Astron. Astrophys. 324, 27-31 (1997)

2. Luminescence of primordial molecules

As mentioned previously, opacity (in the narrow molecular lines, for example) and peculiar motion of the protoclouds results in CBR disturbances. This can be more easily seen in the rest frame of the proto-object. In this frame the CBR becomes non-isotropic and out of thermal equilibrium. >From the side towards which the protocloud moves, the temperature of the CBR will be higher than average and from the back side it will be lower. After reflection, the photons are distributed isotropically in this frame. This leads to non-isotropic distribution in our frame. That is the explanation of the principal role of opacity. But the amplitude of the effect depends on the peculiar velocity and spectral index of the reflecting radiation (Dubrovich, 1977, Maoli et al., 1994). This effect corresponds to the elastic scattering between the molecules and photons, i.e. the total number of the photons does not change. But in fact all molecules have a quite complicated energy level structure. This allows for the possibility of a non-elastic process. It is the well-known luminescence process which, for example, plays an important role in the formation of radiation from a reflection nebula. In our case a "hot spot" in the CBR (in the rest frame of the cloud) plays the role of a "star" for the reflection nebula. This consideration is just a phenomenological one. For the full description of this process, the equations of the photon transfer must be solved. Here we won't do this, but will only make simple estimates.

Let us consider this process in detail. Taking into account only those chemical elements (the most abundant) which are predicted by the pure Big Bang model - H, He, D, ³ He, Li, and their ions, we can list the most probable molecules in the primordial matter at z = 100-200: H₂, H , HD, HD , HeH , LiH, LiH , H₂ D , ³ He⁴ He . All other molecules should be considered more critically, because their appearance is caused by some non-standard circumstances: non-equilibrium nuclear synthesis at the early times (z= ), or star formation at z = 200-300, etc. But, as was mentioned before, the pure model of the Universe is the most probable one. So, we won`t consider any other molecules here.

For obtaining the greatest interaction between molecules and photons, two values are important - the cross-section for scattering and the concentration (or relative abundance) of this molecule. The first parameter depends on the specific quantum structure of the molecule - its symmetry and charge. The second one depends on the abundance of the chemical elements of which it is composed and on the rate of the appropriate chemical reactions. According to these constraints, we should take into consideration only those molecules which have a large enough dipole moment and relatively high abundance. These are: HD , HeH , LiH, H₂ D , ³ He⁴ He . The molecules H₂ and H have no dipole moment, while HD has a rather small dipole moment and the abundance of D is not high enough. LiH has a very low potential of dissociation and so its abundance in a hot Universe is very small. So, we can expect only a small number of molecules to be visible from the early Universe: HD , HeH , LiH, H₂ D , ³ He⁴ He .

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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