Carbon Isotope Ratios in Molecular Clouds and Circumstellar Envelopes from Millimeter Observations of CN

0 2 4 6 8 10 12 14 16 Dgc (kpc) 12 C / 1 3 C Molecular Clouds: Observations of isotopic ratios in the interstellar medium provide an avenue for a quantitative assessment of stellar nucleosynthesis. For example, the 12C/13C-isotope ratio is also considered an important tracer because it reflects the relative degree of primary to secondary processing in stars. Carbon-12 is predicted to be formed in the triple alpha reaction found in massive stars (Timmes, Woosley and Weaver 1995) and supernovae. Carbon-13 is a reaction intermediate in the carbon-nitrogen-oxygen (CNO)-cycle, which occurs in Asymptotic Giant Branch (AGB) stars (Pagel 1997). In the course of the third “dredge-up”, the intermediate product, 13C, is mixed into the expanding envelope of the star, where it is then ejected into the interstellar medium (ISM). Numerous studies have been conducted towards molecular clouds throughout the galaxy (Wilson and Rood 1994, Penzias 1980, Langer and Penzias 1990, and Henkel et al. 1982). One of the most common methods of measuring the ratio is to compare line intensities of the carbon-12 and carbon-13 isotopomers of common molecules such as CO, H2CO, and HCO+. However, favorable transitions of very abundant molecules are often saturated, and therefore relative line intensities are not an accurate indicator of isotope ratios. In the past, this problem has been dealt with by using molecular line modeling or the double isotope ratios. Molecular line measurements taken over a period of several decades indicate that the 12C/13C ratio steadily increases with distance from the galactic center (e.g. Wilson and Rood 1994). In order to further examine 12C/13C ratios, we have conducted new observations of the N=1 0 and N=2 1 transitions of 12CN and 13CN at 1.2 and 3 mm towards Galactic molecular clouds. This work is an extension of a study previously conducted by Savage et al. (2002). CN is a unique tracer of this ratio because of hyperfine structure present within a given rotational transition which enables an accurate assessment of the line opacity (Savage et al. 2002). Thus, more accurate values of the 12C/13C-isotope ratio can be determined. Circumstellar Envelopes: In order to decipher the origins of the two isotopes throughout the galaxy, CN is being investigated in circumstellar envelopes of various types of stars. Observations are currently being conducted towards Hydrogen-deficient stars, supergiants, Li-rich stars, and typical Carbon stars.