Dual-band Unified Exploration of three CMZ Clouds (DUET)

Cloud-wide census of continuum sources showing low spectral indices

¹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
² Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Cologne, Germany
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
⁵ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, R.O.C.
⁶ Center of Astronomy and Gravitation, National Taiwan Normal University, Taipei 116, Taiwan
⁷ Department of Astronomy, University of Florida, PO Box 112055, Florida, USA
⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, PR China
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China
¹¹ Cosmic Origins Of Life (COOL) Research DAO, https://coolresearch.io
¹² Department of Physics, University of Connecticut, 196A Auditorium Road, Unit 3046, Storrs, CT 06269, USA
¹³ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester M13 9PL, UK
¹⁴ Department of Physics and Astronomy, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA
¹⁵ Haystack Observatory, Massachusetts Institute of Technology, 99 Milsstone Road, Westford, MA 01886, USA
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK

^★ Corresponding authors: xinglu@shao.ac.cn; kwang.astro@pku.edu.cn

Received: 23 December 2024
Accepted: 20 March 2025

Abstract

Context. The Milky Way’s central molecular zone (CMZ) has been measured to form stars ten times less efficiently than in the Galactic disk, based on emission from high-mass stars. However, the CMZ’s low-mass (⩽2 M_⊙) protostellar population, which accounts for most of the initial stellar mass budget and star formation rate (SFR), is poorly constrained observationally due to limited sensitivity and resolution.

Aims. We aim to perform a cloud-wide census of the protostellar population in three massive CMZ clouds.

Methods. We present the Dual-band Unified Exploration of three CMZ Clouds (DUET) survey, targeting the 20 km s⁻¹ cloud, Sgr C, and the dust ridge cloud “e” using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 and 3 mm. The mosaicked observations achieve a comparable resolution of 0.^′′2–0.^′′3 (∼2000 au) and a sky coverage of 8.3–10.4 arcmin², respectively.

Results. We report 563 continuum sources at 1.3 mm and 330 at 3 mm, respectively, and a dual-band catalog with 450 continuum sources. These sources are marginally resolved at a resolution of 2000 au. We find a universal deviation (>70% of the source sample) from commonly used dust modified blackbody (MBB) models, characterized by either low spectral indices or low brightness temperatures.

Conclusions. Three possible explanations are discussed for the deviation. (1) Optically thick class 0/I young stellar objects (YSOs) with a very small beam filling factor can lead to lower brightness temperatures than what MBB models predict. (2) Large dust grains with millimeter or centimeter in size have more significant self-scattering, and frequency-dependent albedo could therefore cause lower spectral indices. (3) Free-free emission over 30 μJy can severely contaminate dust emission and cause low spectral indices for milliJansky sources, although the number of massive protostars (embedded UCHII regions) needed is infeasibly high for the normal stellar initial mass function. A reliable measurement of the SFR at low protostellar masses will require future work to distinguish between these possible explanations.