Probing outflow physics through CH

Springer Link (Chiba Institute of Technology)

Abstract

Chemical correlations between molecules provide powerful diagnostics to probe the physical conditions of protostellar outflows. In particular, the relationship between methanol (CH3OH) and methyl cyanide (CH3CN) offers a promising tool to investigate the chemistry and irradiation environment of shocked gas. In this Letter, we use the CH3OH/CH3CN abundance ratio to constrain the physical properties of the outflow driven by the Class 0 protostar S68N using ALMA Band 3 and Band 6 observations. Assuming local thermodynamic equilibrium (LTE), we derived excitation temperatures of 50–60 K and column densities of 2–3 × 1013 cm−2 for CH3CN and 3–5 × 1015 cm−2 for CH3OH. The resulting CH3OH/CH3CN abundance ratio is nearly constant along the outflow, with values of ∼100–200, similar to those found in other protostellar environments. Using an up-to-date astrochemical model, we tested whether the gas-phase formation of CH3CN can account for the observed ratios. We found that they could only be reproduced under the assumption of enhanced cosmic-ray ionisation rates: ζCR up to ∼10−14 s−1. These results suggest that the CH3OH–CH3CN correlation can be used as a probe of the irradiation conditions in protostellar outflows. Further studies are required to explore the contribution of grain-surface formation of CH3CN that could lead to a lower ζCR, and to analyse a larger sample of sources.

Authors 7

1. L. Giani

   Arcetri Astrophysical Observatory

   Affiliation as printed

   INAF , Osservatorio Astrofisico di Arcetri , Largo E. Fermi 5 , 50125 Firenze , Italy

2. D. Zaremba

   University of Victoria

   Affiliation as printed

   Department of Physics and Astronomy , University of Victoria , PO Box 3055 ,

3. M. De Simone

   European Southern Observatory · Arcetri Astrophysical Observatory

   Affiliation as printed

   European Southern Observatory , Karl-Schwarzschild-Strasse 2 , D-85748 Garching bei München , Germany

   INAF , Osservatorio Astrofisico di Arcetri , Largo E. Fermi 5 , 50125 Firenze , Italy

4. Łukasz Tychoniec Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , 2300 RA Leiden , The Netherlands

5. L. Podio

   Arcetri Astrophysical Observatory

   Affiliation as printed

   INAF , Osservatorio Astrofisico di Arcetri , Largo E. Fermi 5 , 50125 Firenze , Italy

6. C. Codella

   Arcetri Astrophysical Observatory

   Affiliation as printed

   INAF , Osservatorio Astrofisico di Arcetri , Largo E. Fermi 5 , 50125 Firenze , Italy

   STN CSC , Victoria , BC V8W 3P6 , Canada

7. A. Somigliana

   Max Planck Institute for Astronomy

   Affiliation as printed

   Max-Planck-Institut für Astronomie (MPIA) , Königstuhl 17 , 69117 Heidelberg , Germany