The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations

TL;DR

This study compares theoretical models of acoustic mode amplitudes with CoRoT observations of HD 49933, validating the excitation model for a star hotter and metal-poor compared to the Sun, while noting discrepancies at high frequencies.

Contribution

It demonstrates the effectiveness of a stochastic excitation model in predicting mode amplitudes for HD 49933, highlighting the importance of accurate chemical composition assumptions.

Findings

Model predictions agree with observations within error bars at most frequencies.

Assuming the star's actual metal abundance improves model-data agreement.

Discrepancies at high frequency suggest areas for model refinement.

Abstract

From the seismic data obtained by CoRoT for the star HD 49933 it is possible, as for the Sun, to constrain models of the excitation of acoustic modes by turbulent convection. We compare a stochastic excitation model described in Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star that is rather metal poor and significantly hotter than the Sun. Using the mode linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation rates computed in Paper I, we derive the expected surface velocity amplitudes of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic approximation relating the mode amplitudes in intensity to those in velocity, we derive the expected values of the mode amplitude in intensity. Our amplitude calculations are within 1-sigma error bars of the mode surface velocity spectrum derived with the HARPS spectrograph. The same is found with the mode amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other hand, at high frequency, our calculations significantly depart from the CoRoT and HARPS measurements. We show that assuming a solar metal abundance rather than the actual metal abundance of the star would result in a larger discrepancy with the seismic data. Furthermore, calculations that assume the ``new'' solar chemical mixture are in better agreement with the seismic data than those that assume the ``old'' solar chemical mixture. These results validate, in the case of a star significantly hotter than the Sun and Alpha Cen A, the main assumptions in the model of stochastic excitation. However, the discrepancies seen at high frequency highlight some deficiencies of the modelling, whose origin remains to be understood.