Grid-based seismic modelling at high and low signal-to-noise ratios - HD 181420 and HD 175272

摘要

Context. Recently, the CoRoT target HD 175272 (F5V), which shows a weak signal of solar-like oscillations, was modelled by a differential asteroseismic analysis (Ozel et al. 2013, A&A, 558, A79) relative to a seismically similar star, HD 181420 (F2V), for which there is a clear signature of solar-like oscillations. The results provided by Ozel et al. indicate the possibility of HD 175272 having subsolar mass, while being of the order of 1000 K hotter than the Sun. This seems unphysical – standard stellar evolution theory generally does not predict solar-metallicity stars of subsolar mass to be hotter than about 6000 K – and calls for a reanalysis of this star. Aims. We aim to compare the performance of differential asteroseismic analysis with that of grid-based modelling. Methods. We use two sets of stellar model grids and two grid-fitting methods to model HD 175272 and HD 181420 using their effective temperatures, metallicities, large frequency separations (Δν), and frequencies of maximum oscillation power (νmax) as observational constraints. Results. We find that we are able to model both stars with parameters that are both mutually compatible and comparable with other modelling efforts. Hence, with modest spectroscopic and asteroseismic inputs, we obtain reasonable estimates of stellar parameters. In the case of HD 175272, the uncertainties of the stellar parameters from our grid-based modelling are smaller, and hence more physical, than those reported in the differential analysis. For both stars, the models have significantly lower values of νmax than the reported observed values. Furthermore, when using the asymptotic large frequency separation as opposed to the scaling relation to compute Δν, we find that our modelling results are significantly more self-consistent when νmax is ignored. Conclusions. Grid-based modelling is a useful tool even in cases of weak solar-like oscillations. It provides more precise and more realistic results than obtained with differential seismology. The difference in the observed and modelled values of νmax indicates that the four observational constraints are not fully consistent with the stellar models used here, with νmax most likely to be the inconsistent constraint for these two stars.