FAUST

XX. The chemical structure and temperature profile of the IRAS 4A2 hot corino at 20–50 au

¹ DIFA, Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² ESO, Karl Schwarzchild Str. 2, 85748 Garching bei München, Germany
³ Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
⁴ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ National Radio Astronomy Observatory, 1011 Lopezville Rd, Socorro, NM 87801, USA
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, A.P. 3-72 (Xangari), 8701 Morelia, Mexico
⁸ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
⁹ David Rockefeller Center for Latin American Studies, Harvard University, 1730 Cambridge Street, Cambridge, MA 02138, USA
¹⁰ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
¹¹ The Institute of Physical and Chemical Research (RIKEN), 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
¹² Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
¹³ Leiden Observatory, Leiden University, PO Box 9513, 23000 RA Leiden, The Netherlands
¹⁴ Center for Astrochemical Studies, Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
¹⁵ Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁶ Centro de Astrobiología (CAB), INTA-CSIC, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁷ NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁸ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
¹⁹ Steward Observatory, 933 N Cherry Ave., Tucson, AZ 85721, USA
²⁰ The Graduate University for Advanced Studies (SOKENDAI), Shonan Village, Hayama, Kanagawa 240-0193, Japan

^★ Corresponding author; jenny.frediani@astro.su.se; marta.desimone@eso.org

Received: 10 September 2024
Accepted: 24 January 2025

Abstract

Context. Young low-mass protostars often possess hot corinos, which are compact, hot, and dense regions that are bright in interstellar complex organic molecules (iCOMs). In addition to their prebiotic role, iCOMs can be used as a powerful tool to characterize the chemical and physical properties of hot corinos.

Aims. Using ALMA/FAUST data, our aim was to explore the iCOM emission at <50 au scale around the Class 0 prototypical hot corino IRAS 4A2.

Methods. We imaged IRAS 4A2 in six abundant common iCOMs (CH₃OH, HCOOCH₃, CH₃CHO, CH₃CH₂OH, CH₂OHCHO, and NH₂CHO), and derived their emitting sizes. The column density and gas temperature for each species were derived at 1σ from a multiline analysis by applying a non-LTE approach for CH₃OH, and LTE population or rotational diagram analysis for the other iCOMs. Thanks to the unique estimates of the absorption from foreground millimeter dust toward IRAS 4A2, we derived for the first time unbiased gas temperatures and column densities.

Results. We resolved the IRAS 4A2 hot corino, and found evidence for a chemical spatial distribution in the inner 50 au, with the outer emitting radius increasing from ∼22–23 au for NH₂CHO and CH₂OHCHO, followed by CH₃CH₂OH (∼27 au), CH₃CHO (∼28 au), HCOOCH₃ (∼36 au), and out to ∼40 au for CH₃OH. Combining our estimate of the gas temperature probed by each iCOM with their beam-deconvolved emission sizes, we inferred the gas temperature profile of the hot corino on scales of 20–50 au in radius, and found a power-law index q of approximately –1.

Conclusions. We observed, for the first time, a chemical segregation in iCOMs of the IRAS 4A2 hot corino, and derived the gas temperature profile of its inner envelope. The derived profile is steeper than when considering a simple spherical collapsing and optically thin envelope, hinting at a partially optically thick envelope or a gravitationally unstable disk-like structure.