# Summary of K2 Program GO16099

Title: Characterization of Be stars in K2 fields

PI: Emilio, Marcelo (UNIVERSIDADE ESTADUAL DE PONTA GROSSA)
CoIs: Andrade, Laerte; Armstrong, James D.; Janot-Pacheco, Eduardo; Rabello-Soares, Maria Cristina

We propose the characterization of Be stars in K2 fields 14, 15 and 16. Classical Be stars are rapid rotators of spectral type B and luminosity class V-III which have an equatorially concentrated circumstellar disk. The disk often exhibits Balmer emission lines and an infrared excess fed by discrete mass loss events called outbursts. In spite of their high V sin i, rapid rotation alone cannot explain  the ejection episodes as most Be stars do not rotate at their critical rotation rates. High-resolution, high signal-to-noise spectroscopic observations have been analyzed to demonstrate short-term variations are rather common among early-type Be stars. The observed line profile variability (LPV) is characterized by moving bumps traveling from blue to red across the line profile on timescales ranging from minutes to a few days. Non-radial pulsations (NRP) have been proposed as an explanation of the LPV observed in hot stars. NRP produce LPV thanks to the combination of the Doppler displacement of stellar surface elements with their associated temperature variations due to the compression/expansion caused by the passage of waves through the photosphere. NRP could be the additional mechanism required for a rapidly rotating B star to become a Be star, that is to trigger the Be phenomenon by means of mass ejection(Rivinius et al. 1998).  

Huat et al. (2009) found for the Be HD 49330 from CoRoT and ground-based spectroscopic data an increase in amplitude for g-modes and a decrease for p-modes just before an outburst. Neiner et al. (2012) discovered gravito-inertial modes in the Be star HD 51452. They found that the mode amplitudes were linked to the presence of mini outbursts. In general, hot Be stars display nonradial pressure pulsation modes, whereas cooler ones display gravity modes. Despite those empirical evidences, the physical processes responsible for the outbursts and mass loss in Be stars are still poorly understood. Dziembowski et al. (2007) showed that high-order g-modes driven by the kappa mechanism are unstable in the region of the HR diagram occupied by Be stars. The predicted radial velocity amplitude due to an l=3 g-mode is ~ 20 km/s for a star with 4 solar masses, with pulsation periods in the range ~0.5 - 1.1 days. For more massive and hotter stars like HD 171219, unstable modes are found with periods of 0.5-2 days. 

Pulsation analysis of a star requires the measurement of frequencies with both high photometric precision and high time-frequency resolution. Only space missions provide both conditions. Since NRP in Be stars have periods near 1.0 cycles per day, Kepler observations are especially important to avoid the frequency aliases due to the day/night cycle of ground observations. Another advantage of Kepler data is the continuous observations for months with a 30 min cadence that improves the chances to observe outburst events in our targets. Stellar seismology made a great leap forward thanks to the MOST, CoRoT and Kepler satellites. We chose targets in the K2 cycle 5 fields from those already discovered as well as new targets identified by ground photometry and spectroscopy made in the present year. Our targets have Kp between 7 and 14. Long cadence (30 min) is desirable for this research. We aim to have both Kepler photometric time-series and simultaneous ground spectroscopic observations (when possible) to measure Be periodicities and understand the Be phenomenon. Our proposal fits the Small category, with 20 targets in total. Our group have experience with asteroseismology and have already worked in the same analysis with CoRoT and K2 data (cycles 3 and 4).

Be stars, as easy-to-observe objects giving a privileged peek into the structure of rapidly rotating stars, are therefore among the best suited laboratories to investigate important problems in contemporary astrophysics, like the effects of rapid rotation on stellar evolution.

# Targets requested by this program that have been observed (3)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
211433013, 137.605977, 12.140855, 14.017, GO16016_LC|GO16065_LC|GO16080_LC|GO16089_LC|GO16096_LC|GO16099_LC|GO16080_SC
211495446, 130.432738, 13.074995, 14.03, GO16016_LC|GO16051_LC|GO16080_LC|GO16089_LC|GO16096_LC|GO16099_LC|GO16051_SC|GO16080_SC
211941698, 133.083414, 19.350775, 9.327, GO16056_LC|GO16099_LC