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Molecular oxygen in interstellar clouds

Priscilla Marechal

DEMIRM, Observatoire de Paris, FRANCE


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 .


1. Introduction

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)).

2. Modelling the chemistry and the rotational excitation of oxygen

2.1. The steady state model

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 :

2.2. Level population of oxygen

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.

2.3. Results

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 .

3. The observations of the 234 GHz line of in L134N

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.

4. Conclusion

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.


YERAC 94 Account
Wed Feb 22 20:13:39 GMT 1995