2012A&A...543A..88B

The nature of embedded accretion disks around forming high-mass stars is one of the missing puzzle pieces for a general understanding of the formation of the most massive and luminous stars. We want to dissect the small-scale structure of the dust continuum and kinematic gas emission toward two of the most prominent high-mass disk candidates. Using the Plateau de Bure Interferometer at ∼1.36mm wavelengths in its most extended configuration we probe the dust and gas emission at ∼0.3'', corresponding to linear resolution elements of ∼800AU. Even at that high spatial resolution NGC 7538IRS1 remains a single compact and massive gas core with extraordinarily high column densities, corresponding to visual extinctions on the order of 10⁵mag, and average densities within the central 2000AU of ∼2.1x10⁹cm^–3 that have not been measured before. We identify a velocity gradient across in northeast-southwest direction that is consistent with the mid-infrared emission, but we do not find a gradient that corresponds to the proposed CH₃OH maser disk. The spectral line data toward NGC 7538IRS1 reveal strong blue- and red-shifted absorption toward the mm continuum peak position. While the blue-shifted absorption is consistent with an outflow along the line of sight, the red-shifted absorption allows us to estimate high infall rates on the order of 10^–2M_☉/yr. Although we cannot prove that the gas will be accreted in the end, the data are consistent with ongoing star formation activity in a scaled-up low-mass star formation scenario. Compared to that, NGC 7538S fragments in a hierarchical fashion into several sub-sources. While the kinematics of the main mm peak are dominated by the accompanying jet, we find rotational signatures from a secondary peak. Furthermore, strong spectral line differences exist between the sub-sources which is indicative of different evolutionary stages within the same large-scale gas clump. NGC 7538IRS1 is one of the most extreme high-mass disk candidates known today. The large concentration of mass into a small area combined with the high infall rates are unusual and likely allow continued accretion. While the absorption is interesting for the infall studies, higher-excited lines that do not suffer from the absorption are needed to better study the disk kinematics. In contrast to that, NGC 7538S appears as a more typical high-mass star formation region that fragments into several sources. Many of them will form low- to intermediate-mass stars. The strongest mm continuum peak is likely capable to form a high-mass star, however, likely of lower mass than NGC 7538 IRS1.