High 12 C/ 13 C isotopic ratios toward G+0.693-0.027: Evidence for gas inflow to the Central Molecular Zone

Context. Isotopic ratios are essential tools for understanding Galactic chemical evolution and reconstructing the star formation history of the Milky Way, as different isotopes are produced through distinct stellar nucleosynthesis processes. While the 12 C/ 13 C ratio is known to increase with galactocentric distance in the Galactic disk, measurements in the Central Molecular Zone (CMZ), which are often made more difficult by high line optical depth effects, have historically yielded low values (~3–30). Aims. The aim of this work is to determine the initial 12 C/ 13 C isotopic ratio for the parent material of the CMZ molecular cloud G+0.693–0.027. By employing optically thin molecular tracers and applying corrections for isotopic fractionation effects, we provide a more accurate constraint on the nucleosynthetic enrichment history of the Galactic center. Methods. We used an ultrahigh-sensitivity spectral survey carried out with the IRAM 30m and Yebes 40m telescopes to detect single and double 13 C-isotopologues of HC 3 N and HC 5 N. We analyzed the data and derived the column densities and isotopic ratios, which were then compared with astrochemical models including 13 C- and 15 N-isotopologues to account for potential degree of isotopic fractionation. Results. We derived 12 C/ 13 C ratios of 36.7±1.02 for the main velocity component of HC 3 N (from double isotopologues) and 38.8±1.5 for HC 5 N. These values are significantly higher than typical CMZ measurements from simpler species. Our models suggest low to intermediate fractionation degrees at early timescales (<3×10 4 yr), leading to an inferred initial 12 C/ 13 C ratio of ~48 for the material from which the present-day CMZ molecular clouds formed. Conclusions. The derived ratios (37–48) are consistent with those found at galactocentric distances of 3–5 kpc. This supports a scenario where the CMZ is replenished by gas inflows from the Galactic disk, likely driven by the Galactic bar, with a possible contribution from the accretion of less-processed material from external sources such as dwarf galaxies.