# Summary of K2 Program GO11061

Title: The Relationship Between Pulsations and Effective Temperature in R Coronae Borealis Stars

PI: Clayton, Geoffrey C. (Louisiana State University)
CoIs: De Marco, Orsola; Montiel, Edward J; Saio, Hideyuki; Ramsay, Gavin; Jeffery, Christopher Simon; Welch, Doug

K2 has opened a new avenue for the detailed study of the pulsations of the R Coronae Borealis (RCB) stars. These observations are key to understanding the evolution of the RCB stars because their masses cannot be accurately estimated by other means. The ~75 days of near continuous, high-precision observations are ideal for our planned analysis of the brightness variations of the RCB stars. Recently, V1157 Sgr was the first RCB star to be observed by Kepler during the Field 7 program. The proposed observations will be at least two orders of magnitude more precise than the best that can be done from the ground, where a typical cadence is once per night and uncertainties are > 0.01 mag. These observations will provide a better understanding of the pulsation mechanisms and modes in RCB stars. RCB stars are thought to be ~0.8-0.9 M(Sun) from previous stellar pulsation modeling. These estimated masses agree well with the predicted masses of the merger products of a CO- and a He-WD. Final-flash stars, since they are single white dwarfs, should typically have masses of 0.55-0.6 M(Sun). No cool RCB star, with T(eff) = 5000-7000 K, is known to be a binary so these mass estimates are of great importance to understanding the evolution of these enigmatic stars.

RCB stars show periodic or semi-periodic light and radial velocity fluctuations due to both radial and non-radial pulsations. These stars show pulsation periods in the 40-100 d range. These variations are separate from the large declines in brightness caused by dust forming around the star. The pulsations in RCB stars are thought to arise through strange-mode instabilities. Strange modes occur in stars with high luminosity where radiation pressure dominates. RCB stars comprise a peculiar and rare class of stars that offers an excellent opportunity to reveal crucial insights into the advanced stages of stellar evolution. The first star of this class, R Coronae Borealis (R CrB), was discovered to be variable over two centuries ago. Today, the known population of RCB stars is now ~100 stars in the Galaxy. RCB stars form a class of cool, carbon-rich supergiants that have almost no hydrogen. They undergo extreme, irregular declines in brightness, of up to 8  magnitudes, due to the formation of thick clouds of carbon dust. Two scenarios have been proposed for the origin of an RCB star: the Double Degenerate and the final helium-shell flash models. The former involves the merger of a CO- and a He-WD. In the latter, a star evolving from a planetary nebula central star expands to supergiant size.

V1157 Sgr, observed in Field 7, is the first RCB star ever observed by Kepler. The data will be available at the end of March 2016. We have proposed to observe 8 additional RCB stars in Field 9. Field 11 provides an opportunity to observe 8 additional RCB stars with K2. These new proposed observations will bring our sample of RCB stars with K2 data to 17. We wish to have accurate pulsation periods for as many RCB stars as possible to see if there is a systematic change in the pulsation properties (amplitudes, periods, and multi-periodicities) as a function of effective temperature. We would like to understand how the pulsation/convection coupling affects pulsations in cool stars, and to see the effects of Fe opacity in the hotter RCB stars. The duration of the K2 monitoring is useful since it will cover almost two complete pulsation cycles for an average RCB star. We plan to analyze the pulsations in the new K2 lightcurves using our codes to model the radial and non-radial strange modes. The study of the pulsations of RCB stars will be crucial in helping to distinguish between the WD merger and final flash scenarios.

# Targets requested by this program that have been observed (7)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
222124534, 264.836212, -27.95669, 14.36, GO11061_LC
232201881, 256.421896, -26.834266, 11.595, GO11061_LC
234155797, 259.292552, -20.721026, 11.163, GO11123_LC|GO11071_LC|GO11122_LC|GO11061_LC
235395168, 260.810651, -22.868453, 10.983, GO11071_LC|GO11122_LC|GO11061_LC|GO11021_LC|GO11052_LC
235697900, 259.435386, -29.633387, 12.615, GO11061_LC
235873479, 258.832228, -29.09372, 10.685, GO11061_LC
242181621, 262.599337, -30.141246, 14.118, GO11061_LC