2025A&A...696A.195P

Context. Over the past decades, observational evidence of circumstellar disks around massive protostars has been steadily accumulating. However, there have also been cases of non-detections in high-mass star-forming regions, leaving the role and prevalence of disks around massive protostars still uncertain.
Aims. The aim of this work is to investigate the substructures of the previously identified 2000-au-scale rotating structures around massive protostars and search for the embedded Keplerian disk inside.
Methods. We used high-resolution (∼0.2″) NOrthern Extended Millimeter Array (NOEMA) observations to study the 1.3 mm continuum and molecular line emission of five massive dense cores in the Cygnus-X cloud complex. Four cores host 2000-au-scale rotating structures previously identified as disk candidates in lower-resolution SMA observations, while the remaining core with no evidence for a disk serves as a comparison.
Results. With a resolution of 300 au, the 1.3 mm continuum emission reveals varying levels of fragmentation in our sample, with fragment radii ranging from 150 to 800 AU. The emission of the CO J = 2-1 transition shows that 13 fragments are associated with uni- or bipolar outflows, but only seven are detected in the CH₃CN emission. We find velocity gradients across two fragments perpendicular to the outflow axis and their position-velocity (PV) diagrams along the velocity gradient resemble the Keplerian rotation. Fitting the velocity profiles in the PV diagrams with a Keplerian model, we obtain protostellar masses for the two disks. Both disks have gas masses lower than one third of the protostellar masses and Toomre Q values are higher than 1, indicating that the disks are globally stable. Among the other sources detected in the CH₃CN emission, some show velocity fields indicative of gas flows connecting multiple systems or outflowing gas, while others show no clear velocity gradient.
Conclusions. In this work, we confirm the existence of two small, stable disks in Keplerian-like rotation at scales of 500 au out of four previously identified disk candidates from the SMA observations at coarser resolution. The lack of evidence for Keplerian disks in other disk candidates identified from the SMA data suggests that rotational signatures observed at 2000 au scales do not necessarily imply the presence of Keplerian disks at smaller scales. Therefore, higher-resolution and higher-sensitivity observations are essential to definitively identify Keplerian disks on smaller scales.