Which molecule traces what: Chemical diagnostics of protostellar sources

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: lukasz.tychoniec@eso.org
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
⁶ National Astronomical Observatory of Japan, NAOJ Chile, Alonso de Córdova 3788, Office 61B, 7630422, Vitacura, Santiago, Chile
⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁹ Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama 351-0198, Japan
¹⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA

Received: 1 March 2021
Accepted: 8 July 2021

Abstract

Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation.

Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au).

Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture.

Results. The protostellar envelope is well traced by C¹⁸O, DCO⁺, and N₂D⁺, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH₃OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H₂CO. The cavity walls show tracers of UV-irradiation such as C₂H, c-C₃H₂ and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H₂S, SO, OCS, and H¹³CN, which most likely are related to ice sublimation and high-temperature chemistry.

Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.