2026ApJ...997...94W

We present nonsimultaneous Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 and Submillimeter Array (SMA) observations of the HD 166191 disk, which was recently thought to have a collision in its terrestrial planet zone. Both observations detect dust continuum emission, and the ALMA observations detect the ¹²CO J = 3-2 line from the circumstellar disk. We do not detect SiO, a potential indicator of giant collisions, but place a limit on the total SiO mass in the system. Unlike as previously observed in the IR, we do not find evidence for variability at millimeter wavelengths when comparing the ALMA continuum observations from 2024 to the precollision SMA observations from 2014. We perform modeling of the CO and continuum visibilities, and find that both the CO and dust are marginally spatially resolved and are contained to within 20 au from the central star. The modeling of the CO suggests that the outer regions of the disk are gas-rich, although further observations are needed to confirm the total gas mass. The evolutionary state of this system has been debated in the literature, and our observations, while not definitive, are generally consistent with the idea that this disk is similar to an evolved protoplanetary or transition/hybrid disk. This could suggest that collisions in the terrestrial planet zone of HD 166191 are occurring while the disk is in a transitional phase, where the inner few au are depleted of gas. This makes HD 166191 an important object for understanding the transition between protoplanetary and debris disks and the stages at which collisions occur.