Massive pre-main-sequence stars in M17

Abstract

Context. Recently much progress has been made in probing the embedded stages of massive star formation, pointing to formation scenarios that are reminiscent of a scaled-up version of low-mass star formation. However, the latest stages of massive-star formation have rarely been observed, as young massive stars are assumed to reveal their photospheres only when they are fully formed. Aims. Using first and second overtone CO bandhead emission and near- to mid-infrared photometry, we aim to characterize the remnant formation disks around five unique pre-main-sequence (PMS) stars with masses 6–12 M⊙ that have constrained stellar parameters thanks to their detectable photospheres. We seek to understand this emission and the disks from which it originates in the context of the evolutionary stage of the studied sources. Methods. We used an analytic disk model, and adopted local thermodynamical equilibrium, to fit the CO bandhead and the dust emission, assumed to originate in different disk regions. For the first time, we modeled the second overtone emission, which helped us to put tighter constraints on the density of the CO gas. Furthermore, we fit continuum normalized bandheads, using models for stellar and dust continuum, and show the importance of this in constraining the emission region. We also included 13CO in our models as an additional probe of the young nature of the studied objects. Results. We find that the CO emission originates in a narrow region close to the star (<1 AU) and under very similar disk conditions (temperatures and densities) for the different objects. This is consistent with previous modeling of this emission in a diverse range of young stellar objects and identifies CO emission as an indicator of the presence of a gaseous inner disk reaching close to the stellar surface. From constraining the location of the inner edge of the dust emission, we find that all but one of the objects have undisrupted inner dust disks. Conclusions. We discuss these results in the context of the positions of these PMS stars in the Hertzsprung-Russel diagram and the CO emission’s association with an early age and high accretion rates in (massive) young stellar objects. We conclude, considering their mass range and the fact that their photospheres are detected, that the M17 PMS stars are observed in a relatively early formation stage. They are therefore excellent candidates for longer wavelength studies to further constrain the end stages of massive star formation.

New Insights on the Accretion Properties of Class 0 Protostars from 2 μm Spectroscopy

“…These studies found that the Brγ emission can be consistent with magnetospheric accretion, but in several Herbig Ae/Be stars the emitting region extends well beyond the magnetospheric accretion region (located typically below 0.1 au), where the line profiles suggest an additional contribution from material in Keplerian rotation at the base of the disk wind and/or from an inner hot gaseous disk (Caratti o Garatti et al 2015;Mendigutía et al 2015;Hone et al 2019). CO overtone emission is generally attributed to a hot (T ∼ 2000-3000 K) and dense (N CO ∼ 10 21 -10 22 cm −2 ) inner disk inside the dust sublimation radius (e.g., Ilee et al 2014;Lyo et al 2017;Poorta et al 2023).…”