A&A 456, 1037-1043 (2006)
DOI: 10.1051/0004-6361:20040294
P. P. Tennekes1,2 - J. Harju1 - M. Juvela1 - L. V. Tóth3,4
1 - Observatory, PO Box 14, 00014 University of Helsinki,
Finland
2 -
Julius Institute, Utrecht University,
The Netherlands
3 -
Department of Astronomy of the Loránd Eötvös University,
Pázmány Péter sétány 1, 1117 Budapest, Hungary
4 -
Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary
Received 18 February 2004 / Accepted 19 June 2006
Abstract
Aims. The purpose of this study is to investigate the distributions of the isomeric molecules HCN and HNC and estimate their abundance ratio in the protostellar core Cha-MMS1 located in Chamaeleon I.
Methods. The core was mapped in the J=1-0 rotational lines of
,
,
and
.
The column densities of
,
,
and
were estimated towards the centre of the core.
Results. The core is well delineated in all three maps. The kinetic temperature in the core, derived from the
(1, 1) and (2, 2) inversion lines, is
K. The
/
column density ratio is between 3 and 4, i.e. similar to values found in several other cold cores. The
/
column density ratio is
7. In case no 15N fractionation occurs in
(as suggested by recent modelling results), the
/
abundance ratio is in the range 30-40, which indicates a high degree of 13C fractionation in
.
Assuming no differential 13C fractionation the
and
abundances are estimated to be
and
,
respectively, the former being nearly two orders of magnitude smaller than that of
.
Using also previously determined column densities in Cha-MMS1, we can put the most commonly observed nitrogenous molecules in the following order according to their fractional abundances:
.
Conclusions. The relationships between molecular abundances suggest that Cha-MMS1 represents an evolved chemical stage, experiencing at present the "late-time'' cyanopolyyne peak. The possibility that the relatively high HNC/HCN ratio derived here is only valid for the 13C isotopic substitutes cannot be excluded on the basis of the present and other available data.
Key words: ISM: individual objects: Chamaeleon-MMS1 - ISM: abundances - ISM: molecules
Hydrogen cyanide,
,
and its metastable isomer hydrogen isocyanide,
,
are commonly used tracers of dense gas in molecular clouds. The
column density has been observed to be similar or
larger than that of
in cold regions, whereas in warm GMC cores
the [HNC]/[HCN] ratio is typically much smaller than unity
(e.g., Goldsmith et al. 1981; Churchwell et al. 1984; Irvine & Schloerb 1984;
Schilke et al. 1992; Hirota et al. 1998).
Gas-phase chemistry models, including both ion-molecule and
neutral-neutral reactions, predict that the
/
abundance ratio
should be close to unity in cold gas (see e.g. Herbst et al. 2000,
where also previous work is reviewed). The two isomers are thought to
form primarily via the dissociative recombination of HCNH+, which
in turn is produced by a reaction between
and C+. The
recombination reaction regulates the abundance ratio despite the
initial production mechanisms of the two isomers, provided that they
are efficiently protonated via reactions with ions, such as H3+and HCO+ (Churchwell et al. 1984; Brown et al. 1989). Recent
theoretical studies corroborate the equal branching ratios for
and
in dissociative recombination (Hickman et al. 2005;
Ishii et al. 2006).
/
column density ratios clearly larger than unity have been
reported: Churchwell et al. 1984 find that [
]/[
]
may be as high
as
10 in some dark clouds cores, and Hirota et al. (1998) find a
ratio of
5 in L1498. The first result quoted is
uncertain because it depends on the assumed degree of 13C
fractionation in HNC, whereas the second value refers to
and
measurements. Nevertheless, both results suggest that the
/
chemistry is not fully understood. Further determinations
of the relative abundances of these molecules towards cores with
well-defined chemical and physical characteristics seem therefore
warranted.
In this paper we report on observations of
and
and some of
their isotopologues towards the dense core Cha-MMS1, which is
situated near the reflection nebula Ced 110 in the central
part of the Chamaeleon I dark cloud. We also derive the gas kinetic
temperature in the core by using the (1, 1) and (2, 2) inversion lines
of
.
Cha-MMS1 was discovered in the 1.3 mm dust
continuum by Reipurth et al. (1996), and was studied in several molecules
by Kontinen et al. (2000). The object is embodied in one of the most
massive "clumps'' in Chamaeleon I identified in the large scale
survey of Haikala et al. 2005 (clump No. 3,
).
Reipurth et al. (1996) suggested that Cha-MMS1 contains
a Class 0 protostar. A FIR source (Ced 110 IRS10) was
detected near the centre of the core by Lehtinen et al. (2001). However,
no centimetre continuum nor near-infrared sources have been found in
its neighbourhood (Lehtinen et al. 2003, and references therein), and
the core therefore represents a very early stage of star formation.
At the same time, its chemical composition has probably reached an
advanced stage (Kontinen et al. 2000).
In Sect. 2 of this paper we describe the observations. In Sect. 3 we
present the direct observational results in the form of spectra and
maps, and derive the
,
,
and
column
densities. By combining our results with previous observations we
derive in Sect. 4 the fractional abundances of several nitrogen
containing molecules in Cha-MMS1, and discuss briefly the
chemical state of the core. In Sect. 5 we summarize our results.
The J=1-0 transitions of
,
,
,
and
at about 90 GHz were observed with the
Swedish-ESO-Submillimeter-Telescope, SEST, located on La Silla in
Chile. The observations took place in December 1990. A 3 mm dual
polarization Schottky receiver was used in the frequency switching
mode. The backend was a 2000 channel acousto optical spectrometer
with an 86 MHz bandwidth. The velocity resolution with this
configuration is about
at 90 GHz. Typical values for the
single sideband system temperatures ranged from 350 K to 450 K. The
half-power beam width of the SEST is about
at the
frequencies observed. Calibration was achieved by the chopper wheel
method. Pointing was checked every 2-3 h towards a nearby SiO
maser source. We estimate the pointing accuracy to have been better
than
during the observations. The focusing was done using
a strong SiO maser.
The (1, 1) and (2, 2) inversion lines of
at about 23 GHz were
observed with the Parkes 64-m radio telescope of the Australian
Telescope National Facility (ATNF) in April 2000. The FWHM of the
telescope is about
at this frequency. The two lines were
observed simultaneously in two orthogonal polarizations in the
frequency switching mode, using the 8192 channel autocorrelator. The
calibration was checked by observing standard extragalactic
calibration sources at different elevations, and by comparing the
ammonia line intensities towards some strong galactic sources also
visible from the Effelsberg 100-m telescope, taking the different beam
sizes into account.
The electric dipole moments of the molecules and the line frequencies
used in this paper are listed in
Table 1. Except for the frequencies of the
and
hyperfine transitions adopted from
Frerking et al. (1979), the parameters are obtained from the Jet
Propulsion Laboratory molecular spectroscopy data base
(http://spec.jpl.nasa.gov), where also the original references
are given. The hyperfine components of
are not listed. The
frequencies given represent the centre frequencies of 18 and 21 individual components of the (1, 1) and (2, 2) lines, respectively,
distributed over a range of about 4 MHz.
Table 1: The permanent dipole moments of the molecules and the rest frequencies of the observed transitions.
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Figure 1:
Integrated
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The integrated
(J=1-0),
(J=1-0) and
(J=1-0)intensity maps of Cha-MMS1 are presented as contour plots in
Figs. 1-3, respectively. The values of the contour levels
are given in the bottom right of each figure. The position of the 1.3 mm dust emission maximum, Cha-MMS1a (Reipurth et al. 1996), is chosen
as the map centre. The coordinates of Cha-MMS1a are
,
.
The grid spacing used in the maps was
.
Also
indicated in these figures are the location of the
maximum
(
,
black dot), and
the positions used for the
(J=1-0) excitation temperature
estimates in the LTE method (open circles, see below). Long
integrations towards the
maximum were made besides
and
also in the J=1-0 lines of
and
.
The five
spectra obtained towards this position are shown in
Fig. 5.
The three maps have differences, but they all show a relatively
compact distribution peaking near Cha-MMS1a. The
emission
with the lowest opacity reflects probably best the distribution dense
gas.
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Figure 2:
Integrated
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Figure 3:
Integrated
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The column densities of the molecules have been determined with two different methods. First, we have derived the optical thicknesses of the lines and the total column densities of the molecules by assuming local thermodynamic equilibrium, LTE. The LTE method involves idealistic assumptions about the homogenity of the cloud. In the next subsection we present modelling of the observed spectra with a Monte-Carlo program using a realistic density distribution.
In the following, we assume that the rotational levels of all five
isotopologues are characterized by the same
,
and that the
core is homogeneous, i.e. the same
is valid for
different locations. The former assumption can be justified by the
similar dipole moments and rotational constants of the molecules. The
second assumption infers that the excitation conditions remain
constant along the line of sight and also between close-lying
positions in the core.
The excitation temperature is derived by fitting first the optical
thickness, ,
to the observed hyperfine ratios of a "clean''
spectrum, i.e. a spectrum with a good S/N and hyperfine
intensity ratios in accordance with the LTE assumption. The
is solved from the antenna equation using observed peak main beam
brightness temperature,
and assuming uniform beam
filling. The fit is possible towards the four positions listed in
Table 2. In other positions with strong signal,
like the
maximum
,
or the
maximum at
the
spectra show
"anomalous'' hyperfine ratios. The smallest error is obtained towards
the offset (
)
near the edge of the core with
K. This
spectrum is shown in
Fig. 4. The other positions listed in
Table 2 give similar values of
but
the errors are larger. Also
column density estimates using the
values for
and
obtained from hyperfine fits are
given in Table 2.
Table 2:
Estimates of the
(J=1-0) excitation temperatures and
the
column densities towards selected positions using hyperfine
structure fits. The coordinates are offsets from the position of
Cha-MMS1a.
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Figure 4:
The
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The
,
,
and
column densities towards the
maximum,
,
have been derived from
the integrated J=1-0 line intensities and the
estimate
derived from the
spectrum shown in
Fig. 4. For
the weakest of the well
separated hyperfine components, F=0-1, with the least optical
thickness has been used in this calculation by taking its relative
strength into account. For
we did not attemp to do a similar
estimate because the hyperfine components overlap and are very likely
optically thick. Overlapping hyperfine components hamper also the
column density estimate, but their optical thicknesses should
be lower than those of
by an order of magnitude or
more.
has no hyperfine structure due to zero nuclear spin of 15N. The results of these calculations are presented in
Table 3.
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Figure 5:
Observations used for the column density estimates.
The position corresponds to the
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Table 3:
Column densities towards the
maximum derived using
the LTE method. The assumed value for
is
5.2+1.0-0.6 K.
a The intensity value is for the weakest hyperfine component F=0-1 which
comprises one ninth of the total intensity of the J=1-0 line.
The following column density ratios can be derived from
Table 3:
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= | ![]() |
|
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= | ![]() |
|
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= | ![]() |
The ammonia column density,
,
and the gas kinetic
temperature,
,
were derived using hyperfine fits to the
inversion lines arising from the rotational states
(J,K) =
(1,1) and (2,2). The spectra are shown in
Fig. 6. The standard method of analysis described
e.g. in Ho et al. (1979) gave the following values:
and
K. Here
it is assumed that the relative populations of all metastable levels
of both ortho- and para-
are determined by thermal equilibrium
at the
(2,2)/(1,1) rotational temperature.
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Figure 6:
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,
,
and
spectra were calculated with the
Monte Carlo radiative transfer program of Juvela (1997). A
constant kinetic temperature and a spherically symmetric density
distribution,
,
were assumed. The selected density
power law conforms with observational results towards star-forming
cores (e.g., Tatematsu et al. 2004). In order to avoid divergence in
the cloud centre, we assume a constant density within a distance
corresponding to 6% of the outer radius of the model cloud. Compared
with the beam size, the constant density region is small and the
results are not very sensitive to the selected value of its
radius. The spectra were convolved with a Gaussian that corresponds to
the size of the beam used in the observations.The simulated spectra
were fitted to observations by adjusting four parameters: the outer
radius of the core, the scaling of gas density, and the fractional
abundances of
and
.
The
fit was optimized for
the weakest hyperfine component F=0-1. The abundances of the other
species were determined by assumed fixed ratios
[12C]/[13C] = 20, as suggested by the LTE column density
determination, and [14N]/[15N] = 280 (terrestrial). The
resulting fractional
and
abundances do not, however,
depend on these assumptions about isotopic ratios. The calculations
were done with
= 12 K, which was derived from the ammonia
observations, and
= 20 K.The lower temperature resulted
in better fit although the difference in the
values was no
more than 25%. The results for the 12 K are shown in
Fig. 7. The outer radius of the model is 60
.
The volume density increases from
cm-3 at
the outer edge to
cm-3 in the centre, and the
molecular hydrogen column density towards the centre is
cm-2.
The fractional
and
abundances resulting from the fit
are
and
,
respectively. The
/
abundance ratio obtained from the simulations is thus 3.8, i.e. close to the value obtained from the LTE method.
The largest discrepancy between the observed and modelled profiles is
seen in
.
It can be explained if the optical depth is larger than
in this particular model. For example, a foreground layer of cold gas
would readily decrease the intensity of the
lines and would
bring the ratios between the hyperfine components closer to the
observed values. The hyperfine ratios are, however, sensitive to the
actual structure of the source (Gonzáles-Alfonso & Cernicharo 1993).
The optical thickness of the
main component is
0.7.
This moderate value suggests that the method used in the previous
subsection results in a slight underestimatation of column densities.
The
and
column densities (fractional abundances
multiplied by
)
are indeed about two times larger than
those presented in the previous section:
,
.
These results suggest,
however, that the LTE method does not lead to crude underestimates of the
column densities of rarer isotopologues of HCN and HNC.
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Figure 7:
Simulated spectra fitted to the observations towards the
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The present observations and the LTE method give reasonable estimates
for the column densities of
,
,
and
near
the centre of Cha-MMS1. Unfortunately the column densities of
the common isotopologues of
and
cannot be directly
determined because of the large optical thickness of their
lines.
is, however, likely to be useful for this purpose.
According to the modelling results of Terzieva & Herbst (2000) hardly any
15N fractionation occurs in dense interstellar clouds. The
isotopic 14N/15N ratio lies probably between 240 and 280,
i.e. the values derived for the local ISM (Lucas & Liszt 1998), and the
terrestrial atmosphere. Using this range and the
column
density listed in Table 3 we obtain
.
Assuming that no differential 13C
fractionation occurs, we get from the
/
ratio derived
above that
.
The dust continuum emission provides the most reliable estimate for
the
column density. Reipurth et al. (1996) measured towards
Cha-MMS1 a total 1.3 mm flux density of 950 mJy from a nearly
circular region with a diameter of about
(bordered by the
lowest white contour in Fig. 8). As this size is similar to the SEST
beamsize at 3 mm, the resulting average intensity and average
can be compared with the column densities derived here.
The dust temperature as derived from ISO observations by
Lehtinen et al. (2001) is 20 K (Cha-MMS1 = Ced 110
IRS10). Due to crowding of sources in this region and the limited
angular resolution of ISOPHOT, this value may be affected by emission
from other sources in its vicinity (see Fig. 2 of
Lehtinen et al. 2001). Therefore, value 20 K is probably an upper
limit. A lower limit is provided by the gas kinetic temperature
12 K derived above. By using the two temperatures and Eq. (1) of
Motte et al. 1998 with
g
appropriate for circumstellar envelopes around Class 0 and Class I
protostars, we get
.
The
corresponding mass range of the dust core obtained from the total flux
given by Reipurth et al. (1996) is
.
The median of the reasonable H2 column density range,
,
yields the following values for the
fractional
,
and
abundances:
,
,
and
.
The column densities of two other nitrogen bearing molecules
towards Cha-MMS1 were determined by Kontinen et al. (2000):
,
and
,
which imply the fractional abundances
and
.
We find that both
and
are about 100 times less abundant
than
,
whereas
is almost equally abundant as
.
The
derived
/
column density ratio is in accordance with
previous observational results (e.g., Tafalla et al. 2002;
Hotzel et al. 2004). The abundance ratios are discussed in Sect. 4.3.
The
column density in this direction is
(Haikala et al. 2005). The fractional
abundance is therefore
or smaller. This upper limit
falls below the "standard'' value
of
Frerking et al. (1982), and suggests CO depletion.
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Figure 8:
The
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The superposition of our
map, the
map of
Kontinen et al. (2000), and the 1.3 mm dust continuum map of
Reipurth et al. (1996) is shown in Fig. 8.
The two molecular line maps have similar angular resolution (about
)
whereas the resolution of the continuum map is
.
Both molecules show rather compact, symmetric
distributions around the dust continuum source,
agreeing
slightly better with dust than
.
Nevertheless, the dense core
can be clearly localized with the aid of
as well as
and
.
This is in contrast with SO, which shows two peaks on both sides
of the core (Kontinen et al. 2000), and with C18O which has a flat
distribution in this region (Haikala et al. 2005), probably due to
depletion. This result agrees with the results of Hirota et al. (2003)
and Nikolic et al. (2003), which suggest that
and
are less
affected by the accretion onto dust grains than some other commonly
observed molecules.
CO depletion and a large
abundance point towards an advanced
stage of chemical evolution. The
/
abundance ratio
agrees with model predictions for dense cores, but does not put
severe constraints on the evolutionary stage (Aikawa et al. 2005).
According to the observational results of Hotzel et al. (2004) this value
should be more appropriate for a star-forming than for a prestellar
core. However, due to slightly different beamsizes the accuracy of the
present determination of the
/
ratio is not sufficient for
distinguishing between the two cases. Kontinen et al. (2000) suggested
that the large
abundance observed in Cha-MMS1
indicates either a so called "late-time'' cyanopolyyne peak associated
with a large degree of depletion according to the model of
Ruffle et al. (1997), or carbon chemistry revival by the influence of a
newly born, so far undetected star. A large ammonia abundance can be
understood in either case. Ammonia production benefits from the
diminishing of CO and can resist depletion longer
(e.g. Nejad & Wagenblast 1999). On the other hand, ammonia can possibly form
on grain surfaces and be released into the gas phase in shocks
(Nejad et al. 1990; Aikawa et al. 2003). As there is no clear evidence
for star-cloud interaction in Cha-MMS1, the alternative that
the core - or the portion traced by the present observations -
represents matured pre-stellar chemistry is more likely.
The behaviour of
and
under these circumstances is not
clear. The main gas-phase production pathway via the HCNH+, which
is effective especially at later stages, requires
and C+ions. The latter can be produced also in deep interiors of molecular
clouds via the cosmic ray ionization of C atoms, or in the reaction
between CO and the He+ ion, the latter mechanism becoming less
important with CO depletion (Nejad & Wagenblast 1999). To our knowledge the
production of
and
on interstellar grains and their
desorption has not been studied.
The models of Aikawa et al. (2005) for collapsing prestellar cores
include the gas-phase production of
,
,
,
and
.
Although the order of fractional abundances,
,
and the
/
abundance ratio derived from the
present observations are well reproduced by this model, especially for
the gravity-to-pressure ratio
impying a rapid collapse and
less marked depletion, the predicted
column densities relative
to
and
are an order of magnitude larger than those
observed here. A likely explanation is that
is not as centrally
peaked than
and
(see Fig. 3 of Aikawa et al. 2005).
Therefore a large fraction of
resides in on the outskirts of the
core, where the density is not sufficient for the collisional
excitation of its rotational levels, and the molecule is not seen in
emission (see also the next section). Another discrepancy is that the
model predicts much lower
column densities relative to ammonia
than observed here. The agreement is slightly better for the model
with overwhelmingly large gravitational potential (
)
than
for the slowly evolving model near hydrostatic equilibrium
(
)
which results in very high degrees of
depletion. Unfortunately the spectral resolution and S/N ratio of the
present data do not allow to study the core kinematics.
As compared to the "principal'' N-bearing molecules
and
,
and
have clearly smaller abundances, and probably only
marginal importance to the nitrogen chemistry in the
core. Nevertheless, the fact that
/
abundance ratio is once
again observed to be larger than unity remains a
problem. Talbi & Herbst (1998) have shown that even though the reaction
between
and C+ may produce a significant amount of the
metastable ion
,
which should produce only
in
electron recombination, the energy released in the former reaction
leads to the efficient transformation into the linear isomer
before the recombination. Likewise, according to
Herbst et al. (2000), the vibrational energies of
and
formed
in the neutral reactions
,
and
are
sufficient to overcome the isomerization barrier, and roughly equal
amounts of both isomers are produced.
The theoretical results presented in Talbi & Herbst (1998) and
Herbst et al. (2000), and more recently by Hickman et al. (2005) and
Ishii et al. (2006), show quite convincingly that the
/
abundance ratio resulting from the chemical reactions known to be
effective in dark clouds cannot differ significantly from unity.
There are two possible explanations for the discrepancy between the
observations and theoretical predictions. Either so far unknown
processes favour
in the cost of
,
or then the derived
/
column density ratio is affected by different degrees
of 13C fractionation in
and
.
In the latter case
13C would about three times more enhanced in
than in
in Cha-MMS1.
The
/
column density ratio 7.4 together with the above
quoted range of 14N/15N ratios imply that the
/
ratio in Cha-MMS1 is in the range 30-40. As the typical
12C/13C isotopic ratio observed in nearby clouds is 60 (Lucas & Liszt 1998) the result suggests considerable 13C
enhancement in
.
An
column density determination would
readily show, according to the results of Terzieva & Herbst (2000), whether
also
has a similar degree of 13C fractionation.
In the only survey so far including all these isotopologues,
Irvine & Schloerb (1984) found very similar
/
and
/
intensity ratios (
1.5) towards TMC-1. This
observation suggests similar 13C fractionation in
and
.
On the other hand, this result might not be universally valid.
Ikeda et al. 2002 determined substantially different
/
column density ratios in the starless dark cloud cores
L1521E, TMC-1, and L1498, listed in order
of increasing
/
ratio (2.6, 6.7, and 11). The
/
column density ratios in these sources increase in
the same order (0.54, 1.7, and 4.5; Hirota et al. 1998). The values
obtained towards TMC-1 are consistent with those of
Irvine & Schloerb (1984), but the results towards L1521E and
L1498 suggest (if the possibility of 15N fractionation
is neglected) that the 13C fractionation in
changes from
source to source, and perhaps the variation in the case of
is
even larger. L1521E is suggested to represent youthful
chemistry with little molecular depletion (Hirota et al. 2002;
Tafalla & Santiago 2004) whereas the position observed in L1498 is
known to be heavily depleted (Willacy et al. 1998). Thus, it appears
that the freeze-out of molecules favour the 13C substituted
isotopologues of
and
.
It seems warranted to further
examine the possibility of their differential 13C fractionation,
although no theoretical arguments are in favour of different gas-phase
production and destruction rates of
and
,
or their
different freezing rates. As no empirical data on this matter is
currently available, the best way to proceed is probably to perform
similar observations as those done by Irvine & Schloerb (1984) towards a
sample of cores with different degree of depletion.
The low
/
and
/
intensity ratios and the line
shapes in the
and
spectra observed towards the centre of
the core require some attention. The intensity ratios suggest very
large optical thicknesses for the
and
lines, and
consequently, deep self-absorption features and "anomalous'' hyperfine
component ratios are to be expected. A clear absorption feature can
be seen in the simulated
spectrum shown in
Fig. 7, although a very low 12C/13C
isotopic ratio is used in order to avoid inverted hyperfine ratios,
which would disagree with the observations. The observed
and
spectra are, however, free from absorption dips, and they
look more like weakened "normal'' lines.
This weakening can probably be attributed to less dense gas
surrounding the core. The
and
molecules in the envelope,
where the density lies below the critical density of their J=1-0lines, function as scatterers of line emission arising from the core,
i.e. they re-emit absorbed photons (into random directions) before
colliding with other particles. The effect is less marked for
13C isotopologues, and the resulting
/
and
/
intensity ratios decrease, without
and
loosing their characteristic line patterns. This model was used by
Cernicharo et al. (1984) to explain
hyperfine anomalies,
particularly the intesity of the intrinsically weakest component
F=0-1 observed towards dark clouds. The
spectrum shown in
Fig. 5 conforms with this explanation: the F=0-1component on the left is stronger than F=1-1 component on the right,
although the optically thin LTE ratio is 1:3.
The scattering of
emission from the core has probably lead to
underestimation of the excitation temperature and column densities
derived using the LTE method in Sect. 3.2. For example, an increase of
1 K in
,
would mean 30% larger column densities, and 3 K
would correspond to a 100% increase in the column density.
Quantitative estimates of the scattering effects are, however, beyond
the scope of this paper, and here we merely note this additional
source of error.
A mapping in the J=1-0 rotational lines of
,
and
shows a compact structure in the direction of the protostellar core
Cha-MMS1. The
map agrees best with the previous 1.3 mm dust continuum map of Reipurth et al. (1996), and the
map of
Kontinen et al. (2000).
The column density determinations of
,
,
and
,
and the results of the quoted two studies allow us to estimate
that the fractional
and
abundances are
and
,
respectively. These are slightly larger than that of
,
but clearly smaller than the
and
abundances (
and
,
respectively). In accordance with
recent results in other dense cores we find that
is about 100 times more abundant than
.
The kinetic temperature derived from
the
lines is about 12 K. The derived fractional abundances
conform with the previous suggestion of Kontinen et al. (2000), that
Cha-MMS1 represents an advanced chemical state, although it
has a large
abundance, which usually is thought to indicate
early times. This situation is likely to have developed under the
circumstances of CO depletion (Ruffle et al. 1997).
The following column density ratios are found:
and
.
Using the latter ratio,
and the assumption based on the modelling results of
Terzieva & Herbst (2000), we estimate that the[
]/[
]
ratio
is between 30 and 40. As there is no
column density
determination available towards this source, the possibility that
differential 13C fractionation causes the observed relatively
high [
]/[
]
ratio cannot be entirely ruled out.
The
and
lines are not much brighter than those of the
13C substituted species, but still show no absorption dips. These
lines are probably weakened by scattering by
and
molecules
residing in less dense gas around the core. As suggested by
Cernicharo et al. (1984), the same process can cause the so called
hyperfine anomalies in dark clouds, and is probably also responsible
for the "too'' bright F=0-1 component of HCN(J=1-0) towards the
centre of Cha-MMS1. The evidence for scattering and the
relatively low
and
column densities derived towards the
core suggest that the
and
distributions are less centrally
peaked than those of
and
,
which would be in agreement
with the model of Aikawa et al. (2005) for a rapidly collapsing core.
Acknowledgements
We thank Wolf Geppert for useful discussions and the anonymous referee for valuable comments on the manuscript. This project was supported by the Academy of Finland, grant Nos. 173727 and 174854. P.P.T. thanks the ERASMUS programme of the European Union, Olga Koningfonds, Utrecht, and Kapteyn Fonds, Groningen.