Peter Pan disks

To collapse into a star, a protostar sheds angular momentum by forming a torus of gas and dust that spins around its equator. Within about 3 million years, almost all the original disk material has either ended up in planets, asteroids, and comets or been evaporated by starlight. The disk disappears.

It was therefore surprising when earlier this year two independent groups reported the discovery of circumstellar disks around two red dwarf stars whose ages both exceed 45 million years. One of the groups named the class of long-lived disks after J. M. Barrie’s fictional boy who never grows up, Peter Pan.

Gavin Coleman and Thomas Haworth of Queen Mary University of London set themselves the goal of accounting for the disks’ longevity. Based on disk models, their newly published analysis looked at two complementary ways a disk can persist: by reducing the effectiveness of processes that cause a disk to dwindle and by starting off wider and with more material than is typical.

Red dwarfs are small and dim. The principal source of photo-evaporation in Peter Pan disks is not the star but UV emission from more luminous neighbors. Coleman and Haworth calculated that the disks would survive only in neighborhoods where the UV background is low, such as the outskirts of low-mass star-forming regions.

Disks also lose material through accretion onto their host star. To make it to the surface of the star, inward-moving material has to transfer its angular momentum to outward-moving material. The higher the disk’s viscosity, the more efficiently the transfer proceeds and the quicker the disk depletes. According to Coleman and Haworth’s analysis, Peter Pan disks do indeed require low viscosity, but not lower than the lowest values observed in disks.

The initial mass of a typical circumstellar disk is no more than 25% of the host star’s. Coleman and Haworth found that Peter Pan disks need a ratio of at least 25%, even 50%. Peter Pan disks also need to start off wider than the 100 AU initial radius of a typical disk.

Coleman and Haworth conclude that Peter Pan disks are rare. Of the requirements for sustaining a Peter Pan disk, the low UV background is the most stringent: A normal UV background snuffs out the disk, as depicted in the figure. However, the requirement for an initially massive, wide disk is consistent with the unusually populous planetary systems found to orbit red dwarfs. The extra material goes into the extra planets. (G. A. L. Coleman, T. J. Haworth, Mon. Not. R. Astron. Soc. 496, L111, 2020 .)