|
|
|
Measurements of
Galactic 12C/13C
Isotope Ratios using CO and CN
New
measurements of 12C/13C ratios in galactic
molecular
clouds have been conducted using the N=1
--> 0 transition
of the CN radical. This species is unique
in that it has
extensive hyperfine structure that can be accurately used to correct
for line
saturation effects. Combined with the
past observations of Savage et al. (2002), the ratios derived from CN
are the
most extensive data set to date for molecular clouds and include
sources that
lie in the range of 0.09-16.41 kpc in distance from the Galactic center
(DGC). The ratios derived from
CN indicate a
gradient with Galactic distance of 12C/13C =
6.01 DGC
+ 12.28. This gradient agrees relatively
closely with those derived from millimeter measurements of CO and H2CO. The average gradient determined from all
three molecules is 12C/13C = 6.21 DGC
+
18.71. CO, CN and H2CO are
synthesized from quite different reactions, and any 13C
fractionation must follow different pathways for these three species. The relatively good agreement between the 12C/13C
ratios of the three molecules, as well as its lack of correlation with
gas
kinetic temperature, suggests that chemical fractionation does not play
a
substantial role in influencing such ratios.
Therefore, the 12C/13C gradient
found in the
Galaxy is a true indicator of Galactic chemical evolution.
The apparent discrepancy between the solar
system (12C/13C = 89) and local interstellar
medium
values (12C/13C = 68) of this ratio may be a
result of 13C
enrichment since the formation of the solar system, as predicted by
recent
models.
Below, 12CN and 13CN spectra
towards S156
showing a 12C/13C ratio of 78.
|
