Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky

Adaptive optics laser guide star systems perform atmospheric correction of stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order laser-correction. The requirement of a sufficiently bright guide star in the field-of-view to correct tip-tilt limits sky coverage. Here we show an improvement to effective seeing without the need for nearby bright stars, enabling full sky coverage by performing only laser-assisted wavefront correction. We used Robo-AO, the first robotic AO system, to comprehensively demonstrate this laser-only correction. We analyze observations from four years of efficient robotic operation covering 15,000 targets and 42,000 observations, each realizing different seeing conditions. Using an autoguider (or a post-processing software equivalent) and the laser to improve effective seeing independent of the brightness of a target, Robo-AO observations show a 39+/-19% improvement to effective FWHM, without any tip-tilt correction. We also demonstrate that 50% encircled-energy performance without tip-tilt correction remains comparable to diffraction-limited, standard Robo-AO performance. Faint-target science programs primarily limited by 50% encircled-energy (e.g. those employing integral field spectrographs placed behind the AO system) may see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6 figure

Multiplicity of the Galactic Senior Citizens: A high-resolution search for cool subdwarf companions

Cool subdwarfs are the oldest members of the low mass stellar population. Mostly present in the galactic halo, subdwarfs are characterized by their low metallicity. Measuring their binary fraction and comparing it to solar metallicity stars could give key insights into the star formation process early in the history of the Milky Way. However, because of their low luminosity and relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs have suffered from small sample sizes and incompleteness. Previous surveys have suggested that the binary fraction of red subdwarfs is much lower than for their main sequence cousins. Using the highly efficient RoboAO system, we present the largest yet high-resolution survey of subdwarfs, sensitive to angular separations, down to 0.15 arcsec, and contrast ratios, up to 6 magnitude difference, invisible in past surveys. Of 344 target cool subdwarfs, 40 are in multiple systems, 16 newly discovered, for a binary fraction of 11.6 percent and 1.8 percent error. We also discovered 6 triple star systems for a triplet fraction of 1.7 percent and 0.7 percent error. Comparisons to similar surveys of solar metallicity dwarf stars gives a 3 sigma disparity in luminosity between companion stars, with subdwarfs displaying a shortage of low contrast companions.Comment: 13 pages, 10 figures, submitted to Ap

CHARACTERIZATION OF EXOPLANETS AND STELLAR SYSTEMS WITH NEW ROBOTS

Large astronomical surveys find thousands of interesting transient events, such as exoplanets. Beyond detection, these surveys are limited in their ability to study the properties of these discoveries. In particular, a common problem with wide-field surveys is because they observe huge swaths of the sky, their resolution is often quite coarse, leading to source confusion and photometric contamination. In this dissertation, I discuss the use of robotic, high-resolution instruments to confirm and characterize exoplanets and also better understand the demographics of stellar populations. These surveys are only feasible with autonomous instruments due to the order-of-magnitude increase in observational time efficiency gained with automation. I present first the design and construction of Robo-SOAR, a moderate-order NGS-AO system in development for the SOAR telescope. With robotic software adapted from Robo-AO, Robo-SOAR will be capable of observing hundreds of targets a night. With an innovative, low-cost dual knife-edge WFS, similar in concept to a pyramid WFS but with reduced chromatic aberrations, Robo-SOAR can reach the diffraction limit on brighter targets. I then discuss the observations of 348 cool subdwarf stars with Robo-AO, a pilot study for future kilo-target surveys. Cool subdwarfs are remnants of the first population of stars formed in the Milky Way. I find that approximately 12% of cool subdwarfs have binary companions, a multiplicity fraction three times lower than similar dwarf stars. The disparity between the two populations may be evidence of the different environments in which they formed. The lack of companions to cool subdwarfs suggests they may have formed in less dense regions, or over their long lifetimes may have had more disruptive encounters with other stars and the Galactic tide. It is also possible that they are galactic interlopers, and formed in small, less-dense galaxies that merged with the Milky Way. We show that the disparity between cool subdwarf and red dwarf multiplicity is consistent with this scenario. I report the results of the Robo-AO survey of every planetary candidate discovered with Kepler to search for previously unknown nearby stars. These stars contaminate the exquisite photometry of Kepler and can either dilute the transit signal from a real planet, resulting in underestimated radii estimates, or be the source of an astrophysical false positive transit signal. More than half of the over 4000 Kepler planetary candidates have only been observed with Robo-AO. We find 610 stars within 4'' of a planetary candidate host star, and correct the derived radii estimates of the more than 800 planets within these systems. On average, we find that the planetary radii increase by a factor of approximately 1.59 in systems with a detected nearby star. We quantify the probability of association for over 150 multiple systems hosting planets using multi-band photometry. In particular, we examine five planetary candidate host stars with four nearby stars detected by Robo-AO and quantify the probability they are high-order planet-hosting systems. Lastly, I use the results of the Robo-AO Kepler survey to search for evidence of the impact multiple star systems have on planets. The presence of a companion star is believed to have a significant impact on the properties of planetary systems. I find that hot Jupiters are more likely than any other planet to be found in a binary star system. This suggests that stellar companions drive orbital migration of giant planets. I also find that single and multiple-transiting planet systems are equally likely to be found in a binary. I find that KOIs from later data releases are less likely to have a nearby star than systems from earlier data releases, possibly a result of the automation of the Kepler vetting pipeline. I find that KOIs follow trends observed in field stars with respect to the relationship between stellar multiplicity and stellar effective temperature and metallicity.Doctor of Philosoph

Robo-AO Discovery and Basic Characterization of Wide Multiple Star Systems in the Pleiades, Praesepe, and NGC 2264 Clusters

We identify and roughly characterize 66 candidate binary star systems in the Pleiades, Praesepe, and NGC 2264 star clusters based on robotic adaptive optics imaging data obtained using Robo-AO at the Palomar 60" telescope. Only $\sim$10% of our imaged pairs were previously known. We detect companions at red optical wavelengths having physical separations ranging from a few tens to a few thousand AU. A 3-sigma contrast curve generated for each final image provides upper limits to the brightness ratios for any undetected putative companions. The observations are sensitive to companions with maximum contrast $\sim$6$^m$ at larger separations. At smaller separations, the mean (best) raw contrast at 2 arcsec is 3.8$^m$ (6$^m$), at 1 arcsec is 3.0$^m$ (4.5$^m$), and at 0.5 arcsec is 1.9$^m$ (3$^m$). PSF subtraction can recover close to the full contrast in to the closer separations. For detected candidate binary pairs, we report separations, position angles, and relative magnitudes. Theoretical isochrones appropriate to the Pleiades and Praesepe clusters are then used to determine the corresponding binary mass ratios, which range from 0.2-0.9 in $q=m_2/m_1$. For our sample of roughly solar-mass (FGK type) stars in NGC 2264 and sub-solar-mass (K and early M-type) primaries in the Pleiades and Praesepe, the overall binary frequency is measured at $\sim$15.5% $\pm$ 2%. However, this value should be considered a lower limit to the true binary fraction within the specified separation and mass ratio ranges in these clusters, given that complex and uncertain corrections for sensitivity and completeness have not been applied.Comment: Accepted to A

Probability of Physical Association of 104 Blended Companions to Kepler Objects of Interest Using Visible and Near-Infrared Adaptive Optics Photometry

We determine probabilities of physical association for stars in blended Kepler Objects of Interest (KOIs), and find that 14.5%_(-3.4%)^(+3.8%) of companions within ~ 4" are consistent with being physically unassociated with their primary. This produces a better understanding of potential false positives in the Kepler catalog and will guide models of planet formation in binary systems. Physical association is determined through two methods of calculating multi-band photometric parallax using visible and near-infrared adaptive optics observations of 84 KOI systems with 104 contaminating companions within ~ 4". We find no evidence that KOI companions with separations of less than 1" are more likely to be physically associated than KOI companions generally. We also reinterpret transit depths for 94 planet candidates, and calculate that 2.6% ± 0.4% of transits have R > 15R_⊕, which is consistent with prior modeling work

Robo-AO Kepler Survey IV: the effect of nearby stars on 3857 planetary candidate systems

We present the overall statistical results from the Robo-AO Kepler planetary candidate survey, comprising of 3857 high-angular resolution observations of planetary candidate systems with Robo-AO, an automated laser adaptive optics system. These observations reveal previously unknown nearby stars blended with the planetary candidate host star which alter the derived planetary radii or may be the source of an astrophysical false positive transit signal. In the first three papers in the survey, we detected 440 nearby stars around 3313 planetary candidate host stars. In this paper, we present observations of 532 planetary candidate host stars, detecting 94 companions around 88 stars; 84 of these companions have not previously been observed in high-resolution. We also report 50 more-widely-separated companions near 715 targets previously observed by Robo-AO. We derive corrected planetary radius estimates for the 814 planetary candidates in systems with a detected nearby star. If planetary candidates are equally likely to orbit the primary or secondary star, the radius estimates for planetary candidates in systems with likely bound nearby stars increase by a factor of 1.54, on average. We find that 35 previously-believed rocky planet candidates are likely not rocky due to the presence of nearby stars. From the combined data sets from the complete Robo-AO KOI survey, we find that 14.5\pm0.5% of planetary candidate hosts have a nearby star with 4", while 1.2% have two nearby stars and 0.08% have three. We find that 16% of Earth-sized, 13% of Neptune-sized, 14% of Saturn-sized, and 19% of Jupiter-sized planet candidates have detected nearby stars.Comment: Accepted to the Astronomical Journa

Robo-AO Kepler Survey V: The effect of physically associated stellar companions on planetary systems

The Kepler light curves used to detect thousands of planetary candidates are susceptible to dilution due to blending with previously unknown nearby stars. With the automated laser adaptive optics instrument, Robo-AO, we have observed 620 nearby stars around 3857 planetary candidates host stars. Many of the nearby stars, however, are not bound to the KOI. In this paper, we quantify the association probability between each KOI and detected nearby stars through several methods. Galactic stellar models and the observed stellar density are used to estimate the number and properties of unbound stars. We estimate the spectral type and distance to 145 KOIs with nearby stars using multi-band observations from Robo-AO and Keck-AO. We find most nearby stars within 1" of a Kepler planetary candidate are likely bound, in agreement with past studies. We use likely bound stars as well as the precise stellar parameters from the California Kepler Survey to search for correlations between stellar binarity and planetary properties. No significant difference between the binarity fraction of single and multiple planet systems is found, and planet hosting stars follow similar binarity trends as field stars, many of which likely host their own non-aligned planets. We find that hot Jupiters are ~4x more likely than other planets to reside in a binary star system. We correct the radius estimates of the planet candidates in characterized systems and find that for likely bound systems, the estimated planetary candidate radii will increase on average by a factor of 1.77, if either star is equally likely to host the planet. We find that the planetary radius gap is robust to the impact of dilution, and find an intriguing 95%-confidence discrepancy between the radius distribution of small planets in single and binary systems.Comment: 19 pages, 12 figures, submitted to AAS Journal