tokens: The other possibility to explain the location and the age of the massive O3.5If* star RFS8 being nearly 69 pc from the nearest massive cluster is that the star formed in a small cluster, which has not yet been detected. To investigate if there is a cluster around RFS8, we utilize Two Micron All Sky Survey (2MASS) near-infrared JHK s imaging ( Cutri et al. 2003 ). Currently, this is the deepest photometry (allowing for extinction) available in the vicinity of RFS8. We obtain photometry of a region of 5′ surrounding RFS8. The depth of the photometry reaches roughly 16.5 mag in K s. Allowing for an extinction similar to RFS8 (of A V = 6.7), this reaches down to a spectral type of approximately mid-late A. Following the sequential sampling prescription of the stellar initial mass function (IMF) from Kroupa (2001) , we should have been able to detect a few tens of A-type stars, which might have allowed for the formation of such a massive star. This should be sufficient to identify any young cluster (3 Myr following the upper age limit of RFS8). We note that in the case of stochastic sampling of the IMF, there are no restrictions on the mass of the second most massive star in the cluster, that is, there is still a possibility of finding a cluster, but with few BA spectral type stars. From the colour–colour and colour–magnitude diagrams of stars within 5′ of RFS8, with high-quality infrared photometry, we find no evidence for a cluster sequence, and the upper locus of stars in the colour–magnitude diagram have no clear spatial correlation. In addition, we plot the stellar density (Fig. 4 ) of all point sources. From this, we find no evidence for any significant clustering around RFS8. Fig. 4. Stellar density as a function of distance from RFS8 of all infrared 2MASS sources within 5′. The lower axis shows the radius in projected distance. 4. Discussion We have shown that RFS7, a young (∼1 Myr), massive (∼100 M ⊙ ) O2.5If*/WN6 star is located ∼53 pc from the centre of the nearest massive cluster, NGC 3603 YC. Its mass is comparable to the most massive stars found within the cluster ( Roman-Lopes et al. 2016 ). From Gaia astrometry, we find that RFS7 displays a proper motion outside the locus of cluster stars of NGC 3603 YC, with a 2D projected velocity from NGC 3603 YC of 27.1 ± 1.5 km s −1 . It has a high tangential velocity (∼33 km s −1 ), and a possible bow shock located 1 pc from the star. The motion suggests the star was ejected from the cluster 1–2 Myr ago, consistent with the age of the cluster. All this indicates that RFS7 could likely be a runaway from NGC 3603 YC, making it the most massive known runaway in our Galaxy (and comparable to the behemoths found around the R 136 cluster in the Large Magellanic Cloud). A key future study in this respect would be to decipher whether the runaway was dynamically ejected, which might also help us to better understand the total mass and stellar content of the central cluster (to eject a ∼100 M ⊙
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