2015A&A...575A..19R

Gas plays a major role in the dynamical evolution of protoplanetary discs. Its coupling with the dust is the key to our understanding of planetary formation. Studying the gas content is therefore a crucial step towards understanding protoplanetary discs evolution. Such a study can be made through spectroscopic observations of emission lines in the FIR, where some of the most important gas coolants emit, such as the [OI] ³P₁->³P₂ transition at 63.18µm. We aim at characterising the gas content of protoplanetary discs in the intermediate-aged (from the perspective of the disc lifetime) Chamaeleon II (Cha II) star forming region. We also aim at characterising the gaseous detection fractions within this age range, which is an essential step tracing gas evolution with age in different star forming regions. This evolutionary study can be used to tackle the problem of the gas dispersal timescale in future studies. We obtained Herschel-PACS line scan spectroscopic observations at 63µm of 19 Cha II Class I and II stars. The observations were used to trace [OI] and o-H₂O at 63µm. The analysis of the spatial distribution of [OI], when extended, can be used to understand the origin of the emission. We have detected [OI] emission toward seven out of the nineteen systems observed, and o-H₂O emission at 63.32µm in just one of them, Sz 61. Cha II members show a correlation between [OI] line fluxes and the continuum at 70µm, similar to what is observed in Taurus. We analyse the extended [OI] emission towards the star DK Cha and study its dynamical footprints in the PACS Integral Field Unit (IFU). We conclude that there is a high velocity component from a jet combined with a low velocity component with an origin that may be a combination of disc, envelope and wind emission. The stacking of spectra of objects not detected individually in [OI] leads to a marginal 2.6σ detection that may indicate the presence of gas just below our detection limits for some, if not all, of them.