ALMA detections of circumstellar disks in the giant H ii region M17. Probing the intermediate- to high-mass pre-main-sequence population

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Our current understanding is that intermediate- to high-mass stars form in a way similar to low-mass stars, through disk accretion. The expected shorter formation timescales, higher accretion rates, and increasingly strong radiation fields compared to their lower-mass counterparts may lead to significantly different physical conditions that play a role in disk formation, evolution, and the possibility of (sub)stellar companion formation therein. We searched for the mm counterparts of four intermediate- to high-mass ($4-10$ young stellar objects (YSOs) in the giant H ii region M17 at a distance of 1.7 kpc. These objects expose their photospheric spectrum such that their location on the pre-main-sequence (PMS) is well established. They have a circumstellar disk that is likely remnant of the formation process. With ALMA we detected, for the first time, these four YSOs in M17, in Band 6 and 7, as well as four other serendipitous objects. In addition to the flux measurements, the source size and spectral index provide important constraints on the physical mechanism(s) producing the observed emission. We applied different models to estimate the dust and gas mass contained in the disks. All our detections are spatially unresolved, constraining the source size to $<120$,au, and have a spectral index in the range $0.5-2.7$. The derived (upper limits on) the disk dust masses are on the order of a few M_⊕, and estimations of the upper limits on the gas mass vary between 10^-5 and 10^-3 Our modeling suggests that the inner disks of the target YSOs are dust depleted. In two objects (B331 and B268) free-free emission indicates the presence of ionized material around the star. The four serendipitous detections are likely low-mass YSOs. We compared the derived disk masses of our M17 targets to those obtained for YSOs in low-mass star-forming regions (SFRs) and Herbig stars, as a function of stellar mass, age, luminosity, and outer disk radius. The M17 sample, though small, is both the most massive and the youngest sample, yet has the lowest mean disk mass. The studied intermediate- to high-mass PMS stars are surrounded by low-mass compact disks that likely no longer offer a significant contribution to either the final stellar mass or the formation of a planetary system. Along with the four serendipitous discoveries, our findings show the capability of ALMA to probe disks in relatively distant high-mass SFRs, and offer tentative evidence of the influence of the massive star formation environment on disk formation, lifetime, and evolution.