In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD 169142 , we present far-IR observations obtained with the PACS instrument onboard the Herschel Space Observatory .
This work is part of the Open Time Key Project GASPS , which is aimed at studying the evolution of protoplanetary discs .
To constrain the gas properties in the outer disc , we observed the star at several key gas-lines , including [ OI ] 63.2 and 145.5 \mu m , [ CII ] 157.7 \mu m , CO 72.8 and 90.2 \mu m , and o-H _ { 2 } OÂ 78.7 and 179.5 \mu m. We only detect the [ OI ] 63.2 \mu m line in our spectra , and derive upper limits for the other lines .
We complement our data set with PACS photometry and ^ { 12 / 13 } CO data obtained with the Submillimeter Array .
Furthermore , we derive accurate stellar parameters from optical spectra and UV to mm photometry .
We model the dust continuum with the 3D radiative transfer code MCFOST Â and use this model as an input to analyse the gas lines with the thermo-chemical code ProDiMo .
Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU , but this is not a unique solution .
Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions .
Our results are inconsistent with a gas-poor disc with a large UV excess ; a gas mass of 5.0 \pm 2.0 \times 10 ^ { -3 } M _ { \odot } is still present in this disc , in agreement with earlier CO observations .