EDP Sciences
Free Access
Volume 441, Number 1, October I 2005
Page(s) 181 - 194
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361:20042149

A&A 441, 181-194 (2005)
DOI: 10.1051/0004-6361:20042149

Interstellar $\mathsf{^{12}}$C/ $\mathsf{^{13}}$C ratios through CH $^+\lambda\lambda $3957,4232 absorption in local clouds: incomplete mixing in the ISM

S. Casassus1, O. Stahl2 and T. L. Wilson3, 4

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 )

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 12CH $^+\lambda\lambda $3957.7,4232.3 and 13CH $^+\lambda\lambda $3958.2,4232.0 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 $\zeta$Oph, 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 $78.27 \pm 1.83$, while the weighted dispersion of our measurements is 12.7, giving a 6.9$\sigma$ scatter. Thus we report on a 6.9$\sigma$ detection of 16.2% root-mean-square variations in the carbon isotopic ratio on scales of ~100 pc: $R= 74.7 \pm 2.3$ in the $\zeta$Oph cloud, while $R = 88.6 \pm 3.0$ toward HD 152235 in the Lupus clouds, $R = 62.2 \pm 5.3$ towards HD 110432 in the Coalsack, and $R = 98.9 \pm 10.1$ toward HD 170740. The observed variations in 13C/12C are the first significant detection of chemical heterogeneity in the local ISM.

Key words: ISM: abundances -- ISM: clouds -- ISM: molecules -- ISM: individual objects: Coalsack -- ISM: individual objects: Lupus cloud -- ISM: individual objects: $\zeta$Oph cloud

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