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Molecular oxygen in
interstellar clouds
Priscilla
Marechal
e-mail:
priscila@mesion.obspm.fr
DEMIRM, Observatoire de Paris, FRANCE
Abstract:
The prospects for detection of interstellar 
depend on its abundance
and its excitation. Our results shown the importance of the 
ratio for
the emmissivity of 
and the role of molecular oxygen in the thermal
balance. 
has not yet been detected in interstellar clouds but we
obtained the first tentative detection of 
.
According to chemical models, 
is one of the most important carriers
of oxygen in molecular clouds, predicted to be as abundant as 
. This
fact encourages attempts to observe 
despite low line strengths of
submillimeter magnetic dipole transitions of this molecule. The balloon-borne
telescope PRONAOS-SMH (Beaudin et al. (1994)) should observe the 368 GHz 
line of 
. To prepare the mission, we must select
as well as possible molecular clouds which should have high intensity in this
line. Thus, I inserted the processes of radiative and collisional transfer
between rotational levels of this molecule in a model which works out chemical
and thermal balance in molecular clouds (Warin et al. (1995)). In addition I briefly
present results obtained during an observing session made at POM-2 telescope
(Castets et al. (1988)) to detect the 234 GHz 
line of the
isotopomer 
(Pagani et al. (1994),Pagani et al. (1995)).
Figure 1: The steady state model of interstellar clouds
The abundances of chemical species and the temperature profile throughout a
molecular cloud are obtained by solving, in a self-consistent way, a radiative
transfer equation for UV photons - taking into account absorption by both
dust and gas - the chemical balance equations and a thermal balance equation.
The atomic and molecular spectral lines are obtained by adding the statistical
equilibrium equations which lead to the fine structure and the rotational
population of the species. The complete process is iterative and is illustrated
in Figure 1.
The chemistry takes into account 136 chemical species and about 2,800
reactions. In the case of 
, the main production route is the
reaction:
is mainly destroyed by photodissociation, photoionization and
collisions with 
and 
:
Figure 2: a) rotational levels of 
, b) radiative transitions and
Einstein coefficients for the first levels 
To obtain the rotational population of 
, we take into account the
following processes: spontaneous emission, stimulated emission and absorption
of the ambient background radiation, excitation and de-excitation by collisions
with 
and 
. The model takes into account the 24 first
rotational levels of 
.
The ground electronic state of 
is a 
state with two
unpaired electrons. So its rotational levels are described by the rotational
quantum number N and the total angular momentum quantum number 
as
illustrated by Figure 2a:
consists of two identical atoms obeying to the Bose-Einstein
statistics so that rotational levels with even value of 
do not occur. Since
is homopolar, it has no permanent electric dipole moment. Hence the
radiative transitions obey to selection rules of magnetic dipole transitions as
shown in Figure 2b for the first levels of the molecule:
The collisional rates are taken from Black & Smith (1984) who derived their
values from experimental data about collisions between 
and 
. These rates are however very uncertain and inclusion in the model of
collision rates deduced from theoretical calculations (Corey et al. (1986)) are
in progress.
Table 1: Oxygen-bearing molecules in ``standard'' molecular clouds models:
represents the visual extinction througout the whole cloud
Figure 3: The 368 GHz line of 
in dense clouds: a) for different
values of 
, b) for different values of the 
ratio
Figure 4: Atomic and molecular cooling rates in a dense cloud with 
.
Figure 5: Cooling by 
in dense clouds for different 
ratio.
Table 1 shows the results of the model for three standard clouds: a
diffuse one, a translucent one and a dense one. The most abundant
oxygen-bearing species are 
, 
and 
. The 368 GHz line
of 
is intended to be observed by PRONAOS balloon and the 234 GHz
line of 
is the one we observe at POM-2. In fact, only dense
clouds can be observed by those telescopes.
Figure 3 displays the intensity profile of the 368 GHz line of 
computed by the model for dense clouds and for some values of the gas
density and the 
ratio. The density does not affect so much the
emissivity of 
. The integration time for a 3
detection is
between 20 minutes and 1 hour with actual characteristics of PRONAOS-SMH
receiver (
). The 
ratio is a
parameter which causes dramatic changes on the abundance and the emissivity of
. The integration time varies between a few minutes for 
=0.1 and several years for 
=1.
The cooling of a molecular cloud, plotted in Figure 4, occurs through
fine structure emission of atoms and ions and rotational transitions of
molecules. The model takes into account cooling by 
, 
, 
, 
, 
, 
, 
and 
.
For a standard 
ratio of 0.4, the cooling by 
in the center
of dense clouds, shown in Figure 5, is about 30%. It could be as
great as 80% for 
and lower than 1% for 
.
Table 2: Comparison of the 
/CO ratio in 3 different positions in
L134N
Figure 6: Observations of the 
transition of
in the dark cloud L134N. Axes represent the usual LSR
velocity (
) and brightness temperature (K).
Observations were made in several runs at POM-2, a 2.5 meter antenna, on
Plateau de Bure (France). The instrument works with an SIS mixer of temperature
. The observations are made with the
autocorrelator backend 18 MHz bandwidth (
resolution) using frequency switching by 
.
In addition to its low line strength, 
is a rare isotopomer
species. So we need very long time of integration - over 100 hours of
effective time on the source - to get a 
detection towards L134N.
Figure 6 shows the result of the observations of 3 positions in the
cold dense molecular cloud, L134N. The central position in L134N (Pagani et al. (1994))
shows three peaks at 1.3, 2.5 and 3.4 
. At the two other
positions of L134N (Pagani et al., submitted), there are only two peaks at 1.3
and 2.8 
and their spacing is different. So they are not a fine
structure emission or a symmetric top triplet from another species. Moreover,
the 
ratio deduced from observations (Table 2) are
consistent with our predicted value of about 0.24 for a standard dense model
(Table 1). Those facts would reinforce the case for molecular oxygen
detection.
The modelling of emissivity of 
shows that the detection of the 368
GHz line of 
in dense clouds is feasible by a project like
PRONAOS-SMH provided the 
ratio is about, or less than, the standard
value of 0.4. The balloon might fly during twenty or thirty hours and such
clouds could be detected between a few minutes and one hour. We have also seen
that 
has a non-negligible role in thermal balance of dense clouds.
Models do not generally take this fact into account.
The low signal-to-noise ratio and the presence of several lines prevented us
from reaching definite conclusions but we have some evidence for
detection in L134N. This source certainly stands out as a
very peculiar source for its chemico-physical behaviour for which the
observations still reveal exciting possibilities.
- Beaudin et al., 1994,
24
European Microwave Conference proceedings, Vol. III,
1955.
- Black J.H. & Smith P.L., 1984,
ApJ, 277, 562.
- Castets A., Lucas R., Lazareff B. et al., 1988,
A&A, 194, 340.
- Corey, Alexander & Schaeffer, 1986,
J.Chem.Phys., 85, 2726.
- Pagani L., Langer W.D. & Castets A., 1994,
A&A, 274, L13.
- Pagani L., Marechal P., Castets A., Langer W.D., 1995,
submitted.
- Warin S., Benayoun J.J., Viala Y.P., 1995,
in preparation.
YERAC 94 Account
Wed Feb 22 20:13:39 GMT 1995
