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Can we do asteroseismology on F stars?
Tim Bedding (University of Sydney), D. Compton (University of Sydney), J. Guzik (Los Alamos National Laboratory), W. Ball (University of Sydney), S. Murphy (University of Sydney), et al.
Procyon (F5 IV-V) was the first star in which solar-like oscillations were detected (Brown et al 1991). It has been observed at least ten times since then but after 25 years, we still are not able to identify which modes are being observed. This is because the mode lifetimes are significantly shorter than in the Sun, causing the pattern in the Fourier spectrum to be blurred to the extent that adjacent l=0 and l=2 modes overlap. This "F-star problem" was further highlighted by HD 49933 (F3 V), which was the first CoRoT star to be published (Appourchaux et al. 2008). Many other F stars have now been observed with CoRoT and Kepler, and some progress has been made (e.g., White et al. 2012), but it is still unclear whether we will ever be able to apply asteroseismology to them. An important example is theta Cyg (F3 V), which is the brightest star to fall on the Kepler detectors (Guzik et al. 2016). There are surely great possibilities to apply the power of seismology to F stars, if only we could solve the problem of mode identification. Meanwhile, at slightly hotter temperatures on the main sequence, many early F stars show beautiful gamma-Dor g modes. Four years of Kepler observations have revealed a stunning variety of behaviour, including regular and irregular period spacings, as well as rotationally split multiplets (Kurtz et al 2014, Bedding et al 2015, van Reeth et al 2015, etc.). These stars show no sign of solar-like high-order p modes, but perhaps there is a transition region? In F stars that show solar-like oscillations, how can we decide which modes are which? The so-called surface correction term appears to be very different than in Sun-like stars, but surely there must be a gradual transition as we move up in temperature? We can also ask at what temperature the solar-like modes shut off, and become replaced by g modes. What about the granulation background that drives the solar-like modes? Is there any overlap with the g-mode pulsators, or is this a sudden discontinuity? This talk will review our progress in answering these questions, and will include new observational results (e.g. theta Cyg) and new theoretical models, in an attempt to address some of these issues.