First spectroscopic identification of the main sequence in Westerlund 1

¹ Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), Ctra. Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
² Departamento de Física Teórica, Universidad Autónoma de Madrid (UAM), Campus de Cantoblanco, 28049 Madrid, Spain
³ Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Alicante, Carretera de San Vicente s/n, 03690 San Vicente del Raspeig, Spain
⁴ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁵ Istituto Nazionale di Astrofisica (INAF) - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁶ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 6997801, Israel
⁷ European Space Agency (ESA), ESA Office; Space Telescope Science Institute, 3700 San Martin Drive ; Baltimore, MD 21218, USA
⁸ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁹ Square Kilometre Array Observatory (SKAO), Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 November 2025
Accepted: 24 February 2026

Abstract

Context. Being the most massive known young stellar cluster in the Milky Way, Westerlund 1 (Wd1) constitutes an ideal benchmark for understanding the evolution of massive stars. However, the cluster age remains highly controversial (∼4-10 Myr), hindering the use of Wd1 as a reference for massive star evolution. One of the main issues is a high foreground extinction, which has thus far prevented the detection of the main sequence.

Aims. Using infrared (IR) spectroscopy, we seek to detect the cluster’s main sequence for the first time, to characterise the Hertzsprung-Russell diagram (HRD), and to use the cluster’s turn-off to obtain a robust age estimate.

Methods. We obtained multi-epoch, near-IR VLT/KMOS spectroscopic observations of Wd1 to map its population of massive stars. The spectra of ∼110 members were analysed with CMFGEN models to derive the stellar parameters, populate the cluster HRD, and compare it with isochrones from evolutionary models.

Results. Our observations returned 47 new spectroscopically identified cluster members, with spectral types O9-B1 III-V. The cluster turn-off indicates an age of 5.5 ± 1.0 Myr at a distance of 4.23_−0.21^+0.23 kpc, displaying a moderate degree of coevality. We demonstrate that our estimate of the age of Wd1 is robust against reasonable changes in the distance and extinction law, as well as the adopted rotational velocity and metallicity of the stellar isochrones. We further find that ∼65% of the OB stars with multi-epoch coverage exhibit radial-velocity variability.

Conclusions. The IR observations of the unevolved stellar population support a single episode of star formation with an age of ∼5.5 Myr, reinforcing its potential as a benchmark for massive star evolution and providing a reference sample for future binary population studies.