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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 ∼4Myr, with evidence of a circumstellar disc extending out to ∼540AU. 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 μm line and upper limits for several other far infrared lines. We complement this with continuum data and ground-based observations of the 12CO 3-2, 2-1 and 13CO J = 1-0 line transitions, as well as an upper limit for the H2 0-0 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. [ABSTRACT FROM AUTHOR]

We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo radiative transfer code MCFOST to calculate a grid of ∼300 000 circumstellar disc models, systematically varying 11 stellar, disc and dust parameters including the total disc mass, several disc shape parameters and the dust-to-gas ratio. For each model, dust continuum and line radiative transfer calculations are carried out for 29 far-infrared, sub-mm and mm lines of [O i], [C ii], 12CO and o/p-H2O under five inclinations. The grid allows us to study the influence of the input parameters on the observables, to make statistical predictions for different types of circumstellar discs and to find systematic trends and correlations between the parameters, the continuum fluxes and the line fluxes. The model grid, comprising the calculated disc temperature and chemical structures, the computed spectral energy distributions, line fluxes and profiles, will be used in particular for the data interpretation of the HERSCHEL open time-key program GASPS. The calculated line fluxes show a strong dependence on the assumed ultraviolet excess of the central star and on the disc flaring. The fraction of models predicting [O i] and [C ii] fine-structure lines fluxes above HERSCHEL/PACS and SPICA/SAFARI detection limits is calculated as a function of disc mass. The possibility of deriving the disc gas mass from line observations is discussed. [ABSTRACT FROM AUTHOR]

We investigate the evolution of accretion luminosity Lacc and stellar luminosity in pre-main-sequence stars. We make the assumption that when the star appears as a Class II object, the major phase of accretion is long past and the accretion disc has entered its asymptotic phase. We use an approximate stellar evolution scheme for accreting pre-main-sequence stars based on Hartmann, Cassen & Kenyon. We show that the observed range of values between 0.01 and 1 can be reproduced if the values of the disc mass fraction at the start of the T Tauri phase lie in the range 0.01–0.2, independent of stellar mass. We also show that the observed upper bound of is a generic feature of such disc accretion. We conclude that as long as the data uniformly fill the region between this upper bound and observational detection thresholds, then the degeneracies between age, mass and accretion history severely limit the use of this data for constraining possible scalings between disc properties and stellar mass. [ABSTRACT FROM AUTHOR]