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Adaptive Optics Images of Kepler Objects of Interest

All transiting planets are at risk of contamination by blends with nearby, unresolved stars. Blends dilute the transit signal, causing the planet to appear smaller than it really is, or produce a false positive detection when the target star is blended with eclipsing binary stars. This paper reports on high spatial-resolution adaptive optics images of 90 Kepler planetary candidates. Companion stars are detected as close as 0.1 arcsec from the target star. Images were taken in the near-infrared (J and Ks bands) with ARIES on the MMT and PHARO on the Palomar Hale 200-inch. Most objects (60%) have at least one star within 6 arcsec separation and a magnitude difference of 9. Eighteen objects (20%) have at least one companion within 2 arcsec of the target star; 6 companions (7%) are closer than 0.5 arcsec. Most of these companions were previously unknown, and the associated planetary candidates should receive additional scrutiny. Limits are placed on the presence of additional companions for every system observed, which can be used to validate planets statistically using the BLENDER method. Validation is particularly critical for low-mass, potentially Earth-like worlds, which are not detectable with current-generation radial velocity techniques. High-resolution images are thus a crucial component of any transit follow-up program.


INTRODUCTION
Two methods have been responsible for the vast majority of extrasolar planet discoveries. Radial velocity observations are used to determine the planetary mass from the amplitude of the shift in spectral lines due to the planet's gravitational pull on the star, while transit photometry gives the planetary radius based on the amount of light blocked as the planet transits across the host star. Both of these methods, however, are vulnerable to unresolved light from nearby stars, whether the contaminating star is a bound companion or a chance alignment. When close companions fall within the same spectral slit or photometric aperture as the target, the resulting blend distorts the derived planetary parameters and sometimes creates false positive signals.
Accounting for nearby stars is particularly important for transiting planets that lack corresponding radial velocity measurements and hence have no confirmation of planetary mass. This paper focuses on the transiting planet candidates identified by the Kepler space mission, currently numbering over 2300 (Batalha et al. 2012). Many of these objects do not have mass estimates from radial velocity measurements because of the amount of observing time required, particularly for small planets around relatively faint stars. Current radial velocity instruments cannot detect planets around the size of the Earth, and thus are not useful for confirming objects of this size that have recently been found with transit photometry (e.g. Fressin et al. 2012). Another method is needed to confirm these types of planets.
The best way to account for nearby stars is provided by high-resolution imaging. A variety of methods have been used to examine planet-hosting stars, such as lucky imaging (e.g. Daemgen et al. 2009), speckle imaging (e.g Howell et al. 2011), and adaptive optics (this work). High-resolution images are used to (1) rule out false positive scenarios caused by background blends (e.g. Hartman et al. 2011), (2) estimate the dilution of the transit light curve caused by additional faint stars in the aperture (e.g. Buchhave et al. 2011;Demory et al. 2011), and (3) confirm statistically that a candidate is probably a planet even without radial velocity measurements, by calculating the likelihood of all false positive scenarios using the BLENDER method (e.g. Torres et al. 2011;Fressin et al. 2011;Ballard et al. 2011;Fressin et al. 2012).

False positives
For transiting planet detections, the main source of false positives is a blend of the target star with an eclipsing binary star. The Kepler team searches for many signatures of such blends, include examining the light curves for V-shapes, discrepancies between odd and even transits, and signs of secondary occultations that are too deep to be planetary (Batalha et al. 2010). In addition, the center of light for each target is tracked in and out of transit. If the centroid shifts significantly, this indicates that the source of the transit signal is around a different star. At a minimum, the derived planetary parameters must be completely reevaluated, sometimes resulting in a false positive (Jenkins et al. 2010). This extensive vetting effort is partly why the candidates announced by the Kepler team are thought to have a low false positive rate, variously estimated from 5 − 20% (Borucki et al. 2011;Morton & Johnson 2011;Wolfgang & Laughlin 2011). Finding which of the announced candidates are actually false positives requires extensive follow-up efforts, including ground-and spacebased high-resolution imaging.
For the purposes of this paper, a false positive is defined as a transit signal that is not produced by a planetary-mass object around the proposed target star. Some false positives may actually be larger planets around a fainter star that is blended with the brighter target star.

Dilution corrections
Additional stars in the photometric aperture will dilute the transit signal and distort the measured planetary radius. The amount of dilution depends on the brightness of the background star and how much of its light falls in the aperture. A Kepler pixel is about 4 ′′ on a side, while a typical Kepler aperture is 12 ′′ across, and existing catalogs typically do not contain companions more than a few magnitudes fainter or closer than a few arcseconds. Thus, it is critical to probe as close to the star as possible to find nearby companions that might challenge the identity and parameters of the star hosting the transit signature.
It is particularly useful to obtain dilution corrections in more than one wavelength. An important component of the Kepler follow-up is transit observations with the near-infrared Spitzer satellite. Most false-positive blend scenarios produce a color-dependent transit-like signature. Thus, an object is more likely to be a planet if it has the same depth at both the Kepler bandpass and, for instance, at 4.5 µm with Spitzer. If the depth of the transit does vary with wavelength, however, it is vital to have resolved images of nearby stellar companions to test whether the depth variation is more consistent with a false positive or with a genuine planetary transit that is being diluted by the companion star(s).

Planetary validation
If no companions are detected with direct imaging, we can place strict limits on any remaining companions using BLENDER. The BLENDER method has been developed to combine the constraints on false positive scenarios placed by all available radial velocity, photometric, and imaging measurements (Torres et al. 2011). Strong limits on the allowed magnitude difference of undetected stars within a few tenths of an arcsecond are particularly useful. The probability of a false positive scenario scales directly with the area in which a background blend can exist within the limits placed by AO imaging. If the combined probability of any allowed background blend is much less than the planet occurence frequency, the candidate is said to be statistically validated.
Validation is particularly critical for the smallest objects, which cannot be confirmed using radial velocity techniques. Several planets from 1-2 R E have been statistically validated using the BLENDER method, include Kepler-9d (Torres et al. 2011), Kepler-10c (Fressin et al. 2011), Kepler-19b (Ballard et al. 2011, Kepler-22b (Borucki et al. 2012), and Kepler-20e and f (Fressin et al. 2012). This paper reports on observations of 90 candidate planetary objects, or Kepler Objects of Interest (KOIs). The observations are described in Section 2. The limits placed on observable companions are reported in Section 3. A list of all companion stars within 6 ′′ of the targets is reported in Section 4, along with a discussion of the frequency and implications of the companion stars.

OBSERVATIONS AND DATA ANALYSIS
All observations were made during the 2009 and 2010 seasons (roughly May to November). Data were obtained over four nights on the MMT and seven nights on Palomar, as listed in Table 1, with observations of 90 unique KOIs.
When possible, objects were observed in both J and Ks bands; many of these objects have also been observed using speckle imaging in optical wavelengths (Howell et al. 2011). Observations in multiple wavelengths are particularly useful for estimating the spectral type of any detected companions, and for converting the observed delta-magnitude into Kepler's broad optical band (Kp). All observations in the this paper are relative photometry, so the absolute J and Ks magnitudes are found by adding the 2MASS catalog magnitude to the relative magnitudes observed by ARIES and PHARO. The resulting absolute magnitudes are converted to Kpband magnitudes using the fifth-order polynomial fits for dwarf stars presented in Appendix A of Howell et al. (2012): (1) and (2) The single-filter conversions in Equations 1 and 2 yield Kp magnitude estimates that are accurate to approximately 0.6-0.8 mag, and are used for all magnitude limit calculations. For objects detected in both J and Ks, a better estimate of the Kp magnitude can be obtained by using the dual-filter conversions, which yield Kp magnitudes accurate to about 0.05 mag (Howell et al. 2012).
Observations with ARIES were taken on four nights between Nov 2009 and Sep 2010. The Arizona Infrared imager and Echelle Spectrograph (ARIES) on the 6.5m MMT telescope can provide diffraction-limited imaging in the J and Ks bands (McCarthy et al. 1998). ARIES is fed by the adaptive secondary AO system. Most KOIs were imaged in the f/30 mode, with a field of view of 20 ′′ × 20 ′′ and a resolution of 0.02085 ′′ per pixel. In poor seeing, the f/15 mode is used, with a field of view of 40 ′′ × 40 ′′ and a resolution of 0.0417 ′′ per pixel. The adaptive optics system in all cases guided on the primary target. The median width of the central cores of the point spread functions were 0.25 ′′ at J and 0.14 ′′ at Ks, with a best resolution of 0.1 ′′ at J and 0.09 ′′ at Ks. Under good conditions in May 2010 (uncorrected seeing of 0.5 ′′ at Ks), the Strehl ratios were measured at 0.3 in Ks and 0.05 in J.
For each KOI, at least one set of 16 images on a 4-point dither pattern were observed in both J and Ks. In May and Sep 2010, a random jitter was added to the dither position, which had steps of 2 or 4 ′′ . Integration times varied from 0.9 to 30 s depending on stellar magnitude; in some cases, more than one set of 16 images were taken. The images for each filter were calibrated using standard IRAF procedures 6 , and combined and sky-subtracted using the xmosaic function in the xdimsum package. The images taken in 2009-2010 have a slight differential rotation in them, which was too small to require correction near the target, but causes stars near the edges of the field to be smeared out when stacked. The orientations of the fields are estimated from the dither pattern, and are only accurate to within a few degrees.

PHARO
Near-infrared adaptive optics imaging was obtained on the nights of 07-10 Sep 2009 and 01-03 July 2010 UT with the Palomar Hale 200-inch telescope and the PHARO near-infrared camera (Hayward et al. 2001) behind the Palomar adaptive optics system (Troy et al. 2000). PHARO, a 1024 × 20124 HgCdTe infrared array, was used in 0.0251 ′′ pixel −1 mode yielding a field of view of 25 ′′ . The KOIs observed in 2009 were imaged only in the J filter (λ 0 = 1.25 µm) while the KOIs observed in 2010 were imaged in the in both the J and K s (λ 0 = 2.145 µm) filters. All the data were collected in a standard 5-point quincunx dither pattern of 5 ′′ steps interlaced with an off-source (60 ′′ ) sky dither pattern. Individual integrations times varied depending on the brightness of the KOIs, from 1.4 to 69 s, and were aimed at detecting sources 9 magnitudes fainter than the target in J and 8 magnitudes fainter in Ks (5σ). The individual frames were reduced with a custom set of IDL routines written for the PHARO camera and were combined into a single final image. In all cases, the adaptive optics system guided on the primary target itself. The median width of the central cores of the point spread functions were 0.08 ′′ at J and 0.1 ′′ at Ks, with a best resolution at 0.05 ′′ at J and 0.09 ′′ at Ks. The Strehl ratio for good images is 0.1-0.15 in J and 0.35-0.5 in Ks.

DETECTION LIMITS
6 http://iraf.noao.edu/ All objects were identified by manual inspection, which was more efficient at weeding out spurious signals and artifacts than automatic detection methods. The magnitude of a companion was estimated using the IRAF routine phot using a 5 pixel aperture (large enough to capture most of the point spread function, or PSF, without also including light from all but the closest companions). In a few cases, PSF fitting was used on very close companions (such as K00098, separation=0.3 ′′ ).
Limits on undetected stars are estimated as follows. A series of concentric annuli are drawn around the star, and the standard deviation of the background counts is calculated for each annulus. A star is considered detectable if its peak signal is more than 5 times the standard deviation above the background level. The magnitude of this star is reported as the detection limit at the distance of the center of the annulus. Limits are reported for distances from 0.1-4 ′′ in Table 2. The 4 ′′ level can also be applied toward more distant objects.
The innermost detectable object is a function of the observed PSF of the target star. The best FWHM achieved for targets in J band was 0.1 ′′ for ARIES and 0.05 ′′ for PHARO, while both instruments reached 0.09 ′′ in Ks. However, poor weather and problems with the AO systems often caused excursions well above that level. The magnitude limits for each KOI are shown in Table 2. The limits on a few examples are shown in Figure 3, along with a scatter plot of the companions detected near all targets.

FREQUENCY AND IMPLICATIONS OF COMPANIONS
Additional faint stars are common near Kepler targets. Over half (53/90, or nearly 60%) of the targets imaged have at least one companion within 6 ′′ . All of the stars with companions are listed in Table 3, while a list of the relative magnitudes, distances and position angles is in Table 4. Many of these objects are very faint (down to 10 magnitudes fainter in Ks, and typically even fainter visible magnitudes), and so have little dilution effect on the Kepler light curves. Being able to say for certain that there are no brighter objects present lends confidence to the stated planetary parameters.
Close companions, within 2 ′′ , are of particular concern, since they are within the same Kepler pixel as the target, and may not produce a detectable centroid shift. Of the objects presented here, 20% of objects imaged have at least one companion within 2 ′′ , and 7% have one within 0.5 ′′ . The images of twelve KOIs with detected companions between 0.5-2 ′′ are shown in Figure 1, while six objects with companions closer than 0.5 ′′ are shown in Figure 2.
This subset of planetary candidates with close stellar companions should be carefully examined for false positives; the list in Table 4 Demory et al. 2011), andK00975 (aka Kepler-21, Howell et al. 2012). For some objects, such as K00097, the corrections were relatively minor, at the level of a few percent. However, for K00098, the companion was within 0.3 ′′ and only a few tenths of a magnitude fainter, and the dilution corrections were substantial: the radius increased by 10%, the mass by 60% (since the stellar spectra were also blended), and the density changed by 25% (Buchhave et al. 2011). Without high resolution images, we would have had a very inaccurate picture of this planet.
Not surprisingly, given the generally red colors of faint objects, more apparent binaries occur in the infrared than in the visible. High-resolution optical speckle images al by Howell et al. (2011) found that only 10/156 stars, or 6.4%, had companions within 2 ′′ and down to 4 magnitudes fainter. (No attempts have been made to correct for the selection biases in the objects that were selected for follow-up observations.) It is unknown which, if any, of the detected companions are bound to their targets. At present, we lack the time baseline needed to detect proper motion for any of the closest companions. Two statistical arguments can be made to argue that many of the closest stars are likely to be bound. The first is to note that if all of the companions detected were unconnected background or foreground objects, then we would expect to find 9 times as many objects within 6 ′′ as within 2 ′′ , However, we actually find only 3 times as many objects (53 vs. 18), and the ratio would be smaller if it included the the closest objects that are missed because they are within the stellar PSF.
The second statistical argument is to compare the Galactic latitudes of KOIs with detected companions within 2 and 6 ′′ , as shown in Figure 4. The AO targets with companions within 6 ′′ are somewhat more likely to appear at low Galactic latitudes, indicating that some of them are likely background blends, but no such correlation can be seen with the (admittedly small) sample of close companions (within 2 ′′ ). Thus many of the closest objects may be part of physical binary systems, but further observations are required to determine which ones they may be.

CONCLUSIONS
High-resolution, adaptive optics images of 90 Kepler planetary candidates have been obtained. A list of all companions within 6 ′′ is provided, with measured magnitudes in J and/or Ks band and calculated Kepler magnitudes. Limits on additional companions from 0.1 to 4 ′′ are also given for each target, and can be used to calculate the probability of a remaining undetected blend.
Roughly 20% of the objects imaged have at least one companion within 2 ′′ , and 7% have one within 0.5 ′′ . Over half have a more distant companion at a distance of 6 ′′ . Although small number statistics apply, the objects within 2 ′′ appear uncorrelated with Galactic latitude, making it more likely that they represent physically bound (though still distant) companion stars.