ISOTOPIC RATIOS OF O-18/O-17 IN THE GALACTIC CENTRAL REGION

The O-18/O-17 isotopic ratio of oxygen is a crucial measure of the secular enrichment of the interstellar medium by ejecta from high-mass versus intermediate-mass stars. So far, however, there is a lack of data, particularly from the Galactic center (GC) region. Therefore, we have mapped typical molecular clouds in this region in the J = 1-0 lines of (CO)-O-18 and (CO)-O-17 with the Delingha 13.7 m telescope (DLH). Complementary pointed observations toward selected positions throughout the GC region were obtained with the IRAM 30 m and Mopra 22 m telescopes. (CO)-O-18/(CO)-O-17 abundance ratios reflecting the O-18/O-17 isotope ratios were obtained from integrated intensity ratios of (CO)-O-18 and (CO)-O-17. For the first time, (CO)-O-18/(CO)-O-17 abundance ratios are determined for Sgr C (V similar to-58 km s(-1)), Sgr D (V similar to 80 km s(-1)), and the 1 degrees.3 complex (V similar to 80 km s(-1)). Through our mapping observations, abundance ratios are also obtained for Sgr A (similar to 0 and similar to 50 km s(-1) component) and Sgr B2 (similar to 60 km s(-1)), which are consistent with the results from previous single-point observations. Our frequency-corrected abundance ratios of the GC clouds range from 2.58 +/- 0.07 (Sgr D, V similar to 80 km s(-1), DLH) to 3.54 +/- 0.12 (Sgr A, similar to 50 km s(-1)). In addition, strong narrow components (line width less than 5 km s(-1)) from the foreground clouds are detected toward Sgr D (-18 km s(-1)), the 1 degrees.3 complex (-18 km s(-1)), and M+ 5.3-0.3 (22 km s(-1)), with a larger abundance ratio around 4.0. Our results show a clear trend of lower (CO)-O-18/(CO)-O-17 abundance ratios toward the GC region relative to molecular clouds in the Galactic disk. Furthermore, even inside the GC region, ratios appear not to be uniform. The low GC values are consistent with an inside-out formation scenario for our Galaxy.