An updated stellar census of the Quintuplet cluster

Abstract

Context. The Quintuplet is one of the most massive young clusters in the Galaxy. As a consequence it offers the prospect of constraining stellar formation and evolution in extreme environments. However, current observations suggest that it comprises a remarkably diverse stellar population that is difficult to reconcile with an instantaneous formation event. Aims. To better understand the nature of the cluster we aim to improve observational constraints on the constituent stars. Methods. In order to accomplish this goal we present HST/NICMOS+WFC3 photometry and VLT/SINFONI+KMOS spectroscopy for ∼ 100 and 71 cluster members, respectively. Results. The Quintuplet appears far more homogeneous than previously expected. All supergiants are classified as either O7-8 Ia or O9-B0 Ia, with only one object of earlier (O5 I-III) spectral type. These stars form a smooth morphological sequence with a cohort of seven early-B hypergiants and six luminous blue variables and WN9-11h stars, which comprise the richest population of such stars of any stellar aggregate known. In parallel, we identify a smaller population of late-O hypergiants and spectroscopically similar WN8-9ha stars. No further H-free Wolf-Rayet (WR) stars were identified, resulting in a 13:1 ratio for WC/WN stars. A subset of the O9-B0 supergiants are unexpectedly faint, suggesting they are both less massive and older than the greater cluster population. Conclusions. Due to an uncertain extinction law, it is not possible to quantitatively determine a cluster age via isochrone fitting. Nevertheless, we find an impressive coincidence between the properties of cluster members preceding the H-free WR phase and the evolutionary predictions for a single, non-rotating 60M ⊙ star, implying an age of ∼ 3.0 − 3.6Myr. Neither the late O-hypergiants nor the low luminosity supergiants are predicted by such a path; we suggest that the former either result from rapid rotators or are the products of binary driven mass-stripping, while the latter may be interlopers. The H-free WRs must evolve from stars with an initial mass in excess of 60M ⊙ but it appears difficult to reconcile their observational properties with theoretical expectations. This is important since one would expect the most massive stars within the Quintuplet to be undergoing core-collapse/SNe at this time; since the WRs represent an evolutionary phase directly preceding this event,their physical properties are crucial to understanding both this process and the nature of the resultant relativistic remnant. As such, the Quintuplet provides unique observational constraints on the evolution and death of the most massive stars forming in the local, high metallicity Universe.

The Arches cluster revisited

“…Notably, the lack of H-free WRs, BHGs, and LBVs strongly argues against ages much larger than this. Comparison to the Quintuplet also suggest an upper limit to the age of the Arches of 3.6 Myr (Clark et al 2018), consistent with the above results. Unfortunately these estimates still bracket the ages at which one might expect the first SNe to occur for very massive stars (≥120 M ).…”

“…Liermann et al 2009). Of these, the faintest, potentially single stars have m F205W ∼ 11.3−11.7 mag, while those found in binaries are signficantly brighter still, ranging up to m F205W ∼ 7.2 mag (due to the formation of hot dust in the wind collision zone; Clark et al 2018). In comparison we have spectral classifications for all stars to a limit of m F205W ∼ 12.41 mag (F31, Table A.1), strongly suggesting that no WC stars are present within the Arches at this time unless subject to particularly extreme differential reddening.…”

“…Trivially, the lack of cool super/hypergiants suggests an age of <5 Myr (Clark et al 2005). Fortuitously, despite suffering from a similarly uncertain extinction, a more stringent constraint is provided by the Quintuplet cluster, which hosts a substantial ∼O7-B0 supergiant population -while lacking the O4-6 supergiants found in the Arches -as well as a wealth of early B hypergiants and LBVs (Figer et al 1999b;Liermann et al 2009;Clark et al 2018). These populations are absent from the Arches, suggesting that it is a younger system.…”

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…BHG, LBV, sgB[e], YHG, and RSG). Surprisingly, only one LBV is identified (Wd1-243; Clark & Negueruela 2004), compared to large numbers of confirmed and candidate systems within, for example, the Quintuplet (Clark et al 2018b) 12 . Intriguingly, Wd1-243 also has a rather moderate mass-loss rate (Ritchie et al 2009b;Fenech et al 2018) in comparison to other (candidate) LBVs such as P Cygni (Najarro 2001), AG Car (Groh et al 2009) and HDE 316285 where, unlike Wd1-243, the wind densities are sufficient to drive the Paschen series into emission.…”

“…No such simple scheme can be constructed for Wd1 due to an absence of transitional WN/WC stars and the range of WN spectral sub-types observed. Similarly the almost complete lack of H-free WN stars within the Quintuplet complicates interpretation of evolution from the BHG/LBV/WN9-11h to WC phase at very high initial masses ( 60 M ; Clark et al 2018b); we return to these observational findings in Sect. 4.4.…”