THE 12C/13C RATIO IN THE GALACTIC CENTER: IMPLICATIONS FOR GALACTIC CHEMICAL EVOLUTION AND ISOTOPE CHEMISTRY

Observations from a spectral-line survey of Sgr B2(N) of the $^{12}$C and $^{13}$C isotopologues of H$_2$CS, CH$_3$CCH, NH$_2$CHO, CH$_2$CHCN, and CH$_3$CH$_2$CN have been analyzed to more accurately establish the $^{12}$C/$^{13}$C ratio in this cloud. The wide spectral coverage has enabled an accurate evaluation of the $^{12}$C/$^{13}$C ratios in these low abundance molecules, based on numerous transitions. The lines typically exhibited two distinct velocity components at 64 and 73 km s$^{-1}$. The $^{12}$C/$^{13}$C ratio was found to be in the range 18-24 for all 5 molecules, for optically thin transitions, with an average value of 20.5, and did not significantly vary between the two velocity components. The Galactic gradient has been revised to be $^{12}$C/$^{13}$C = 6.08(0.48) $D_{GC}$ + 15.7(2.9). Furthermore, the $^{12}$C/$^{13}$C ratio did not change with substitution site on the molecule. Therefore, there appears to be very little chemical fractionation or isotope-selective photodissociation occurring in Sgr B2(N), and the $^{12}$C/$^{13}$C ratios are a true reflection of the isotopic abundances generated by stellar nucleosynthesis