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Other available formats: PDF | gzipped Postscript
Lithium Trail Leads Back to the Big Bang
Sean G. Ryan (Dept. of Physics and Astronomy, The Open University)
Studies
of lithium with the high resolution spectrographs UES and UCLES on the
WHT and AAT have revealed a slow but steady rise in the Galactic lithium
abundance from the earliest stars in the halo of the Galaxy up until the
epoch at which the old disk population formed. After this period, prolific
Li nucleosynthesis is apparent. Long-standing uncertainties over the contributions
of galactic chemical evolution and the destruction of Li can now be addressed,
and the primordial lithium abundance and ultimately the universal baryon
density 
B can be
inferred more reliably. The observations may also help clarify the roles
of AGB stars and novae in more recent periods of galactic chemical enrichment.
Introduction
Studies by many workers following Spite & Spite's (1981, 1982) discoveries of an almost uniform Li abundance in the old stars of the Galaxy (e.g. Rebolo, Molaro, & Beckman, 1988) supported the interpretation of this abundance as the primordial one, at worst "hardly altered" (Spite & Spite, 1982). This interpretation required that any depletion of the stellar surface Li from a higher initial abundance be minimal. While ample evidence existed of Li destruction in some stars, the lack of a spread in halo dwarf Li abundances provided empirical evidence that destruction may have been minimal in these objects (e.g. Boesgaard & Steigman, 1985).
Classical stellar evolution models
(e.g. Deliyannis, Demarque, & Kawaler, 1990) fitted this interpretation,
showing that Li destruction in metal-deficient dwarfs with shallow surface
convective zones would be minimal (
0.05dex). However, the same models failed to explain numerous Population
I star observations, and an alternative class of models possessing extra
mixing implied that considerable Li depletion as high as 1dex could have
occurred in the halo stars. Other dissenting voices were heard. Deliyannis,
Pinsonneault, & Duncan (1993) argued that there was a non-negligible
spread in the Li abundances of the halo dwarfs that would not be consistent
with a perfectly primordial composition. Thorburn (1994) found an even
greater intrinsic spread 
0.10 dex, and moreover claimed, as did Norris, Ryan, & Stringfellow
(1994), that the abundances depended on both Teff and [Fe/H].
Such dependences were contrary to the notion of a unique Li abundance in
the halo stars, and thus undermined the association of the observed Li
abundance(s) with the primordial one.
The Intrinsic Spread of 7Li
Ryan, Norris, & Beers (1999)
set out to obtain a highly homogeneous data set on a sample occupying only
a narrow range of Teff, [Fe/H], and evolutionary type. Restricting
their sample to 6000K
Teff
6400K and –3.5 [Fe/H]
–2.5, applying double-blind data analysis techniques, obtaining multiple
high-resolution, high-S/N UCLES observations of the targets, and using
multiple temperature indicators to minimise random errors, they achieved
a formal abundance error as low as err
0.033 dex per star. These results have considerably higher precision than
most previous Li measurements (for which typically err
0.06–0.08 dex).
Excluding one previously known
ultra-Li-deficient star, the objects exhibited a total observed spread obs=
0.053 dex, considerably less than that found by Thorburn (1994). However,
this 0.053 dex was found to be dominated by an underlying metallicity dependence,
and the spread of the Li abundances about this trend is a mere obs=0.031
dex, and Gaussian in form. This corresponds to the spread in Li abundance
at a given metallicity. Comparing this with the formal measurement errors
of err =0.033 dex
leads to the conclusion that the intrinsic spread in the stars must be
negligible.
The very narrow spread of Li abundances constrains the impact of extra-mixing in so far as extra-mixing models predict a spread in the final Li abundances of a population of stars. The rotationally-induced mixing models of Pinsonneault et al. (1999) suggested that Li depletion by ~0.2–0.4 dex existed. As Figure 1 shows, the new data with their narrower spread (at a given metallicity) rule out rotationally-induced mixing models that exhibit even 0.1 dex median depletion. Previous results that gave contrary conclusions can be understood as arising from a mixture of stars having different metallicities and by poorer quality data.
 |
| Figure 1. Spread in Li abundance A(Li) (at a given metallicity [Fe/H] = –2.8 after compensation for the [Fe/H] dependence of Li), compared with predictions for a rotationally-induced mixing model exhibiting a median depletion of 0.1dex. The absence of stars in the tail of the theoretical distribution indicates that this particular model overestimates Li destruction. [ JPEG | TIFF ] |
The Underlying Li vs [Fe/H] Trend
Although Li GCE during the halo-forming era has often been ignored, we should not be surprised that it exists. With modern CCDs and large aperture telescopes, even small levels of 7Li enrichment can be measured. Moreover, it is consistent with recently measured 6Li abundances (Smith, Lambert, & Nissen, 1993,1998; Hobbs & Thorburn, 1997; Cayrel et al., 1999, Deliyannis & Ryan, 2000).
To examine Li GCE, Ryan et al.
(2000b) obtained UES and UCLES data on 18 halo stars with –2.0
[Fe/H] –1.0, in the same temperature
range as the metal-poor sample, and compared the abundances with several
GCE models. The Fields & Olive (1999a,b) model includes the most likely
Population II sources of Li, the 
-process
in supernovae and GCR spallation, and the models of Romano et al. (1999)
incorporate many Population I sources, primarily the -process,
AGB stars, and novae. Figure 2 shows the hybrid;
it provides a reasonable fit to the data over the history of the Galaxy
from [Fe/H] = –3.5 to –0.6. Mismatches at the highest metallicities may
be due to the exclusion of the newly studied cool bottom processing of
Li, and/or to sizeable uncertainties in the theoretical Li yields of novae.
 |
| Figure 2. Evolution of Li with metallicity. Observations are for halo stars having Teff>5800K, to avoid lower-mass stars with Li depletion and to reduce the heterogeneity of the sample, and Population I stars from sources indicated. The model (solid curve) is a hybrid using the Population II contribution of Fields & Olive (1999a,b; Ryan et al., 2000a) with Population I evolution from Romano et al. (1999). [ JPEG | TIFF ] |
The Primordial Li Abundance and Uncertainties
The new constraints on the stellar
destruction of Li and its production over Galactic history permit a new
calculation of the primordial Li abundance taking into account a wide range
of random and systematic uncertainties. Beginning with the observed abundance
for the most metal-poor stars, we apply corrections for the inferred GCE
contribution (with uncertainties) and for stellar depletion. Uncertainties
in the stellar temperature scales remain one of the largest sources of
error, and in this analysis we adopt the zero-point of the IRFM scale of
Alonso, Arribas, & Martínez-Roger (1996). We associate an uncertainty
of ±0.08 dex with this choice, in recognition of the remaining difficulties
in the temperature scales for halo dwarfs. These and the other effects
lead us to infer a primordial abundance A(Li)p=2.09 
dex, where the uncertainties resemble 2
limits (Ryan et al., 2000a). The baryon-to-photon ratios corresponding
to this range are in excellent agreement with those inferred from estimates
of the primordial He abundance, but the range of deuterium measurements
published in recent years present more of a challenge. Disfavoured high
D/H values(~20×10–5) are compatible with the He and Li
values, whereas the lowest values (~3.4×10–5) are barely
so. However, even slightly larger D/H values around ~5×10–5
are in tolerable agreement with the 7Li and 4He values
(Ryan et al., 2000a). In any event, the inferred baryon density range,
continues to lie 10 times below
the value of M
inferred from recent cosmological studies (e.g. Efstathiou et al., 1999).
Acknowledgments
This work represents collaborations involving Prof J. E. Norris (Australian National University), Dr T. C. Beers (Michigan State University), Dr K. A. Olive (University of Minnesota), Dr B. D. Fields (University of Illinois at Urbana-Champaign), Dr T. Kajino (National Astronomical Observatory of Japan), Ms. D. Romano (SISSA, Trieste), Dr. F. Matteucci (University of Trieste), and Ms K. Rosolankova (The Open University, & St Hilda's College, Oxford), whose contributions are gratefully acknowledged.
References:
Email contact: Sean Ryan (S.G.Ryan@open.ac.uk)
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