Shocking Sgr B2 (N1) with its own outflow

A new perspective on segregation between O- and N-bearing molecules

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: labusch@mpifr-bonn.mpg.de
² Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 22 June 2023
Accepted: 2 October 2023

Abstract

Aims. Because studies on complex organic molecules (COMs) in high-mass protostellar outflows are sparse, we want to investigate how a powerful outflow, such as that driven by the exciting source of the prominent hot core Sagittarius B2(N1), influences the gas molecular inventory of the surrounding medium with which it interacts. Identifying chemical differences to the hot core unaffected by the outflow and what causes them may help to better understand molecular segregation in other star-forming regions.

Methods. We made use of the data taken as part of the 3 mm imaging spectral-line survey Re-exploring Molecular Complexity with ALMA (ReMoCA). We studied the morphology of the emission regions of simple and complex molecules in Sgr B2 (N1). For a selection of twelve COMs and four simpler species, spectra were modelled under the assumption of local thermodynamic equilibrium and population diagrams were derived at two positions, one in each lobe of the outflow. From this analysis, we obtained rotational temperatures and column densities. Abundances were subsequently compared to predictions of astrochemical models and to observations of L1157-B1, a position located in the well-studied outflow of the low-mass protostar L1157, and the source G+0.693-0.027 (G0.693), located in the Sgr B2 molecular cloud complex, which are other regions whose chemistry has been impacted by shocks.

Results. Integrated intensity maps of SO and SiO emission reveal a bipolar structure with blue-shifted emission dominantly extending to the south-east from the centre of the hot core and red-shifted emission to the north-west. The morphology of both lobes is complex but can roughly be characterised by an emission component at a larger opening angle, containing most of the emission, and narrower features. The wider-angle component is also prominently observed in emission of S-bearing molecules and species that only contain N as a heavy element, including COMs, but also CH₃OH, CH₃CHO, HNCO, and NH₂CHO. Rotational temperatures are found in the range of ~ 100–200 K. Abundances of N-bearing molecules with respect to CH₃OH are enhanced in the outflow component compared to N1S, a position that is not impacted by the outflow. A comparison of molecular abundances with G+0.693–0.027 and L1157-B1 does not show any correlations, suggesting that a shock produced by the outflow impacts Sgr B2 (N1)’s material differently or that the initial conditions were different.

Conclusions. The short distance of the analysed outflow positions to the centre of Sgr B2 (N1) lead us to propose a scenario in which a phase of hot-core chemistry (i.e. thermal desorption of ice species and high-temperature gas-phase chemistry) preceded a shock wave. The subsequent compression and further heating of the material resulted in the accelerated destruction of (mainly O-bearing) molecules. Gas-phase formation of cyanides seems to be able to compete with their destruction in the post-shock gas. The abundances of cyanopolyynes are enhanced in the outflow component pointing to (additional) gas-phase formation, possibly incorporating atomic N sourced from ammonia in the post-shock gas. To confirm such a scenario, chemical shock models need to be run that take into account the pre- and post-shock conditions of Sgr B2 (N1). In any case, the results provide new perspectives on shock chemistry and the importance of the environment in which it occurs.