[Paper Review] The Herschel view of Gas in Protoplanetary Systems (GASPS). First comparisons with a large grid of models

C. Pinte, P. Woitke+50 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|May 20, 2010

Astrophysics and Star Formation Studies23 references16 citations

TL;DR

This paper presents a grid of 300,000 radiative transfer and chemical models (DENT) combining MCFOST and ProDiMo to interpret Herschel GASPS observations of protoplanetary disc gas. It shows that [OI] 63 μm emission, combined with CO rotational lines and continuum data, is essential for breaking degeneracies and estimating disc gas masses, especially when CO lines saturate at high masses.

ABSTRACT

The Herschel GASPS Key Program is a survey of the gas phase of protoplanetary discs, targeting 240 objects which cover a large range of ages, spectral types, and disc properties. To interpret this large quantity of data and initiate self-consistent analyses of the gas and dust properties of protoplanetary discs, we have combined the capabilities of the radiative transfer code MCFOST with the gas thermal balance and chemistry code ProDiMo to compute a grid of 300 000 disc models (DENT). We present a comparison of the first Herschel/GASPS line and continuum data with the predictions from the DENT grid of models. Our objective is to test some of the main trends already identified in the DENT grid, as well as to define better empirical diagnostics to estimate the total gas mass of protoplanetary discs. Photospheric UV radiation appears to be the dominant gas-heating mechanism for Herbig stars, whereas UV excess and/or X-rays emission dominates for T Tauri stars. The DENT grid reveals the complexity in the analysis of far-IR lines and the difficulty to invert these observations into physical quantities. The combination of Herschel line observations with continuum data and/or with rotational lines in the (sub-)millimetre regime, in particular CO lines, is required for a detailed characterisation of the physical and chemical properties of circumstellar discs.

Motivation & Objective

• To interpret the large dataset from the Herschel GASPS Key Program on gas in protoplanetary discs using a comprehensive grid of physical models.
• To test the predictive power of the DENT model grid against actual Herschel observations of far-IR lines and continuum emission.
• To identify empirical diagnostics that can reliably estimate disc gas mass and gas-to-dust ratios despite observational degeneracies.
• To assess the relative roles of stellar UV, X-ray, and UV excess in heating disc gas across different stellar types (Herbig vs. T Tauri).
• To evaluate the complementarity of [OI] 63 μm and low-J CO lines in probing warm inner disc regions and outer disc components.

Proposed method

• Coupled the MCFOST radiative transfer code with the ProDiMo code to simulate gas thermal balance and chemistry in circumstellar discs.
• Generated a grid of approximately 300,000 disc models covering a wide range of stellar types, disc ages, and physical parameters.
• Used the DENT grid to predict line fluxes for key far-IR transitions, including [OI] 63 μm and CO J=3→2 at 867 μm.
• Compared model predictions with actual Herschel observations to validate model trends and identify key diagnostics.
• Assessed the impact of self-shielding on CO abundance calculations, acknowledging limitations in the treatment.
• Combined observational diagnostics (e.g., [OI] 63 μm, CO J=3→2, continuum) to break parameter degeneracies in disc mass estimation.

Experimental results

Research questions

• RQ1 How do UV and X-ray irradiation from different stellar types (Herbig vs. T Tauri) affect the heating of gas in protoplanetary discs?
• RQ2 To what extent can [OI] 63 μm emission be used as a reliable proxy for disc gas mass, especially when CO lines saturate?
• RQ3 What is the role of continuum emission and CO rotational lines in constraining the physical and chemical structure of discs?
• RQ4 How do degeneracies between gas mass, gas-to-dust ratio, and optical depth affect the inversion of line observations into physical parameters?
• RQ5 Can the combination of [OI] 63 μm and (sub-)mm CO lines overcome the saturation problem in high-mass discs?

Key findings

• Photospheric UV radiation is the dominant gas-heating mechanism for Herbig stars, while UV excess and/or X-rays dominate for T Tauri stars.
• The [OI] 63 μm line is highly sensitive to warm gas in the inner 10–30 AU and serves as a crucial diagnostic for breaking degeneracies in disc parameter estimation.
• A significant fraction of discs with far-IR excess are expected to be detected in [OI] 63 μm by Herschel, due to the strong correlation between continuum excess and line flux.
• Most discs in the sample are gas-rich, with gas-to-dust mass ratios exceeding 10, though individual estimates are highly uncertain due to scatter and optical depth effects.
• CO J=3→2 line fluxes saturate at high gas masses, limiting their use for mass estimation in massive discs, but this degeneracy can be overcome by combining with [OI] 63 μm or lower-abundance isotopologues like 13CO and C18O.
• The combination of [OI] 63 μm and low-J CO lines provides complementary views of the inner and outer disc regions, especially when resolved spatially.