The abundance of C¹⁸O and HDO in the envelope and hot core of the intermediate mass protostar NGC 7129 FIRS 2^⋆

¹ Observatorio Astronómico Nacional (OAN,IGN), Apdo 112, 28803 Alcalá de Henares, Spain
e-mail: a.fuente@oan.es
² School of Physics & Astronomy, E.C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
³ Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
⁴ Centro de Astrobiología (CSIC/INTA), Laboratory of Molecular Astrophysics, Ctra. Ajalvir km. 4, 28850 Torrejón de Ardoz, Spain
⁵ Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
⁶ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
⁷ Joint ALMA Offices, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
¹⁰ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹¹ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 33271 Floirac Cedex, France
¹² CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France

Received: 18 November 2011
Accepted: 7 February 2012

Abstract

Context. NGC 7129 FIRS 2 is a young intermediate-mass (IM) protostar, which is associated with two energetic bipolar outflows and displays clear signs of the presence of a hot core. It has been extensively observed with ground based telescopes and within the WISH guaranteed time Herschel key program.

Aims. This paper is dedicated to the modeling of the C¹⁸O and HDO lines in NGC 7129 FIRS 2. Our goal is to investigate the chemistry in the envelope and hot core of this IM protostar.

Methods. We present new observations of the C¹⁸O 3 → 2 and the HDO 3₁₂ → 2₂₁ lines towards NGC 7129 FIRS 2. Combining these observations with Herschel data and modeling their emissions, we constrain the C¹⁸O and HDO abundance profiles across the protostellar envelope. In particular, we derive the abundance of C¹⁸O and HDO in the hot core.

Results. The intensities of the C¹⁸O lines are well reproduced assuming that the C¹⁸O abundance decreases through the protostellar envelope from the outer edge towards the centre until the point where the gas and dust reach the CO evaporation temperature (≈20–25 K) where the C¹⁸O is released back to the gas phase. Once the C¹⁸O is released to the gas phase, the modelled C¹⁸O abundance is found to be  ≈ 1.6 × 10^-8, which is a factor of 10 lower than the reference abundance. This result is supported by the non-detection of C¹⁸O 9 → 8, which proves that even in the hot core (T_k > 100 K) the CO abundance must be 10 times lower than the reference value. Several scenarios are discussed to explain this C¹⁸O deficiency. One possible explanation is that during the pre-stellar and protostellar phase, the CO is removed from the grain mantles by reactions to form more complex molecules. Our HDO modeling shows that the emission of HDO 3₁₂ → 2₂₁ line is maser and comes from the hot core (T_k > 100 K). Assuming the physical structure derived by Crimier et al. (2010), we determine a HDO abundance of  ~0.4−1 × 10^-7 in the hot core of this IM protostar.

Conclusions.Herschel data combined with ground based observations have allowed us to estimate the C¹⁸O and HDO abundance in the protostellar envelope and hot core of an IM protostar. The HDO abundance in the hot core is  ~0.4−1 × 10^-7, similar to that found in the hot corinos NGC 1333 IRAS 2A and IRAS 16293−2422. The C¹⁸O abundance, at  ≈ 1.6 × 10^-8, is a factor of 10 lower than the reference value.