Solar-like oscillations and magnetic activity of the slow rotator EK Eridani

Abstract

Aims. We aim to understand the interplay between non-radial oscillations and stellar magnetic activity and test the feasibility of doing asteroseismology of magnetically active stars. We investigate the active slow rotator EK Eri which is the likely descendant of an Ap star. Methods. We analyze 30 years of photometric time-series data, 3 years of HARPS radial velocity monitoring, and 3 nights of highcadence HARPS asteroseismic data. We construct a high-S /N HARPS spectrum that we use to determine atmospheric parameters and chemical composition. Spectra observed at different rotation phases are analyzed to search for signs of temperature or abundance variations. An upper limit on the projected rotational velocity is derived from very high-resolution CES spectra. Results. We detect oscillations in EK Eri with a frequency of the maximum power of ν max = 320 ± 32 μHz, and we derive a peak amplitude per radial mode of ≈0.15 m s −1 , which is a factor of ≈3 lower than expected. We suggest that the magnetic field may act to suppress low-degree modes. Individual frequencies can not be extracted from the available data. We derive accurate atmospheric parameters, refining our previous analysis, finding T eff = 5135 ± 80 K, log g = 3.39 ± 0.12, and metallicity [M/H] = +0.02 ± 0.04. Mass and radius estimates from the seismic analysis are not accurate enough to constrain the position in the HR diagram and the evolutionary state. We confirm that the main light variation is due to cool spots, but that other contributions may need to be taken into account. We tentatively suggest that the rotation period is twice the photometric period, i.e., P rot = 2P phot = 617.6 d, and that the star is a dipole-dominated oblique rotator viewed close to equator-on. We conclude from our derived parameters that v sin i < 0.40 km s −1 and we show that the value is too low to be reliably measured. We also link the time series of direct magnetic field measurements available in the literature to our newly derived photometric ephemeris.

Global asteroseismic properties of solar-like oscillations observed by Kepler: a comparison of complementary analysis methods

“…We have correlated the fraction of stars for which we detected oscillations with the stellar parameters from the KIC and have found a significant reduction in the proportion of solar‐like oscillators with effective temperatures close to T eff ∼ 5500 K, the temperature range that separates the distributions of main‐sequence stars from subgiants. This is in agreement with the findings of Chaplin et al (2011) who suggest that stars in this region may show interesting evolutionary effects in their stellar dynamos (Gilliland 1985; Dall et al 2010), which would manifest in the surface magnetic activity and, therefore, decrease the detectability of oscillations.…”

First detection of a weak magnetic field on the giant Arcturus: remnants of a solar dynamo?

“…The maximum mean longitudinal component measured via Zeeman effect has a strength of 0.65 ± 0.26 G. Our magnetic field measurements endorse an intraseasonal activity variability revealed by other indicators, satisfying a 208-day period. Combined with the observations of Brown et al (2008), we propose that a diminishing solar-like dynamo is the origin of the magnetic activity, excluding the possibilities of a remnant fossil field as in EK Eri (Dall et al 2010, and references therein), or the local small-scale dynamos driven by large convection cells, as suggested in the case of the supergiant Betelgeuse (Aurière et al 2010). The deepening of the convection zone and additional rotational slowdown during the RGB phase may alter the characteristics of an active dynamo mode into a turbulent, or shear-driven αΩ-dynamo acting between the rapidly rotating (helium-)core and the outer shells (Nordhaus et al 2008).…”

Metal-rich SX Phe stars in theKeplerfield

“…and for most of the other stars, the FT and GOF values are consistent. Plots similar to the bottom-right contour graph, where the 'banana-shaped' contours reflect the rela- Saar & Osten (1997), have been presented by Ryans et al (2002), Dall et al (2010), and Bruntt et al (2010a). Since the GOF v sin i values agree extremely well with those derived using the 'rcros' FT method (see §2.4.1) the latter were adopted, and only the ζRT retained.…”