Interstellar C toward HD 210121^*

¹ LUTH and FRE 2462 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon, France e-mail: Evelyne.Roueff@obspm.fr
² LESIA and FRE 2461 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon, France e-mail: Paul.Felenbok@obspm.fr
³ Centre for Astrophysics and Space Science, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden e-mail: jblack@oso.chalmers.se
⁴ ISO Data Center, ESA Research and Scientific Support Department, PO Box 50727, 28080 Madrid, Spain e-mail: cgry@iso.vilspa.esa.es

Corresponding author: E. Roueff, Evelyne.Roueff@obspm.fr

Received: 9 August 2001
Accepted: 7 January 2002

Abstract

We report the detection of the 405 nm band of interstellar C₃ in absorption toward HD 210121. The abundance of triatomic carbon is approximately 1/17 of that of diatomic carbon in the same diffuse molecular cloud. Rotational levels of C₃ up to are seen in this cloud. The rotational excitation of C₃ in the interstellar medium may reflect a competition between inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. The abundance of C₃ is compared with chemical models. Attention is called to molecular properties that need to be better determined.