Testing external photoevaporation in the ?? -Orionis cluster with spectroscopy and disk mass measurements

Context. The evolution of protoplanetary disks is regulated by an interplay of several processes, either internal to the system or related to the environment. As most of the stars and planets, including our own Solar System, have formed in massive stellar clusters that contain OB-type stars, studying the effects of UV radiation on disk evolution is of paramount importance. Aims. For this work, we tested the impact of external photoevaporation on the evolution of disks in the mid-age (~3–5 Myr) σ-Orionis cluster by conducting the first combined large-scale UV to IR spectroscopic and millimeter-continuum survey of this region. Methods. We studied a sample of 50 targets located at increasing distances from the central, massive OB system σ-Ori. We combined new spectra obtained with VLT/X-shooter, used to measure mass accretion rates and stellar masses, with new and previously published ALMA measurements of disk dust and gas fluxes and masses. Results. We confirm the previously found decrease in Mdust in the inner ~0.5 pc of the cluster. This is particularly evident when considering the disks around the more massive stars (≥0.4 M⊙), where those located in the inner part (<0.5 pc) of the cluster have Mdust about an order of magnitude lower than the more distant ones. About half of the sample is located in the region of the Ṁacc versus Mdisk expected by models of external photoevaporation, namely showing shorter disk lifetimes than expected for their ages. The shorter disk lifetimes is observed for all targets with a projected separation from σ-Ori < 0.5 pc, proving that the presence of a massive stellar system affects disk evolution. Conclusions. External photoevaporation is a viable mechanism to explain the observed shorter disk lifetimes and lower Mdust in the inner ~0.5 pc of the σ-Orionis cluster, where the effects of this process are more pronounced. Follow-up observations of the low stellar mass targets are crucial to constrain disk dispersion timescales in the cluster and to confirm the dependence of the external photoevaporation process with stellar host mass. This work confirms that the effects of external photoevaporation are significant down to at least impinging radiation as low as ~104 G0.