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Fig. 6


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Hertzsprung-Russell diagram displaying the MIST pre-main-sequence tracks (Dotter 2016) for stellar masses between 6 and 20 M, with the solid black line on the left denoting the ZAMS and the gray dashed line on the right indicating the birthline. The objects in this study, along with the one M17 PMS star from RT17 without CO band-head emission (B337), are shown together with the >4 M Herbig stars from the Vioque et al. (2022) sample. Also included are the two RT17 objects (B289 and B215) close to the ZAMS without inner dust or gaseous disk signatures, but with MIR excess. The M17 PMS stars appear to represent a younger stage of formation, consistent with the high detection rate of CO bandhead emission in their spectra and their high extinction. For comparison, the estimated CO detection rate representative for >4 M Herbig stars is 13–17% (see text).

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