Context. Observations and analysis of solar-type oscillations in red-giant stars is an emerging aspect of asteroseismic analysis with a number of open questions yet to be explored. Although stochastic oscillations have previously been detected in red giants from both radial velocity and photometric measurements, those data were either too short or had sampling that was not complete enough to perform a detailed data analysis of the variability. The quality and quantity of photometric data as provided by the CoRoT satellite is necessary to provide a breakthrough in observing p-mode oscillations in red giants. We have analyzed continuous photometric time-series of about 11 400 relatively faint stars obtained in the exofield of CoRoT during the first 150 days long-run campaign from May to October 2007. We find several hundred stars showing a clear power excess in a frequency and amplitude range expected for red-giant pulsators. In this paper we present first results on a sub-sample of these stars. Aims. Knowing reliable fundamental parameters like mass and radius is essential for detailed asteroseismic studies of red-giant stars. As the CoRoT exofield targets are relatively faint (11−16 mag) there are no (or only weak) constraints on the stars' location in the H-R diagram. We therefore aim to extract information about such fundamental parameters solely from the available time series. Methods. We model the convective background noise and the power excess hump due to pulsation with a global model fit and deduce reliable estimates for the stellar mass and radius from scaling relations for the frequency of maximum oscillation power and the characteristic frequency separation. Results. We provide a simple method to estimate stellar masses and radii for stars exhibiting solar-type oscillations. Our method is tested on a number of known solar-type pulsators.

The Microvariability and Oscillations of Stars (MOST) satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies P 2.8 cycles day À1 with amplitudes of a few millimagnitudes (mmag) and less. The frequency range embraces gand p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive postYmain-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between 20 M at log T eA % 4:41 and 15 M at log T eA % 4:36 are roughly consistent with the observed frequency range. Excitation by the Fe bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The MOST light curve of HD 163899 shows that such a reflection of g-modes actually occurs and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from Cyg variables. Such g-modes have great potential for asteroseismology.

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We present a study of white light flares from the active M5.5 dwarf Proxima Centauri using the Canadian microsatellite MOST. Using 37.6 days of monitoring data from 2014 and 2015, we have detected 66 individual flare events, the largest number of white light flares observed to date on Proxima Cen. Flare energies in our sample range from 10 29 -10 31.5 erg,. The flare rate is lower than that of other classic flare stars of similar spectral type, such as UV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth. Proxima Cen does have an unusually high flare rate given its slow rotation period, however. Extending the observed power-law occurrence distribution down to 10 28 erg, we show that flares with flux amplitudes of 0.5% occur 63 times per day, while superflares with energies of 10 33 erg occur ∼8 times per year. Small flares may therefore pose a great difficulty in searches for transits from the recently announced 1.27 M ⊕ Proxima b, while frequent large flares could have significant impact on the planetary atmosphere.

Abstract:We demonstrate that a seismic analysis of stars in their earliest evolutionary phases is a powerful method to identify young stars and distinguish their evolutionary states. The early star that is born from the gravitational collapse of a molecular cloud reaches at some point sufficient temperature, mass and luminosity to be detected. Accretion stops and the pre-main sequence star that emerges is nearly fully convective and chemically homogeneous. It will continue to contract gravitationally until the density and temperature in the core are high enough to start nuclear burning of hydrogen. We show that there is a relationship between detected pulsation properties for a sample of young stars and their evolutionary status illustrating the potential of asteroseismology for the early evolutionary phases. One Sentence Summary:We interpret the pulsational properties of young stars that have not yet entered their nuclear burning phases and show that a seismic analysis can be used to determine the young stars' evolutionary states. Main Text:The earliest phases in the lives of stars determine their future fate. For example, the production of chemical elements, which are used to trace the history of Galactic evolution, depends on the initial mass and metallicity of young stars. Also, the angular momentum obtained during the birth process is vital for the further stellar life. Therefore, understanding the physical processes that occur during the earliest stages of stellar evolution is essential.During the early evolutionary phases, following the star's appearance on the birthline (1, 2, 3), the central temperatures and densities are not yet high enough to initiate significant nuclear burning, hence, the early star derives all of its energy (heat and radiative) from the release of gravitational potential energy. The interior structure undergoes several changes as the temperature rises and the opacities, in general, decrease (1). Initially, the higher opacities and the outward transportation of the released gravitational energy force the star to be nearly fully convective. But, as the star heats up and the opacities drop, a radiative core develops. At this point the star leaves the Hayashi track (4), follows the Henyey track (5), moving leftward to hotter surface temperatures in the evolutionary diagram. Eventually, the star's central regions become hot enough for nuclear burning to occur. Models predict that for intermediate-mass stars (i.e., with 1.5 to 8 solar masses, M ! ), just before the star reaches this nuclear burning stage, called the main-sequence (MS) phase, there is a "bump" or brief period of nuclear burning where carbon is "burned" by the CNO cycle (a chain of hydrogen burning reactions catalyzed by the presence of C, N, and O) until C and N reach nuclear burning equilibrium. As this happens, the star's core briefly becomes convective and the star's luminosity increases. Shortly thereafter, the star resumes its contraction onto the zero-age main sequence (ZAMS), when nuclear burning of hydrogen re-ign...