Transcript Slide 1

Modeling of Deep Submillimeter Images of the Pre-protostellar
Core L1498
1,2
2
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Miranda K. Nordhaus , Yancy L. Shirley , Neal J. Evans II , & Jonathan M. C. Rawlings
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1 – Rensselaer Polytechnic Institute, 2 – NRAO, 3 – University of Texas at Austin, 4 – University College London
I. 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.
III. 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 3σ 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 nonaxisymmetric 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.).
II. 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 fivepoint 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
Archibald et al. (2002) between τcso and the
skydip determined opacity at 850 μm (τ850),
τ850=(3.99±0.02)(τcso–0.004±0.001),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 τ450 between hourly skydips.
We find a flux of 2.30±0.4 Jy and 14.7±2.5 Jy at 850 & 450 mm respectively in a 120”
aperture. The resulting submm spectral index is 2.9±0.6, 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.
IV. 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
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 > 2 mag.
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.