A&A 454, L71-L74 (2006)
DOI: 10.1051/0004-6361:20065387
LETTER TO THE EDITOR
L. K. Haikala1 - M. Juvela1 - J. Harju1 - K. Lehtinen1 - K. Mattila1 - M. Dumke2
1 - Observatory,
PO Box 14, University of Helsinki, Finland
2 -
European Southern Observatory, Casilla 19001, Santiago, Chile
Received 7 April 2006 / Accepted 3 June 2006
Abstract
The feasibility of observing the
(3-2) spectral
line in cold clouds with the APEX telescope has been tested. As the
line at 329.330 GHz lies in the wing of a strong atmospheric H2O
absorption it can be observed only at high altitude observatories.
Using the three lowest rotational levels instead of only two helps to
narrow down the physical properties of dark clouds and globules. The
centres of two
maxima in the high latitude low mass star forming
region CG 12 were mapped in
(3-2) and the data were analyzed
together with spectral line data from the SEST. The
(3-2)/
(2-1) ratio in the northern
maximum, CG 12-N, is 0.8, and in the southern maximum, CG 12-S,
2. CG 12-N is modelled
as a 120
diameter (0.4 pc) cold core with a mass of 27
.
A small size maximum with a narrow, 0.8
,
(3-2)
spectral line with a peak temperature of
K was detected
in CG 12-S. This maximum is modelled as a 60
-80
diameter (
0.2 pc) hot (
K)
1.6
clump. The source lies on the axis of a highly
collimated bipolar molecular outflow near its driving source. This is
the first detection of such a compact, warm object in a low mass star
forming region.
Key words: clouds - ISM molecules - ISM: structure - radio lines - ISM: individual objects: CG 12, NGC 5367
The two lowest rotational transitions of
,
and
have
been used extensively for studies of dark clouds and globules. Also
the J=3-2 lines of
and
can be routinely observed. In
contrast, there are very few published observations of the J=3-2transition. Besides the nine point
(3-2) map of S106 (Little et al. 1995) and mapping of the centre of the pre-stellar cloud core
L1689B (Jessop & Ward-Thompson 2001) only pointed observations are available
(e.g., Jørgensen et al. 2002, 2004; Schöier et al. 2002; Lee
et al. 2003). The addition of the
(3-2) transition to the lower
transitions would, however, substantially improve the accuracy of physical
parameters derived from
data as the relative line intensities
depend heavily on the cloud temperature and density.
The SEST telescope has been used to map numerous southern molecular clouds
in
(1-0) and (2-1). The new Atacama Pathfinder Experiment
(APEX
) provides an
excellent opportunity to complement these data with
(3-2)
observations. The APEX beam size at 329 GHz,
19
,
is
about equal to the SEST beam size at the
(2-1) frequency,
24
.
This simplifies the comparison of APEX and SEST data.
In this Letter we demonstrate the use of
(3-2), (2-1) and (1-0)
observations for radiative transfer modelling in the case of Cometary
Globule 12, CG 12, which is a high-latitude star forming region at a
distance of 630 pc (Williams et al. 1977). The head of the cloud
contains a highly collimated bipolar outflow (White et al. 1993) and
two compact
maxima separated by about 3' (Haikala & Olberg 2006,
Paper I). The centres of these maxima were mapped in
(3-2).
Based on these observations a simple model is derived for both cores.
The observations were made with APEX on Aug. 21,
2005 in good weather (PWV 0.7 mm). The
(3-2) line at
329 330.546 MHz was observed with the APEX-2A SIS DSB receiver. The
zenith optical depth was
0.27 and the DSB system temperature
ranged from 180 K to 280 K. The receiver signal sideband lies in the
wing of an atmospheric H2O absorption line whereas the mirror
sideband centered at 335.330 GHz is at a more transparent
frequency. The difference in the atmospheric opacities was estimated
using an atmospheric model and was taken into account in the
calibration.
Two 5 by 5 point maps with 10
spacing were obtained in the
position switching mode. Off positions (+5
in azimuth for
CG 12-N and +5
in azimuth and +5
in elevation for
CG 12-S) were outside the globule at the time of the observations. CG 12-N
was observed at elevations
to
and CG 12-S at
to
.
Integration time of 20 s was used and
calibration was done every 10 to 30 min. Each position was
observed two or three times. The map centre positions were integrated
for a total of 250 s. The 1 GHz bandwidth of the MPIfR Fourier transform
spectrometer was divided into 16 384 channels of 61 kHz (
55
at 329 GHz).
Most of the observed spectra contain ripple due to variations in the
atmospheric emission, reflections in the telescope optics and
instability of the receiver. In the data reduction the possible low
frequency ripple was first fit with a sinusoidal baseline where-after
possible higher frequency ripple was removed by masking the
corresponding frequency in Fourier transform space if possible. Finally a
first order baseline was fit to the spectra around the source velocity
and subtracted. The resulting rms of the spectra is by 15%
higher than the value expected according to the radiometer formula
indicating that the possible non white noise due to, e.g., remaining
baseline ripple, does not dominate the spectral noise. The spectra
were transformed to the main beam brightness temperature scale using a
main beam efficiency of 0.7. All the spectral line intensities and
line integrals reported in this paper are on the
scale.
![]() |
Figure 1:
The Apex
![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 2:
As Fig. 1 for CG 12-S. The
tick line is at -6.2
![]() ![]() |
Open with DEXTER |
![]() |
Figure 3:
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The hanning smoothed
(3-2) spectra in CG 12-N and CG 12-S are shown
in Figs. 1 and 2 together with the
SEST
(2-1) and (1-0) observations (Paper I). The area mapped
in
(3-2) in each maximum covers only the
SEST HPBW at 109 GHz,
47
,
and the emission continues
beyond the map boundaries.
Maps of the
(3-2) line integral
are shown in Fig. 3 superposed on
SEST molecular line and mm continuum
maps and a NIR Ks (NTT SOFI) image.
CG12-N: in the southern part of the CG 12-N map the line profiles
and intensities of all the observed
transitions are similar.
Elsewhere both the
(3-2) line profile and line peak velocity
vary more than in the (2-1) transition. The maximum is shifted to SE
from the
(2-1) (and also (1-0)) maximum and is located near the
faint IRAS point source 12546-3941 (Fig. 3).
The three
transitions together with the CS (3-2) and
(2-1) lines in the CG 12-N map centre position are shown in Fig.
4. The
lines are asymmetric but the profiles
are similar. An unpublished
(1-0) spectrum obtained in this
position shows that the
(1-0) line is optically thin. The weak
(
K) CS line peaks at the same velocity as the
lines. The
line is redshifted with respect to the
and CS
lines.
CG12-S: the
(3-2) emission peaks 10
West of the
CG 12-S map centre position. In contrast with CG 12-N, the
(3-2)
line is significantly stronger than
(2-1) in the map centre,
West and South-West.
The maximum lies at the NW edge of the dense core outlined by the
high density tracers (
(2-1), CS (3-2)) and the 1.2 mm
continuum and on the axis of the outflow.
It is likely that
the driving source of the molecular outflow (White 1993) lies at the
Southern tip of the cone-like NIR nebulosity seen in the Ks
image at
.
The nature of the driving source is not known.
In the
(3-2) map centre position two
velocity components
(-6.4
and -6.2
,
Paper I) are blended and the lines are
skewed (Fig. 4). Only the -6.4
component
emits strongly in
,
CS (2-1), CS (3-2) and
(1-0)
(Paper I). However, in
the -6.2
component is the
strongest. A two component gaussian fit with centre velocities fixed
to -6.4
and -6.2
gives 7.6 K, 3.7 K and 1.7 K,
respectively, for the
(3-2), (2-1) and (1-0) peak temperatures
for the component centred at -6.2
.
In
(3-2) this component extends beyond
the mapped area in the South-West (Fig. 2). The
source is elongated in the NE-SW direction but is hardly resolved
perpendicular to this direction.
The
(3-2)/
(2-1) ratio (
)
is
0.8 and 2 (the -6.2
component) in CG 12-N and
CG 12-S, respectively. Assuming LTE, these ratios would correspond to
of
10 K in CG 12-N and >30 K in CG 12-S.
Many of the
line ratios in pre- and protostellar cores
(Jørgensen et al. (2005) and references therein) are similar to those
observed in CG 12-N. These line ratios were modelled with a "drop''
model which assumes a strong molecular depletion in a cold shell (<30 K)
between
the heating source and the outermost part of the envelope.
The intensity of the
lines in IRAS 16293-2422 is as high as
observed in CG 12-S and
is 1.3 (Schöier et al. 2003). The
lines are, however, optically thick and, e.g., the CS(5-4) line is
stronger than the
(3-2) line. In CG 12-S the CS(5-4) line is not
detected (Fig. 4).
![]() |
Figure 4:
![]() ![]() ![]() |
Open with DEXTER |
It is evident from Figs. 1 and 2
that considerable fine structure in
(3-2) is present both in
intensity and velocity in the two mapped areas. However, the signal-to-noise
ratio of the spectra is not high enough for a more refined
analysis of the source structure than presented in Sect.
3. We therefore restrict our further analysis to
constructing models which
reproduce the observed line ratios and intensities in the
(3-2)
map central positions (Fig. 4) where the signal-to-noise
ratio is best. In CG 12-S only the -6.2
component
is considered.
The reproducibility of the
(1-0) and (2-1) spectral line
intensities at SEST has been very good. The lines were also observed
simultaneously and therefore the observed relative line intensities
should be quite accurate. No such record is available for the APEX
telescope and the receiver. We therefore assume a calibration accuracy
of 20%.
Radiative transfer modelling was done with the Monte Carlo method
(Juvela 1997) assuming a spherically symmetric geometry. The C18O
maps were used to estimate the cloud radii, 60
for CG 12-N and
35
for CG 12-S. The density
distribution adopted as a starting point was
truncated at cloud radius.
Power law temperature distributions were tested with
temperature either increasing or decreasing towards the cloud
centre. The model was fitted to the intensities and line widths of the
observed C18O spectra (CG 12-N) and to line temperatures fitted
for the -6.2
component (CG 12-S).
The spectra around the central position were
used to constrain the radial density profile. The free parameters of
the fit were the size of the inner region with constant density and
temperature, and the parameters of linear scalings that were applied
separately to the density and temperature values. The model spectra
were convolved with gaussian beams that correspond to the resolution
of the observations. Constant C18O abundance
of
was used. The quality of the fit depends only weakly on
the abundance which does, on the other hand, directly affect the mass
estimates.
The resulting radial density and temperature profiles are shown in
Fig. 5. For CG 12-N the -value is close to one. However, in the
case of CG 12-Sthe
-value is over five, mainly
because the predicted J=1-0 intensity is only half of the
observed value. Best fits in CG 12-Swere obtained with models where the kinetic
temperature increased inwards. One could obtain reasonable fits even
with isothermal models. CG 12-N is modelled with a 120
diameter
cloud where the temperature decreases from 12 K down to 6 K at the
edge. The CG 12-Sdata can only be fit with a small size, hot
clump. The modelled total gas column density in the APEX beam and the cloud
mass are
and 27
for CG 12-N and
and 1.6
for CG 12-S, respectively.
The models presented above should be considered only as
indicative. Spherical
symmetry was assumed and the
(3-2) maps cover only the very
centres of the maxima. Furthermore, the cloud sizes are close to the
telescope beam size so that the beam convolution has a large effect on
observed and calculated line ratios. Especially the CG 12-Sfit is
sensitive to the beam convolution. Different combinations of source
size, temperature and density produce acceptable fits. The central
density cannot be increased much as the CS lines would become
stronger than observed. Common to all of these solutions is the small
size, a radius of 30
to 40
,
and high central
temperature, 80 K to 200 K. The problem is, however, not unique for
this source but inherent to all observing and modeling small size
(compared to beam size) sources. The source beamfilling factor
becomes an important modelling parameter.
If the outer parts of the CG 12-N cloud envelope are heated up, e.g, by
interstellar radiation field (e.g., Snell 1981), this has very
small effect to the fit at the cloud centre position. The gas
column density in the cloud envelope is small compared to the
central density from where the bulk of the emission is coming. The
LTE mass (
K) calculated from the
(2-1) data within
60
from the centre of CG 12-N is 25
which is in
agreement with the modeled mass of 27
.
![]() |
Figure 5:
The radial density (lower curve) and temperature profiles of
CG 12-N and CG 12-S. The horizontal axis shows the distance from the clump centre.
One arc-min corresponds to 0.18 pc or
![]() |
Open with DEXTER |
The LTE mass of the -6.2
velocity component in CG 12-S
calculated from
(2-1) data within
35
from the
(3-2) map centre is 4.1
for
K.
However, for
K the mass would be three times higher.
The LTE method assumes that all energy levels are thermalized
and this is clearly not the case in the CG 12-Smodel.
The addition of the
(3-2) transition to the
data on the two lower transitions restricts strongly the parameter
space of possible cloud models.
The
(3-2) observations of CG 12-N confirm the cold temperature of this
core. The
(3-2) line profile varies more over the mapped
region than the
(2-1) profile. The
(3-2) line is therefore
probably better suited for studying the small-scale stucture of
molecular clouds than the lower transitions of this molecule.
The detection of high, 10 K,
(3-2) line temperatures in
CG 12-Swas unexpected and forced us to reconsider the temperature and
density structure of the material traced by
(2-1) and (1-0)
presented in Paper I. The small, hot clump lies projected on the
edge of a dense core traced by
.
It also lies on the axis of a
highly collimated bipolar molecular outflow near its driving
source. This suggests that the two are related. As the
lines from
the hot spot are narrow and appear at the same radial velocity as the
parent cloud, they probably do not originate in shocked gas. Neither
is direct radiative heating from the strong IRAS point source
13547-3944 likely. The heating mechanism and the relation of the hot
spot to the collimated outflow remain therefore unspecified. Further
studies of this phenomenom seem warranted as they may lead to a better
understanding of the interaction between newly born stars and the
surrounding molecular material.
Better spatial resolution can be achieved by
observing still higher
transitions
which should be detectable
in CG 12-S. The sizes of the CG 12
maxima are well suited for the
future ALMA interferometer.