Heliocentric X–Y distribution of OCs (crosses) younger than 30 Myr and HMSFRs (dots) from Reid et al. (2014), used to fit the spiral arms. Different colours correspond to different arms. The assignments to each arm are computed using a GMM. Solid black lines are the fitted spiral arms with the parameters in Table 1, while shaded regions account for 1σ uncertainties. Dash-dotted lines correspond to the spiral arms defined by HMSFRs only. The Galactic centre is towards positive X and the Galactic rotation direction is towards positive Y.