PENELLOPE. IV. A comparison between optical forbidden lines and H2 UV lines in the Orion OB1b and σ-Ori associations

Context. Observing the spatial distribution and excitation processes of atomic and molecular gas in the inner regions (<20 au) of young (<10 Myr) protoplanetary disks helps us to understand the conditions for the formation and evolution of planetary systems.
Aims: In the framework of the PENELLOPE and ULLYSES projects, we aim to characterize the atomic and molecular component of protoplanetary disks in a sample of 11 classical T Tauri stars of the Orion OB1 and σ-Orionis associations.
Methods: We analyzed the flux-calibrated optical forbidden lines and the fluorescent ultraviolet H₂ progressions using spectra acquired with ESPRESSO at VLT, UVES at VLT, and HST-COS. Line morphologies were characterized through Gaussian decomposition. We then focused on the properties of the narrow low-velocity (full width half maximum <40 km s⁻¹ and |υ_p| < 30 km s⁻¹) component (NLVC) of the [O I] 630 nm line and compared them with those of the UV-H₂ lines.
Results: We found that the [O I]630 NLVC and the UV-H₂ lines are strongly correlated in terms of peak velocities, full width at half maximum values, and luminosity. Assuming that the line width is dominated by Keplerian broadening, the [O I]630 NLVC originates from a disk region between 0.5 and 3.5 au, while that of UV-H₂ originates in a region from 0.05 to 1 au. The luminosities of [O I]630 NLVC and UV-H₂ correlate with an accretion luminosity with a similar slope, as well as with the luminosity of the C IV154.8, 155 nm doublet. We discuss such correlations in the framework of the currently suggested excitation processes for the [O I]630 NLVC.
Conclusions: Our results can be interpreted in a scenario in which the [O I]630 NLVC and UV-H₂ have a common disk origin with a partially overlapped radial extension. We also suggest that the excitation of the [O I] NLVC is mainly induced by stellar far-ultraviolet continuum photons, than being of mostly thermal origin. This study demonstrates the potential of contemporaneous wide-band highresolution spectroscopy in linking different tracers of protoplanetary disks.