Carbon reservoirs in protoplanetary systems surveyed across the galaxy

High-resolution near-infrared observations of carbon monoxide (CO) gas toward young stellar objects (YSOs) enable precise evaluation of carbon and oxygen isotopes in these systems, which in turn lead to valuable insights into protoplanetary processes [1-6]. Comparisons made between solar system materials and young stellar gas further the understanding of phenomena that could have affected chemistry in the early solar nebula, including CO self-shielding [3,6], supernova enrichment [4], and the potential interplay between CO ice and gas reservoirs [5,6]. Such astronomical observations also enable the study of potential differences in chemical pathways between isolated cores and low-mass binary systems separated by a few hundred AU [5]. In contrast to low-mass YSOs, massive YSOs (>~ 8 solar-masses, ~ 10^3 to 10^5 solar-luminosities) trace high-UV fields, permitting observations over a significant range of Galactocentric radii (R_(GC)). Further, since precise abundances of gas-phase ^(12)C^(16)O (^(12)CO) and ^(13)C^(16)O (^(13)CO) isotopologues can be compared to ice-phase ^(12)C/^(13)C from CO and CO_2 ice reservoirs along a single line of sight toward massive YSOs, these are particularly valuable targets in evaluating protoplanetary and prebiotic carbon chemistry. Here we present results from a completed phase of our Keck survey of massive YSOs at R_(GC) from ~0.01 kpc (at the Galactic Center) to ~ 9.7 kpc (just beyond the local solar neighborhood).