Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades

Abstract

The most powerful tests of stellar models come from the brightest stars in the sky, for which complementary techniques, such as astrometry, asteroseismology, spectroscopy, and interferometry can be combined. The K2 Mission is providing a unique opportunity to obtain high-precision photometric time series for bright stars along the ecliptic. However, bright targets require a large number of pixels to capture the entirety of the stellar flux, and bandwidth restrictions limit the number and brightness of stars that can be observed. To overcome this, we have developed a new photometric technique, that we call halo photometry, to observe very bright stars using a limited number of pixels. Halo photometry is simple, fast and does not require extensive pixel allocation, and will allow us to use K2 and other photometric missions, such as TESS, to observe very bright stars for asteroseismology and to search for transiting exoplanets. We apply this method to the seven brightest stars in the Pleiades open cluster. Each star exhibits variability; six of the stars show what are most-likely slowly pulsating Bstar (SPB) pulsations, with amplitudes ranging from 20 to 2000 ppm. For the star Maia, we demonstrate the utility of combining K2 photometry with spectroscopy and interferometry to show that it is not a 'Maia variable', and to establish that its variability is caused by rotational modulation of a large chemical spot on a 10 d time scale.

K2 photometry and HERMES spectroscopy of the blue supergiant ρ Leo: rotational wind modulation and low-frequency waves

“…Following the idea of scattered-light Kepler photometry of the blue supergiant HD 188209 in the nominal Field-of-View of the mission (Aerts et al 2017a), the technique of halo photometry was developed and is meanwhile well established for the follow-up K2 mission White et al 2017). We hence proposed ρ Leo for K2 halo photometry during Campaign 14 of the mission.…”

“…Long-term ground-based photometry of mmag-level precision (e.g., van Genderen 1989;Lamers et al 1998, and references therein) and spectroscopy of km s −1 precision (e.g., Markova et al 2005;Simón-Díaz et al 2010;Martins et al 2015;Kraus et al 2015) devoted to blue supergiants typically had sparse sampling and led to aliasing and high noise levels in the Fourier spectra. As a result, the interpretation of blue supergiant variability from groundbased data remained limited.…”

Photometry and spectroscopy of massive stars observed during K2 Campaign 8

“…Taken the values of v sin i and radius found in the paper and 1σ uncertainties, the star inclination is between i = 27°and i = 66°. Figure 6 shows the posterior probability density function for the inclination (White et al 2017). A model with the two frequencies mentioned above give an excellent fit to the light curve.…”

The K2 Bright Star Survey. I. Methodology and Data Release

“…The 'TV-min' halo method was first described by White et al (2017) and applied to the Pleiades' Seven Sisters. It was also applied to Aldebaran with further developments by Farr et al (2018).…”