Issue 16 - April 2016
NN-EXPLORE: Accurate Stellar Characterization for Kepler Mission Exoplanet Host Stars
By Verne Smith

The Kepler space mission has had an enormous impact on observational studies of exoplanet systems as a result of its mission to determine how common exoplanetary systems are, and to measure the distribution in the sizes and orbits of the planets they contain. Kepler spent more than four years continuously observing a star field straddling the constellations of Cygnus and Lyra before a failure in the spacecraft’s attitude control system rendered it unable to continue the observations. Nevertheless, Kepler discovered well over four thousand exoplanet candidates, and revealed a bewildering variety of planetary system architectures (the orbital spacings, masses, and compositions of exoplanets). In addition, the “K2” extended mission, which repurposes the Kepler telescope to conduct ~80-day surveys of fields along the ecliptic plane, is underway and adding new exoplanet candidates to the list every day. But confirming that these candidates are truly exoplanets and understanding the processes that drive their formation and evolution requires a variety of ground-based follow up observations.

Hand-in-hand with determining the nature and characteristics of transiting exoplanet systems (such as exoplanet masses and densities) goes the need to understand the detailed characteristics of their host stars. It is sometimes said that “one can only know the exoplanet as well as one knows the host star.” The depth of a transit signal reflects the amount of light the planet blocks out when it passes in front of its star. As such, it only tells us the ratio of the exoplanet radius to the radius of the host star. This means that we can only know the size of the exoplanet accurately if we know the size of the star accurately. Likewise, the masses of exoplanets are typically determined by carefully measuring the back and forth “wobble” of the host star caused by the gravitational tug of the planet orbiting around it. Here again, we can only know the mass of the planet if we know the mass of the host star very well. Thus, exoplanet studies are inextricably tied to stellar astrophysics.

Stellar radii and masses can be determined through a combination of stellar spectroscopy and stellar modeling. Spectroscopy provides accurate values of the star’s effective temperature and its surface gravity, which is a measure of the surface gas pressure. These quantities can then be combined with stellar models to provide the requisite stellar masses and radii. The more accurately these stellar parameters can be determined, the more accurately exoplanet properties can be determined.

High-resolution stellar spectroscopy also reveals the detailed chemical compositions of host stars, which is crucial to understanding connections between planetary formation and stellar chemistry. The chemical composition of a star reflects the chemistry of the interstellar cloud from which the star and its planets formed, since the abundances of key metals, such as iron, magnesium, or silicon, or the ratio of carbon to oxygen in the star affects the nature of the planetary system architecture. High-resolution spectroscopy is an excellent tool with which to derive both accurate stellar physical parameters and detailed chemical abundances.

As part of the NASA-NSF EXoplanet Observational REsearch (NN-EXPLORE) program, our team is using the WIYN/Hydra echelle spectrograph (Kitt Peak, Tucson, Arizona) to provide high-resolution (R~25,000) spectra of Kepler exoplanet host stars that we then analyze to determine accurate stellar parameters and detailed chemical compositions. The accompanying figure shows a small piece of a Hydra spectrum for the near-solar twin exoplanet host star Kepler 452. In July 2015, NASA reported that this star hosts a planet only 60% larger than the Earth in a 385-day orbit, placing it squarely in the habitable zone. This is exactly the type of star our team is targeting and analyzing. The solid curves show our theoretical spectral modeling used to fit the observed stellar spectrum, with these models providing the fundamental stellar parameters and detailed chemical compositions.

A WIYN/Hydra spectrum of Kepler-452 (red open circles) was obtained by us, along with synthetic spectra (solid curves). Kepler 452 is a solar-type star (spectral type G2) hosting an Earth-size planet in the habitable zone. This is a very small piece of the observed spectrum used to show a sample of absorption line profiles, with their chemical identifications indicated. This particular wavelength region is shown in order to highlight 3 lines of neutral magnesium (labeled Mg I), as magnesium plays a key role in determining the structure of rocky planets. The solid lines represent model fits to the observed data both with and without the absorption of neutral magnesium. The fits illustrate the quality of both the observed spectrum and the theoretical fit to it, and the clear contribution of neutral magnesium (Mg 1) to the spectrum.

The NN-EXPLORE partnership will take advantage of the full National Optical Astronomy Observatory (NOAO) share of the 3.5-m Wisconsin, Indiana, Yale, and NOAO (WIYN) telescope on Kitt Peak. It will provide the community with the tools and access to conduct ground-based observations that advance exoplanet science, with particular emphasis on Kepler, K2, and (eventually) TESS follow-up observations. It will also provide observations that inform future NASA missions, such as the James Webb Space Telescope (JWST) and the Wide Field Infrared Survey Telescope (WFIRST) mission.

NASA will manage an exoplanet-targeted Guest Observer program with existing instrumentation. Call for proposals are issued by NOAO and are due on the normal semester schedule. Deadlines are the last day in September for the following “A” semester (February 1 – July 31) and the last day in March for the following “B” semester (August 1 – January 31). Proposal submission information can be found at http://ast.noao.edu/observing/proposal-info.

Learn more about NN-EXPLORE partnership and Guest Observer Program here: http://exep.jpl.nasa.gov/NNExplore/.

Verne V. Smith
NOAO System Science Center Director
National Optical Astronomy Observatory