Constraints on Galactic center activity: A search for enhanced Galactic center lithium and boron

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 grou nd-state hyperfine-structure transitions of Li I (S-2(1/2); F = 2-1; 803 MH z) and B I (P-2(1/2); F = 2-1; 732 MHz). We used the 43 m NRAO radio telesc ope to search for enhanced Galactic center (GC) Li and B expected from mode ls of Galactic activity. We did not detect Li I or B I and obtained upper l imits 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)(G C) < 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 mas s loss from AGB stars (only for Li), spallation reactions, and SN nu-nucleo synthesis, 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 produc ed via spallation reactions from a cosmic-ray proton flux phi(p)(t) with th e same energy and trapping as in the disk, then [integral phi(p)(t)dt](GC) < 13[integral phi(p)(t)dt](disk). Comparing our results to AGN models, we c onclude that the GC has not had an extended period of AGN activity containi ng a large cosmic-ray flux (L-CR less than or equal to 10(44) ergs s(-1) fo r 10(8) yr), a large low-energy cosmic-ray flux (less than 100 times the di sk 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 significantl y enhance the abundances of Li and B, our results imply that there are no s ources of D in the GC or Galaxy. Therefore, all the Galactic D originated f rom the infall of primordial matter with the current D/H reduced by astrati on and mixing.