Why One-dimensional Models Fail in the Diagnosis of Average Spectra from Inhomogeneous Stellar Atmospheres
Abstract
We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular, we construct four different one-dimensional stratifications from a single snapshot of a magnetohydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height and three by averaging over surfaces of constant optical depth at 500 nm. Using these models, we calculate continuum and atomic and molecular line intensities and their center-to-limb variations. From an analysis of the emerging spectra, we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are nonlinearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; nonlinearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; and the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere and is therefore inherently unreliable.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2011
- DOI:
- arXiv:
- arXiv:1101.2643
- Bibcode:
- 2011ApJ...736...69U
- Keywords:
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- line: formation;
- radiative transfer;
- stars: atmospheres;
- Sun: granulation;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 27 pages, 9 figures