We present detailed model fits to observations of the disc around the Herbig Ae star HD 163296 .
This well-studied object has an age of \sim 4 Myr , with evidence of a circumstellar disc extending out to \sim 540 AU .
We use the radiation thermo-chemical disc code ProDiMo to model the gas and dust in the circumstellar disc of HD 163296 , and attempt to determine the disc properties by fitting to observational line and continuum data .
These include new Herschel/PACS observations obtained as part of the open-time key program GASPS ( Gas in Protoplanetary Systems ) , consisting of a detection of the [ Oi ] 63 \mu m line and upper limits for several other far infrared lines .
We complement this with continuum data and ground-based observations of the ^ { 12 } CO 3-2 , 2-1 and ^ { 13 } CO J=1-0 line transitions , as well as the H _ { 2 } S ( 1 ) transition .
We explore the effects of stellar ultraviolet variability and dust settling on the line emission , and on the derived disc properties .
Our fitting efforts lead to derived gas/dust ratios in the range 9-100 , depending on the assumptions made .
We note that the line fluxes are sensitive in general to the degree of dust settling in the disc , with an increase in line flux for settled models .
This is most pronounced in lines which are formed in the warm gas in the inner disc , but the low excitation molecular lines are also affected .
This has serious implications for attempts to derive the disc gas mass from line observations .
We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM levels .
Using a stellar and UV excess input spectrum based on a detailed analysis of observations , we find that the all observations are consistent with the previously assumed disc geometry .