Publication Abstracts

Helling et al. 2008

Helling, C., A. Ackerman, F. Allard, M. Dehn, P. Hauschildt, D. Homeier, K. Lodders, M. Marley, F. Rietmeijer, T. Tsuji, and P. Woitke, 2008: A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres. Mon. Not. Roy. Astron. Soc., 391, 1854-1873, doi:10.1111/j.1365-2966.2008.13991.x.

The atmospheres of substellar objects contain clouds of oxides, iron, silicates, and other refractory condensates. Water clouds are expected in the coolest objects. The opacity of these "dust" clouds strongly affects both the atmospheric temperature-pressure profile and the emergent flux. Thus any attempt to model the spectra of these atmospheres must incorporate a cloud model. However the diversity of cloud models in atmospheric simulations is large and it is not always clear how the underlying physics of the various models compare. Likewise the observational consequences of different modeling approaches can be masked by other model differences, making objective comparisons challenging. In order to clarify the current state of the modeling approaches, this paper compares five different cloud models in two sets of tests. Test case 1 tests the dust cloud models for a prescribed L, L-T, and T-dwarf atmospheric (temperature T, pressure p, convective velocity vconv)-structures. Test case 2 compares complete model atmosphere results for given (effective temperature Teff, surface gravity log g). All models agree on the global cloud structure but differ in opacity-relevant details like grain size, amount of dust, dust and gas-phase composition. These models can loosely be grouped into high- and low-altitude cloud models whereas the first appear generally redder in near-infrared colours then the later. Comparisons of synthetic photometric fluxes translate into an modelling uncertainty in apparent magnitudes for our L-dwarf (T-dwarf) test case of 0.25 ≲ Δm ≲ 0.875 (0.1 ≲ Δm ≲ 1.375), taking into account the Two-Micron All Sky Survey, the UKIRT WFCAM, the Spitzer IRAC and VLT VISIR filters with UKIRT WFCAM being the most challenging for the models. Future developments will need closer links with laboratory astrophysics, and a consistent treatment of the cloud chemistry and turbulence.

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BibTeX Citation

@article{he01200l,
  author={Helling, C. and Ackerman, A. and Allard, F. and Dehn, M. and Hauschildt, P. and Homeier, D. and Lodders, K. and Marley, M. and Rietmeijer, F. and Tsuji, T. and Woitke, P.},
  title={A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres},
  year={2008},
  journal={Mon. Not. Roy. Astron. Soc.},
  volume={391},
  pages={1854--1873},
  doi={10.1111/j.1365-2966.2008.13991.x},
}

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RIS Citation

TY  - JOUR
ID  - he01200l
AU  - Helling, C.
AU  - Ackerman, A.
AU  - Allard, F.
AU  - Dehn, M.
AU  - Hauschildt, P.
AU  - Homeier, D.
AU  - Lodders, K.
AU  - Marley, M.
AU  - Rietmeijer, F.
AU  - Tsuji, T.
AU  - Woitke, P.
PY  - 2008
TI  - A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres
JA  - Mon. Not. Roy. Astron. Soc.
VL  - 391
SP  - 1854
EP  - 1873
DO  - 10.1111/j.1365-2966.2008.13991.x
ER  -

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