Spectroscopically resolved far-IR observations of the massive star-forming region G5.89–0.39

摘要

Context. The fine-structure line of atomic oxygen at 63 μm ([OI]63μm) is an important diagnostic tool in different fields of astrophysics: it is for example predicted to be the main coolant in several environments of star-forming regions (SFRs). However, our knowledge of this line relies on observations with low spectral resolution, and the real contribution of each component (photon-dominated region, jet) in the complex environment of SFRs to its total flux is poorly understood. Aims. We investigate the contribution of jet and photon-dominated region emission, and of absorption to the [OI]63μm line towards the hot gas around the ultra-compact Hii region G5.89–0.39 and study the far-IR line luminosity of the source in different velocity regimes through spectroscopically resolved spectra of atomic oxygen, [CII], CO, OH, and H2O. Methods. We mapped G5.89–0.39 in [OI]63μm and in CO(16–15) with the GREAT receiver onboard SOFIA. We also observed the central position of the source in the ground-state OH 2Π3/2 J = 5/2 → J = 3/2 triplet and in the excited OH 2Π1/2 J = 3/2 → J = 1/2 triplets with SOFIA. These data were complemented with APEX CO(6–5) and CO(7–6) maps and with Herschel/HIFI maps and single-pointing observations in lines of [CII], H2O, and HF. Results. The [OI] spectra in G5.89–0.39 are severely contaminated by absorptions from the source envelope and from different clouds along the line of sight. Emission is detected only at high velocities, and it is clearly associated with the compact north-south outflows traced by extremely high-velocity emission in low-J CO lines. The mass-loss rate and the energetics of the jet system derived from the [OI]63μm line agree well with previous estimates from CO, thus suggesting that the molecular outflows in G5.89–0.39 are driven by the jet system seen in [OI]. The far-IR line luminosity of G5.89–0.39 is dominated by [OI] at high-velocities; the second coolant in this velocity regime is CO, while [CII], OH and H2O are minor contributors to the total cooling in the outflowing gas. Finally, we derive abundances of different molecules in the outflow: water has low abundances relative to H2 of 10-8?10-6, and OH of 10-8. Interestingly, we find an abundance of HF to H2 of 10-8, comparable with measurements in diffuse gas. Conclusions. Our study shows the importance of spectroscopically resolved observations of the [OI]63μm line for using this transition as diagnostic of star-forming regions. While this was not possible until now, the GREAT receiver onboard SOFIA has recently opened the possibility of detailed studies of the [OI]63μm line to investigate the potential of the transition for probing different environments.