Variability of M giant stars based on Kepler photometry: general characteristics

Abstract

M giants are among the longest-period pulsating stars which is why their studies were traditionally restricted to analyses of low-precision visual observations, and more recently, accurate ground-based data. Here we present an overview of M giant variability on a wide range of time-scales (hours to years), based on analysis of thirteen quarters of Kepler longcadence observations (one point per every 29.4 minutes), with a total time-span of over 1000 days. About two-thirds of the sample stars have been selected from the ASAS-North survey of the Kepler field, with the rest supplemented from a randomly chosen M giant control sample.We first describe the correction of the light curves from different quarters, which was found to be essential. We use Fourier analysis to calculate multiple frequencies for all stars in the sample. Over 50 stars show a relatively strong signal with a period equal to the Kepleryear and a characteristic phase dependence across the whole field-of-view. We interpret this as a so far unidentified systematic effect in the Kepler data. We discuss the presence of regular patterns in the distribution of multiple periodicities and amplitudes. In the period-amplitude plane we find that it is possible to distinguish between solar-like oscillations and larger amplitude pulsations which are characteristic for Mira/SR stars. This may indicate the region of the transition between two types of oscillations as we move upward along the giant branch.

The variability behaviour of CoRoT M-giant stars

“…This is the largest detailed study of M giants observed by CoRoT, and it provides an opportunity for comparison with previous works based on space missions like CoRoT and Kepler (e.g., Lebzelter 2011;Bányai et al 2013;Huber et al 2014;Hartig et al 2014) and on ground-based observations (e.g., Alard et al 2001;Wray et al 2004;Tabur et al 2010).…”

“…2.4), we defined the variability amplitude as half of the difference between the maximum and minimum of Eq. (2) in the phase diagram of the main period, which is similar to the amplitude definitions used by De Medeiros et al (2013) and Bányai et al (2013). This fit was then applied to the LC domain and was subtracted from the time series (pre-whitening), thereby yielding a new Lomb-Scargle periodogram.…”

“…For each LC, we selected all the independent periods that exhibited a significance level greater than 99%, i.e., a false alarm probability (FAP) less than 0.01. Therefore, a collective set of independent periods for each target was retained to perform an analysis that was similar to that of Tabur et al (2010) and Bányai et al (2013), where we retained the first three periods above the FAP, as shown in Table 2. The FAP was computed based on the Eaton et al (1995) method, which consists of randomising the temporal bins of the original LC and computing the resulting power spectra (see also Affer et al 2012).…”

“…15 of Tabur et al (2010). The blue line between the two blue crosses represents the area defined by Bányai et al (2013), selected from the top panel of Fig. 3 of Huber et al (2011), where solar-like oscillations are expected to be found.…”

“…In this regard, Stello et al (2014) detected non-radial modes in early M giants, as previously found by Mosser et al (2013) for less luminous giants. Bányai et al (2013) performed a detailed study of the variability of M-type giants using Kepler data as well. They affirm that they could distinguish between solar-like oscillations and larger amplitude pulsations.…”