Cluster Ages to Reconstruct the Milky Way Assembly (CARMA)

I. The final word on the origin of NGC 6388 and NGC 6441

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: davide.massari@inaf.it
² atlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
³ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
⁴ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Tenerife, Spain
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
⁶ INAF – Osservatorio Astronomico di Abruzzo, Via M. Maggini, 64100 Teramo, Italy
⁷ INFN – Sezione di Pisa, Universitá di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁰ Universidad de Granada, Departamento de Física Teórica y del Cosmos, Campus Fuente Nueva, Edificio Mecenas, 18071 Granada, Spain
¹¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
¹² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 26 June 2023
Accepted: 27 September 2023

Abstract

We present CARMA, the Cluster Ages to Reconstruct the Milky Way Assembly project, the aim of which is to determine precise and accurate age measurements for the entire system of known Galactic globular clusters (GCs) and to use them to trace the most significant merger events experienced by the Milky Way. The strength of CARMA relies on the use of homogeneous photometry, theoretical isochrones, and statistical methods, which will enable us to define a systematic uncertainty-free chronological scale for the complete sample of Milky Way GCs. In this paper, we describe the CARMA framework in detail, and present a first application on a sample of six metal-rich GCs, with the aim being to unequivocally elucidate the debated origin of NGC 6388 and NGC 6441. Our results demonstrate that this pair of clusters is coeval with another four systems that have a clear in situ origin. Moreover, their location in the age–metallicity plane matches that occupied by in situ field stars. The accurate age comparison enabled by the CARMA methodology rules out the possibility that NGC 6388 and NGC 6441 were accreted as part of a past merger event.