# Summary of K2 Program GO15076

Title: Searching for Planets around Bright Binary ``Twin Stars with K2

PI: Teske, Johanna (Carnegie Observatories)
CoIs: Shectman, Stephen; Crane, Jeffrey D; Simon, Joshua; Butler, R. Paul

A new, growing body of work aims to revisit how planet formation may alter host star composition by examining differences between stars that do/do not host planets. Wide binary stars provide an ideal laboratory for investigating the effects of planet formation on stellar composition. Assuming the stars formed in the same environment/from the same material, any differences in composition between the binary components could be related to planet formation.  Furthermore, studying ``twin'' stars (”Teff d 100 K, ”log g d 0.1 dex, ”[Fe/H] d 0.03 dex) allows for very precise relative abundance measurements, as systematic uncertainties that usually dominate such analyses are so similar that they essentially cancel out. The remaining observational noise can then be pushed down with very high S/N, high-resolution spectra. I have started an observational program targeting known exoplanet host star binary systems, and have led three studies of twin planet hosting stars, including in my most recent paper the detection of three new long period giant planets via radial velocity observations. Studying twin star abundance differences may prove to be the most useful for constraining the effects of planet formation on stars, but we need more data to understand their significance; only eight twin binary host star systems are currently known. 

Here I propose to observe HD 134439 and HD 134440 in K2 Campaign 15 to search for transiting planets in this well-characterized, bright twin binary system. Even one more twin binary planet hosting system would be a significant addition to the current sample. My collaborators have RV observations of these stars going back 20 years, and although the data do not yet allow for firm planet detection, they suggest a planet with a ~3d period around HD 134440. Given this suggested period and minimum mass, and the estimated radius of the star, the transit probability of such a planet is between ~10% and 20%. If transiting planets are found around either star in this system, we already have RV data to constrain the planet masses. We also already have high-resolution spectra of both stars, so I can perform a high precision abundance analysis to determine if the stars show different chemical abundances and examine whether this could be due to planet formation. While these stars abundances have been measured in the past, no such high precision, differential study has been conducted, which is necessary to tease out the very small compositional differences that could be due to (different types of) planet formation.

The HD 134439/40 system is intriguing for additional reasons, besides being a bright twin binary. First, these stars are significantly metal-poor ([Fe/H]~1.5), yet also low in ±-elements like Mg, Si, Ca, and Ti, unlike many metal-poor halo stars. This abundance pattern, combined with their UVW motions, suggest an extra-Galactic origin for this common proper-motion pair, the low-± ratios being consistent in particular with formation in a low/irregular star formation rate environment. Second, the low volatile elemental abundances (e.g., C, N, O) versus refractory element abundances (e.g., Ti, Fe, Mg) are consistent with claims of chemical pollution of the stellar atmosphere via planetesimal/refractory accretion. Third, lithium is enhanced in HD 134439 but undetected in HD 134440; this has also been suggested as a signature of planets, via increased mixing in the star from rotational braking and/or more mixing from tidal forces. Thus, (1) promising signs of planet formation have already been detected in HD 134439/40, and (2) if planets were detected around either star, it would be the most metal-poor planetary system ever discovered (and likely has an extra-Galactic origin).

# Targets requested by this program that have been observed (2)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
249914684, 227.554037, -16.462922, 9.164, GO15010_LC|GO15025_LC|GO15068_LC|GO15063_LC|GO15076_LC|GO15009_LC|GO15065_LC|GO15010_SC
249921958, 227.554533, -16.379407, 8.923, GO15010_LC|GO15025_LC|GO15068_LC|GO15063_LC|GO15001_LC|GO15076_LC|GO15009_LC|GO15065_LC|GO15010_SC