Research

HIP 41378 f is a sub-Neptune exoplanet with an anomalously low density. Its long orbital period and deep transit make it an ideal candidate for detecting oblateness photometrically. We present a new cross-platform, GPU-enabled code, greenlantern, suitable for computing transit light curves of oblate planets at arbitrary orientations. We then use the Markov Chain Monte Carlo method to fit K2 data of HIP 41378 f, specifically examining its transit for evidence of oblateness and obliquity. We find that the flattening of HIP 41378 f is f ≤ 0.889 at the 95% confidence level, consistent with a rotation period of P_rot ≥ 15.3 hr. In the future, high-precision data from JWST have the potential to tighten such a constraint and can differentiate between spherical and flattened planets.

ADS, arXiv, ApJL

Hydrodynamical simulations of protoplanetary disk dynamics are useful tools for understanding the formation of planetary systems, including our own. Approximations are necessary to make these simulations computationally tractable. A common assumption when simulating dust fluids is that of a constant Stokes number, a dimensionless number that characterizes the interaction between a particle and the surrounding gas. Constant Stokes number is not a good approximation in regions of the disk where the gas density changes significantly, such as near a planet-induced gap. In this paper, we relax the assumption of a constant Stokes number in the popular FARGO3D code using semianalytic equations for the drag force on dust particles, which enables an assumption of constant particle size instead. We explore the effect this change has on disk morphology and particle fluxes across the gap for both outward- and inward-drifting particles. The assumption of constant particle size, rather than constant Stokes number, is shown to make a significant difference in some cases, emphasizing the importance of the more accurate treatment.

ADS, arXiv, ApJ

To date, at least three comets — 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) — have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H₂O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H₂O of unity, and one model reaches a CO/H₂O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H₂O ice. In light of this, a population of CO-ice-enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own solar system and in extrasolar systems, than previously expected.

ADS, arXiv, ApJ

The transiting planet candidate KOI 1843.03 (0.6 Earth radii, 4.245 hour orbital period, 0.46 solar mass host star) has the shortest orbital period of any planet yet discovered. Here we show, using the first three-dimensional interior structure simulations of ultra-short-period tidally distorted rocky exoplanets, that KOI 1843.03 may be shaped like an American football, elongated along the planet-star axis with an aspect ratio of up to 1.79. Furthermore, for KOI 1843.03 to have avoided tidal disruption (wherein the planet is pulled apart by the tidal gravity of its host star) on such a close-in orbit, KOI 1843.03 must be as iron-rich as Mercury (about 66% by mass iron compared to Mercury"s 70% by mass iron, Hauck et al. 2013). Of the ultra-short-period (orbital period < 1 day) planets with physically-meaningful constraints on their densities characterized to date, just under half (4 out of 9) are iron-enhanced. As more are discovered, we will better understand the diversity of rocky planet compositions and the variety of processes that lead to planetary iron enhancement.

ADS, arXiv, ApJ

The composition of a protoplanetary disk is set by a combination of interstellar inheritance and gas and grain surface chemical reactions within the disk. The survival of inherited molecules, as well as the disk in situ chemistry depends on the local temperature, density and irradiation environment, which can change over time due to stellar and disk evolution, as well as transport in the disk. We address one aspect of this coupling between the physical and chemical evolution in disks by following accretion streamlines of gas and small grains in the disk midplane, while simultaneously taking the evolving star into account. This approach is computationally efficient and enables us to take into account changing physical conditions without reducing the chemical network. We find that many species are enhanced in the inner disk midplane in the dynamic model due to inward transport of cosmic-ray driven chemical products, resulting in, e.g., orders of magnitude hydrocarbon enhancements at 1 au, compared to a static disk. For several other chemical families, there is no difference between the static and dynamic models, indicative of a robust chemical reset, while yet others show differences between static and dynamic models that depend on complex interactions between physics and chemistry during the inward track. The importance of coupling dynamics and chemistry when modeling the chemical evolution of protoplanetary disks thus depends on what chemistry is of interest.

ADS, arXiv, ApJ

It is well-known that the light curve of a transiting planet contains information about the planet"s orbital period and size relative to the host star. More recently, it has been demonstrated that a tight constraint on an individual planet"s eccentricity can sometimes be derived from the light curve via the "photoeccentric effect," the effect of a planet"s eccentricity on the shape and duration of its light curve. This has only been studied for large planets and high signal-to-noise scenarios, raising the question of how well it can be measured for smaller planets or low signal-to-noise cases. We explore the limits of the photoeccentric effect over a wide range of planet parameters. The method hinges upon measuring g directly from the light curve, where g is the ratio of the planet"s speed (projected on the plane of the sky) during transit to the speed expected for a circular orbit. We find that when the signal-to-noise in the measurement of g is < 10, the ability to measure eccentricity with the photoeccentric effect decreases. We develop a "rule of thumb" that for per-point relative photometric uncertainties σ = {1e-3, 1e-4, 1e-5}, the critical values of planet-star radius ratio are Rp/R⋆ ≈ {0.1, 0.05, 0.03} for Kepler-like 30-minute integration times. We demonstrate how to predict the best-case uncertainty in eccentricity that can be found with the photoeccentric effect for any light curve. This clears the path to study eccentricities of individual planets of various sizes in the Kepler sample and future transit surveys.

ADS, arXiv, ApJ

Kepler has revolutionized the study of transiting planets with its unprecedented photometric precision on more than 150,000 target stars. Most of the transiting planet candidates detected by Kepler have been observed as long-cadence targets with 30 minute integration times, and the upcoming Transiting Exoplanet Survey Satellite (TESS) will record full frame images with a similar integration time. Integrations of 30 minutes affect the transit shape, particularly for small planets and in cases of low signal-to-noise. Using the Fisher information matrix technique, we derive analytic approximations for the variances and covariances on the transit parameters obtained from fitting light curve photometry collected with a finite integration time. We find that binning the light curve can significantly increase the uncertainties and covariances on the inferred parameters when comparing scenarios with constant total signal-to-noise (constant total integration time in the absence of read noise). Uncertainties on the transit ingress/egress time increase by a factor of 34 for Earth-size planets and 3.4 for Jupiter-size planets around Sun-like stars for integration times of 30 minutes compared to instantaneously-sampled light curves. Similarly, uncertainties on the mid-transit time for Earth and Jupiter-size planets increase by factors of 3.9 and 1.4. Uncertainties on the transit depth are largely unaffected by finite integration times. While correlations among the transit depth, ingress duration, and transit duration all increase in magnitude with longer integration times, the mid-transit time remains uncorrelated with the other parameters. We provide code in Python and Mathematica for predicting the variances and covariances on this website.

ADS, arXiv, ApJ

Thousands of exoplanets have been discovered to date; most orbit stars that are similar to our Sun (FGK dwarfs) or cooler (M dwarfs). Detecting planets orbiting hotter stars (A dwarfs) is a challenge because hot stars have rotationally-broadened spectral features and large radii. Accumulating a statistical sample of well-characterized planets orbiting A stars is important to constrain trends in planet occurrence and orbital properties as a function of stellar mass. Throughout its four years of operation, the Kepler mission monitored a few thousand hot stars (T_eff > 7000 K) with sufficient photometric precision to detect the transits of Jupiter-size planets. We characterize the main sequence A stars with transiting planet candidates detected by Kepler. We identify likely A stars in the Kepler Input Catalog (KIC) by their stellar effective temperatures, derived from KIC grizJHK photometry using the empirical relations from Boyajian et al. (2013). To verify the classification of a subset of these stars, we measure their spectra using Palomar DBSP and collect high-resolution images with Keck NIRC2. We determine the physical parameters of the transiting planets' orbits by fitting the Kepler transit light curves with Markov Chain Monte Carlo methods. By constraining the semi-major axis and eccentricity distributions of planets orbiting A stars, we gain insights into the formation and tidal evolution of planets in a relatively uncharted region of the H-R diagram.

ADS

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