[Paper Review] The warm gas atmosphere of the HD 100546 disk seen by Herschel (Evidence of a gas-rich, carbon-poor atmosphere?)

S. Bruderer, E. F. van Dishoeck+2 more|arXiv (Cornell University)|Jan 23, 2012

Astrophysics and Star Formation StudiesPhysics and Astronomy146 references114 citations

TL;DR

This study uses Herschel/PACS observations of HD 100546 to model the warm gas atmosphere in its protoplanetary disk, combining radiative transfer, chemical networks, and gas energetics. It finds that a high gas/dust ratio and low volatile carbon abundance best reproduce the CO ladder, [O I], and [C II] emission, indicating a gas-rich, carbon-poor atmosphere with $ T_{\text{gas}} \gg T_{\text{dust}} $, likely due to UV heating and carbon locking in refractory species.

ABSTRACT

(Abridged) Context. With the Herschel Space Observatory, lines of simple molecules (C+, O, and CO) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based [CI], all principle forms of carbon can be studied. The absence of neutral carbon [CI], which is predicted by models to be strong, can then be interpreted together with [CII] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species by the method of chemical-physical modeling. We explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic fine-structure lines of [OI] and either [CI] or [CII]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with Tgas>>Tdust. The high-J CO observable with PACS are dominated from regions within some tens of AU. Conclusions. Only a warm atmosphere with Tgas>>Tdust can reproduce the CO ladder. The CO ladder together with [O I] and the upper limit to [CI] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [CII]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [OI], are in closer agreement with observations of [CII], but overproduce [CI]. Due to the uncertain origin of the [CII] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

Motivation & Objective

• To determine the physical and chemical conditions in the warm gas atmosphere of the HD 100546 protoplanetary disk using Herschel observations.
• To test whether observed high- $J$ CO lines and atomic fine-structure lines ([O I], [C II], [C I]) can be reproduced by self-consistent physical-chemical models.
• To constrain the gas/dust ratio, volatile carbon abundance, and dust properties by comparing model predictions with multi-line observations.
• To assess the origin of [C II] emission, which may arise from a remnant envelope or foreground material.
• To understand the spatial origin and excitation of CO rotational lines across different $J$-levels.

Proposed method

• A 2D axisymmetric radiative transfer model is used to compute line emission from CO, [O I], [C II], and [C I], coupled with a detailed chemical network.
• Gas temperature is calculated self-consistently by solving the energy balance equation, including heating from UV radiation and cooling from molecular lines.
• The model includes dust reprocessing, UV shielding, and non-thermal H2 formation with a temperature-dependent sticking coefficient.
• The dust opacity law is varied (e.g., $ R_V = 5.5 $) to test penetration depth of UV radiation and its impact on gas heating and chemistry.
• Line profiles and fluxes are computed for different disk configurations, including varying gas/dust ratios, carbon abundances, and grain size distributions.
• Benchmark tests validate the model’s consistency, and the results are compared to Herschel-PACS and ground-based data on CO and [C I] lines.

Experimental results

Research questions

• RQ1 What gas temperature structure is required to reproduce the observed CO rotational ladder in HD 100546?
• RQ2 Can models with different gas/dust ratios and volatile carbon abundances reproduce the observed CO, [O I], [C II], and [C I] line fluxes?
• RQ3 What is the spatial origin of the high- $J$ CO lines detected by Herschel, and how do they compare to low- $J$ lines observed from the ground?
• RQ4 Why is [C I] not detected despite model predictions of strong emission, and how does this affect the interpretation of carbon chemistry?
• RQ5 To what extent can the [C II] emission be attributed to the disk versus a remnant envelope or foreground material?

Key findings

• A warm gas atmosphere with $ T_{\text{gas}} \gg T_{\text{dust}} $ is required to reproduce the high- $J$ CO lines, as models with $ T_{\text{gas}} = T_{\text{dust}} $ underpredict them by orders of magnitude.
• Models with a high gas/dust ratio (100) and low volatile carbon fraction ($ \delta_{\text{C}} = 0.05 $) reproduce the CO ladder, [O I], and [C II] emission, though they slightly overpredict [C I] relative to observations.
• Models with a low gas/dust ratio (20) and higher volatile carbon ($ \delta_{\text{C}} = 0.5 $) better match the [C I] upper limit but underpredict [C II] flux.
• The [C II]/[C I] line ratio is lower in high gas/dust ratio models, bringing it closer to the observed upper limit, and is further reduced by larger grains or absence of PAHs in the outer disk.
• The highest- $J$ CO line (J=30-29) originates from radii of ~20–50 AU, mid- $J$ lines (e.g., J=16-15) from ~40–90 AU, and low- $J$ lines from the outer disk (>100 AU).
• High- $J$ CO lines are predicted to be significantly broader than low- $J$ lines due to higher turbulence and velocity gradients in the inner disk.