2018A&A...612A.111G

Context. Atmospheric properties of F-, G- and K-type stars can be measured by spectral model fitting or with the analysis of equivalent width (EW) measurements. These methods require data with good signal-to-noise ratios (S/Ns) and reliable continuum normalisation. This is particularly challenging for the spectra we have obtained with the CORALIE echelle spectrograph for FGK stars with transiting M-dwarf companions. The spectra tend to have low S/Ns, which makes it difficult to analyse them using existing methods.
Aims. Our aim is to create a reliable automated spectral analysis routine to determine T_eff, [Fe/H], Vsini from the CORALIE spectra of FGK stars.
Methods. We use wavelet decomposition to distinguish between noise, continuum trends, and stellar spectral features in the CORALIE spectra. A subset of wavelet coefficients from the target spectrum are compared to those from a grid of models in a Bayesian framework to determine the posterior probability distributions of the atmospheric parameters.
Results. By testing our method using synthetic spectra we found that our method converges on the best fitting atmospheric parameters. We test the wavelet method on 20 FGK exoplanet host stars for which higher-quality data have been independently analysed using EW measurements. We find that we can determine T_eff to a precision of 85K, [Fe/H] to a precision of 0.06dex and Vsini to a precision of 1.35km/s for stars with Vsini≥5km/s. We find an offset in metallicity ~=-0.18dex relative to the EW fitting method. We can determine logg to a precision of 0.13dex but find systematic trends with T_eff. Measurements of logg are only reliable enough to confirm dwarf-like surface gravity (logg~=4.5).
Conclusions. The wavelet method can be used to determine T_eff, [Fe/H], and Vsini for FGK stars from CORALIE echelle spectra. Measurements of log g are unreliable but can confirm dwarf-like surface gravity. We find that our method is self consistent, and robust for spectra with S/N≳40.