2026/07/07 by L. Colzi, O. Sipilä, I. Jiménez-Serra +4 · 1 voice
Physics and Astronomy · #astro-ph.EP #astro-ph.GA #astro-ph.SR
paper · pdf · doi:10.1051/0004-6361/202660705
arxiv published 2026/07/07 · arxiv updated 2026/07/08
Isotopic ratios are key tracers of Galactic chemical evolution because different isotopes are synthesized through distinct stellar nucleosynthesis processes. While the 12C/13C ratio increases with galactocentric distance across the Galactic disc, measurements in the Central Molecular Zone (CMZ) have historically yielded low values (∼3-30), often affected by high optical depths. We aim to determine the initial 12C/13C ratio of the parent material of the CMZ molecular cloud G+0.693-0.027 using optically thin molecular tracers and correcting for isotopic fractionation. We analyzed an ultra-high-sensitivity spectral survey obtained with the IRAM 30m and Yebes 40m telescopes, detecting single and double 13C isotopologues of HC3N and HC5N. Column densities and isotopic ratios were derived and compared with astrochemical models including 13C- and 15N-isotopologues to quantify isotopic fractionation. We derive 12C/13C ratios of 36.7±1.0 for HC3N (using double 13C isotopologues) and 38.8±1.5 for HC5N, significantly higher than previous CMZ estimates based on simpler molecules. The models indicate low to intermediate isotopic fractionation at early times (<3×104 yr), implying an initial 12C/13C ratio of ∼48 for the gas from which present-day CMZ molecular clouds formed. The inferred range (37-48) is consistent with values observed at Galactocentric distances of 3-5 kpc, supporting a scenario in which the CMZ is replenished by gas inflows from the Galactic disc driven by the Galactic bar, with a possible contribution from less chemically processed material accreted from external systems such as dwarf galaxies.