# Summary of K2 Program GO14096

Title: SAMPLINg: Searching for low AMPlitude puLsatIoNs in hot sud-dwarf OB stars

PI: Johnston, Cole (KU Leuven)
CoIs: Aerts, Conny Clara

The existence of hot, compact, core helium burning sub-dwarf O and B (sdOB) objects is currently thought to be explained through different channels of binary evolution, which results in the stripping of the hydrogen envelope of an evolving star near the onset of core helium burning. The resulting population of sdOB stars consists of apparently single (merger) and both close (Common Envelope) and wide (stable Roche Lobe Over Flow) binary systems. Currently, the formation of those close binary systems is explained through competing, and poorly constrained, physical descriptions of the common-envelope phase of evolution. As they are directly the result of common-envelope evolution, the properties of close binary sdOB stars are highly sensitive to the mechanics of this phase of evolution. Additionally, sdOB stars are observed to exhibit both pressure (p-) and gravity (g-) mode pulsations, making them ideal targets for detailed seismic modeling, which enables precise and independent determination of the stellar parameters sensitive to the physics of their formation. 
Unfortunately, these objects are both rare and relatively faint. As a result, only a handful had been observed with the nominal Kepler mission, which had a stationary observing field. Furthermore, only a select few of those have had their rich pulsation spectrum exploited for seismic modeling. Most regrettably, Kepler did not observe any of the hottest sdOB pulsators exhibiting p-modes. Their interior structure could therefore not be tuned from space asteroseismology and compared with the one of the cooler g-mode sdB pulsators. Here, we plan to remedy this situation. We propose 19 objects for observation with K2 to search for pulsations for follow-up ground based observations with the MAIA multicolor photometer.
The high precision, quasi-uninterrupted observations provided only by K2 are essential for our science goals aimed at the detailed study of low amplitude compact pulsators such as sdOB stars. Moreover, the high quality data allows for the accurate detection and characterization of pulsations down to the milli-magnitude level, as well as the accurate subtraction of any binary signal present in the lightcurve. Finally, while the nominal Kepler mission saw success with sdOB stars, these observations will serve as a proof of concept for the highly anticipated TESS mission, which is restricted to shorter observing campaigns as is the K2 mission. 
The team present at the host institute of the PI has the experience and means to effectively and efficiently exploit the data provided by the K2 mission. Using these K2 data, we aim to search for pulsations in the proposed targets, and if detected use them to investigate the interior structure of these targets with detailed seismic modeling. The methodology we will employ hinges upon what type of signal is present. If binarity is detectable in the lightcurve, the methodology will consist of an iterative binary modeling and frequency analysis routine to as completely as possible model the binarity without the influence of any other signal, while simultaneously attempting to detect pulsations. If no binary signal is present in the lightcurve, we will proceed directly to the frequency analysis to search for pulsations. If pulsations are detected but cannot be identified with the K2 data, ground-based, multicolor observations will be obtained with MAIA, owned by the host institute, for mode identification of the dominant mode(s) and subsequent detailed seismic modeling synergistically using the MESA stellar modeling code and GYRE pulsation code.
	For two objects that fall within the theoretical p-mode instability strips, we request short-cadence observations as it is required to sample the high frequencies (+400 d-1) where pulsations are observed. Long-cadence observations are requested for the characterization of the remaining objects as potential binaries or g-mode pulsators to improve statistics on these objects.

# Targets requested by this program that have been observed (6)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
201535046, 163.117447, 1.062752, 14.439, GO14016_LC|GO14096_LC|GO14089_LC|GO14080_LC|GO14096_SC|GO14080_SC
248411044, 162.511777, -0.010237, 13.555, GO14096_LC|GO14051_LC|GO14089_LC|GO14080_LC|GO14099_LC|GO14096_SC|GO14051_SC|GO14080_SC
248422838, 158.803172, 0.458058, 16.272, GO14016_LC|GO14096_LC|GO14089_LC|GO14080_LC|GO14080_SC
248588851, 155.836676, 5.57942, 16.331, GO14016_LC|GO14096_LC|GO14051_LC|GO14089_LC|GO14080_LC|GO14046_LC|GO14051_SC|GO14080_SC
248748173, 158.957034, 9.431051, 16.378, GO14016_LC|GO14096_LC|GO14051_LC|GO14089_LC|GO14080_LC|GO14065_LC|GO14051_SC|GO14080_SC
248761152, 161.912937, 9.740084, 17.093, GO14016_LC|GO14096_LC|GO14051_LC|GO14089_LC|GO14080_LC|GO14051_SC|GO14080_SC