Institute in General
   Events / Jobs
   Public Relations
   Research Groups
   Internal Pages
Search Telephone list Feedback Impressum Links
last change 2007 February 1, R. Arlt
Stellar physics
   Carbon monoxide in the solar atmosphere

New insights from Chemo-Radiation-Hydrodynamics simulations

Carbon monoxide (CO) has been observed in the solar atmosphere for more than 30 years. The discovery of strong CO lines in the early 1970s provoked a crisis in our understanding of the thermal structure of the outer layers of the Sun and solar-type stars: The very existence of CO molecules in significant amounts requires much lower gas temperatures than predicted by the standard models of the solar atmosphere.

Due to its strong infrared line emission, carbon monoxide is known to be an efficient cooling agent that can actively affect the thermal state of the atmospheric gas itself. Therefore, the idea that CO is capable of inducing large horizontal temperature differences in the solar atmosphere has attracted considerable attention during the past three decades.

The scenario is as follows: A small perturbation towards lower temperature would lead to the formation of more CO molecules, whose infrared lines would further cool the gas and thus induce the formation of even more CO, ultimately leading to a "cooling catastrophe". CO could thus enforce a very cool equilibrium temperature in quiet regions where radiative equilibrium conditions prevail. In more active regions with a critical amount of mechanical heating, e.g. through to the dissipation of acoustic waves, CO molecules are destroyed and the atmosphere switches over to a hot state.

In an effort to verify that this cooling instability is really at work in the Sun, and is responsible for the existence of cool patches in the lower chromosphere, Sven Wedemeyer-Böhm (Kiepenheuer-Institut für Sonnenphysik, Freiburg) and Matthias Steffen (Astrophysikalisches Institut Potsdam, AIP) have carried out the hitherto most sophisticated numerical simulations of this process, using the computer code COBOLD. This code, developed partly at the AIP, combines time-dependent hydrodynamics, detailed radiative transfer including CO line cooling, and a non-equilibrium chemical reaction network into a 2- or 3-dimensional model of the solar atmosphere.

Temperature distribution in a vertical cross-section in a COBOLD simulation of the solar atmosphere, showing the violent dynamics of the outer layers.

3-dimensional distribution of carbon monoxide (blue) and temperature (red) in a COBOLD simulation of the solar atmosphere.


[Stellar physics program]

[AIP home page]


Two-dimensional slices for an exemplary time step a) logarithmic gas temperature log T, b) fraction of carbon atoms bound in CO, c) net radiative heating rate for the simulation without CO line cooling, and d) the difference of radiative heating rates between the runs with and without CO cooling.



Main result: If the atmospheric dynamics is artificially suppressed, the CO cooling process works in principle and drives the atmosphere to a very cool radiative equilibrium state, as expected. However, the radiative relaxation of the atmosphere to a cool state is a slow process that takes several 1000 s. The situation is completely different in the fully dynamical simulations. Here the effect of CO cooling is found to be almost negligible. This is not so surprising since the dynamical timescales in the solar atmosphere model are of the order of 30 seconds, much too short to permit the radiative relaxation to a cool state.

The authors conclude that the beautiful idea to explain the very cool temperature regions observed in the solar atmosphere as a result of a thermal instability due to carbon monoxide as a cooling agent seems to be invalid. Rather, the new calculations indicate that it is the violent dynamics of the outer model atmosphere that produces the inhomogeneous thermal structure by means of shock wave heating and expansion cooling. A similar conclusion probably holds for other solar-type stars.




Dr Matthias Steffen
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 371

[Previous highlights]      [Press releases]     [Image archive]