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Abundances of Light Elements B Proc. Natl. Acad. Sci. USA Vol. 90, pp. 4789-4792, June 1993 Colloquium Paper This paper was presented at a coUoquium entitled "Physical Cosmology," organized by a committee chaired by David N. Schramm, held March 27 and 28, 1992, at the National Academy of Sciences, Irvine, CA. Abundances of light elements B. E. J. PAGEL NORDITA, Blegdamsvej 17, 2100 Copenhagen 0, Denmark ABSTRACT Recent developments in the study of abun- cosmic abundance at any one time is a lower limit to the dances of light elements and their relevance to cosmological primordial abundance, which is itself a steeply falling func- nucleosynthesis are briefly reviewed. The simplest model, tion of the baryon density parameter. Studies of meteorites based on standard cosmology and particle physics and assum- and planetary atmospheres indicate a 2H/H ratio between 2 ing homogeneous baryon density at the relevant times, contin- and 3 x 10-5 at the formation of the Solar System, whereas ues to stand up well. Copernicus and International Ultra-violet Explorer (IUE) observations of interstellar Lyman lines have given values The "standard" Big Bang nucleosynthesis (SBBN) model that are mostly between 0.8 and 2 x 10-5 in the present-day (1-6), which assumes a homogeneous baryon density at the interstellar medium (5, 6). Recently a very accurate deter- relevant times, standard cosmology, and particle physics, mination has been made using the Hubble Space Telescope makes definite predictions of primordial abundances as a along the line of sight to Capella giving a 2H/H ratio of (1.65 function of a single free parameter: the ratio of baryons to + 0.15) x 10-5 (15). Because Ly-a is the only Lyman line photons, which has remained unchanged since the epoch of accessible to the Hubble Space Telescope (except at high positron-electron annihilation a few seconds after the Big redshifts), and this line is already saturated Bang and is related through the known temperature of the severely along the microwave background to the mean density ofbaryons in the relatively short sight line of 13 parsecs to Capella, it will be universe today. Previous discussions (4-10) have shown that a long time before one can tell how much, if any, of the the best fit to what has been deduced about primordial substantial dispersion apparent in the Copernicus data is real. abundances from extrapolations of cosmochemical and as- 3He is detected in the Sun (16) and in meteorites; its trophysical data concerning 2H, 3He, 4He, and 7Li occurs abundance was (1.5 + 0.3) x 10-5 relative to hydrogen at the within a quite narrow range of this density parameter corre- formation of the Solar System (5, 6), and it has also been sponding to flboh2oo in the range 0.010-0.015, where ilbo is the detected in Galactic HII regions with substantial abundance present-day contribution of baryons to the cosmological variations from one region to another (17). A large abundance density in units of the Einstein-de Sitter closure density and of the order of 10-3 or more was recently detected in the h1oo is the Hubble expansion parameter in units of 100 planetary nebula NGC 3242 (18), confirming the theoretical km s-l Mpc-1 or (1010 years)-1 [1 parsec (pc) = 3.09 x 1016 expectation that 3He not only survives but also is freshly m], which has a value somewhere between about 0.5 and 1. produced in advanced stages of evolution of at least some These results have been very successfully used, furthermore, stars. Because 3He is the main product resulting from the to predict upper bounds on the mean life offree neutrons and destruction of 2H when diffuse interstellar material is cycled on the number of light neutrino families (6, 11). Some through stars, and some ofit survives further stellar process- reservations about SBBN, however, have motivated inves- ing and may even be added to, it is possible to place a quite tigations of more complicated models involving either non- firm upper limit of 10-4 on the primordial ratio (2H + 3He)/H standard particle physics or baryon density inhomogeneities (refs. 4-6; see also ref. 19), which in turn gives the most following the quark-hadron phase transition (12). These stringent available lower bound to the cosmological baryon reservations include the idea that perhaps Qb = 1 [which is density. actually excluded even in inhomogeneous models (13)], the possibility (discussed below) that the primordial helium mass Lithium-7 fraction Yp might be significantly less than the minimum of 0.235 or 0.236 (6, 14) required for compatibility within SBBN For the preferred range of the baryon density parameter, theory with the upper limit on primordial (2H + 3He)/H, and SBBN predicts a primordial 7Li abundance with 7Li/H = (1 the possibility that there might be a primordial "floor" to 2) x 10-10, only an order of magnitude below the amount (analogous to those displayed by 4He and 7Li) to the abun- seen in young stars and meteorites. For any other value ofthe dance of heavier elements (e.g., Be) that are not predicted to density, or in models with inhomogeneous baryon density, have detectable primordial values within SBBN theory. In the predicted primordial abundance is still higher. This con- what follows I report and comment on some of the most trasts with the abundances of all heavier elements in the very recent developments in studies ofthe distribution ofelements old stars belonging to the halo population of the Galaxy, that are relevant to these questions. which are lower than in the Sun and other stars belonging to the Galactic disk by factors of tens, hundreds, and even Deuterium and Helium-3 thousands. The discovery of Li with the cosmologically predicted abundance in such stars (20) was (apparently at 2H is of special interest because it is destroyed rather than least) a triumph for SBBN theory but also somewhat sur- created in all plausible astrophysical processes so that its prising because moderately old disk stars like the Sun, with a similar effective temperature, have by now destroyed most The publication costs of this article were defrayed in part by page charge of the Li in their surface layers by convective mixing with payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. Abbreviation: SBBN, standard Big Bang nucleosynthesis. 4789 Downloaded by guest on September 28, 2021 4790 Colloquium Paper: Pagel Proc. Natl. Acad. Sci. USA 90 (1993) hotter layers below. In halo dwarf and subgiant stars, with the photospheric value by a factor of2 (39). The upshot ofthis just a few exceptions (21, 22), the Li abundance is remarkably work, together with earlier work on more metal-rich stars (26, constant over a wide range of stellar effective temperatures 40), is that Be tracks 0 in abundance quite closely (and Fe (23, 24), falling offas a result ofdeeper convection zones only more roughly), which (i) indicates that there is no trace of a at much lower effective temperatures than that below which cosmological "floor" to Be at the level of a few times 10-14 it is substantially depleted in all but the youngest stars of the atoms per H atom but (ii) presents a challenge to conven- disk, and it is quite independent of the abundance of heavier tional cosmic ray spallation theory, which led to a suggestion elements or "metallicity" for [Fe/H] s -1.8 (the square that some third process might be involved (41). brackets indicate the logarithmic abundance relative to so- An clue to the of Be in the Galactic halo lar). At higher metallicities, a large range sets in due to important origin varying amounts of depletion, but with an upper envelope is provided by the B abundance that has so far been measured suggestive of an additional component that varies paripassu using the Hubble Space Telescope in HD 140283 and two with Fe (9, 24). other metal-deficient stars giving a B/Be ratio of 10 (42). This whole pattern provides circumstantial evidence that The expected B/Be ratio from spallation is somewhere in the we are indeed seeing in the halo stars something very close range of 8-15, depending on the energy spectrum. A flatter to the primordial 7Li abundance with relatively little effect of spectrum than that prevailing today can account for a B/Be depletion. Such a view is compatible with standard models of ratio as low as 10 and at the same time remove the embar- stellar evolution (25) that neglect rotational effects and other rassment of relative overproduction of Li pointed out in ref. such complications and may quite possibly apply to most halo 34, and such a spectrum with the enhanced path length stars (though not to the Sun). On the other hand, rotational required to compensate the low abundance of C, N, 0 nuclei models with a much higher degree of destruction could also can be made plausible on the (still ad hoc) hypothesis that apply (26), in which case the agreement with SBBN would be cosmic rays were confined in the early Galaxy, leading to merely a (strange) coincidence (27). energy-dependent ionization losses (43) (Fig. 1). Alterna- tively, spallation could have taken place in the immediate Beryllium supernova environment (37). It seems, therefore, that spal- lation is indeed the most likely source ofboth Be and B in the SBBN predicts no significant primordial abundances for any Galactic halo. element heavier than 7Li, but some versions of inhomoge- neous models have predicted significant amounts of heavier Helium elements, especially for large values Of fb (28), although this is not confirmed in later versions (29).
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