Constraints on Galactic Center Activity: A Search for Enhanced Galactic Center Lithium and Boron

Abstract

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 -rst search for the ground-state hyper-ne-structure transitions of Li I F 2E1; 803 MHz) and B I (2S 1@2 ; F 2E1; 732 MHz). We used the 43 m NRAO radio telescope to search for enhanced Galactic (2P 1@2 ; 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 ) 1016 cm~2, (Li/H) 3.9 ) 10~8, N(B I) 2.2 ) 1018 cm~2, and (B/H) 9.2 ) 10~6 for the dense 20 km s~1 Sgr A molecular cloud where our largest sources of uncer- tainties are Li I/Li, B I/B, and N(H). Our observations imply (Li/H) GC 22 (Li/H) disk , (Li/H) GC 39 For a simple (Li/H) diskvspallation , (B/H) GC 1.2 ) 104 (B/H) disk , (B/H) GC 1.5 ) 104 (B/H) diskvspallation . model combining mass loss from AGB stars (only for Li), spallation reactions, and SN l-nucleosynthesis, we estimate (13 times enhancement) and (10 times (Li/H) GC 1.3 ) 10~8 (B/H) GC 7.4 ) 10~9 enhancement). If Li is primarily produced via spallation reactions from a cosmic-ray proton Nux / p (t) with the same energy and trapping as in the disk, then (/ Comparing our / p (t)dt) GC 13(/ / p (t)dt) disk . results to AGN models, we conclude that the GC has not had an extended period of AGN activity containing a large cosmic-ray Nux ergs s~1 for 108 yr), a large low-energy cosmic-ray Nux (L CR 1044 (less than 100 times the disk Nux), or a large c-ray Nux ergs s~1 for 109 yr). Furthermore, (L c 1042 since any Galactic deuterium production will signi-cantly 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 orig- inated from the infall of primordial matter with the current D/H reduced by astration and mixing. Subject headings: Galaxy: center E Galaxy: evolution E ISM: abundances E radio lines: general E radio lines: ISM