Broad N₂H⁺ Emission toward the Protostellar Shock L1157-B1

C. Codella , S. Viti , C. Ceccarelli (1) , B. Lefloch (1) , M. Benedettini , G. Busquet , P. Caselli , F. Fontani , A. Gómez-Ruiz , L. Podio (1) , M. Vasta

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

We present the first detection of N₂H⁺ toward a low-mass protostellar outflow, namely, the L1157-B1 shock, at ~0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30 m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. Analysis of this emission coupled with HIFI CHESS multiline CO observations leads to the conclusion that the observed N₂H⁺(1-0) line originated from the dense (>=10⁵ cm^-3) gas associated with the large (20''-25'') cavities opened by the protostellar wind. We find an N₂H⁺ column density of a few 10¹² cm^-2 corresponding to an abundance of (2-8) × 10^-9. The N₂H⁺ abundance can be matched by a model of quiescent gas evolved for more than 10⁴ yr, i.e., for more than the shock kinematical age (sime2000 yr). Modeling of C-shocks confirms that the abundance of N₂H⁺ is not increased by the passage of the shock. In summary, N₂H⁺ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10⁴ cm^-3, and then further compressed and accelerated by the shock.

Keywords: ISM: abundances; ISM: jets and outflows; ISM: molecules; Astrophysics - Astrophysics of Galaxies

Domains: Sciences of the Universe [physics]

C. Codella, S. Viti, C. Ceccarelli, B. Lefloch, M. Benedettini, et al.. Broad N₂H⁺ Emission toward the Protostellar Shock L1157-B1. The Astrophysical Journal, 2013, 776, ⟨10.1088/0004-637X/776/1/52⟩. ⟨insu-03620889⟩