MID-INFRARED IMAGING OF ORION BN/KL WITH THE KECK I TELESCOPE
Daniel Y. Gezari1, William C. Danchi1, Lincoln Greenhill2, Eli Dwek1 and Frank Varosi3
1NASA/Goddard
Space Flight Center
2Harvard
Smithsonian Center for Astrophysics
3University
of Florida- Gainsville
INTRODUCTION:
We have made array camera images of the 15 x 20 arcsec field of view containing the
BN/KL infrared complex at eight mid-infrared wavelengths between 4.8 and 20.0 m
with ~0.3 arcsec spatial resolution. The observations show three times higher detail
than the best previous work (c.f., Gezari 1992, Gezari, Backman and Werner 1998).
The images were made at the Keck I telescope at Mauna Kea in December 1999,
October 2000, and December 2001, using the Keck LWS facility spectrograph in the
imaging mode, each with typically 1 minute of integration time. Each of the final
images was assembled from a dozen individual exposures using the MOSAIC image
processing software we developed at Goddard (Varosi and Gezari 1993;
http://astrolab.gsfc.nasa.gov/mosaic). Observing procedures and data reduction
techniques for high background array image data are discussed by Gezari et al.
(1992).
The final images are used to model the temperature, opacity and bolometric
luminosity distributions of the emitting dust in the Orion BN/KL infrared complex, as
well as the dust extinction in the line of sight.
Figure 3: 12.4 um image of BN/KL obtained Gezari’s array camera on the 3.0-meter
IRTF telescope, for comparison with the Keck image (Figure 2). Dramatic increase in
spatial detail can be seen, particularly in the vicinity of IRc2/IRc7 (see Figure 5).
DUST EMISSION-EXTINCTION MODEL:
The observed infrared intensity I from each source element of solid angle  with
source opacity S() total line of sight (LOS) extinction LOS() is
I = B(T) e-LOS() [1 - e-S() ]
= B(T) e-LOS() S()
for S() << 1
(1)
(2)
Dust opacities for the emitting and absorbing grains were calculated for grain radii a
ranging from 2 x 10-4 to 5 x 10-1 m, assuming grain size number distribution
n(a)  a-3.5 and characterized by optical constants from Draine and Lee (1984) and
Draine (1985). However, the source dust composition is a priori unknown, and could
be either silicon- or carbon-rich. The abundance ratio therefore must be chosen to
represent a range of mixtures between all-silicate and all-graphite grains, and the
parameters are varied to achieve the best fit to the observed spectra.
MODEL RESULTS:
Several new results emerge from the model calculations:
The compact IRc sources are all local peaks in the emitting dust temperature
distribution. Essentially all the compact IRc sources in the central parsec coincide
with local temperature enhancements in the model temperature distribution results.
Temperatures in the field range from about 200 - 1300 K (see Figure 8d).
The IRc source positions are local minima in the emitting dust opacity distribution
(Figure 8c). The emitting dust opacity differs dramatically from the temperature
distribution. The two significant peaks in dust emission opacity S() occur at BN
and KL. The opacity and temperature distributions are generally complementary.
Figure 4: Identifications of the prominent compact IRc sources (circle), near
infrared stars (asterisx) and radio continuum point sources (cross) superimposed
on the 20 um image by Gezari, Backman and Werner (1998).
Figure 1: 12.4 um image of BN/KL obtained with the Keck I telescope. Pixel size = 0.086 arcsec, resolution ~ 0.3 arcsec limited by
seeing and mosaicing errors. New detail is resolved here in the familiar sources IRc2, IRc7 and KL (IRc4) and “n” (see Figure 5).
Figure 2: Contour map of the BN/KL field shown in Figure 1. Image brightness contours are logarithmic and range from
0.1 – 150 janskys/arcsec2 between the sky noise level and the peak 12.4 um brightness at the BN Object. The newly
discovered mid-infrared companion object is seen prominently about 1.5”W and 0.8”S of BN (see also Danchi et al. 2003).
The extinction opacity distribution LOS() of cool absorbing grains in the line-ofsight (Figure 8b) shows more structure than the directly observed 9.8 m silicate
feature strength distribution. Typical extinction opacity at 12.4 m is about LOS ~ 2
across the extended ridge, peaking at LOS = 2.5 at IRc2 and “I”.
The model luminosity distribution (Figure 8a) is very similar to the mid-infrared
brightness distribution. The compact sources IRc2 and IRc7 are moderately
luminous,
(L ~ 500 L), and they make a significant contribution to the energy budget of
infrared complex. The luminosity over the extended BN/KL plateau is generally
about 100 L arcsec-2. The total luminosity integrated over the 15 x 25 arcsec region
modeled is about 104 L. This represents roughly half the total luminosity of the
entire BN/KL infrared complex. (These results are preliminary and will be revised as
part of an ongoing modeling effort with additional observational data).
DISCUSSION:
If an extraordinary compact object located near “I” was the dominant source of the
observed infrared luminosity of the complex, the temperature distribution should
show evidence for a centralized radiation field, and the temperature distribution
would be expected to decrease radially at least as T()  r-2. The influence of a
centrally concentrated luminosity source should be seen in the model results if such a
phenomenon exists. The modeled temperature distribution (Figure 6b) is quite
smooth, except at BN and near “I”. The influence of the luminous sources IRS3 and
IRS7 has not figured prominently in the generally held view of BN/KL.
Alternately, if embedded stellar luminosity sources were distributed within the dust
clouds they would be expected to produce local temperature "hot spots" at the stellar
source positions. The temperature distribution is also well correlated with the midinfrared brightness distribution, especially near the compact IRS sources, and the
luminosity distribution and the mid-infrared brightness distributions are nearly
identical. These results suggest the presence of imbedded, luminous objects at the
IRc source positions, which heat the extended dust clouds surrounding them. The
fact that the density of the emitting dust is anti-correlated with the mid-infrared
emission at the IRS source positions also supports this view.
Figure 5: Four individual “speckle” sub-images of IRc2, IRc7, “n” and KL (IRc4). These are
short (0.1 sec) exposures which freeze seeing motion and reveal additional detail which is not
seen when several such exposures are averaged in a longer integration to obtain a final image.
Figure 8(a): Modeled total source luminosity distribution, in
units of Larcsec-2. The peak luminosity occurs at BN (L = 3000
L). A luminous point source appears at the southern tip of IRc2
which is identified with compact radio continuum HII region “I”.
Figure 8(b): Modeled extinction opacity LOS in the line of
sight, including both circumstellar and interstellar
contributions. The extinction is fairly smooth, but shows
moderate enhancements associated with the bright sources.
Figure 8(c): Modeled source emission opacity S ranges between
0.1 – 1.3. The densest dust is at KL, a cool cloud which is
probably externally heated.
Figure 8(d): Modeled emitting dust temperature, ranging from
T = 100 – 1200 K. The dust temperature is generally
complementary to the source emission opacity. The hottest
peaks occur at BN and near IRc2, suggesting heating of the
dust by embedded luminous sources.
CONCLUSIONS:
From the mid-infrared array data and the model results we conclude that: 1) the
compact sources IRc2, IRc7, “n” and BN are all local hot spots in the BN/KL dust
clouds, 2) KL is a very dense, cool cloud which is probably not internally heated,
3) the local dust density peaks are not correlated with the IRc sources, suggesting that
the IRc sources must contain imbedded stars. 4) Compact HII region and radio
continuum source “I” near IRc2 and the BN Objects are the dominant luminosity
sources in the BN/KL infrared complex.
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Figure 6: Three-color composite near infrared image in J (blue),
FeII (green) and H2 (red) by Allen and Burton (1993) showing
BK/KL and the Trapezium cluster stars (center, white). The
region we imaged (Figure 1) is outlined by the black rectangle.
The yellow dot near the top of the rectangle is BN. The streamers
radiating away from a position near BN are purported to be the
remnants of an extraordinary explosive event.
Figure 7: Our 12.4 m image of the BN/KL region overlaid
with contours of ammonia (3,3) and point sources of 3.6 cm
continuum emission, water and OH maser emission, 3.8 m near
stars. The strongest NH3 emission coincides with the obscured
region just south of IRc2 and the ionizing radio continuum point
source - compact HII region “I”. Note the 3.6 cm continuum
point source near BN is at the same location as the new midinfrared companion to BN, source B2 (see Danchi et al. 2003).