Solar-like oscillations with low amplitude in the CoRoT target HD 181906

Context: The F8 star HD 181906 (effective temperature ~6300K) was observed for 156 days by the CoRoT satellite during the first long run in the centre direction. Analysis of the data reveals a spectrum of solar-like acoustic oscillations. However, the faintness of the target (m_v=7.65) means the signal-to-noise (S/N) in the acoustic modes is quite low, and this low S/N leads to complications in the analysis. Aims: To extract global variables of the star as well as key parameters of the p modes observed in the power spectrum of the lightcurve. Methods: The power spectrum of the lightcurve, a wavelet transform and spot fitting have been used to obtain the average rotation rate of the star and its inclination angle. Then, the autocorrelation of the power spectrum and the power spectrum of the power spectrum were used to properly determine the large separation. Finally, estimations of the mode parameters have been done by maximizing the likelihood of a global fit, where several modes were fit simultaneously. Results: We have been able to infer the mean surface rotation rate of the star (~4 microHz) with indications of the presence of surface differential rotation, the large separation of the p modes (~87 microHz), and therefore also the ridges corresponding to overtones of the acoustic modes.

💡 Research Summary

The paper presents a comprehensive asteroseismic analysis of the F‑type star HD 181906, observed by the CoRoT satellite during its first long run toward the centre of the Galaxy. Over a continuous 156‑day interval, high‑precision photometric time series were obtained for a relatively faint target (V ≈ 7.65), resulting in a power spectrum where the solar‑like acoustic (p‑mode) signal is buried in a low signal‑to‑noise background. The authors tackled this challenge by combining several complementary techniques.

First, the light curve was detrended and calibrated, after which a Fourier power spectrum was computed. A wavelet transform was applied to the time‑frequency domain to reveal rotational modulation caused by surface active regions (spots). Spot‑fitting models, assuming one or more rotating spots, yielded an average surface rotation frequency of about 4 µHz (corresponding to a period of roughly 2.9 days) and an inclination angle near 45°, with hints of differential rotation inferred from the spread of the rotational peaks.

To extract the global seismic parameter – the large frequency separation (Δν) – the authors employed two independent diagnostics. The autocorrelation of the power spectrum highlighted a regular spacing of peaks, while the power‑spectrum‑of‑the‑power‑spectrum (PSPS) amplified this periodicity. Both methods converged on a large separation of approximately 87 µHz, a value that directly informs the mean stellar density via the well‑known scaling relation Δν ∝ √(M/R³).

Having established the global parameters, the team performed a global maximum‑likelihood fit of the oscillation spectrum. Their model simultaneously included several radial orders for angular degrees l = 0, 1, 2, each described by a central frequency, amplitude, and linewidth (mode lifetime). The background was modeled with a combination of white noise and a granulation‑like component to account for stellar activity. Parameter estimation was carried out using Bayesian sampling (MCMC), allowing robust determination of uncertainties. The fitted mode amplitudes were found to be only about 20 % of solar values, reflecting the low intrinsic signal strength and the faintness of the star. Mode lifetimes were on the order of a few days, comparable to or slightly shorter than solar values.

In summary, the study successfully extracted key asteroseismic quantities from a low‑S/N dataset: a mean rotation rate of ~4 µHz, an inclination angle of ~45°, a large separation of ~87 µHz, and the identification of the l = 0, 1, 2 ridge structure in the power spectrum. The work demonstrates that, even for relatively faint CoRoT targets, a combination of wavelet analysis, autocorrelation/PSPS techniques, and global likelihood fitting can yield reliable measurements of both surface rotation and interior acoustic properties. These results not only enrich our knowledge of HD 181906’s internal structure but also provide a methodological blueprint for future asteroseismic investigations with missions such as Kepler, TESS, and PLATO, where many targets will present similarly low signal‑to‑noise conditions.