The Galactic centre G+0.633-0.0604 molecular cloud: A new astrochemical gold mine. I. Gas physical properties

In the central molecular zone (CMZ) environment, shocks play a key role in both triggering star formation and providing chemical enrichment. The Sgr B2 complex is a prime template, hosting several massive protoclusters (N, M, and S) and the northern G+0.693 cloud, which exhibits shock-induced prestellar signatures. We report on G+0.633-0.0604, a new shock-dominated and chemically rich cloud at the southern edge of Sgr B2, where it has been proposed that the next star formation episodes will occur. We characterise its physical properties and the shocks shaping it. We analysed data from new high-sensitivity broadband spectral surveys using the Yebes 40m, IRAM 30m, and APEX radio telescopes, covering a ∼100 aggregated bandwidth across the ∼31-275 , , , , and several isotopologues of to infer kinetic temperatures and densities. We complementarily used 3 range. We present analyses of CH3CCH CH3CN HC3N HNCO CO H2 IRAM 30m mosaics (∼13 , , and to probe G+0.633's environment. ∼32times32 pc2) of Sgr B2 in HC3N HNCO C2H5OH We identify three velocity components towards G+0.633: a narrow main one (C1, v_ LSR ∼48.5̨ms; ∼55-90 ∼(3-7)times10^ and two broader, much fainter components at higher velocities, C2 (∼61̨ms; ∼13̨ms) and C3 (∼89̨ms; ∼18̨ms), all of which show similar properties (T_ kin N_ H2 22 , cm ^ -2 , n_ H2 ∼(0.5-2.5)times10^ 4 , cm ^ -3 ) and extended distributions. C1 delineates G+0.633 physically, and corresponds to a peak in . This supports a shock-driven origin, likely rooted in the cloud–cloud collision shaping Sgr B2 and also traced by C2, which mainly extends north to G+0.693. C3 is kinematically unlinked and related to large-scale CMZ dynamics. Of the three, C1 may be consistent with a very early protocluster phase, but this is yet to be confirmed. HNCO G+0.633 emerges as a new shock-dominated CMZ cloud with a physical resemblance to G+0.693, providing another unique laboratory to investigate how shocks drive molecular complexity and regulate the onset of cluster formation in the CMZ.