Abstract
In spite of the huge advances in exoplanet research provided by the NASA Kepler Mission, there remain only a small number of transit detections around evolved stars. Here we present a reformulation of the noise properties of red-giant stars, where the intrinsic stellar granulation, and the stellar oscillations described by asteroseismology play a key role. The new noise model is a significant improvement on the current Kepler results for evolved stars. Our noise model may be used to help understand planet detection thresholds for the ongoing K2 and upcoming TESS missions, and serve as a predictor of stellar noise for these missions. As an application of our noise model, we explore the minimum detectable planet radii for red giant stars, and find that Neptune sized planets should be detectable around low luminosity red giant branch stars.
Highlights
Red giants, stars near the end of their life – which have exhausted fuseable hydrogen in the stellar core, and bloated massively compared to their main-sequence radii – are a relatively new focus for photometric exoplanet research
We present a reformulation of the noise properties of red-giant stars, where the intrinsic stellar granulation and the stellar oscillations described by asteroseismology play a key role
As an application of our noise model, we explore the minimum detectable planet radii for red giant stars, and find that Neptune-sized planets should be detectable around low-luminosity red giant branch stars
Summary
Stars near the end of their life – which have exhausted fuseable hydrogen in the stellar core, and bloated massively compared to their main-sequence radii – are a relatively new focus for photometric exoplanet research. Asteroseismology may be used to discriminate between stars either ascending the red giant branch (RGB), or in the Helium core burning ‘red clump’ (RC) phase (Bedding et al 2011) This is important for the possible detection, and existence, of close-in planets. The ability to detect a planetary transit is limited by multiple factors, the primary factor being the depth of the transit, which is directly related to the relative size of planet and host star. Another more subtle issue is the noise properties of the host star, which in cool main-sequence, sub-giant and red-giant stars can contain contributions from various stellar signals indicative of granulation, oscillations and activity.
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