Astron. Astrophys. 361, 704-718 (2000)

2. Observed molecules

We discuss here the spectroscopic and chemical properties of the molecules included in this study. Table 1 lists their chemical formulae, structure types (hfs means that the lines have hyperfine structure; o/p and that the molecule has ortho and para or A-symmetry and E-symmetry forms, respectively), permanent electric dipole moments () along the relevant molecular axises, and rotational constants (A, B and C). The data were extracted mainly from the JPL catalogue ²(Poynter & Pickett 1985); exceptions are the CH₃OH data (Sastry et al. 1981; Anderson et al. 1990) and the value for the dipole moment of CCS (Murukami 1990).

Table 1. Spectroscopic properties of the observed molecules.

2.1. Spectroscopic properties

Carbon monosulphide, CS , is a linear molecule with a simple rotational spectrum; the states are denoted by the total angular momentum quantum number J. The simplicity of the spectrum is due to the facts that in the electronic ground state, the electrons in the CS molecule have paired spins and possess no orbital angular momentum. Hence, the ground state is denoted . In addition, the C and S nuclei have no spin.

The dicarbon sulphide radical, CCS , and the sulphur monoxide radical, SO , have more complicated spectra than CS. The electrons in the electronic ground states of CCS and SO have no net orbital angular momentum, but the total spin is non-zero due to two electrons having unpaired spins. Thus, the spin quantum number is and the electronic ground state is denoted (e.g. Gordy & Cook 1970, Sect. 4.2). The electron spin is coupled to the weak magnetic field arising from the rotation of the molecule. Since S=1, for each molecular rotational level with the rotational angular momentum quantum number , the total angular momentum quantum number, J, has three possible values: , or . Consequently, the rotational levels are split into triplets, whereas for N=0 only one state () exists (e.g. Yamamoto et al. 1990).

Hydrogen isocyanide, HNC , is a linear molecule. The nitrogen nucleus has a non-zero spin, and therefore possesses an electric quadrupole moment. This interacts with the electric field gradient of the molecule, which depends on the rotation. Consequently, the nuclear spin () is coupled to the molecular rotational angular momentum, J, to yield the total angular momentum, F (Gordy & Cook 1970, Sect. 9.4). The resulting hyperfine splitting of the rotational lines of HNC is smaller than for its isomer HCN. For example, for the lowest rotational transition () the and components are separated from the main component by -0.27 and km s^-1 respectively (Frerking et al. 1979). Hence, the hyperfine structure is usually not resolved in observations due to Doppler broadening in the source and the insufficient spectral resolution of the spectrometers available.

Cyanoacetylene, HC₃N , is also a linear molecule. Due to its high moment of inertia, it has a small rotational constant (B) and its rotational transitions lie relatively closely in frequency. Many of these lines are located in the commonly observed frequency bands, and it is often used in multi-transition studies. Like HNC, it has hyperfine structure due to the nuclear spin of nitrogen. For higher J-values, however, the relative energy differences between different F-levels are very small, and so no hyperfine structure is seen in the spectra observed here.

Methyl acetylene, CH₃CCH , is a low dipole moment, symmetric top molecule. Its energy levels are described by two quantum numbers J and K. The former represents the total angular momentum and the latter is the projection of J on the symmetry axis of the molecule. The lowest energy state in each K-ladder is J=K. CH₃CCH exists in two chemically different forms, the so called A and E symmetry species, depending on the relative orientations of the spins of the three hydrogen nuclei in the CH₃ group. For the A-symmetry species, the K quantum numbers are multiples of 3 (i.e. ; n=0,1,2,...), while the E-symmetry species has and ; n=0,1,2,... . The ground-state energies for A and E CH₃CCH differ by 8.02 K, E lying higher in energy. There are no allowed electric dipole transitions between two states belonging to different K-ladders. Consequently, the relative populations of two K-ladders are determined by collisional transitions and therefore depend on the kinetic temperature. In fact, the so-called `rotational temperature', which can be determined from a single observation of a multiplet (the lines lie closely in frequency), is considered to be a good approximation of the kinetic temperature (e.g. Bergin et al. 1994).

Cyclo-propenylidene, c-C₃H₂ , is one of the few cyclic molecules detected in interstellar space. It is a slightly asymmetric, almost oblate top. C₃H₂ has ortho - and para -states originating from the two possible relative orientations of the spins of the two hydrogen nuclei (having each). In the ortho-state the spins are parallel (), whereas in the para-state the spins are anti-parallel (). Since the ortho- and para-forms cannot be interchanged in electric dipole transitions or easily in chemical reactions, they can be seen as two different chemical compounds. The ground-state of the ortho-form () lies 2.35 K higher than that of the para-form (). The electric dipole transitions of c-C₃H₂ are of b-type. This means that the quantum numbers describing the projection of J on the molecular axes and axes, and , change according to the rule , (see Vrtilek et al. 1987).

Diazenylium, N₂H⁺ , is a linear molecular ion. Its rotational transitions have hyperfine structure due to the coupling of the spins of the two nitrogen nuclei (,) to the rotational angular momentum (J). The energy levels are labelled using the quantum numbers J, and F, where and (e.g. Caselli et al. 1995; Gordy & Cook 1970, Sect. 9.5). The transition observed here has seven well resolved hyperfine components.

Methanol, CH₃OH , is a slightly asymmetric top (e.g. Townes & Schawlow 1975, chapters 4 & 12). It has a rather complicated rotational spectrum, since it exhibits so called hindered internal rotation: the hydroxyl (OH) group can rotate with respect to the methyl (CH₃) group. This gives rise to two different forms of CH₃OH between which both radiative and collisional transitions are forbidden: the A-symmetry and E-symmetry forms. Chemical reactions that might convert A to E or vice versa are believed to occur only on very long time scales. Therefore the two forms of CH₃OH can be treated as chemically distinct species. The ground-state of the E-symmetry form () lies 6.95 K higher than that of the A-symmetry form (). The relevant quantum numbers describing the rotational spectrum are J and k, with . The electric dipole transitions observed here are of a-type, i.e. they obey the selection rule . Like other symmetric or slightly asymmetric tops, CH₃OH may provide useful information about the gas temperature.

2.2. Chemistry

The buildup of cyanopolyynes proceeds mainly via reactions between complex hydrocarbon ions and nitrogen atoms (Herbst & Leung 1989). HC₃N is the simplest cyanopolyyne, and its abundance correlates well with heavier cyanopolyynes (e.g. Federman et al. 1990). Due to its large dipole moment, as well as being chemically an early-time species, HC₃N should be a useful probe of the conditions in pre-stellar cloud cores.

The main reactions leading to HC₃N and CH₃CCH involve either the C₃H or C₃H ions (Huntress & Mitchell 1979). The neutral reactions involving C₂H₂ and CCH suggested recently by Turner et al. 1998and Turner et al. 1999do not change this fact, since these species are also derivatives of the aforementioned molecular ions. Therefore, HC₃N and CH₃CCH abundances are expected to be well correlated.

Reactions of nitrogen-bearing ions, such as HCNH⁺, with neutral hydrocarbons may also contribute to the production of cyanopolyynes (Herbst & Leung 1989). Since HCNH⁺ is the precursor ion of HCN and HNC (Herbst 1978), a correlation between these isomeric molecules and cyanopolyynes is possible. In this paper, we shall not address the as yet unresolved problem of the variation of the HNC/HCN abundance ratio from source to source (for a recent overview see e.g. Talbi & Herbst 1998). We instead used HNC merely to survey the extension of the core CrA C, where HC₃N was not detected. This should be as abundant as HCN but easier to detect due to the smaller hyperfine splitting,

In their survey towards dense cores of dark clouds, Suzuki et al. 1992found a strong correlation between the CCS and HC₃N column densities, and, using data from Cox et al. 1989, a weaker correlation between CCS and C₃H₂. They explained these correlations by the production of CCS in reactions between hydrocarbons and S⁺, which should be abundant in regions where also carbon-chain molecules are formed (Prasad & Huntress 1982). The formation of CS also involves S⁺ and occurs in the very early stages of chemical evolution. Thereafter, CS is mainly recycled via the thioformyl ion, HCS⁺, and its abundance is considered to be rather constant in time (Nejad & Wagenblast 1999).

The correlation between HC₃N and CCS is due to fact that C₃H dominates the production of both molecules. HC₃N is related to C₃H₂ via the neutral-neutral reaction (Herbst & Leung 1989; Nejad & Wagenblast 1999). On the other hand, complex carbon chain molecules and CCS show no correlation with ammonia, NH₃ (e.g. Little et al. 1979; Suzuki et al. 1992). This can be explained by the fact that the production of NH₃ involves molecular nitrogen, N₂, which is formed in neutral-neutral reactions and becomes abundant in the later stages of cloud evolution when carbon chain molecules have already been lost in reactions with ions such as He⁺, H⁺ and H (Suzuki et al. 1992). N₂H⁺ is also formed from N₂ by and its abundance has been found to follow closely that of NH₃ (Hirahara et al. 1995; Nejad & Wagenblast 1999). Therefore, if the CCS/NH₃ abundance ratio can be used as an age indicator as suggested by Suzuki et al. 1992, the same should be true for CCS/N₂H⁺. The depletion of CO onto grain surfaces further increases the N₂H⁺ abundance by making H available for non-carbon bearing species to react with (Nejad & Wagenblast 1999).

SO is destroyed by neutral carbon and, like N₂H⁺, it is formed in neutral-neutral reactions and benefits from the freezing-out of CO. Its abundance therefore increases slowly and probably remains low as long as the gas is rich in neutral atomic carbon. The SO abundance first increases when carbon is consumed in the production of CO, i.e. at late stages in the chemical evolution (Bergin & Langer 1997; Nejad & Wagenblast 1999). The late peaking of SO and the stability of CS (until it too is frozen out) has led to the suggestion that the SO/CS abundance could be used as a chemical clock (Ruffle et al. 1999). Nilsson et al. 2000came to the same conclusion on the basis of pure gas phase chemistry models. However, the SO/CS ratio is also sensitive to the gas-phase O/C ratio. Thus, when observing the SO/CS ratio in two individual clouds, it is not easy to disentangle the effects of time evolution from those of the O/C ratio.

The precursor ion of CH₃OH, , is formed from the radiative association reaction between and H₂O (e.g. Herbst & Leung 1989). The close relation to makes CH₃OH an early-time molecule, as long as gas phase reactions are considered. The large abundances of CH₃OH in high-mass star-forming cores has been explained by evaporation from grain surfaces due to shock heating (e.g. Menten et al. 1988).

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000
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