ALMA observations of the young protostellar system Barnard 1b: Signatures of an incipient hot corino in B1b-S^★

¹ Instituto de Física Fundamental, CSIC, C/ Serrano 123, 28006 Madrid, Spain
e-mail: nuria.marcelino@csic.es
² Sorbonne Université, Observatoire de Paris, Université PSL, École normale supérieure, CNRS, LERMA, 75014 Paris, France
³ Observatorio Astronómico Nacional (OAN, IGN), Apdo 112, 28803 Alcalá de Henares, Spain
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁶ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
⁸ Univ. Lyon, ENS de Lyon, Univ. Lyon1, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69007 Lyon, France

Received: 14 September 2017
Accepted: 17 September 2018

Abstract

The Barnard 1b core shows signatures of being at the earliest stages of low-mass star formation, with two extremely young and deeply embedded proto-stellar objects. Hence, this core is an ideal target to study the structure and chemistry of the first objects formed in the collapse of prestellar cores. We present ALMA Band 6 spectral line observations at ~0.6″ of angular resolution towards Barnard 1b. We have extracted the spectra towards both protostars, and used a local thermodynamic equilibrium (LTE) model to reproduce the observed line profiles. B1b-S shows rich and complex spectra, with emission from high energy transitions of complex molecules, such as CH₃OCOH and CH₃CHO, including vibrational level transitions. We have tentatively detected for the first time in this source emission from NH₂CN, NH₂CHO, CH₃CH₂OH, CH₂OHCHO, CH₃CH₂OCOH and both aGg′ and gGg′ conformers of (CH₂OH)₂. This is the first detection of ethyl formate (CH₃CH₂OCOH) towards a low-mass star forming region. On the other hand, the spectra of the FHSC candidate B1b-N are free of COMs emission. In order to fit the observed line profiles in B1b-S, we used a source model with two components: an inner hot and compact component (200 K, 0.35″) and an outer and colder one (60 K, 0.6″). The resulting COM abundances in B1b-S range from 10⁻¹³ for NH₂CN and NH₂CHO, up to 10⁻⁹ for CH₃OCOH. Our ALMA Band 6 observations reveal the presence of a compact and hot component in B1b-S, with moderate abundances of complex organics. These results indicate that a hot corino is being formed in this very young Class 0 source.