Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...508..729l&link_type=abstract
The Astrophysical Journal, Volume 508, Issue 2, pp. 729-735.
Astronomy and Astrophysics
Astronomy
10
Galaxy: Center, Galaxy: Evolution, Ism: Abundances, Radio Lines: General, Radio Lines: Ism
Scientific paper
The abundances of lithium and boron provide important information about big bang nucleosynthesis, Galactic chemical evolution, stellar evolution, and cosmic-ray spallation reactions. We conducted the first search for the ground-state hyperfine-structure transitions of Li I (^2S_1/2; F = 2-1 803 MHz) and B I (^2P_1/2; F = 2-1 732 MHz). We used the 43 m NRAO radio telescope to search for enhanced Galactic center (GC) Li and B expected from models of Galactic activity. We did not detect Li I or B I and obtained upper limits of N(Li I) < 1.9 x 10^16 cm^-2, (Li/H) < 3.9 x 10^-8, N(B I) < 2.2 x 10^18 cm^-2, and (B/H) < 9.2 x 10^-6 for the dense 20 km s^-1 Sgr A molecular cloud where our largest sources of uncertainties are Li I/Li, B I/B, and N(H). Our observations imply (Li/H)_GC < 22 (Li/H)_disk, (Li/H)_GC < 39 (Li/H)_disk-spallation, (B/H)_GC < 1.2 x 10^4 (B/H)_disk, (B/H)_GC < 1.5 x 10^4 (B/H)_disk-spallation. For a simple model combining mass loss from AGB stars (only for Li), spallation reactions, and SN nu-nucleosynthesis, we estimate (Li/H)_GC = 1.3 x 10^-8 (13 times enhancement) and (B/H)_GC = 7.4 x 10^-9 (10 times enhancement). If Li is primarily produced via spallation reactions from a cosmic-ray proton flux phi_p(t) with the same energy and trapping as in the disk, then [∫ phi_p(t)dt]_GC < 13[∫ phi_p(t)dt]_disk. Comparing our results to AGN models, we conclude that the GC has not had an extended period of AGN activity containing a large cosmic-ray flux (L_CR <= 10^44 ergs s^-1 for 10^8 yr), a large low-energy cosmic-ray flux (less than 100 times the disk flux), or a large gamma-ray flux (L_gamma < 10^42 ergs s^-1 for 10^9 yr). Furthermore, since any Galactic deuterium production will significantly enhance the abundances of Li and B, our results imply that there are no sources of D in the GC or Galaxy. Therefore, all the Galactic D originated from the infall of primordial matter with the current D/H reduced by astration and mixing.
Hobbs Lew M.
Lubowich Donald A.
Turner Barry E.
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