Abstract
Over 2,000 stars were observed for one month with a high enough cadence in order to look for acoustic modes during the survey phase of the Kepler mission. Solar-like oscillations have been detected in about 540 stars. The question of why no oscillations were detected in the remaining stars is still open. Previous works explained the non-detection of modes with the high level of magnetic activity. However, the studied stars contained some classical pulsators and red giants that could have biased the results. In this work, we revisit this analysis on a cleaner sample of 1,014 main-sequence solar-like stars. First we compute the predicted amplitude of the modes. We find that the stars with detected modes have an amplitude to noise ratio larger than 0.94. We measure reliable rotation periods and the associated photometric magnetic index for 684 stars and in particular for 323 stars where the mode amplitude is predicted to be high enough to be detected. We find that among these 323 stars 32% have a magnetic activity level larger than the Sun at maximum activity, explaining the non-detection of p modes. Interestingly, magnetic activity cannot be the primary reason responsible for the absence of detectable modes in the remaining 68% of the stars without p modes detected and with reliable rotation periods. Thus, we investigate metallicity, inclination angle, and binarity as possible causes of low mode amplitudes. Using spectroscopic observations for a subsample, we find that a low metallicity could be the reason for suppressed modes. No clear correlation with binarity nor inclination is found. We also derive the lower limit for our photometric activity index (of 20-30 ppm) below which rotation and magnetic activity are not detected. Finally with our analysis we conclude that stars with a photometric activity index larger than 2,000 ppm have 98.3% probability of not having oscillations detected.
Highlights
Solar-type stars show acoustic oscillations that are intrinsically damped and stochastically excited by near-surface convection (e.g., Goldreich and Keeley, 1977), which is an important ingredient for magnetic activity (Brun and Browning, 2017, references therein).In the Sun, the properties of the acoustic modes are sensitive to the varying magnetic activity (e.g., Woodard and Noyes, 1985; Elsworth et al, 1990; Howe et al, 2015)
They were limited to 1 month of data, which on the one hand hampers the detection of acoustic oscillations and on the other hand, may bias the activity level estimates
The comparison of the different filters provided a list of reliable rotation periods for 412 stars
Summary
Solar-type stars show acoustic oscillations that are intrinsically damped and stochastically excited by near-surface convection (e.g., Goldreich and Keeley, 1977), which is an important ingredient for magnetic activity (Brun and Browning, 2017, references therein).In the Sun, the properties of the acoustic modes are sensitive to the varying magnetic activity (e.g., Woodard and Noyes, 1985; Elsworth et al, 1990; Howe et al, 2015). Already observed in the Sun, such activity-related variations were first discovered in a star other than the Sun, HD 49933 observed by the CoRoT satellite, by García et al (2010) Searching for such signatures of magnetic activity in Kepler data, it was found that other solar-type stars, in particular KIC 8006161 and KIC 5184732, show evidence for a decrease in the mode heights with increasing activity (Kiefer et al, 2017; Salabert et al, 2018; Santos et al, 2018). In the past several indexes were used mostly based on the standard deviation of the time series allowing to measure the variability of the stars This variability can be due to different phenomena, such as oscillations, convection, or magnetic activity (e.g., Basri et al, 2010, 2011; García et al, 2010; Chaplin et al, 2011a). When measuring the magnetic activity index, we need to make sure that the variability is related to magnetic activity and not to other phenomena
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