# Summary of K2 Program GO11072

Title: Asteroseismology discerning the properties of hot horizontal branch stars: Fields 11, 12, and 13

PI: Reed, Mike D (Missouri State University)
CoIs: Jeffery, Christopher Simon; Ostensen, Roy H.; Baran, Andrzej S; Telting, John

We propose to explore core structure of horizontal branch stars by applying asteroseismological tools to K2 data of extreme horizontal branch (spectral class subdwarf B) stars. Subdwarf B (sdB) stars are the atmosphere-stripped cores of horizontal branch stars some of which happen to pulsate, and that allows us to probe their structure. From Kepler's main 
mission, we learned how to identify pulsation modes to constrain structure models from period spacings and frequency multiplets. Period spacings also help to discern internal structure and age- some stars show mode trapping while in others it is almost completely absent. The amount (and location) of the mode trapping sequence is a function of the core structure which evolves with time. By sampling sdB stars of different evolutionary age and envelope mass, we can use the small deviations of period spacings to constrain structure models.Frequency multiplets also tell us rotation periods and orientation of pulsation axes. From this information we can measure rotation profiles and have discovered that rotation periods are long (20-100 days), even for stars with binary periods down to 0.4 days. This is a surprising result as it has been assumed that close binaries quickly tidally lock their components. From K2 Campaign 1, we discovered the first known ellipsoidal variable which rotates subsynchronously. We made significant progress from the original Kepler mission. However, each sdB pulsator we have examined so far has had unique features. This tells us that we have not  completed the characterization of helium-fusing cores across their range of age (from zero-age to terminal-age on the horizontal branch) and temperature (an indicator of the envelope-to-core mass ratio). To fully exploit K1 and K2 data, we need more pulsators to cover the complete range. It would also be a huge benefit to have more examples of pulsators in binaries, as that allows us to more fully explore how angular momentum is conserved and/or transferred during envelope ejection. Thus far, we have discovered one solid-body rotator and one radially-differential rotator. To continue working toward a complete understanding of horizontal branch cores (including mass loss and post-helium-flash evolution), we are proposing 
that K2 obtain short-cadence (SC) observations of a selection of known and likely sdB pulsators. The SC data can fully exploit pulsations and short-period (1 day) binaries. Only K2 provides sufficient accuracy, duty cycle, and duration necessary to fully understand the pulsations. There is no planned mission that can compete with the accuracy of K2. K2 really is the only tool for this work. Our team has extensively explored many aspects of sdB stars, including 
spectroscopic characterization, binary properties, and the application of asteroseismic tools. We have been examining sdB stars since their pulsations were discovered in 1996 and have published over 20 papers from Kepler's main mission.
We will be proposing for about five to eight stars per campaign. We only apply for the most promising targets, both in terms of pulsations and interesting binary properties. We are proposing to observe two binary pulsators, EQ Psc, discovered with K2's engineering data and V1405 Ori, a rare p-mode pulsator in a short-period binary. We have also incorporated students into our work. Many of our papers include undergraduate students as significant contributors. Missouri State University does not have a graduate program, and as a result, undergraduate students have learned to process, analyze, and interpret NASA Satellite data to detect pulsations and signatures of binarity. They have also been involved in pre- and post-Kepler characterization studies, providing them with a broad range of training. Graduate students at European partner institutions will also be working with us; processing and interpreting pulsations as part of their thesis work.

# Targets requested by this program that have been observed (6)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
204613898, 252.512752, -21.611126, 16.771, GO11072_LC
227389858, 262.368289, -18.271804, 13.791, GO11072_LC|GO11072_SC
227441033, 262.953937, -18.145014, 15.1, GO11072_LC|GO11072_SC
230195595, 256.130968, -17.943539, 15.591, GO11072_LC
231422890, 257.12008, -22.294018, 17.071, GO11072_LC
234319842, 260.604997, -19.909844, 12.97, GO11122_LC|GO11072_LC|GO11072_SC