Herschel-PACS observations of discs in the η Chamaeleontis association^⋆

¹ European Space Astronomy Centre (ESA), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: priviere@sciops.esa.int
² European Southern Observatory, Alonso de Cordova 3107, Vitacura Casilla 19001, Santiago 19, Chile
³ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
⁴ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, 5030 Casilla, Valparaíso, Chile
⁵ ICM nucleus on protoplanetary discs, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ ALMA, Avda Apoquindo 3846, Piso 19, Edificio Alsacia, Las Condes, Santiago, Chile
⁸ Depto. Astrofísica, Centro de Astrobiología (CAB, INTA–CSIC), PO Box 78, ESAC Campus, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 22 May 2015
Accepted: 2 October 2015

Abstract

Context. Protoplanetary discs are the birthplace of 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 η 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.

Aims. We characterise the properties of dust and gas in protoplanetary and transitional discs in the η 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.

Methods. We observed 17 members of the η 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 μm, 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.

Results. We detect four out of fifteen sources observed at 70 μm, four out of six at 100 μm, and six out of sixteen at 160 μm. Only one system shows [OI] emission at 63 μm, namely RECX 15 or J0843.3-7905. None of them shows far-IR line emission at any other wavelength. The [OI] emission toward RECX 15 points to the presence of an outflow; however, the emission is not extended. We study H_α emission among η Cha members and conclude that RECX 4, 5, 9, 11, and 15 are actively accreting in at least one epoch.

Conclusions. The SEDs of the discs in η Cha show a variety of shapes, from those in Taurus and in Upper Scorpius to sources showing excess over the Taurus median SED. Furthermore, the SEDs of RECX 3 and RECX 4 are typical of debris discs. The detection fraction for [OI] in η Cha is lower than younger regions like Taurus and Cha II, indicative of an evolutionary trend. The lack of [OI] emission, together with the intermediate values of the IR excess, can be explained by long-lived discs with a flattened geometry or by flared discs with a low UV flux, or by a combination of the two scenarios.