Revisiting the Impact of Stellar Magnetic Activity on the Detectability of Solar-Like Oscillations by Kepler

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 of the stars. However, the sample of stars studied 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 main-sequence solar-like stars that consists of 1,014 stars. First we compute the predicted amplitude of the modes of that sample and for the stars with detected oscillation and compare it to the noise at high frequency in the power spectrum. 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 out of the full sample and in particular for 323 stars where the amplitude of the modes is predicted to be high enough to be detected. We find that among these 323 stars 32% of them have a level of magnetic activity larger than the Sun during its maximum activity, explaining the non-detection of acoustic modes. Interestingly, magnetic activity cannot be the primary reason responsible for the absence of detectable modes in the remaining 68% of the stars without acoustic modes detected and with reliable rotation periods. Thus, we investigate metallicity, inclination angle of the rotation axis, 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 1 arXiv:1907.01415v1 [astro-ph.SR] 2 Jul 2019 Mathur et al. Magnetic activity and oscillationnot 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.

Active red giants: Close binaries versus single rapid rotators

“…7). This latter fact -increased activity leads to lower modes -was previously observed for MS stars (e.g., Chaplin et al 2011;Mathur et al 2019), but also for RGs in close multiple systems (e.g., Derekas et al 2011;Gaulme et al 2014;Benbakoura et al 2020).…”

“…original Kepler mission. We note that this fraction is much higher than the 40% detection rate reported for MS SL stars (Chaplin et al 2011;Mathur et al 2019), whose light curves are dominated by observational noise (Mosser et al 2019). Asteroseismology is thus a very powerful tool to analyze the evolution of stars after the MS as it can be performed on almost all targets of vast samples of stars.…”

“…Investigating that aspect will be part of future works. Last, Mathur et al (2019) report that MS stars with S ph ≥ 0.2% never show SL oscillations. We note that this threshold is not the same for RGs, as we detect oscillations up to levels of about S ph ≈ 1%.…”

“…The first step of our work consists of measuring the fraction of RGs displaying clear rotational modulation, and reassessing the correlation between rotational modulation and suppression of SL oscillations pointed out by García et al (2010), Chaplin et al (2011), Gaulme et al (2014), or Mathur et al (2019, for instance. Our main concern at this step is to avoid observational biases by including targets that are not suitable for our study.…”

Probing the interior physics of stars through asteroseismology

“…Systematic large-scale observational studies with spectroscopy, interferometry, or adaptive optics are required to deduce the cause(s) of the absence of expected oscillations in unknown spectroscopic binaries, e.g., to discriminate between tidal changes of the mode cavities versus dilution of oscillation amplitudes due to contaminating flux from visual companions. Evidence for damping of solarlike oscillations or amplitude suppression due to magnetic activity has also piled up for both single and binary lowmass stars (Mathur et al, 2019).…”