# Summary of K2 Program GO15081

Title: Variability of Stellar Analogs to F3 V Stars theta Cyg and KIC 8462852

PI: Guzik, Joyce Ann (Los Alamos National Laboratory)
CoIs: Jackiewicz, Jason; Gaulme, Patrick

We propose to observe in short cadence several bright main-sequence stars of mid-F spectral type in each of K2 Campaigns 14, 15, and 16.  theta Cyg was the brightest star observed by the original Kepler mission (visual magnitude 4.5), and was observed in short cadence using a custom mask during Quarters 8 and 12-17 (Guzik et al. 2016, ApJ 831, 17 ).  With effective temperature 6697 ± 78 K (spectroscopy), mass 1.39 solar masses (asteroseismology) and radius 1.49 solar radii (interferometry), theta Cyg was expected from pulsation models to show gamma Doradus-type g-mode pulsations with periods of order one day, but no g modes were detected.  However, theta Cyg does exhibit stochastically excited solar-like oscillations with frequency of maximum amplitude 1829 µHz that were used for asteroseismic modeling.

KIC 8462852 (a.k.a. Tabby's star) was observed by Kepler for 16 quarters, however, only in long cadence.  Spectroscopic observations place this star at effective temperature 6750 ± 120 K, with an estimated mass 1.43 solar masses and radius 1.58 solar radii (Boyajian et al. 2016, MNRAS 457, 3988).   KIC 8462852 shows a prominent small-amplitude period of 0.88 day that is probably related to rotation, as well as a longer period (10-20 day) brightness variation.  This star notably shows irregular mysterious flux decreases of up to 20 percent lasting 5 to 80 days, as well as a slow brightness decrease of about a percent throughout the Kepler mission. The causes of these unusual brightness changes are under investigation(Boyajian et al. 2016; Montet and Simon 2016, ApJL 830, L39; Wright and Sigurdssen 2016, ApJL, 829, L3).  Although external causes such as eclipses by comet swarms are a leading explanation, intrinsic stellar brightness variations by an unknown mechanism have not been entirely ruled out.  An asteroseismic analysis of KIC 87462852 cannot be carried out (yet) because of the lack of short-cadence observations that may have revealed solar-like oscillations such as those found in theta Cyg.

We expect that short-cadence K2 observations of relatively bright mid-F main-sequence stars will provide additional pulsating targets for detailed asteroseismic modeling.  Finding stars that exhibit both solar-like observations and gamma Dor g-mode or even delta Sct-like p-mode oscillations would provide constraints on stellar properties, age, and pulsation driving mechanisms.  There may exist analogs of theta Cyg or KIC 8462852 in the Kepler and K2 data archives observed in short cadence, although the Kepler targets are likely to be less bright or have shorter time-series observations than theta Cyg or KIC 8462852.  Solar-like oscillations were first characterized with a partial quarter of Kepler data for theta Cyg, and they have also been measured in short-cadence K2 data (e.g., Lund et al. 2016, MNRAS 463, 200).  The generally higher-amplitude gamma Dor and delta Sct pulsations should be detectable, if they exist.  Perhaps asteroseismic modeling of additional F3 V spectral-type stars will provide clues to the strange photometric variations of KIC 8462852, or at least help to rule out intrinsic brightness variations.  It is even possible that stars with similar anomalous brightness variations may be discovered.  Further observations of such stars would help to define and explain the transition region between solar-like and gamma Doradus pulsators on the main sequence.

Our small-proposal budget request is $20 K for the PI to execute the project.  The possibility exists to leverage support via the New Mexico Consortium for a student shared between the PI institution and New Mexico State University.

# Targets requested by this program that have been observed (3)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
249647239, 234.605653, -19.913182, 6.59, GO15081_LC|GO15063_LC|GO15001_LC|GO15028_LC|GO15021_LC|GO15081_SC
249799715, 237.467899, -17.901947, 6.79, GO15010_LC|GO15081_LC|GO15063_LC|GO15028_LC|GO15021_LC|GO15010_SC|GO15081_SC
249835410, 232.394764, -17.440942, 6.64, GO15007_LC|GO15081_LC|GO15063_LC|GO15001_LC|GO15021_LC|GO15007_SC|GO15081_SC