Modeling of Deep Submillimeter Images of the Pre-protostellar

1
Modeling of Deep Submillimeter Images of the
Pre-protostellar
Core L1498
Miranda K. Nordhaus1,2, Yancy L. Shirley2, Neal
J. Evans II3, Jonathan M. C. Rawlings4
1 Rensselaer Polytechnic Institute, 2 NRAO, 3
University of Texas at Austin, 4 University
College London

• Introduction
• L1498 was one of the first dense cores to be
  initially identified kinematically as a cloud on
  the verge of collapse (Myers Benson 1983, Zhou
  et al. 1994). L1498 was later identified as a
  pre-protostellar core by its submillimeter dust
  emission and lack of an IRAS detection at 100 µm
  (Ward Thompson et al. 1994). Molecular line
  observations (Kuiper, Langer, Velusamy 1996)
  have shown dramatic evidence for the freezing of
  gas phase molecules onto dust grains (CS, CCS,
  etc.). As a result, molecular species cannot
  reliably trace the densities in the center of the
  core. However, optically thin dust emission at
  850 and 450 µm is a good tracer of temperature
  and density in the L1498 core. Knowledge of the
  temperature and density structure is essential to
  correct interpretation of chemical and kinematic
  models of L1498. Therefore, we have made deep
  submillimeter maps of L1498 at 850 and 450 µm
  using SCUBA, the Submillimeter Common User
  Bolometer Array, on the James Clerk Maxwell 15
  meter telescope.
• Observations
• L1498 was observed simultaneously at 850 and 450
  µm during the nights of August 29 and 30, 1998.
  A total of 50 jiggle maps were made toward the
  source. After careful reduction, the total on
  source integration time was 3.55 hours. Previous
  observations showed the source extended in a SE
  to NW direction. Therefore, the chop angle was
  set to a constant position angle of 20 with a
  120 chop throw to avoid chopping onto the
  source. It was also shown that the source
  extended beyond the field of view of one jiggle
  map, so the map was extended using 3 offset
  5-point maps, each with 30 spacing. The final
  map spans 140 in right ascension and
  declination.
• Each map was reduced using the standard SURF,
  SCUBA User Reduction Facility, reduction routines
  (Jenness Lightfoot 1997). Each 64-point jiggle
  map was corrected for chop throw, extinction, and
  sky noise. The 450 µm images were reduced using
  a set intensity scale to ensure consistent
  identification of corrupted maps and proper
  addition of all jiggle maps. The telescope
  pointing was checked every hour, with the largest
  shift being 2.5. Therefore, the standard
  pointing offsets from the five-point maps were
  used to shift and add together the individual
  maps.
• Skydips were performed every hour during both
  nights. These were compared to the opacity
  measured at 225 GHz (tcso) and 350 µm every 10
  minutes from tippers located at the Caltech
  Submillimeter Observatory. Using the
  relationship derived by

• Images
• The peak signal-to-noise was 18 and 13 for the
  850 and 450 µm maps, respectively. We show the
  contour maps with contours spaced by 3s for both
  images. The 850 and 450 µm images appear similar
  with an elongated core with flat intensity
  profiles. A 1200 µm continuum map smoothed to
  20 resolution (M. Tafalla, priv. comm.) is also
  shown for comparison. The FWHM intensity contour
  of the 1200, 850, 450 µm maps are very similar
  and well fit by an ellipse with a major axis of
  197, minor axis of 108, and a position angle of
  122. Furthermore, the 1200 and 850 µm maps are
  non-axisymmetric with a sharper gradient in the
  intensity along the northeast edge compared to
  the southwest ridge. The 1200 µm image was
  observed with MAMBO on IRAM-30m (courtesy M.
  Tafalla, priv. comm.).

• Modeling
• The observations were modeled using a
  one-dimensional radiative transfer code that
  self-consistently calculates the temperature
  distribution for a given density distribution,
  dust opacity (OH5), and ISRF. The code also
  simulates scattering of short wavelength light
  (uv to near-IR). The calculated temperature
  distribution is used to construct intensity
  profiles at 850 and 450 mm and the complete SED
  (170 mm to 1.2 mm) by convolving the model
  intensity profiles with the measured beamshape
  and simulating chopping. The modeling procedure
  is fully described in Evans et al. (2001).
• Evans et al. found that Bonnor-Ebert spheres
  with central densities of 1 x 105 to 106 cm-3 fit
  the observed structure in 3 PPCs (L1512, L1544,
  L1689B) therefore we used Bonnor-Ebert (BE)
  sphere to model the structure of L1498.

The reduced chi-squared intensity profiles for a
grid of models with varying central densities and
outer radii are shown to the right. The deep
blue contour indicate c2r between 0,2
increasing by 2. The low density BE-sphere of
1x104 cm-3 is the best fit to the azimuthally
averaged radial profiles with no strong
constraint on the outer radius. If instead, we
use the sector-averaged profiles, a central
density of 3x104 is also a good fit to the
intensity profiles. The 1x104 model BE sphere
does not produce enough flux to match the
observed SED while the 3x104 model BE sphere
produces too much flux. Since
We find a flux of 2.300.4 Jy and 14.72.5 Jy
at 850 450 mm respectively in a 120 aperture.
The resulting submm spectral index is 2.90.6,
higher than all of the PPCs observed by Shirley
et al. (2000) except for the two cores associated
with L1689A. The mass of the core, assuming a
temperature of 10 K within a 60 radius, is 0.74
Msun. This is very consistent with the virial
mass of 0.67 Msun determined from the N2H
linewidth (see Caselli et al. 2002) and corrected
for the modeled density structure (IV).
Azimuthally-averaged, normalized radial profiles
of the 850 and 450 µm maps are shown right. The
images were re-binned to pixels with half beam
spacing (7 at 850 µm and 3.5 at 450 µm)
corresponding to the Nyquist sampling limit in
the map. The radial profiles are binned at half
beam spacing. The flat intensity plateau is
clearly displayed in both the 850 and 450 µm
profiles. Both profiles are displayed to 98
from the centroid (11, -16). The beam profile
determined from jiggle maps of Uranus is shown as
a dashed line. The solid red lines in the 850 µm
plot indicate the sector-averaged radial profiles
centered along the major and minor axes.
the incident radiation is due to the ISRF, the
overall strength, sisrf, maybe be varied as well
as extinction at the outer radius due to the low
density medium that L1498 is embedded in (Av).
For the 1x104 BE-sphere, the best fit c2r(SED)
are for high values of the strength of the ISRF
with a few Av of extinction. Another possibility
is to scale the overall density structure by a
factor, k, to match the observed flux. The
best-fitted model is a 1x104 BE sphere with the
density scaled up by a factor of 2, Ro 35000
AU, sisrf 1.0, and Av gt 2 mag.
Archibald et al. (2002) between tcso and the
skydip determined opacity at 850 µm (t850),
t850(3.990.02)(tcso0.0040.001),we found
excellent agreement between our skydips and the
scaled 850 µm opacity. Therefore, we used the
scaled values to correct the 850 µm jiggle maps
on the night of August 30. We linearly
interpolated between 850 µm for the night of
August 29. Since the sky opacity at 350 and 450
µm is very sensitive to short term variations in
precipitable water vapor, we used the opacity at
350 µm to monitor the variability of t450 between
hourly skydips.
V. Conclusions We find a Bonnor-Ebert sphere
with central density of 1 3 x 104 cm-3 with
nearly isothermal temperature profile of 10.5 K
is the best fitted density structure to the SCUBA
observations. If the core is embedded in a lower
density medium ( few 102 cm-3), this is a stable
configuration. Therefore, L1498 may represent a
chemically mature, currently stable PPC that is
on the verge of collapse.