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The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H₂–He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H₂O-dominated ices/fluids (water worlds). Planets in the mass range of 10–15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R⊕ range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are “water worlds.”

The first interstellar object to be detected in the Solar System is asteroidal in nature and has a shape unlike any Solar System body, with a length about ten times its width. New asteroid makes a visit Science fiction readers will remember Rendezvous with Rama by Arthur C. Clarke, where a cylindrical spacecraft from outside the Solar System passes through it at high speed. An asteroidal version of such a visitor was discovered in October 2017. Karen Meech and collaborators report observations and characterization of 1I/2017 U1, which they have named 'Oumuamua. This asteroid has a red colour, a 10:1 axis ratio, and spectra show its surface to be similar to those of comets and organic-rich asteroids in our Solar System. The mean radius of 'Oumuamua is approximately 102 metres assuming an albedo of 0.04. Calculations suggest that previous estimates of the frequency of such interstellar bodies were too low. None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space 1 . The high predicted number density 2 of icy interstellar objects (2.4 × 10 −4 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of the Solar System today, but there has been no way of telling whether the Solar System is typical of planetary systems. Here we report observations and analysis of the object 1I/2017 U1 (‘Oumuamua) that demonstrate its extrasolar trajectory, and that thus enable comparisons to be made between material from another planetary system and from our own. Our observations during the brief visit by the object to the inner Solar System reveal it to be asteroidal, with no hint of cometary activity despite an approach within 0.25 astronomical units of the Sun. Spectroscopic measurements show that the surface of the object is spectrally red, consistent with comets or organic-rich asteroids that reside within the Solar System. Light-curve observations indicate that the object has an extremely oblong shape, with a length about ten times its width, and a mean radius of about 102 metres assuming an albedo of 0.04. No known objects in the Solar System have such extreme dimensions. The presence of ‘Oumuamua in the Solar System suggests that previous estimates of the number density of interstellar objects, based on the assumption that all such objects were cometary, were pessimistically low. Planned upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to result in the detection of more interstellar objects in the coming years.

A major bottleneck for transiting exoplanet demographics has been the lack of precise properties for most of the observed stars, as the transit method measures exoplanet radii relative to their host's radii. We live in a golden era of host star characterization because of access to Gaia photometry, parallaxes, and proper motions, large-scale spectroscopic surveys, and ground-based photometric and spectroscopic follow-up. I have used all of this data to sharpen our view of exoplanet demographics. First, I constrained the stellar radii of Kepler targets using Gaia DR2, which allowed the first comprehensive classification of main sequence, subgiant, and giant stars in Kepler target sample; I also identified ~4000 low-mass main sequence binary systems. With these precise stellar radii, I was the first to use Gaia to revise planet radii and incident fluxes and corroborate the existence of the planet radius gap. I discovered planets within the hot sub-Neptunian desert (2.2–3.8 Earth radii, > 650 Earth fluxes), presented an updated census of habitable zone planets, and identified a hot Jupiter inflation trend for Kepler planets. I also performed isochrone modeling for the entire Kepler target sample and produced the Gaia-Kepler Stellar Properties Catalog, the first homogeneous catalog to include stellar ages, in addition to precise radii, masses, and mean stellar densities for Kepler target stars. Using these homogeneously derived properties, I found the first observational evidence of a stellar age dependence of the planet radius gap, where sub-Neptunes (1.8–3.5 Earth radii) become super-Earths (1.0–1.8 Earth radii) on roughly Gyr timescales. This result built upon my previous work, where I measured lithium abundances to separate old and young Kepler stars (using the Hyades's empirical A(Li)-Teff isochrone at ~650 Myr) and discovered that the young planets were statistically larger than the old planets. In addition, I investigated the stellar mass dependence of the planet radius valley and provided stringent constraints that will be required to discern between the theories of core-powered mass-loss and photoevaporation. I also confirmed the existence of planets within the hot sub-Neptunian desert, discovered that most desert planets entered recently because of their host's evolution, investigated Jupiters at low incident fluxes with radii larger than the theoretical maximum, and demonstrated that planets in single and multiple transiting systems share the same age distribution.

Servant leadership has made significant progress over the past four decades. In order to emerge from the fifth decade of research as a prominent leadership theory, the servant leadership academic community needs to address two critical issues: 1) the need for strong theory construction and evaluation, and 2) the use of the term servant. This article reviews the advances within servant leadership, documents the major theory construction and evaluation literature, introduces a vocabulary and criteria for theory construction and evaluation, and highlights the problematic nature of the term servant. The author argues that servant leadership scholars should focus their efforts on strong theory construction and evaluation, in addition to openly debating the use of the term servant in an academically rigorous fashion, to position servant leadership squarely within the mainstream leadership research literature.

Until recently, none of the 750,000 known asteroids and comets were thought to have originated outside our solar system. Many decades of asteroid and comet characterization have yielded formation scenarios that explain the mass distribution, chemical abundances and planetary configuration of today’s solar system, but up until now there has been no way to tell if our solar system is typical. Solar system formation models suggest that orbital migration of the giant planets as they formed ejected a large fraction of the original planetesimals into interstellar space1. The predicted interstellar number density2 of icy interstellar objects of 2.4 × 10−4 au−3 suggested that these should have been detected by surveys, yet none had ever been seen. Here we report on the discovery and characterization of 1I/2017 U1 (‘Oumuamua), the first object known to originate outside our solar system. Follow-up observations and subsequent analysis verified the extrasolar trajectory of ‘Oumuamua. Our observations reveal the object to be asteroidal, with no hint of cometary activity despite an approach within 0.25 au of the Sun. Spectroscopic measurements show that the object’s surface is consistent with comets or organic-rich asteroid surfaces found in our own solar system. Light-curve observations of ‘Oumuamua indicate that the object has an extreme oblong shape, with a 10:1 axis ratio and a mean radius of 102±4 m assuming an albedo of 0.04. Very few objects in our solar system have such an extreme lightcurve. The discovery of ‘Oumuamua suggests that previous estimates of the density of interstellar objects were pessimistically low. Imminent upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to produce more interstellar objects in the upcoming years, creating opportunities to interrogate the mineralogical, elemental or isotopic composition of material from other solar systems.

Organizational executives leverage significant resources to build leadership development programs as part of strategic initiatives, but many of these programs still fail. The organizational strategy and leadership fields have made significant advances and have complementary purposes, yet scholars have not synthesized this literature. The need remains to form a pragmatic conceptual framework and process intended to serve as a theoretical foundation for designing leadership development programs as sustainable competitive advantages (SCAs). This research presents such a leadership framework and process that may serve as a theoretical foundation, analytical lens, and tool used for designing such programs. This descriptive single-case study details how I used the leadership framework and process at Hershey Entertainment & Resorts (HE&R) to design a leadership development program (i.e., Core Values Initiative) as a SCA. HE&R is a private for-profit hospitality and entertainment company that has a unique history and social purpose. The study progressed through three distinct phases. Phase I synthesized the research literature to arrive at a leadership framework and process to guide practitioners tasked with designing leadership development programs as SCAs. Phase II detailed how the leadership framework and related process were successfully used to design the Core Values Initiative at HE&R. The Core Values Initiative proposes to more deeply embed HE&R’s core values throughout the organization by identifying the behaviors of excellence for HE&R’s core values at each of the different organizational levels. HE&R would then meaningfully integrate the behaviors of excellence into their performance management plan, specifically the performance appraisals and compensation plans. Phase III presents the findings of guided interviews with the nine senior leaders who provided their perceptions about the pragmatism of the leadership framework and process, and the logic and plausibility of the Core Values Initiative. The results indicate that the interviewees found the leadership framework and process pragmatic, and they supported the logic and plausibility of the Core Values Initiative. This study provides a strategic understanding of leadership and offers a viable option to practitioners tasked with successfully designing leadership development programs as SCAs.

We measure exoplanet occurrence rate as a function of isochrone and gyrochronology ages using confirmed and candidate planets identified in Q1-17 DR25 Kepler data. We employ Kepler's pipeline detection efficiency to correct for the expected number of planets in each age bin. We examine the occurrence rates for planets with radii \\(0.2 \\leq Rp \\leq 20\\) R\\(_\\oplus\\) and orbital periods \\(0.2 \\leq P \\leq 100\\) days for FGK stars with ages between \\(1.5-8\\) Gyr using the inverse detection efficiency method. We find no significant trend between occurrence rate and stellar ages; a slight, decreasing trend (within \\(1.5-2.5\\) \\(\\sigma\\)) only emerges for low-mass and metal-rich stars that dominate our sample. We isolate the effects of mass and metallicity on the occurrence rate trend with age, but find the results to be inconclusive due to weak trends and small sample size. Our results hint that the exoplanet occurrence rate may decrease over time due to dynamical instability from planet-planet scattering or planet ejection, but accurate ages and larger sample sizes are needed to resolve a clear relation between occurrence rate and age.

Dynamical evolution within planetary systems can cause planets to be engulfed by their host stars. Following engulfment, the stellar photosphere abundance pattern will reflect accretion of rocky material from planets. Multi-star systems are excellent environments to search for such abundance trends because stellar companions form from the same natal gas cloud and are thus expected to share primordial chemical compositions to within 0.03\\(-\\)0.05 dex. Abundance measurements have occasionally yielded rocky enhancements, but few observations targeted known planetary systems. To address this gap, we carried out a Keck-HIRES survey of 36 multi-star systems where at least one star is a known planet host. We found that only HAT-P-4 exhibits an abundance pattern suggestive of engulfment, but is more likely primordial based on its large projected separation (30,000 \\(\\pm\\) 140 AU) that exceeds typical turbulence scales in molecular clouds. To understand the lack of engulfment detections among our systems, we quantified the strength and duration of refractory enrichments in stellar photospheres using MESA stellar models. We found that observable signatures from 10 \\(M_{\\oplus}\\) engulfment events last for \\(\\sim\\)90 Myr in 1 \\(M_{\\odot}\\) stars. Signatures are largest and longest lived for 1.1\\(-\\)1.2 \\(M_{\\odot}\\) stars, but are no longer observable \\(\\sim\\)2 Gyr post-engulfment. This indicates that engulfment will rarely be detected in systems that are several Gyr old.

The Kepler Mission revolutionized exoplanet science and stellar astrophysics by obtaining highly precise photometry of over 200,000 stars over 4 years. A critical piece of information to exploit Kepler data is its selection function, since all targets had to be selected from a sample of half a million stars on the Kepler CCDs using limited information. Here we use Gaia DR2 to reconstruct the Kepler selection function and explore possible biases with respect to evolutionary state, stellar multiplicity, and kinematics. We find that the Kepler target selection is nearly complete for stars brighter than \\(Kp < 14\\) mag and was effective at selecting main-sequence stars, with the fraction of observed stars decreasing from 95% to 60% between \\(14 < Kp < 16\\) mag. We find that the observed fraction for subgiant stars is only 10% lower, confirming that a significant number of subgiants selected for observation were believed to be main-sequence stars. Conversely we find a strong selection bias against low-luminosity red giant stars (\\(R \\approx 3-5 R_\\odot\\), \\(T_{eff} \\approx 5500\\)K), dropping from 90% at \\(Kp = 14\\) mag to below 30% at \\(Kp = 16\\) mag, confirming that the target selection was efficient at distinguishing dwarfs from giants. We compare the Gaia Re-normalized Unit Weight Error (RUWE) values of the observed and non-observed main sequence stars and find a difference in elevated (\\(>\\) 1.2) RUWE values at \\(\\sim\\,5\\,\\sigma\\) significance, suggesting that the Kepler target selection shows some bias against either close or wide binaries. We furthermore use the Gaia proper motions to show that the Kepler selection function was unbiased with respect to kinematics.

Identifying and removing binary stars from stellar samples is a crucial but complicated task. Regardless of how carefully a sample is selected, some binaries will remain and complicate interpretation of results, especially via flux contamination of survey photometry. One such sample is the data from the Gaia spacecraft, which is collecting photometry and astrometry of more than \\(10^{9}\\) stars. To quantify the impact of binaries on Gaia photometry, we assembled a sample of known binary stars observed with adaptive optics and with accurately measured parameters, which we used to predict Gaia photometry for each stellar component. We compared the predicted photometry to the actual Gaia photometry for each system, and found that the contamination of Gaia photometry because of multiplicity decreases non-linearly from near-complete contamination (\\(\\rho \\leq 0''.15\\)) to no contamination (binary projected separation, or \\(\\rho > 0''.3\\)). We provide an analytic relation to analytically correct photometric bias in a sample of Gaia stars using the binary separation. This correction is necessary because the Gaia PSF photometry extraction does not fully remove the secondary star flux for binaries with separations with \\(\\rho \\lesssim 0''.3\\). We also evaluated the utility of various Gaia quality-of-fit metrics for identifying binary stars and found that RUWE remains the best indicator for unresolved binaries, but multi-peak image fraction probes a separation regime not currently accessible to RUWE.