Interstellar C/C ratios through CH3957,4232 absorption in local clouds: incomplete mixing in the ISM

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile e-mail: simon@das.uchile.cl
² Landessternwarte Königstuhl, 69117 Heidelberg, Germany e-mail: O.Stahl@lsw.uni-heidelberg.de
³ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: twilson@eso.org
⁴ Max-Planck-Institut für Radioastronomie, Postfach 2024, 53010 Bonn, Germany

Received: 8 October 2004
Accepted: 2 June 2005

Abstract

The ¹²C/¹³C isotope ratio is a tracer of stellar yields and the efficiency of mixing in the ISM. ¹²CH⁺/¹³CH⁺ is not affected by interstellar chemistry, and is the most secure way of measuring ¹²C/¹³C in the diffuse ISM. R=¹²C/¹³C is 90 in the solar system. Previous measurements of ¹²CH3957.7,4232.3 and ¹³CH3958.2,4232.0 absorption toward nearby stars indicate some variations in ¹²C/¹³C, with values ranging from 40 to 90 suggesting inefficient mixing. Except for the cloud toward ζOph, these R values are strongly affected by noise. With UVES on the VLT we have improved on the previous interstellar ¹²C/¹³C measurements. The weighted ¹²C/¹³C ratio in the local ISM is , while the weighted dispersion of our measurements is 12.7, giving a 6.9σ scatter. Thus we report on a 6.9σ detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ~100 pc: in the ζOph cloud, while toward HD 152235 in the Lupus clouds, towards HD 110432 in the Coalsack, and toward HD 170740. The observed variations in ¹³C/¹²C are the first significant detection of chemical heterogeneity in the local ISM.