The importance of lithium
observations lies on being a probe of cosmological models and also as a
tracer for stellar structure and evolution. New measurements of the lithium
abundance in a sample of 37 field dwarfs of low metallicity was corried
out. Extremely Metal-Deficient (EMD) dwarfs form a much more homogeneous
population with regard to 7Li abundance that stars somewhat
more metal-rich. These EMD dwarfs show Li abundances for surface temperatures
between 5500K and 6300K, which average log N(Li)=2.08 (±0.10). A
trend towards 2.2 is found as Teff approaches 6300K. By comparing
the Li-Teff of the EMD stars with the published curve for the
Hyades, the astronomers show that it is improbable that the warmer EMD
stars have depleted their Li significantly, in spite of their age. They
consider that these stars offer the best opportunity to make a reliable
measurement of the primordial Li abundance, which they estimate to be log
N(Li)=2.2(±0.15).
For somewhat less metal-deficient
stars they find a wider scatter in Li abundances. Li depletion appears
to set in at higher temperatures for the moderately metal-deficient stars
than for the extremely metal-deficient, consistent with metallicity-dependent
depletion rates.
Using a recent theoretical curve
of Li against the universal baryon to photon ratio from a standard Big
Bnag model, and the above cited primordial abundance, n is constrained
to the range 1.2×10-10 <= n <= 8×10-10.
9Be has a special place
in schemes of nucleosynthesis. It is the lightest stable nuclide not synthesized
in the Big Bang and it is destroyed in stellar interiors at temperatures
higher than 3×106K. Nevertheless it has been widely observed
in nature: in the sun, in meteorites, in the interstellar medium and in
a variety of atmospheres of stars of different ages. The currently accepted
explanation for its observed existence is spallation of heavy nuclei, mostly
CNO nuclei, ny cosmic rays in the interstellar medium.
Recently six highly metal-deficient
dwarfs in the spectral range containing the resonance doublet of 9Be
II at 3130 Å were observed. Using the method of spectral synthesis
astronomers evaluated the abundance of 9Be in these objects. They set upper
limits to the 9Be abundance, 9Be/H, below 2.5×10-12.
These are the first reported detections of 9Be in stars with
such low metalicities. From these results and literature values for 9Be
in more metallic stars, they can constrain models for galatic evolution:
those with entail an early burst of 9Be production are ruled
out. The data are used to predict the spallogenic content of 7Li,
6Li,
10B
and 11B in the material from which the sample stars was formed
and hence to limit the galactocentric fraction of 7Li in highly
metal-deficient stars. A firm upper limit of 0.1 dex can be set to the
fraction of measured 7Li in such stars which is either non-primordial,
or is due to spectral blending of 6Li, leaving virtually all
of the measured 7Li as primordial.
|
More
information
ING facilities involved:
-
Isaac Newton Telescope,
using the IDS with either a CCD or a IPCS detector.
Some references:
-
Rebolo, R., Beckman,
J. & Molaro P., 1987, "The lithium abundance in the extremely metal-deficient
dwarf G64-12", Astron. & Astrophys. Lett., 172, 17.
-
Rebolo, R., Molaro,
P., and Beckman, J.E., 1988, "Lithium abundances in metal-deficient dwarfs",
Astron.
& Astrophys., 192, 192
-
Rebolo, R., Molaro,
P., Abia, C. & Beckman, J.E., 1988, "Abundances of 9Be in
a sample of highly metal-deficient dwarfs: implications for early galactic
nucleosynthesis and primordial lithium", Astron. & Astrophys.,
193,
193.
-
Rebolo, R. & Beckman,
J.E., 1988, "Lithium and rotation in the Hyades late F and G stars", Astron.
& Astrophys., 201, 267.
-
"El Berilio medido
en estrellas muy antigua, nuevo soporte a la producción de litio
en el Big Bang", IAC Noticias, May 1987, 3.
|
|