MIDINFRARED IMAGING OF ORION BNKL WITH THE KECK I TELESCOPE

1
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.
• 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(?) ?
  (1)
• B?(T) e-?LOS(?)
  ?S(?)? for ?S(?) ltlt 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.

Figure 3 12.4 um image of BN/KL obtained
Gezaris 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).
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 1.5W and 0.8S
of BN (see also Danchi et al. 2003).
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.
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
mid-infrared companion to BN, source B2 (see
Danchi et al. 2003).
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.