First spectroscopic identification of the main sequence in Westerlund 1

Abstract

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 high foreground extinction, which has so far prevented the detection of the main sequence. Using infrared spectroscopy we seek to detect the cluster's main sequence for the first time, to characterise the Hertzsprung-Russell diagram, and to use the cluster's turn-off to obtain a robust age estimate. We obtained multi-epoch, near-infrared 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 stellar parameters, populate the cluster Hertzsprung-Russell diagram, and compare it with isochrones from evolutionary models. 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+0.23-0.21 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, and 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. Infrared 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.

Figures (11)

• Figure 1: Comparison of the final, telluric-corrected science spectra obtained with the two considered methods for telluric correction, one based on a standard star (blue) and the other using Molecfit (orange). Two wavelength intervals strongly affected by telluric absorption are shown: top, 2.04–2.075 $\mu$m around the He i line at 2.058 $\mu$m; bottom, 2.41–2.45 $\mu$m around Si iv at 2.4265 $\mu$m.
• Figure 2: Normalised $J$- and $H$-band spectra of four newly classified OB members of Westerlund 1, offset vertically for clarity. Object IDs and adopted spectral types are indicated on the left. Principal diagnostic lines used for the spectral classification include Pa $\delta$, Pa $\gamma$, He i 1.083 $\mu$m, He i 1.092 $\mu$m, He i 1.278 $\mu$m, Br 10–12, He ii 1.692 $\mu$m, and He i 1.700 $\mu$m.
• Figure 3: Zoom over the spectra used for the radial‐velocity measurement of Wd1-1048. Coloured lines correspond to epochs as indicated in the legend. Left: DIB 1.527 µm absorption in the five epochs, adopted as wavelength reference (vertical red dashed line marks the rest frame wavelength). Right: He i 1.70 µm line in the five epochs after realigning the spectra. Wd1-1048 is a known RV variable and possible eclipsing binary Ritchie2022.
• Figure 4: Hertzsprung–Russell diagram for the observed sample in Wd1. The colour of the symbols encodes the binary status based on RV measurements: confirmed binaries from Ritchie2022 (red), candidate binaries identified in this work (orange), likely single stars (blue), and stars without RV measurements (grey). The shape of the symbols indicates whether the star had been analysed previously by Ritchie2022 (triangles), while circles mark stars analysed for the first time in this work. $Z=0.02$ isochrones Yusof2022 for ages $\log t\,[{\rm yr}] = 6.0$–7.0 are overplotted (coloured curves).
• Figure 5: Hertzsprung–Russell diagram of Westerlund 1 that combines the $\sim$110 OB stars analysed in this work (black circles) with selected evolved members from the literature. Representative error bars for the blue stars analysed in this work are provided at the bottom. Blue triangles denote WR stars Rosslowe2015orange squares mark yellow and red supergiants Beasor2021; black squares show the YSG and RSG luminosities recalculated in this work (Sect. \ref{['sec:YSGs']}); grey dashed lines connect each star to its value given in Beasor2021, illustrating the offset between both approaches; green diamonds correspond to the blue-straggler hypergiants Wd1-27 and Wd1-30a Clark2019; the luminous-blue-variable Wd1-243 is shown as a yellow cross Ritchie2009; finally the supergiant Wd1-5 appears as a magenta inverted triangle Clark2014. The region where YHGs would lie, following the luminosities proposed by Clark2020, is indicated by an empty black square. All literature luminosities have been rescaled to the distance adopted here of $4.23^{+0.23}_{-0.21}$ kpc. Coloured curves are $Z=0.02$ isochrones Yusof2022 for ages between 1 and 10 Myr, as indicated in the legend.