A Systematic Observational Study On Galactic Interstellar Ratio ¹⁸O/¹⁷O

The interstellar oxygen isotopic ratio of ¹⁸O/¹⁷O can reflect the relative amount of the secular enrichment by ejecta from high-mass versus intermediate-mass stars.Previous observations found a Galactic gradient of¹⁸O/¹⁷O,i.e.,low ratios in the Galactic center and large values in the Galactic disk,which supports the inside-out formation scenario of our Galaxy.However,the observed objects are not many and,in particular,not so many at large galactocentric distances.For this reason,we started a systematic study on Galactic interstellar ¹⁸O/¹⁷O,through observations of C¹⁸O and C¹⁷O multi-transition lines toward a large sample of 286sources?at least one order of magnitude larger than previous ones?,from the Galactic center region to the far outer Galaxy??22 kpc?.In this article,we present our observations of J=1-0lines of C¹⁸O and C¹⁷O,with the 12m antenna of the Arizona Radio Observatory?ARO12m?and the IRAM30m telescope.Among our IRAM30m sample of 50 targets,we detected successfully both C¹⁸O and C¹⁷O 1-0 lines for 34 sources.Similarly,our sample of 260 targets for ARO12m observations resulted in the detection of both lines for 166 sources.The C¹⁸O optical depth effect on our ratio results,evaluated by fitting results of C¹⁷O spectra with hyperfine components(assuming?_C18O=4?_C18Oand our RADEX non-LTE model calculation for the strongest source,was found to be insignificant.Beam dilution does not seem to be a problem either,which was supported by the fact of no systematic variation between the isotopic ratio and the heliocentric distance,and consistent measured ratios from two telescopes for most of those detected sources.With this study we obtained¹⁸O/¹⁷O isotopic ratios for a large sample of molecular clouds with different galactocentric distances.Our results,though there are still very few detections made for sources in the outer Galaxy,confirm the apparent ¹⁸O/¹⁷O gradient of ¹⁸O/¹⁷O=?0.10±0.03?R_GC+?2.95±0.30?,with a Pearson's rank correlation coefficient R=0.69.This is supported by the newest Galactic chemical evolution model including the impact of massive stellar rotators and novae.Our future J=2-1 and J=3-2 observations of C¹⁸O and C¹⁷O toward the same sample would be important to determine their physical parameters?opacities,abundances,etc.?and further determine accurately the Galactic radial gradient of the isotopic ratio¹⁸O/¹⁷O.