[Paper Review] The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations

R. Samadi, H.‐G. Ludwig+8 more|arXiv (Cornell University)|Oct 21, 2009

Stellar, planetary, and galactic studies2 references20 citations

TL;DR

This study tests a stochastic excitation model for solar-like oscillations in the metal-poor, hot star HD 49933 by comparing theoretical mode amplitudes—derived from CoRoT's measured linewidths and Paper I's excitation rates—with observed amplitudes from HARPS and CoRoT. The model agrees well with observations at low frequencies but overestimates amplitudes at high frequencies (ν ≳ 1.9 mHz), highlighting limitations in the current modeling framework, particularly regarding scale length separation and metallicity dependence.

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.

Motivation & Objective

• To validate the stochastic excitation model of acoustic modes in a star significantly hotter and more metal-poor than the Sun.
• To test whether theoretical mode amplitudes match observed amplitudes in velocity and intensity for HD 49933.
• To assess the impact of surface metal abundance and solar chemical mixture on predicted mode amplitudes.
• To evaluate the reliability of the calibrated quasi-adiabatic approximation in linking velocity and intensity amplitudes.
• To identify potential deficiencies in the current modeling framework, particularly at high frequencies.

Proposed method

• Theoretical mode excitation rates from Paper I are used, based on the star's specific parameters including [Fe/H] = -0.37.
• Mode linewidths from CoRoT observations are used to derive expected surface velocity amplitudes via the balance between excitation and damping.
• A calibrated quasi-adiabatic approximation is applied to convert velocity amplitudes to intensity fluctuations (δL/L), validated against helioseismic data.
• Predicted amplitudes in velocity and intensity are compared with HARPS and CoRoT observations, respectively, within 1-σ error bars.
• Sensitivity tests are performed using solar metal abundance and the 'new' vs. 'old' solar chemical mixtures to assess their impact on predictions.
• The role of turbulent convection and the scale length separation assumption in the excitation model is examined as a possible source of high-frequency discrepancies.

Experimental results

Research questions

• RQ1 Does the stochastic excitation model accurately predict observed mode amplitudes in HD 49933, a star hotter and more metal-poor than the Sun?
• RQ2 How does the surface metal abundance of HD 49933 affect the predicted mode amplitudes compared to assuming solar metallicity?
• RQ3 To what extent does the choice of solar chemical mixture (old vs. new) influence the model predictions?
• RQ4 Is the calibrated quasi-adiabatic approximation valid for linking velocity and intensity amplitudes in this star?
• RQ5 What causes the systematic overestimation of mode amplitudes at high frequencies (ν ≳ 1.9 mHz)?

Key findings

• Theoretical surface velocity amplitudes agree with HARPS observations within 1-σ error bars for frequencies below 1.9 mHz.
• Mode amplitudes in intensity derived from the model are consistent with CoRoT data within 1-σ for the same low-frequency range.
• At high frequencies (ν ≳ 1.9 mHz), the model overestimates both velocity and intensity amplitudes compared to observations.
• Assuming solar metal abundance instead of the actual [Fe/H] = -0.37 leads to a 35% higher amplitude peak and up to a factor-of-two overestimation at lower frequencies.
• Using the 'new' solar chemical mixture (Asplund et al. 2005) results in amplitudes about 15% lower than with the 'old' mixture (Grevesse & Noels 1993), but this difference is smaller than current seismic uncertainties.
• The calibrated quasi-adiabatic relation successfully scales velocity amplitudes to intensity fluctuations at the current level of observational precision.