A new look at disk winds and external photoevaporation in the <i>σ</i>-Orionis cluster

Context . Disk winds play a crucial role in the evolution of protoplanetary disks. Typical conditions for star and planet formation are in regions with intermediate or strong UV radiation fields produced by massive stars. In these environments, internally or externally driven winds can occur. The σ -Orionis cluster is the ideal site to study disk winds under these conditions; its outer parts, exposed only to mild UV fields, can be used to study disk evolution, while its innermost regions can be used to study the effect of external irradiation. Aims . Our goal is to study disk winds in the σ -Orionis cluster by looking at the properties of optical forbidden lines, and comparing them with other star-forming regions at different ages, to search for potential signatures of disk evolution and external photoevaporation. Methods . We analyzed the [OI] λ 6300, [NII] λ 6583, and [SII] λ 6731, λ 6716 lines using high-resolution MIKE spectra for a sample of 27 classical T Tauri stars and complemented by intermediate-resolution X-shooter data. We decomposed the line profiles into multiple Gaussian components. We calculated luminosities, line ratios, and kinematic properties of these components. Results . We find that the [OI] λ 6300 line luminosity and kinematic properties for our σ -Orionis sample are similar to those found in low-mass star-forming regions. The frequency of single-component [OI] λ 6300 line profiles reflects the expected evolutionary stage given the intermediate age of σ -Orionis (~3–5 Myr). This points to internal processes contributing to the line emission. However, the highly irradiated disks in the cluster do not follow the accretion luminosity-[OI] λ 6300line luminosity relation found in low-mass star-forming regions, and all exhibit single-component line profiles. Line ratios of highly ionized species of [NII] and [SII] show higher ratios than typical values found in sources in low-mass star-forming regions. These are interpreted as signatures of external photoevaporation. Conclusions . We show the potential of using multiple forbidden emission lines to study both internally and externally driven disk winds. In the case of σ -Orionis, the innermost regions are clearly affected by external irradiation, as evidenced by the lack of correlation in the accretion-[OI] luminosity relation. The broad line widths of close-in sources, however, indicate a possible contribution from internal processes, such as magnetohydrodynamical winds and/or internal photoevaporation. This suggests a coevolution of internal and external winds in the σ -Orionis disks, while pointing toward a new way to disentangle these processes.