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Astronomy

The cosmic origin of deuterium

Most of the deuterium in the Universe was thought to be created during the first three minutes after the Big Bang, along with other light elements such as hydrogen, helium and lithium. All the elements heavier than carbon were subsequently synthesized inside stars. Stars burn up primordial deuterium to form heavier elements, a process known as astration. The fact that deuterium is only ever destroyed and not created by stars makes this element a good indicator of past star formation. So, regions where there has been a high rate of stellar activity should be depleted in deuterium, unless they are resupplied with primordial gas (that is, gas with a chemical composition unchanged since the Big Bang).

On page 1025of this issue, Lubowich et al.1 report the discovery of deuterium near the centre of our Galaxy. Their measurement of deuterium (more specifically the ratio of deuterium to hydrogen, D/H) in a molecular cloud near the Galactic Centre provides crucial information about the amount of astration that has occurred over the lifetime of our Galaxy and consequently about the primordial abundance of deuterium.

The primordial abundance of deuterium is very sensitive to the density of ordinary matter (baryons) in the early Universe and helps us to establish whether the Universe will remain open or eventually close. This is why any means of deriving the primordial deuterium abundance is important for cosmology. There are two ways of finding the primordial abundance. The first requires measurement of the abundance of deuterium in distant (high-redshift) objects, namely young protogalaxies. But these measurements only give a lower limit for the primordial deuterium because the protogalaxies also contain metals (in astrophysics, metals are all the elements heavier than helium), indicating that at least some stellar processing has already taken place. The available measurements provide a rather low value2 for the primordial abundance of deuterium, (D/H)P≈3.5×10−5, although much higher values have been suggested3. For this reason, it is important to have an independent way of estimating the primordial abundance.

Another route is to estimate the amount of astration taking place over the lifetime of a galaxy by means of models of chemical evolution. Such models try to reconstruct the chemical history of the interstellar gas in a galaxy since the Big Bang. They do this by assuming a specific history for star formation and taking into account stellar processing, in particular the processed and unprocessed material that stars release into the interstellar medium, either during their lifetime or when they die as supernovae.

Possible sources of material from outside and inside the system, such as gas flows into and out of the galaxy, are also considered. Successful models for the chemical evolution of our Galaxy suggest that the stellar disk formed by infall of mainly primordial gas, which built up faster in the inner than in the outer regions4. This process appears to continue today, at least in the outer regions, with evidence of gas infall from observations of incoming high-velocity clouds of neutral hydrogen5. The primordial abundance of deuterium is usually estimated from chemical evolution models of the local Galactic disk. Constraints on these models are the observed abundance of deuterium in the Solar System6 and the local interstellar medium7.

Most chemical evolution models of the local disk8,9 suggest a primordial abundance of deuterium, (D/H)P≈(3.0–5.0)×10−5, in close agreement with the low values of D/H measured in distant (very young) objects. But these models contain many assumptions. The abundance of deuterium in the Galactic Centre is a good test for the models because, in the absence of recent infall of primordial gas, deuterium would be completely removed from the gas, unlike in the local interstellar medium.

Models devised for the centre of the Galaxy reveal that the Galactic bulge (a spherical region near the centre) formed more quickly and therefore had a greater star-formation rate than the local disk. As a result, the gas in the bulge has been enriched in heavy elements more efficiently than in the rest of the disk. The bulge and the Galactic Centre are indeed rich in metals, as demonstrated by the high iron content (almost ten times the Solar value) measured in some central stars10. A chemical evolution model11 explaining such abundances also predicts a very low abundance of deuterium at the present time (D/H≈3.2×10−11) when a primordial abundance in the range (3.0–5.0)×10−5 is assumed.

The abundance of deuterium measured by Lubowich et al.1 in the Galactic Centre is the lowest ever measured in the Milky Way (D/H≈1.7×10−6). Nonetheless it is much larger than that predicted by models of the Galactic Centre. But the low value predicted for the centre neglects the possible infall of gas containing primordial deuterium at later times, which would increase the present D/H abundance. So the abundance measured by Lubowich et al. is consistent with a high rate of deuterium astration (relative to the local disk), but also requires recent infall of primordial or metal-poor material into the Galactic Centre.

When these effects are taken into account, the deuterium abundance measured by Lubowich et al. is consistent with a (D/H)P≈(3.0–5.0)×10−5, in line with previous results. For these primordial values of the deuterium abundance, the standard Big Bang model predicts that the baryonic density is only a small fraction (3–5%) of the critical density necessary to close the Universe. This low baryon fraction has important cosmological implications if compared with the higher fraction (15–25%) of baryons measured in galaxy clusters. On one hand it implies that most of the matter in the Universe is non-baryonic dark matter. On the other hand, it indicates that the total density of matter is only one third of the critical density needed to close the Universe.

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Correspondence to Francesca Matteucci.

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Matteucci, F. The cosmic origin of deuterium. Nature 405, 1009–1010 (2000). https://doi.org/10.1038/35016682

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