Deuterium in the Diffuse Interstellar Medium
Astrochemistry of Cosmic Phenomena
We review the observational status in evaluating the interstellar deuterium abundance and show that the most reasonable value is of the order of 10~5. Although in general agreement with the standard Big Bang nucleosynthesis, the situation is still unclear and deserves much more observations. Introduction It is largely accepted that the main site of formation of the light elements 2 H, *He, 'He, 7 Li is during the first few minutes of the Universe, in the frame of the so-called standard Big Bang
... d standard Big Bang nucleosynthesis. Among these, deuterium is the most sensitive to primordial conditions, and determining its primordial abundance is therefore of great cosmological significance (see e.g. ). Since the formation of the Galaxy, the evolution of the deuterium abundance X|> is relatively straightforward because it is simply burned when passing through stars. It is predicted that this Galactic astration should decrease Xp by no more than a factor of ~ 2, along with a negative gradient from the Galactic center to the edge small enough to expect a constant D/H ratio in the solar neighbourhood . Although this simplest approach has not yet been proved to be wrong, one should keep in mind that several poorly known parameters and/or unknown pregalactic events could strongly affect these predictions. Deuterium Observations Prior to 1972, deuterium was only measured on the Earth, either in the ocean or inside meteorites, at a ~ 1.5 10 -4 level. Then, it was observed indirectly through J He in the solar wind or directly in planets, in the interstellar medium through deuterated molecules (see respectively Owen and Tielens in these proceedings) or through its atomic form, and in the atmosphere of massive stars. Concerning stars, it has been shown by  that in Canopus, D/H ^ 5.5 10~7, while in the main sequence star a Pav, D/H ^ 10~5  . This latter result implies that deuterium was either also destroyed in a main sequence star, in contradiction with existing model predictions, or that the present value of D/H is at most 10~5 with the important consequences which will be discussed later. 85 P. D. Singh (ed.), Astrochemistry of Cosmic Phenomena, 85-90.