Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks

TL;DR

This study investigates how the environment in Cygnus OB2, especially UV radiation and stellar density, influences the rapid dissipation of protoplanetary disks, highlighting photoevaporation as the dominant process over stellar encounters.

Contribution

It provides the first detailed analysis of disk dissipation mechanisms in Cygnus OB2, emphasizing the dominant role of photoevaporation due to UV radiation over close stellar encounters.

Findings

Disks dissipate faster in regions with intense UV radiation and high stellar density.

Photoevaporation is the primary disk dissipation mechanism in Cygnus OB2.

Close stellar encounters have negligible impact on disk dissipation.

Abstract

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygnus OB2 and we study its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cygnus OB2, and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.