Investigating the A-Type Stars Using Kepler Data

Abstract

Rotation is a key physical process operating in the A stars. We present a method by which rotation periods might be measured with Fourier transforms of Kepler data, potentially removing the requirement of spectroscopic observations to acquire similar information. Angular rotational velocities reach their maximum at about A5, but slowly rotating A stars are also seen and these tend to be chemically peculiar. For Ap stars, rotational braking is understood, but too few progenitors are observed. A review of a rare class of stars, the 'sn' stars, leads us to suggest these may contribute to the 'missing' Ap progenitors. For Am stars, we find the tidal braking mechanisms proposed in the literature induce mixing that is incompatible with observed abundance anomalies. At the other end of the scale are the λ Boo stars, whose rotation velocities are above average. The two main theories for the origin of their metal underabundances are discussed and it is suggested that both of them imply the λ Boo stars contain a high fraction of pulsators -a suggestion that is backed up by observations in the literature. Many λ Boo stars also have circumstellar material, suggesting they are potential planet hosts. This, and the use of asteroseismology to study their interiors, are two excellent reasons to adopt them as prime targets for detailed investigation with Kepler.Pulsation is a common phenomenon in A stars. The δ Sct stars receive wide attention but the fraction of stars that pulsate at the 50 µmag level is shown to be only 56 per cent. The non-δ Sct stars in the δ Sct instability strip receive far less attention. Some of these stars, without appreciable granulation or stellar winds, ii are probably the least variable objects on the HR diagram. It is shown that they have the potential to be among the most peculiar Am stars. Their investigation has led to two important conclusions: (1) the presence of γ Dor pulsation in non-δ Sct stars in the δ Sct instability strip may inhibit the development of the expected Am peculiarities; and (2) chemically normal, non-δ Sct stars in the δ Sct instability strip do exist at the µmag level.Fourier transforms are invaluable tools in the field of stellar pulsation. TheKepler Space Telescope is providing data of exquisite precision, and thus more detail is seen in the Fourier transform than ever before. Truly understanding the properties of the data is fundamental to their successful utilisation. Through statistical analysis of noise levels in over 20 000 stars, granulation is concluded to be visible in Fourier transforms of stars cooler than 7500 K. Another property investigated is the Nyquist frequency. We found that periodically modulated sampling on board Kepler allows distinction between real pulsation frequencies and Nyquist aliases, even when those real frequencies exceed the Nyquist frequency of the data. This discovery opens up study of many hundreds of stars previously thought to have insurmountable Nyquist ambiguities in their data.iii ContentsAbstract ii In Don Kurtz, I have h...

CoRoT photometry and STELLA spectroscopy of an eccentric, eclipsing, and spotted HgMn binary with sub-synchronized rotation

“…The light variation is therefore most likely caused by surface inhomogeneities on both components. These are typically attributed to the combined effects of a chemically peculiar atmosphere, magnetic fields, accretion from the circumbinary medium, and the dredge-up of low angular-momentum material due to a mild ellipticity effect that may also generate a local surface temperature gradient (e.g., Murphy 2015). A spot model with four longitudinally equidistant (circular) spots on one of the two stellar components fits the observed light curve to the measurement precision.…”

Gaia-derived luminosities ofKeplerA/F stars and the pulsator fraction across the δ Scuti instability strip

“…6b shows the distribution of SNR. For the noise estimate we used the 95th percentile of Fourier amplitudes, following Murphy (2014). Traditionally in analyses of classical pulsators, peaks are considered significant if their SNR exceeds 4 (Breger et al 1993), i.e.…”

“…Helium gravitationally settles out of the partial ionization zone where p modes are driven, hence Am stars are expected (and observed) to have a low pulsator fraction (Breger 1970;Kurtz 1989;Smalley et al 2011Smalley et al , 2017. Suggestions that the nonpulsators in the instability strip are actually Ap/Am stars have naturally followed (Murphy 2014;Murphy et al 2015).…”

Gaia's view of the λ Boo star puzzle

“…The pressure-mode (p mode) oscillations of δ Sct stars are driven by the opacity mechanism operating on the He ii partial ionisation zone. Murphy (2014) argued that the delivery of fresh helium at the surface, combined with the typically rapid rotation of λ Boo stars, acts to prevent helium from gravitationally settling in these stars. The driving zone is therefore well-stocked with helium, which Murphy (2014) argued would lead to stronger oscillation amplitudes in these stars, and/or a higher fraction of them being pulsators, compared to an otherwise identical sample of chemically normal stars.…”

“…Murphy (2014) argued that the delivery of fresh helium at the surface, combined with the typically rapid rotation of λ Boo stars, acts to prevent helium from gravitationally settling in these stars. The driving zone is therefore well-stocked with helium, which Murphy (2014) argued would lead to stronger oscillation amplitudes in these stars, and/or a higher fraction of them being pulsators, compared to an otherwise identical sample of chemically normal stars. We have therefore collated pulsation properties from the literature (Antonello & Mantegazza 1982;Paunzen et al 1997Paunzen et al , 1998Paunzen et al , 2002Paunzen et al , 2014Paunzen et al , 2015.…”