ALMA-IMF

XVIII. The assembly of a star cluster: Dense N₂H⁺ (1–0) kinematics in the massive G351.77 protocluster

¹ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
² Franco-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Universidad de Chile, Santiago, Chile
³ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán 58089, Mexico
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA
⁶ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
¹⁰ Departments of Astronomy and Chemistry, University of Virginia, Charlottesville, VA 22904, USA
¹¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹² Astronomy Department, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹³ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain
¹⁴ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain
¹⁵ Institut d’Estudis Espacials (IEEC), Esteve Terradas 1, Edifici RDIT, Ofic. 212 Parc Mediterrani de la Tecnologia (PMT) Campus del Baix Llobregat – UPC 08860 Castelldefels (Barcelona), Catalonia, Spain
¹⁶ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, UMR 5804, 33615 Pessac, France
¹⁷ NRC Herzberg Astronomy and Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁸ Instituto Argentino de Radioastronomía (CCT-La Plata, CONICET; UNLP; CICPBA), C.C. No. 5, 1894, Villa Elisa, Buenos Aires, Argentina
¹⁹ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China
²⁰ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
²¹ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France
²² Observatoire de Paris, PSL University, Sorbonne Université, LERMA, 75014 Paris, France
²³ School of Physics and Astronomy, Yunnan University, Kunming 650091, PR China
²⁴ Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan
²⁵ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^★ Corresponding author; nsandovalgarrido@gmail.com

Received: 12 October 2024
Accepted: 26 February 2025

Abstract

ALMA-IMF observed 15 massive protoclusters capturing multiple spectral lines and the continuum emission. Here, we focus on the massive protocluster G351.77 (~2500 M_⊙, estimated from single-dish continuum observations) located at 2 kpc. We trace the dense gas emission and kinematics with N₂H⁺ (1–0) at ~4 kau resolution. We estimate an N₂H⁺ relative abundance of ~(1.66 ± 0.46) × 10⁻¹⁰. We decompose the N₂H⁺ emission into up to two velocity components, highlighting the kinematic complexity in the dense gas. By examining the position-velocity (PV) and position-position-velocity (PPV) diagrams on small scales, we observe clear inflow signatures (V-shapes) associated with 1.3 mm cores. The most prominent V-shape has a mass inflow rate of ~13.45 × 10⁻⁴ M_⊙ yr⁻¹ and a short timescale of ~11.42 kyr. We also observe V-shapes without associated cores. This suggests both that cores or centers of accretion exist below the 1.3 mm detection limit, and that the V-shapes may be viable tracers of very early accretion and star formation on ~4 kau scales. The large-scale PV diagram shows that the protocluster is separated into two principal velocity structures separate by ~2 km s⁻¹. Combined with smaller-scale DCN and H₂CO emission in the center, we propose a scenario of larger-scale slow contraction with rotation in the center based on simple toy models. This scenario is consistent with previous lines of evidence, and leads to the new suggestion of outside-in evolution of the protocluster as it collapses. The gas depletion times implied by the V-shapes are short (~0.3 Myr), requiring either very fast cluster formation, and/or continuous mass feeding of the protocluster. The latter is possible via the Mother Filament that G351.77 is forming out of. The remarkable similarities in the properties of G351.77 and the recently published work in G353.41 indicate that many of the physical conditions inferred via the ALMA-IMF N₂H⁺ observations may be generic to protoclusters.