We present new continuum and line observations , along with modelling , of the faint ( 6 - 8 ) Myr old T Tauri star ET Cha belonging to the \eta Chamaeleontis cluster .
We have acquired Herschel/Pacs photometric fluxes at 70 \mu m and 160 \mu m , as well as a detection of the [ OI ] 63 \mu m fine-structure line in emission , and derived upper limits for some other far-IR OI , CII , CO and o-H _ { 2 } O lines .
These observations were carried out in the frame of the Open Time Key Programme GASPS , where ET Cha was selected as one of the science demonstration phase targets .
The Herschel data is complemented by new simultaneous Andicam B - K photometry , new Hst/Cos and Hst/Stis UV-observations , a non-detection of CO J = 3 \to 2 with Apex , re-analysis of a Ucles high-resolution optical spectrum showing forbidden emission lines like [ OI ] 6300 Å , [ SII ] 6731 Å and 6716 Å , and [ NII ] 6583 Å , and a compilation of existing broad-band photometric data .
We used the thermo-chemical disk code ProDiMo and the Monte-Carlo radiative transfer code MCFOST to model the protoplanetary disk around ET Cha .
The paper also introduces a number of physical improvements to the ProDiMo disk modelling code concerning the treatment of PAH ionisation balance and heating , the heating by exothermic chemical reactions , and several non-thermal pumping mechanisms for selected gas emission lines .
By applying an evolutionary strategy to minimise the deviations between model predictions and observations , we find a variety of united gas and dust models that simultaneously fit all observed line and continuum fluxes about equally well .
Based on these models we can determine the disk dust mass with confidence , M _ { dust } \approx ( 2 - 5 ) \times 10 ^ { -8 } M _ { \odot } whereas the total disk gas mass is found to be only little constrained , M _ { gas } \approx ( 5 \times 10 ^ { -5 } -3 \times 10 ^ { -3 } ) M _ { \odot } .
Both mass estimates are substantially lower than previously reported .
In the models , the disk extends from 0.022 AU ( just outside of the co-rotation radius ) to only about 10 AU , remarkably small for single stars , whereas larger disks are found to be inconsistent with the CO J = 3 \to 2 non-detection .
The low velocity component of the [ OI ] 6300 Å emission line is centred on the stellar systematic velocity , and is consistent with being emitted from the inner disk .
The model is also consistent with the line flux of H _ { 2 } v = 1 \to 0 S ( 1 ) at 2.122 \mu m and with the [ OI ] 63 \mu m line as seen with Herschel/Pacs .
An additional high-velocity component of the [ OI ] 6300 Å emission line , however , points to the existence of an additional jet/outflow of low velocity 40 - 65 km/s with mass loss rate \approx 10 ^ { -9 } M _ { \odot } / yr .
In relation to our low estimations of the disk mass , such a mass loss rate suggests a disk lifetime of only \sim 0.05 - 3 Myr , substantially shorter than the cluster age .
If a generic gas/dust ratio of 100 was assumed , the disk lifetime would be even shorter , only \sim 3000 yrs .
The evolutionary state of this unusual protoplanetary disk is discussed .