Lithium solar abundance

SNII events alone explain the observed solar abundance distribution between oxygen and chromium. This can be taken as a major theoretical achievement. Complementary sources of hydrogen, helium, lithium, beryllium, boron, carbon and nitrogen are required, and these have been identified. They are the Big Bang, cosmic rays and intermediate-mass stars. Around iron and a little beyond, we must invoke a contribution from type la supernovas (Pig. 8.5). These must be included to reproduce the evolution of iron abundances, a fact which suggests... 

Natural isotopes of lithium and their solar abundances... 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the Sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed or usually assumed to be identical to that of the Sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the Sun s interior by nuclear reactions. The composition of the Sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy and, accordingly, the relative abundances of elements in the Sun are referred to as "cosmic abundances". Although the cosmic abundance pattern is commonly seen in other stars, there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as is the case with neutron stars. The best estimation of solar abundances is based on data from optical spectroscopy and meteorite studies and in some cases extrapolation and nuclear theory. The measured solar abundances are listed in Fig. 2-1 and Table 2-1. It is believed to be accurate to about 10% for the majority of elements. The major features of the solar abundance distribution are a strong decrease in the abundance of heavier elements, a large deficiency of Li, Be, and B, and a broad abundance peak centered near Fe. The factor of 10 higher... 

Fig. 2. (Left panel) evolutionary tracks using FST in the logTefj vs. log g plane (solid line non gray models with rph = 10 by Montalban et al.,2004) and 2D calibrated MLT (dashed line).(Right panel) Lithium evolution for the solar mass with different assumptions about convection and model atmospheres. The dotted line at bottom represents today s solar lithium abundance. MLT models with AH97 model atmospheres down to Tph = 10 and 100 are shown dotted for cum = 1 and dash-dotted for cpr, = 1.9. The Montalban et al. (2004) MLT models with Heiter et al. (2002) atmospheres down to Tph = 10 (lower) and 100 (upper) are dashed The continuous lines show the non gray FST models for rph = 10 and 100, and, in between, the long dashed model employing the 2D calibrated MLT.

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Lithium is an element of interest in astrophysics, owing to the marked variations in Li abundance and isotopic composition of materials emanating from different sources in the cosmos. The development of the terrestrial or Solar system Li isotopic compositions, which, to the limit of our current rmderstanding, are quite different from values predicted for nucleosynthetic or interstellar Li, remains a puzzle for active astronomical research. 

Beryllium-10 P-decays to 10B with a half-life of 1.5 Myr. Beryllium and boron (along with lithium) are several orders of magnitude less abundant than the other light elements in the solar system because, except for 7Li, they are not produced in stars. They are produced when high-energy cosmic rays, mostly protons, fragment atomic nuclei into small pieces in a process called spallation. Beryllium-10 is constantly being produced at low levels by spallation in the solar system, and its abundance in bulk meteorites is used to estimate the amount of time that they were exposed to cosmic rays as small bodies (their cosmic-ray... 

The observed lines gives the stellar photospheric abundance via the standard stellar atmospheric technique. For stars with a surface temperature T > 5500 K and a metallicity less than about l/20th the solar metallicity, the abundances pratically show no dispersion (the famous lithium plateau for such stars.)... 

From the isotopic decomposition ofnormal lithium onefinds thatthemass-6 isotope, 6Li, is the lesser abundant of lithium s two isotopes 7.5% of terrestrial Li. Lithium presents some of the most interesting abundance questions in astrophysics (see also 7Li). Using the total abundance of elemental Li = 57.1 per million silicon atoms in solar-system matter, this isotope has... 

Relative to silicon, the total elemental lithium is even 140 times less abundantin the solar photosphere than on Earth or inmeteorites, as recorded by the solar spectrum. Since the Li/Si element abundance ratio in the meteorites should also have entered the Sun when it formed, one concludes that the Sun must be destroying its initial lithium supply as it ages. This occurs at the base of the surface convection zone of the Sun. The bottom of that zone lies at a depth that is about 1/4 of the Sun s radius. Here the high temperatures (a few million degrees kelvin, MK) that solar-surface nuclei experience is hot enough to destroy lithium, especially 6Li, by nuclear interactions with protons (6Li + p —3He + 4He). Those proton-induced nuclear reactions destroy 6Li much more readily than they do 7Li because the quantum probabilities of the reaction are greater than for 7Li. As a result, to deplete elemental lithium by a factor of 140 in the Sun... 

It is appropriate to begin this lecture with a diagram from the review of Shapiro Silberberg, 1970, which compares the abundances of elements in the cosmic radiation with solar system abundances. This classic measurement is one of the foundations of cosmic-ray physics. The elements lithium, beryllium and boron are quite abundant among cosmic rays even though they constitute only a tiny fraction of the material in the solar system and the interstellar medium. This fact is understood largely as the result of spallation of the... 

Figure 8. A compilation of the lithium abundance data from stellar observations as a function of metallicity. N(Li) = 1012(Li/H) and [Fe/H] is the usual metallicity relative to solar. Note the Spite Plateau in Li/H for [Fe/H] — 2.

In some recipes describing the galactic evolution of the Li abundance, synthesis by novae is an important ingredient for example, Romano et al. (2001) ascribe 18 % of the solar system s lithium to production in novae. Novae again tap the 3He reservoir of the low mass star which feeds the white dwarf whose surface is the site for the novae explosions. A feature of Li-synthesis by novae is that the 7Be is ejected before it decays... 

The light and fragile elements lithium, beryllium, and boron (LiBeB) are not primarily produced in primordial or stellar nucleosynthesis. In fact, the abundance curve in O Fig. 12.13 shows a huge dip (almost a gap, actually) for the mass numbers 8-11, reflecting the scarcity of LiBeB-nuclei in the solar system. Only the nuclide Li can be produced both in primordial (see Sect. 12.3) and in stellar nucleosynthesis (see Sect. 12.4.2), whereas the nuclides Li, Be, B, and B are almost pure spallation products of heavier elements. 

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