The ALMA-ATOMS survey: A sample of weak hot core candidates identified through line stacking

¹ School of Physics and Astronomy, Yunnan University, Kunming 650091, PR China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁴ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
⁵ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
⁶ Department of Physics, Faculty of Science, Kunming University of Science and Technology, Kunming 650500, PR China
⁷ University of Chinese Academy of Sciences, Beijing 100049, PR China
⁸ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Stree, Urumqi, Xinjiang 830011, PR China
⁹ Institute of Astrophysics, School of Physics and Electronical Science, Chuxiong Normal University, Chuxiong 675000, PR China
¹⁰ School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
¹¹ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, PR China
¹² Center for Astrophysics, Guangzhou University, Guangzhou 510006, PR China
¹³ Departamento de Astronomía, Universidad de Chile, Las Condes, 7591245 Santiago, Chile
¹⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China
¹⁶ Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong-gu, Daejeon 34055, Republic of Korea
¹⁷ University of Science and Technology, Korea (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
¹⁸ Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan
¹⁹ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
²¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²² Xinjiang Key Laboratory of Radio Astrophysics, 150 Science 1Street, Urumqi, Xinjiang 830011, PR China
²³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
²⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway

^★ Corresponding authors: qin@ynu.edu.cn;liutie@shao.ac.cn;liuxunchuan001@gmail.com

Received: 26 October 2024
Accepted: 27 March 2025

Abstract

Context. Hot cores represent critical astrophysical environments for high-mass star formation, distinguished by their rich spectra of organic molecular emission lines. Nevertheless, comprehensive statistical analyses of extensive hot core samples remain relatively scarce in current astronomical research.

Aims. We aim to utilize high-angular-resolution molecular line data from the Atacama Large Millimeter and Submillimeter Array (ALMA) to identify hot cores, with a particular focus on weak-emission candidates, and to provide one of the largest samples of hot core candidates to date.

Methods. We propose to use spectral stacking and imaging techniques of complex organic molecules (COMs) in the ALMA-ATOMS survey, including line identification and weights, segmentation of line datacubes, resampling, stacking and normalization, moment 0 maps, and data analysis, to search for hot core candidates. The molecules involved include CH₃OH, CH₃OCHO, C₂H₅CN, C₂H₅OH, CH₃OCH₃, CH₃COCH₃, and CH₃CHO. We classify cores with dense emission of CH₃OH and at least one molecule from the other six molecules as hot core candidates.

Results. In addition to the existing sample of 60 strong hot cores from the ALMA-ATOMS survey, we have detected 40 new weak candidates through stacking. All hot core candidates display compact emission from at least one of the other six COM species. For the strong sample, the stacking method provides molecular column density estimates that are consistent with previous fitting results. For the newly identified weak candidates, all species except CH₃CHO show compact emission in the stacked image, which cannot be fully resolved spatially. These weak candidates exhibit column densities of COMs that are approximately one order of magnitude lower than the ones of the strong sample. The entire hot core sample, including the weak candidates, reveals tight correlations between the compact emission of CH₃OH and other COM species, suggesting they may share a similar chemical environment for COMs, with CH₃OH potentially acting as a precursor for other COMs. Among the 100 hot cores in total, 43 exhibit extended CH₃CHO emission spatially correlated with SiO and H¹³CO⁺, suggesting that CH₃CHO may form in widely distributed shock regions.

Conclusions. The molecular line stacking technique is used to identify hot core candidates in this work, leading to the identification of 40 new hot core candidates. Compared to spectral line fitting methods, it is faster and more convenient, and enables weaker hot cores to be detected with greater sensitivity.