We present the analysis of four M dwarf stars -plus one M giant that seeped past our selection criteria- observed in Cycle 3 of Kepler Guest Observer program (GO3) in a search for intrinsic pulsations. Stellar oscillations in M dwarfs were theoretica
lly predicted by Rodriguez-Lopez et al. (2012) to be in the range ~20-40 min and ~4-8 h, depending on the age and the excitation mechanism. We requested Kepler short cadence observations to have an adequate sampling of the oscillations. The targets were chosen on the basis of detectable rotation in the initial Kepler results, biasing towards youth.The analysis reveals no oscillations attributable to pulsations at a detection limit of several parts per million, showing that either the driving mechanisms are not efficient in developing the oscillations to observable amplitudes, or that if pulsations are driven, the amplitudes are very low. The size of the sample, and the possibility that the instability strip is not pure, allowing the coexistence of pulsators and non-pulsators, prevent us from deriving definite conclusions. Inmediate plans include more M dwarfs photometric observations of similar precision with Kepler K2 mission and spectroscopic searches already underway within the Cool Tiny Beats Project (Anglada-Escude et al. 2014, Berdi~nas et al. 2014) with the high-resolution spectrographs HARPS and HARPS-N.
The overstability of the fundamental radial mode in M dwarf models was theoretically predicted by Rodriguez-Lopez et al. (2012). The periods were found to be in the ranges ~25-40 min and ~4-8 h, depending on stellar age and excitation mechanism. We h
ave extended our initial M dwarf model grid in mass, metallicity, and mixing length parameter. We have also considered models with boundary conditions from PHOENIX NextGen atmospheres to test their influence on the pulsation spectra. We find instability of non-radial modes with radial orders up to k=3, degree l=0-3, including p and g modes, with the period range extending from 20 min up to 11 h. Furthermore, we find theoretical evidence of the potential of M dwarfs as solar-like oscillators.
We have explored the possibility of driving pulsation modes in models of sdO stars in which the effects of element diffusion, gravitational settling and radiative levitation have been neglected so that the distribution of iron-peak elements remains u
niform throughout the evolution. The stability of these models was determined using a non-adiabatic oscillations code. We analysed 27 sdO models from 16 different evolutionary sequences and discovered the first ever sdO models capable of driving high-radial order g-modes. In one model, the driving is by a classical kappa-mechanism due to the opacity bump from iron-peak elements at temperature ~200,000 K. In a second model, the driving result from the combined action of kappa-mechanisms operating in three distinct regions of the star: (i) a carbon-oxygen partial ionization zone at temperature ~2 10^6 K, (ii) a deeper region at temperature ~2 10^7 K, which we attribute to ionization of argon, and (iii) at the transition from radiative to conductive opacity in the core of the star.
A spectroscopic analysis of SDSS J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields Teff = 70 000 +/- 5000 K and log g = 5.25 +/- 0.30, together with a most likely type of K3V for the s
econdary star. We compare our results with atmospheric parameters derived by Fontaine et al. (2008) and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. It therefore seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having log g > 5.3 to be more likely to be unstable and capable of driving pulsation in the observed frequency range.
