A New Look at Disk Winds and External Photoevaporation in the $σ$-Orionis Cluster

Abstract

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. The $σ$-Orionis cluster is the ideal site to study disk winds under these conditions; its outer parts can be used to study disk evolution, while its innermost regions to study the effect of external irradiation. For this, we analyze the $\rm [OI]\,λ$6300, $\rm [NII]\,λ$6583, and $\rm [SII]\,λ$6731,$λ$6716 lines using high-resolution MIKE spectra of 27 classical T Tauri stars and complemented by intermediate-resolution X-shooter data. We decompose the line profiles into multiple Gaussian components. We calculated luminosities, line ratios, and kinematic properties of these components. We found that the $\rm [OI]\,λ$6300 line luminosity and kinematic properties are similar to those found in low-mass star-forming regions (SFRs). The frequency of single-component $\rm [OI]\,λ$6300 line profiles reflects the expected evolutionary stage given the intermediate age of $σ$-Orionis. This points to internal processes contributing to the line emission. However, the highly irradiated disks do not follow the accretion - [OI] luminosity relation found in low-mass SFRs, 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 low-mass SFRs. The innermost regions of $σ$-Orionis are clearly affected by external irradiation, evidenced by the lack of correlation in the accretion - [OI] luminosity relation. The broad line widths of close-in sources, however, indicate a contribution from internal processes, such as magnetohydrodynamical winds and/or internal photoevaporation. This suggests a coevolution of internal and external winds in $σ$-Orionis.