Draft version January 22, 2021 Typeset using LATEX twocolumn style in AASTeX62 The NASA High-Resolution Speckle Interferometric Imaging Program: Validation and Characterization of Exoplanets and Their Stellar Hosts Steve B. Howell,1 Nicholas J. Scott,1 Rachel A. Matson,2 Mark E. Everett,3 Elise Furlan,4 Crystal L. Gnilka,1 David R. Ciardi,4 and Kathryn V. Lester1 1NASA Ames Research Center, Moffett Field, CA 94035, USA 2U.S. Naval Observatory, 3450 Massachusetts Avenue NW, Washington, D.C. 20392, USA 3NSF's National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA 4NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA (Received January 22, 2021; Accepted 19 Jan, 2021) Submitted to Frontiers - Space Science and Astronomy - Exoplanets: The Effect of Stellar Multiplicity on Exoplanetary Systems ABSTRACT Starting in 2008, NASA has provided the exoplanet community an observational program aimed at obtaining the highest resolution imaging available as part of its mission to validate and characterize exoplanets, as well as their stellar environments, in search of life in the universe. Our current program uses speckle interferometry in the optical (320-1000 nm) with new instruments on the 3.5-m WIYN and both 8-m Gemini telescopes. Starting with Kepler and K2 follow-up, we now support TESS and other space- and ground-based exoplanet related discovery and characterization projects. The im- portance of high-resolution imaging for exoplanet research comes via identification of nearby stellar companions that can dilute the transit signal and confound derived exoplanet and stellar parameters. Our observations therefore provide crucial information allowing accurate planet and stellar properties to be determined. Our community program obtains high-resolution imagery, reduces the data, and provides all final data products, without any exclusive use period, to the community via the Exoplanet Follow-Up Observation Program (ExoFOP) website maintained by the NASA Exoplanet Science Insti- tute. This paper describes the need for high-resolution imaging and gives details of the speckle imaging program, highlighting some of the major scientific discoveries made along the way. 1. INTRODUCTION pixel photometric apertures, it is possible that more The study of exoplanets is one of the most important than one star is measured, and thus the transit mea- topics in astrophysics today. Starting over a decade ago, surement becomes even more uncertain or unreliable. in support of the NASA Kepler mission, a program pro- To support exoplanet discovery, spectroscopic follow- viding follow-up observations began. It became clear up observations consisted of medium- and high-resolution as Kepler was nearing launch, that the 4 arcsec pix- work using reconnaissance spectra at the start and then els (Borucki et al. 2010) as well as the many possible large telescope efforts once specific validation steps were confounding events which could imitate exoplanet tran- passed (Furlan et al. 2018). Likewise, imaging observa- sit events (e.g., Brown et al. 2011; Santerne et al. 2013) tions were performed ranging from standard native see- arXiv:2101.08378v1 [astro-ph.EP] 21 Jan 2021 would require follow-up observations from ground-based ing CCD imaging and lucky imaging to high-resolution telescopes in order to validate and characterize any dis- observations (Furlan et al. 2017). These latter consisted covered transit candidates. In addition, for transit ob- of both Infrared Adaptive Optics (IR/AO) observations servations it is crucial to know the stellar properties well, using Lick, Palomar, and Keck and optical speckle inter- since the planet radius depends directly on the stellar ferometric imaging using WIYN and Gemini telescopes. radius. Also, given the relatively large pixels and multi- As new exoplanet transit missions such as K2 (Howell et al. 2014), and the currently operating missions TESS (Ricker et al. 2015) and CHEOPS (Benz et al. 2020) Corresponding author: Steve B. Howell come along, follow-up high-resolution (sub-arcsecond) [email protected] imaging continues to be needed and in larger amounts than before. While Gaia can resolve companions down 2 Howell et al. Figure 1. A schematic timeline of NASA and ESA exoplanet related space missions and the ground-based follow-up telescopes NASA directly participates in. to near 1.0 arcsec and a bit closer using additional observations over time, e.g., EDR3; Fabricius et al. arXiv:2012.06242), it does not reach the spatial reso- lution of speckle imaging. Additionally, other exoplanet search techniques such as radial velocity (Kane et al. 2019) and ground-based small telescope transit surveys (Bakos et al. 2007) also benefit from speckle imaging of any candidate systems. Finally, the next wave of exoplanet space telescope missions will soon be upon us (Figure 1); missions covering larger and deeper sky areas such as PLATO (transits), and those hoping to obtain detailed exoplanet science such as the James Figure 2. Artist concept of the Ariel space telescope. Webb Space Telescope (JWST; transit spectroscopy, emission spectroscopy, direct imaging of exoplanets) and in order to assess their multiplicity, and secondly, imag- the Nancy Grace Roman Space Telescope (formally the ing of microlens sources to aid in the characterization of Wide Field Infrared Survey Telescope [WFIRST]; direct the source and lens stars. imaging and microlensing planets), as well as complete This paper provides an overview of the NASA high spectroscopic characterization of exoplanet atmospheres resolution speckle imaging program. Exoplanet tran- with Ariel (Figure 2). Anywhere high-resolution imag- sit and radial velocity studies mainly focus on (A)F to ing is needed, including for future missions such as LU- M stars, however, our speckle imaging techniques have VOIR, HabEX, or OST, our speckle program will be been used for research programs related to stars of all valuable. By that time, it is hoped that speckle imag- spectral and luminosity classes, extended objects, and ing will be an integral part of the 30-m ground-based solar system bodies. These applications are not dis- telescope system, providing angular resolutions near 5 cussed further in this report. Section 2 discusses the mas. need for high-resolution imaging, §3 presents the NASA For JWST, our program will provide high resolution mechanism to engage the exoplanet community, §4 and imaging in support of targeted exoplanets and their host §5 give an overview of the instrumentation used, the stars. Roman will make use of speckle imaging to sup- community program, and data produced in this pro- port exoplanet research in two main ways: First, to vet gram, §6 lists some of the major scientific discoveries and fully characterize direct exoplanet imaging targets Speckle Interferometry 3 the speckle program has made in relation to exoplanet is indeed multiple, and we know that about 40-50% of host star multiplicity, and finally we summarize in §7. exoplanet host stars have one or more stellar companions (Horch et al. 2014; Matson et al. 2018), then knowledge 2. THE NEED FOR HIGH-RESOLUTION IMAGING of the brightness and type of any companion stars are crucial in order to properly assess the exoplanet and host Survey telescopes, such as Kepler and TESS, cover star properties. a wide field of view, but have large pixels on the sky. Ciardi et al.(2015); Wang et al.(2015); Furlan and Kepler (and K2) had 4 arcsec/pixel values in their focal Howell(2017); Deacon et al.(2016) and Ziegler et al. plane and TESS has 20 arcsec/pixel. These large pixels (2018), for example, have shown that the presence of gather all the light from any stars present within the third-light will mean that the planet radius determined extracted photometric apertures. If a transit-like event from the transit depth alone is incorrect, the planet is detected, it is not immediately obvious which star in will always be larger than estimated from the transit the pixel (or actually in the pixels) used for light curve depth, at times so large as to lose planet status. Furlan construction is the cause of the event. Thus, the status and Howell(2017) noted that such third-light properties as a real exoplanet transit candidate remains in question will also decrease the mean density of the planet, pos- until some form of validation is carried out. sibly turning a terrestrial exoplanet into an ice giant as Telescopes such as Hubble have great spatial reso- well as causing atmospheric scale height calculations to lution, but they come at the cost of a small field of be flawed. These same two authors (Furlan and How- view and large over-subscription rates for observational ell 2020) also showed how the lack of knowledge of a proposals. While space has the advantage of stable companion star could cause measured stellar properties, observing conditions and no atmospheric effects, high- such as metal content and log g, to be incorrectly de- resolution imaging from the ground must make use of rived from an analysis of the star's spectrum. Use of clever means to attempt to \remove" the blurring effects the knowledge of a companion (or not) allows a proper of the atmosphere. IR/AO uses (laser) guide stars and characterization of both the exoplanet and stellar prop- deformable mirrors while speckle interferometry freezes erties. High resolution knowledge of the scene around the atmospheric distortions using many short exposures host stars will remain an important diagnostic for fu- and reconstructs these into diffraction limited images ture transit, direct imaging, microlens, and atmospheric using specialized software techniques. spectroscopy exoplanet missions. Figure 3 illustrates these points for the case of KOI- As an aside, ROBO-AO is another high-resolution 1002 imaged from a typical ground-based telescope, Ke- imaging technique used in the optical wavelength range.