MNRAS, 339, 133
Smith, M. D. & Rosen, A. 2005a, MNRAS, 357, 579
Smith, M. D. & Rosen, A. 2005b, MNRAS, 357, 1370
Tamura, M., Gatley, I., Joyce, R. R., et al. 1991, ApJ, 378, 611
Terquem, C., Eisl�offel, J., Papaloizou, J. C. B., & Nelson, R. P.
1999, ApJ, 512, L131
Vacca, W. D., Garmany, C. D., & Shull, J. M. 1996, ApJ, 460,
914
V�olker, R., Smith, M. D., Suttner, G., & Yorke, H. W. 1999,
A&A, 343, 953
Weigelt, G., Balega, Y. Y., Preibisch, T., Schertl, D., & Smith,
M. D. 2002, A&A, 381, 905
Weigelt, G., Beuther, H., Hofmann, K. ., et al. 2005, ArXiv
Astrophysics e�prints
Weigelt, G. & Wirnitzer, B. 1983, Optics Letters, 8, 389
Weigelt, G. P. 1977, Optics Communications, 21, 55
Werner, M. W., Becklin, E. E., Gatley, I., et al. 1979, MNRAS,
188, 463
Wu, Y., Wei, Y., Zhao, M., et al. 2004, A&A, 426, 503
Wynn�Williams, C. G., Becklin, E. E., &Neugebauer, G. 1974,
ApJ, 187, 473
List of Objects
`NGC 7538' on page 1
`NGC 7538 IRS1' on page 1
`NGC 7538 IRS2' on page 1

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 15
Fig. 2. Figure a) shows the Spitzer/IRAC 4.5 �m image with the position of the infrared sources (IRS1 to 11 and
2MASS sources) and H 2 O masers marked. Furthermore, the position of the sub�millimeter (450 �m and 850 �m)
clumps reported by Reid & Wilson (2005) are shown. The position of the 2MASS sources 2MASS 23135808+6130484,
2MASS 23131660+6128017, 2MASS 23134351+6129372, 2MASS 23144651+6129397, 2MASS 23131691+6129076,
2MASS 23130929+6128184, and 2MASS 23133184+6125161 are labeled explicitly. Figures b) and c) show color�composites
produced with two triplets of the four IRAC bands. The intensity of each image was scaled logarithmically. North is up and east
to the left.

16 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Fig. 3. Bispectrum speckle image with identified point sources (triangles) atop marked. The astrometry for the point�sources was
performed using the high�resolution BTA image, whereas the image shown was reconstructed from MMT data. The contours
trace 0.25%, 0.5%, 0.75%, 1.0%, 1.25%, and 1.5% of the peak intensity. The inset on the upper left shows a reconstruction of
the vicinity of IRS2 using a resolution of 80 mas (BTA data). In the lower left, IRS1 is shown using a different color table,
emphasizing the elongation of the IRS1 Airy disk (MMT data) overplotted with the 15 GHz radio continuum (the contours
show �1, 1, 2.5, 5, 10, 20, ..., 90% of the peak flux) and the position of the OH (circles) and methanol (crosses) masers (image
from Hutawarakorn & Cohen 2003 using data from Gaume et al. 1995). In the lower right we show the integrated brightness of
the methanol masers as presented by Pestalozzi et al. (2004, contour levels of 1, 3, 5, 10, 30, 50, 70, and 90% of the peak flux
density are shown).

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 17
Fig. 5. Illustration showing the
outflow directions in the vari�
ous tracers. The CO contours
by Kameya et al. (1989, red and
blue) are overlaid on the H 2 map
(greyscale) by Davis et al. (1998).
The orientation of the conjectural
methanol maser disk (green), the
fan�shaped structure detected in
our K'�band image (orange), and
the averaged direction of H 2 (red
arcs) are shown schematically.
The arrows indicate the direction
prependicular to the alignment of
the methanol masers (green), the
orientation of the inner (< 0.5 ## )
and outer (> 0.5 ## ) core detected
in the 1.0, 1.3, and 6.0 cm radio
continuum (white), and the direc�
tion along which the IRS1 Airy
disk was found to be elongated
(MIR: De Buizer & Minier 2005;
NIR: this paper).

18 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Fig. 6. Mosaic showing the IRS1--3 cluster at various wavelengths. Beside the speckle K # �band image (also marked as red box)
and IRAC images, data from Scoville et al. (1986, CO), Kameya et al. (1989, CO), Bloomer et al. (1998, J, H, K, H 2 , [Fe II]),
Davis et al. (1998, H 2 ), and De Buizer & Minier (2005, 11.7 �m, 18.3 �m) was incorporated.

Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2 19
Fig. 7. Left: Illustration of the analytic precession model presented in Sec. 4.4.3. Right: MMT speckle image overplotted with
the trajectory of ejecta from a precessing outflow projected onto the plane of the sky (thick blue line) as described by the analytic
precession model. For the counter�clockwise precession, the parameters P prec = 280 yrs, # = 40 # , # = -30 # , = 0 # (precession
axis within the plane of the sky), and # 0 = 0 # were used. In order to simulate a finite collimation of the flow, we varied both
# and by �5 #
, yielding the trajectory given by the thin blue lines. The red contours show the 15 GHz radio continuum map
by Hutawarakorn & Cohen 2003 (using data by Gaume et al. 1995).

20 Kraus et al.: Outflows from the high�mass protostars NGC 7538 IRS1/2
Fig. 8. a), b): Synthetic images (H 2 1 # 0, 2 # 1; CO R(1), R(5)) from our ZEUS�3D molecular hydrodynamic simulation,
shown for two different projections. In Figure a) the images are also shown after a convolution with a Gaussian which roughly
resembles the resolution obtained in our speckle observation (Figure 3). Figure c) shows the total gas column density and the H 2
column density for the same projections. Finally, in d) we show channel maps of the CO outflow for four velocity bins. Each
velocity bin has a width of 5 km s -1 and the central velocity is given by the number on the left of each image (in km s -1 ). The
two numbers on the right of each image indicate the log of the maximum integrated luminosity in any single element in the image
(Top, in erg s -1 ) and the log of the total integrated luminosity in the entire velocity bin (Bottom).
The angle # gives the angle between the z axis and the LOS, corresponding to a rotation around the precession axis (x axis).
Please see the text (Sec. 4.4.4) for a description of the complete model parameters.