Nascent chemical complexity in the prestellar core IRAS 16293E: Complex organics and deuterated methanol

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
² Physikalisch-Meteorologisches Observatorium Davos und Weltstrahlungszentrum (PMOD/WRC), Dorfstrasse 33, 7260 Davos Dorf, Switzerland
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0106, Japan
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

^★ Corresponding authors: sscibell@nrao.edu; maria.drozdovskaya.space@gmail.com

Received: 7 January 2025
Accepted: 4 August 2025

Abstract

Context. Prestellar cores represent early sites of low-mass (M ≤ few M_⊙) star and planet formation and provide insight into the initial chemical conditions of complex organic molecules (COMs). Deuterated COMs trace the degree of molecular inheritance and/or reprocessing, as high deuteration in protostellar systems suggests COMs forming during the prestellar stage when deuteration was enhanced.

Aims. Within the L1689N molecular cloud, the prestellar core IRAS 16293E sits 90" eastward of the chemically rich IRAS 162932422 A and B protostellar system. A unique view of star formation inside a common natal cloud, IRAS 16293A, B, and E all show some of the highest levels of deuteration in the interstellar medium, with a number of D/H ratios 10⁵ times higher than solar. We investigated the deuteration levels of the simplest COM, methanol (CH₃OH), in IRAS 16293E for the first time.

Methods. Using the Arizona Radio Observatory (ARO) 12 m telescope, we targeted favorable transitions of CH₂DOH, CHD₂OH, ¹³CH₃OH, and several higher-complexity COMs (including acetaldehyde, CH₃CHO, methyl formate, HCOOCH₃, and dimethyl ether, CH₃OCH₃) in the 3 mm band. Follow-up observations with the Yebes 40 m telescope provided additional transitions in the 7 mm (Q band).

Results. We report the first detections of these COMs and deuterated methanol in the prestellar core IRAS 16293E and used our observations to calculate excitation temperatures, column densities, and relative abundance ratios. Striking similarities were found between relative molecular ratios and D/H values when we compared IRAS 16293E to the A and B protostars, in addition to to a heterogeneous sample of other prestellar cores, protostars, and comet 67P/Churyumov-Gerasimenko.

Conclusions. Our results support the idea that there is a limited amount of chemical reprocessing of COMs when prestellar cores collapse and heat up during the protostellar phase.