Herschel-PACS observations of discs in the Eta Chamaeleontis association

Abstract

Protoplanetary discs are the birthplace for planets. Studying protoplanetary discs is the key to constraining theories of planet formation. By observing dust and gas in associations at different ages we can study the evolution of these discs, their clearing timescales, and their physical and geometrical properties. The stellar association Eta Cha is peculiar; some members still retain detectable amounts of gas in their discs at the late age of 7 Myr, making it one of the most interesting young stellar associations in the solar neighbourhood. We characterise the properties of dust and gas in protoplanetary and transitional discs in the Eta Cha young cluster, with special emphasis on explaining the peculiarities that lead to the observed high disc detection fraction and prominent IR excesses at an age of 7 Myr. We observed 17 members of the Eta Cha association with Herschel-PACS in photometric mode and line spectroscopic mode. A subset of members were also observed in range spectroscopic mode. The observations trace [OI] and $H2O emissions at 63.18 and 63.32 microns, respectively, as well as CO, OH, CH+ and [CII] at different wavelengths for those systems observed in range mode. The photometric observations were used to build complete spectral energy distributions (SEDs) from the optical to the far-IR. High-resolution multi-epoch optical spectra with high signal-to-noise ratios were also analysed to study the multiplicity of the sources and look for further gas (accreting) and outflow indicators.