Gas and dust in the star-forming region ρ Oph A

II. The gas in the PDR and in the dense cores^⋆,⋆⋆

¹ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
e-mail: bem@astro.su.se
² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: rene.liseau@chalmers.se

Received: 29 June 2017
Accepted: 25 August 2017

Abstract

Context. The evolution of interstellar clouds of gas and dust establishes the prerequisites for star formation. The pathway to the formation of stars can be studied in regions that have formed stars, but which at the same time also display the earliest phases of stellar evolution, i.e. pre-collapse/collapsing cores (Class -1), protostars (Class 0), and young stellar objects (Class I, II, III).

Aims. We investigate to what degree local physical and chemical conditions are related to the evolutionary status of various objects in star-forming media.

Methods. ρ Oph A displays the entire sequence of low-mass star formation in a small volume of space. Using spectrophotometric line maps of H₂, H₂O, NH₃, N₂H⁺, O₂, O I, CO, and CS, we examine the distribution of the atomic and molecular gas in this dense molecular core. The physical parameters of these species are derived, as are their relative abundances in ρ Oph A. Using radiative transfer models, we examine the infall status of the cold dense cores from their resolved line profiles of the ground state lines of H₂O and NH₃, where for the latter no contamination from the VLA 1623 outflow is observed and line overlap of the hyperfine components is explicitly taken into account.

Results. The stratified structure of this photon dominated region (PDR), seen edge-on, is clearly displayed. Polycyclic aromatic hydrocarbons (PAHs) and O I are seen throughout the region around the exciting star S 1. At the interface to the molecular core 0.05 pc away, atomic hydrogen is rapidly converted into H₂, whereas O I protrudes further into the molecular core. This provides oxygen atoms for the gas-phase formation of O₂ in the core SM 1, where X(O₂) ~ 5 × 10^-8. There, the ratio of the O₂ to H₂O abundance [X(H₂O) ~ 5 × 10^-9] is significantly higher than unity. Away from the core, O₂ experiences a dramatic decrease due to increasing H₂O formation. Outside the molecular core ρ Oph A, on the far side as seen from S 1, the intense radiation from the 0.5 pc distant early B-type star HD 147889 destroys the molecules.

Conclusions. Towards the dark core SM 1, the observed abundance ratio X(O₂)/X(H₂O) > 1, which suggests that this object is extremely young, which would explain why O₂ is such an elusive molecule outside the solar system.