Hydrodynamical simulations of convection-related stellar micro-variability^*,**

II. The enigmatic granulation background of the CoRoT target HD 49933

¹ GEPI, Observatoire de Paris, CNRS, Univ. Paris 7, 92195 Meudon Cedex, France e-mail: Hans.Ludwig@obspm.fr
² LESIA, Observatoire de Paris, CNRS (UMR 8109), Univ. Paris 6, Univ. Paris 7, 92195 Meudon Cedex, France
³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institut d'Astrophysique Spatiale, Univ. Paris 11, CNRS (UMR 8617), 91405 Orsay, France
⁵ Institut d'Astrophysique et de Géophysique de l'Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium

Received: 23 February 2009
Accepted: 17 April 2009

Abstract

Context. Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions.

Aims. We aim to model the photometric granulation background of HD 49933 as well as the Sun, and compare the predictions to the measurements obtained by the CoRoT  and SOHO satellite missions.

Methods. We construct hydrodynamical model atmospheres representing HD 49933 and the Sun, and use a previously developed scaling technique to obtain the observable disk-integrated brightness fluctuations. We further performed exploratory magneto-hydrodynamical simulations to gauge the impact of small scale magnetic fields on the synthetic light-curves.

Results. We find that the granulation-related brightness fluctuations depend on metallicity. We obtain a satisfactory correspondence between prediction and observation for the Sun, validating our approach. For HD 49933, we arrive at a significant over-estimation by a factor of two to three in total power. Locally generated magnetic fields are unlikely to be responsible, otherwise existing fields would need to be rather strong to sufficiently suppress the granulation signal. Presently suggested updates on the fundamental stellar parameters do not improve the correspondence; however, an ad-hoc increase of the HD 49933 surface gravity by about 0.2 dex would eliminate most of the discrepancy.

Conclusions. We diagnose a puzzling discrepancy between the predicted and observed granulation background in HD 49933, with only rather ad-hoc ideas for remedies at hand.