Chemical templates of the Central Molecular Zone

Shock and protostellar object signatures under Galactic Center conditions

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Bonn, Germany
³ Department of Physics and Astronomy, University College London, Gower Street, London, UK
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850, Torrejón de Ardoz, Madrid, Spain
⁵ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
⁶ Department of Physics and Astronomy, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA
⁷ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁸ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
¹¹ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
¹³ State Key Laboratory of Radio Astronomy and Technology, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
¹⁴ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA
¹⁵ Observatorio Astronómico de Quito, Observatorio Astronómico Nacional, Escuela Politécnica Nacional, 170403 Quito, Ecuador
¹⁶ Institute for Advanced Study, 1 Einstein Drive, Princeton, NJ 08540, USA
¹⁷ National Radio Astronomy Observatory, 1011 Lopezville Road, Socorro, NM 87801, USA
¹⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹⁹ Star and Planet Formation Laboratory, Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
²⁰ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
²¹ Institute of Space Studies of Catalonia (IEEC), 08860 Barcelona, Spain
²² Green Bank Observatory, P.O. Box 2, Green Bank, WV 24944, USA
²³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁴ Leiden Institute of Chemistry, Leiden University, PO box 9502, 2300 RA Leiden, The Netherlands

^★ Corresponding author: dutkowska@strw.leidenuniv.nl

Received: 30 June 2025
Accepted: 12 August 2025

Abstract

Context. The Central Molecular Zone (CMZ) of the Milky Way exhibits extreme conditions, including high gas densities, elevated temperatures, enhanced cosmic-ray ionization rates, and large-scale dynamics. This makes it a perfect laboratory for astrochemical studies. With large-scale molecular surveys revealing increasing chemical and physical complexity in the CMZ, it is essential to develop robust methods to decode the chemical information embedded in this extreme region.

Aims. A key step to interpreting the molecular richness found in the CMZ is building chemical templates tailored to its diverse conditions. In particular, understanding how CMZ environments affect shock and protostellar chemistry is crucial. The combined impact of high ionization, elevated temperatures, and dense gas remains insufficiently explored for observable tracers.

Methods. For this study, we utilized UCLCHEM, a gas-grain time-dependent chemical model, to link physical conditions with their corresponding molecular signatures and identify key tracers of temperature, density, ionization, and shock activity. To achieve this, we ran a grid of models of shocks and protostellar objects representative of typical CMZ conditions, focusing on 24 species, including complex organic molecules.

Results. Shocked and protostellar environments show distinct evolutionary timescales (≲10⁴ vs. ≳10⁴ years); 300 K emerges as a key temperature threshold for chemical differentiation. We find that cosmic-ray ionization and temperature are the main drivers of chemical trends. HCO⁺, H₂CO, and CH₃SH trace ionization, while HCO, HCO⁺, CH₃SH, CH₃NCO, and HCOOCH₃ show consistent abundance contrasts between shocks and protostellar regions over similar temperature ranges.

Conclusions. We characterized the behavior of 24 species in protostellar and shock-related environments. While our models underpredict some complex organics in shocks, they reproduce observed trends for most species, supporting scenarios involving a need for recurring shocks in Galactic Center clouds and enhanced ionization toward Sgr B2(N2). Future work should assess the role of shock recurrence and metallicity in shaping chemistry.