# Summary of K2 Program GO13112

Title: Understanding classical Be stars thanks to K2s Fields 11 and 13

PI: Neiner, Coralie (Observatoire de Paris-Meudon)
CoIs: Saio, Hideyuki; Oksala, Mary Elizabeth; Mathis, Stephane; Lee, Umin; Buysschaert, Bram D

Classical Be stars are rapidly-rotating pulsating hot stars that host a circumstellar disk created from matter ejected by the star through sporadic events. How these ejections occur is not understood yet, but they are most likely related to the very rapid stellar rotation and their pulsations. The correlation between pulsations and the ejections of matter was first proposed by Rivinius et al. (2001) and firmly established by Huat et al. (2009) thanks to CoRoT observations.
CoRoT observed a few bright Be stars and allowed us to progress significantly on this issue. In particular, sub-inertial gravito-inertial (gi) modes, which are stochastically-excited in the convective core, have been detected in HD51452 (Neiner et al. 2012). In addition, stochastically-excited low-frequency g modes have been observed with CoRoT in HD49330 during an outburst (Huat et al. 2009). These g modes are only visible during the outburst and probably excited just below the surface, while the kappa-driven p modes get destabilized at that time. It was not expected that stochastically-excited gi modes could be observed in hot stars (Samadi et al. 2010). However, Be stars are very rapid rotators and stochastic excitation is enhanced by rapid rotation, through the Coriolis acceleration. This has been demonstrated analytically (Mathis et al. 2014) and observed in numerical simulations (Rogers et al. 2013). Such stochastic modes are thus probably present in all rapidly rotating hot stars.
In addition, Lee (2013) showed that gi modes excited by the kappa mechanism transport angular momentum and could play a role in the Be phenomenon. Our recent work shows that transport by trapped sub-inertial waves may be sustained in rapidly rotating stars thanks to the stronger stochastic excitation by turbulent convective flows. Moreover, sub-inertial gi modes have very low frequencies and therefore they transport more angular momentum than modes with higher frequencies. We thus proposed that this mechanism allows to transport angular momentum from the convective core of Be stars to their surface. The accumulation of angular momentum just below the surface of Be stars increases the surface velocity, which then reaches the critical velocity needed to eject material from the star (Neiner et al. 2013; Lee et al. 2015).
Unfortunately, the nominal Kepler mission did not observe classical Be stars and only 14 Be stars were observed so far in K2 fields. We propose to observe the additional 4 classical Be stars available in Fields 11 and 13, in long cadence mode. We have already developed tools to construct light-curves from the provided pixel data and to correct for any remaining effects of the roll of the satellite (Buysschaert et al. 2015). The data of previous fields are currently being analyzed and will be combined with the Fields 11-13 data, to test the scenario devised from CoRoT observations.
Our first goal is to investigate whether the observed classical Be stars show stochastic gi modes in addition to the kappa-driven g and p pulsation modes, to confirm that these gi modes are common in Be stars and are a signature of the Be phenomenon. Second, we will apply the seismic modelling tool we recently developed to model the kappa-driven and stochastic gi modes. We will then be able to compute the transport of angular momentum from the core of the Be stars to their surface. If this angular momentum transport is sufficient, it would confirm our proposed scenario that gi modes are those that enhance the transport of momentum, leading the surface layer to reach critical velocity and thus igniting the ejection of material to the Keplerian circumstellar disk.
In addition, for 2 Be stars observed with CoRoT it has been possible to derive the extent of the convective core (Neiner et al. 2012). This is very important to quantify the effect of rapid rotation on the internal structure of stars. We expect that this will also be possible for most classical Be stars observed with K2.

# Targets requested by this program that have been observed (2)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
247169098, 71.394528, 19.328305, 8.671, GO13112_LC|GO13071_LC|GO13122_LC|GO13101_LC|GO13024_LC|GO13059_LC|GO13077_LC|GO13082_LC|GO13052_LC|GO13127_LC
247745384, 75.566223, 24.029163, 9.144, GO13112_LC|GO13071_LC|GO13122_LC|GO13002_LC|GO13101_LC|GO13095_LC|GO13059_LC|GO13082_LC|GO13052_LC|GO13127_LC