arXiv subjects

David M. Kipping

Publications and source records attributed to David M. Kipping.

At least 55 records · Page 3Linked to original sources

arXiv2013

The Hunt for Exomoons with Kepler (HEK): II. Analysis of Seven Viable Satellite-Hosting Planet Candidates

From the list of 2321 transiting planet candidates announced by the Kepler Mission, we select seven targets with favorable properties for the capacity to dynamically maintain an exomoon and present a detectable signal. These seven candidates were identified through our automatic target selection (TSA) algorithm and target selection prioritization (TSP) filtering, whereby we excluded systems exhibiting significant time-correlated noise and focussed on those with a single transiting planet candidate of radius less than 6 Earth radii. We find no compelling evidence for an exomoon around any of the seven KOIs but constrain the satellite-to-planet mass ratios for each. For four of the seven KOIs, we estimate a 95% upper quantile of M_S/M_P<0.04, which given the radii of the candidates, likely probes down to sub-Earth masses. We also derive precise transit times and durations for each candidate and find no evidence for dynamical variations in any of the KOIs. With just a few systems analyzed thus far in the in-going HEK project, projections on eta-moon would be premature, but a high frequency of large moons around Super-Earths/Mini-Neptunes would appear to be incommensurable with our results so far.

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arXiv2012

An Analytic Model for Rotational Modulations in the Photometry of Spotted Stars

Photometric rotational modulations due to starspots remain the most common and accessible way to study stellar activity. In the Kepler-era, there now exists precise, continuous photometry of ~150,000 stars presenting an unprecedented opportunity for statistical analyses of these modulations. Modelling rotational modulations allows one to invert the observations into several basic parameters, such as the rotation period, spot coverage, stellar inclination and differential rotation rate. The most widely used analytic model for this inversion comes from Budding (1977) and Dorren (1987), who considered circular, grey starspots for a linearly limb darkened star. In this work, we extend the model to be more suitable in the analysis of high precision photometry, such as that by Kepler. Our new freely available Fortran code, macula, provides several improvements, such as non-linear limb darkening of the star and spot, a single-domain analytic function, partial derivatives for all input parameters, temporal partial derivatives, diluted light compensation, instrumental offset normalisations, differential rotation, starspot evolution and predictions of transit depth variations due to unocculted spots. Through numerical testing, we find that the inclusion of non-linear limb darkening means macula has a maximum photometric error an order-of-magnitude less than that of Dorren (1987), for Sun-like stars observed in the Kepler-bandpass. The code executes three orders-of-magnitude faster than comparable numerical codes making it well-suited for inference problems.

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arXiv2012

The Detection and Characterization of a Nontransiting Planet by Transit Timing Variations

The Kepler Mission is monitoring the brightness of ~150,000 stars searching for evidence of planetary transits. As part of the "Hunt for Exomoons with Kepler" (HEK) project, we report a planetary system with two confirmed planets and one candidate planet discovered using the publicly available data for KOI-872. Planet b transits the host star with a period P_b=33.6d and exhibits large transit timing variations indicative of a perturber. Dynamical modeling uniquely detects an outer nontransiting planet c near the 5:3 resonance (P_c=57.0d) of mass 0.37 times that of Jupiter. Transits of a third planetary candidate are also found: a 1.7-Earth radius super-Earth with a 6.8d period. Our analysis indicates a system with nearly coplanar and circular orbits, reminiscent of the orderly arrangement within the solar system.

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arXiv2012

The Transits of Extrasolar Planets with Moons

The search for extrasolar planets is strongly motivated by the goal of characterizing how frequent habitable worlds and life may be within the Galaxy. Whilst much effort has been spent on searching for Earth-like planets, large moons may also be common, temperate abodes for life as well. The methods to detect extrasolar moons, or "exomoons" are more subtle than their planetary counterparts and in this thesis I aim to provide a method to find such bodies in transiting systems, which offer the greatest potential for detection. Before one can search for the tiny perturbations to the planetary signal, an understanding of the planetary transit must be established. Therefore, in Chapters 3 to 5 I discuss the transit model and provide several new insights. Chapter 4 presents new analytic expressions for the times of transit minima and the transit duration, which will be critical in the later search for exomoons. Chapter 5 discusses two sources of distortion to the transit signal, namely blending (with a focus on the previously unconsidered self-blending scenario) and light curve smearing due to long integration times. I provide methods to compensate for both of these effects, thus permitting for the accurate modelling of the planetary transit light curve. In Chapter 6, I discuss methods to detect exomoons through their gravitational influence on the host planet, giving rise to transit timing and duration variations (TTV and TDV). The previously known TTV effect is updated with a new model and the associated critical problems are outlined. I then predict a new effect, TDV, which solves these problems, making exomoon detection viable. Chapter 7 presents a feasibility study for detecting habitable-zone exomoons with Kepler, where it is found that moons down to 0.2 Earth masses are detectable. Finally, conclusions and future work are discussed in Chapter 8.

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arXiv2012

LUNA: An algorithm for generating dynamic planet-moon transits

It has been previously shown that moons of extrasolar planets may be detectable with the Kepler Mission, for moon masses above ~0.2 Earth masses Kipping et al. 2009c. Transit timing effects have been formerly identified as a potent tool to this end, exploiting the dynamics of the system. In this work, we explore the simulation of transit light curves of a planet plus a single moon including not only the transit timing effects but also the light curve signal of the moon itself. We introduce our new algorithm, LUNA, which produces transit light curves for both bodies, analytically accounting for shadow overlaps, stellar limb darkening and planet-moon dynamical motion. By building the dynamics into the core of LUNA, the routine automatically accounts for transit timing/duration variations and ingress/egress asymmetries for not only the planet, but also the moon. We then generate some artificial data for two feasibly detectable hypothetical systems of interest: a i) prograde and ii) retrograde Earth-like moon around a habitable-zone Neptune for a M-dwarf system. We fit the hypothetical systems using LUNA and demonstrate the feasibility of detecting these cases with Kepler photometry.

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arXiv2012

The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project

Two decades ago, empirical evidence concerning the existence and frequency of planets around stars, other than our own, was absent. Since this time, the detection of extrasolar planets from Jupiter-sized to most recently Earth-sized worlds has blossomed and we are finally able to shed light on the plurality of Earth-like, habitable planets in the cosmos. Extrasolar moons may also be frequent habitable worlds but their detection or even systematic pursuit remains lacking in the current literature. Here, we present a description of the first systematic search for extrasolar moons as part of a new observational project called "The Hunt for Exomoons with Kepler" (HEK). The HEK project distills the entire list of known transiting planet candidates found by Kepler (2326 at the time of writing) down to the most promising candidates for hosting a moon. Selected targets are fitted using a multimodal nested sampling algorithm coupled with a planet-with-moon light curve modelling routine. By comparing the Bayesian evidence of a planet-only model to that of a planet-with-moon, the detection process is handled in a Bayesian framework. In the case of null detections, upper limits derived from posteriors marginalised over the entire prior volume will be provided to inform the frequency of large moons around viable planetary hosts, eta-moon. After discussing our methodologies for target selection, modelling, fitting and vetting, we provide two example analyses.

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arXiv2011

A Novel Method to Photometrically Constrain Orbital Eccentricities: Multibody Asterodensity Profiling (MAP)

We present a novel method to determine eccentricity constraints of extrasolar planets in systems with multiple transiting planets through photometry alone. Our method is highly model independent, making no assumptions about the stellar parameters and requiring no radial velocity, transit timing or occultation events. Our technique exploits the fact the light curve derived stellar density must be the same for all planets transiting a common star. Assuming a circular orbit, the derived stellar density departs from the true value by a predictable factor, Psi, which contains information on the eccentricity of the system. By comparing multiple stellar densities, any differences must be due to eccentricity and thus meaningful constraints can be placed in the absence of any other information. The technique, dubbed "Multibody Asterodensity Profiling" (MAP), is a new observable which can be used alone or in combination with other observables, such as radial velocities and transit timing variations. An eccentricity prior can also be included as desired. MAP is most sensitive to the minimum pair-combined eccentricity e.g. (e1 + e2)[min]. Individual eccentricity constraints are less stringent but an empirical eccentricity posterior is always derivable and is freely available from transit photometry alone. We present a description of our technique using both analytic and numerical implementations, followed by two example analyses on synthetic photometry as a proof of principle. We point out that MAP has the potential to constrain the eccentricity, and thus habitability, of Earth-like planets in the absence of radial velocity data, which is likely for terrestrial-mass objects.

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arXiv2011

Detection of visible light from the darkest world

We present the detection of visible light from the planet TrES-2b, the darkest exoplanet currently known. By analysis of the orbital photometry from publicly available Kepler data (0.4-0.9 microns), we determine a day-night contrast amplitude of (6.5 +/- 1.9 ppm), constituting the lowest amplitude orbital phase variation discovered. The signal is detected to 3.7 sigma confidence and persists in six different methods of modelling the data and thus appears robust. In contrast, we are unable to detect ellipsoidal variations or beaming effects, but we do provide confidence intervals for these terms. If the day-night contrast is interpreted as being due to scattering, it corresponds to a geometric albedo of Ag = 0.0253 +/- 0.0072. However, our models indicate that there is a significant emission component to day-side brightness, and the true albedo is even lower (<1%). By combining our measurement with Spitzer and ground-based data, we show that a model with moderate redistribution (Pn ~ 0.3) and moderate extra optical opacity (kappa' ~ 0.3-0.4) provide a compatible explanation to the data.

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arXiv2011

HAT-P-31b,c: A Transiting, Eccentric, Hot Jupiter and a Long-Period, Massive Third-Body

We report the discovery of HAT-P-31b, a transiting exoplanet orbiting the V=11.660 dwarf star GSC 2099-00908. HAT-P-31b is the first HAT planet discovered without any follow-up photometry, demonstrating the feasibility of a new mode of operation for the HATNet project. The 2.17 Mj, 1.1Rj planet has a period P = 5.0054 days and maintains an unusually high eccentricity of e = 0.2450+/-0.0045, determined through Keck, FIES and Subaru high precision radial velocities. Detailed modeling of the radial velocities indicates an additional quadratic residual trend in the data detected to very high confidence. We interpret this trend as a long-period outer companion, HAT-P-31c, of minimum mass 3.4Mj and period >2.8 years. Since current RVs span less than half an orbital period, we are unable to determine the properties of HAT-P-31c to high confidence. However, dynamical simulations of two possible configurations show that orbital stability is to be expected. Further, if HAT-P-31c has non-zero eccentricity, our simulations show that the eccentricity of HAT-P-31b is actively driven by the presence of c, making HAT-P-31 a potentially intriguing dynamical laboratory.

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arXiv2011

Analysis of Kepler's short-cadence photometry for TrES-2b

We present an analysis of 18 short-cadence (SC) transit lightcurves of TrES-2b using quarter 0 (Q0) and quarter 1 (Q1) from the Kepler Mission. The photometry is of unprecedented precision, 237ppm per minute, allowing for the most accurate determination of the transit parameters yet obtained for this system. Global fits of the transit photometry, radial velocities and known transit times are used to obtain a self-consistent set of refined parameters for this system, including updated stellar and planetary parameters. Special attention is paid to fitting for limb darkening and eccentricity. We place an upper limit on the occultation depth to be <72.9ppm to 3-sigma confidence, indicating TrES-2b has the lowest determined geometric albedo for an exoplanet, of Ag<0.146. We also produce a transit timing analysis using Kepler's short-cadence data and demonstrate exceptional timing precision at the level of a few seconds for each transit event. With 18 fully-sampled transits at such high precision, we are able to produce stringent constraints on the presence of perturbing planets, Trojans and extrasolar moons. We introduce the novel use of control data to identify phasing effects. We also exclude the previously proposed hypotheses of short-period TTV and additional transits but find the hypothesis of long-term inclination change is neither supported nor refuted by our analysis.

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arXiv2011

An Independent Analysis of Kepler-4b through Kepler-8b

We present two independent, homogeneous, global analyses of the transit light curves, radial velocities and spectroscopy of Kepler-4b through Kepler-8b, with numerous differences over the previous methods. These include: 1) improved decorrelated parameter fitting set, 2) new limb darkening coefficients, 3) time-stamps modified to BJD for consistency with RV, 4) two different methods for compensating for the integration-time of Kepler LC-data, 5) best-fit secondary-eclipse depths and excluded upper limits, 6) fitted mid-transit times, durations, depths and baseline fluxes for individual transits. We make several new determinations: 1) We detect a secondary eclipse for Kepler-7b of depth (47+/-14)ppm and significance 3.5-sigma. We conclude reflected light is a more plausible origin than thermal emission and determine a geometric albedo of Ag=(0.38+/-0.12). 2) An eccentric-orbit model for the Neptune-mass planet Kepler-4b is marginally detected to 2-sigma confidence with e=(0.25+/-0.12). If confirmed, this would place Kepler-4b in a similar category as GJ436b and HAT-P-11b as an eccentric, Neptune-mass planet. 3) We find marginal (2-sigma) evidence for Kepler-5b's secondary eclipse of depth (26+/-17)ppm. The most plausible explanation is reflected light caused by a planet with Ag=(0.15+/-0.10). 4) A 2.6-sigma peak in Kepler-6b TTV periodogram is detected and is not easily explained as an aliased frequency. We find that a resonant/non-resonant perturber, Trojan or exomoon all provide inadequate explanations and the most likely source is stellar rotation. 5) We find different impact parameters relative to the discovery papers in most cases, but generally self-consistent. 6) We constrain the presence of resonant planets, exomoons and Trojans using transit timing.

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arXiv2010

How to Weigh a Star Using a Moon

We show that for a transiting exoplanet accompanied by a moon which also transits, the absolute masses and radii of the star, planet and moon are determinable. For a planet-star system, it is well known that the density of the star is calculable from the lightcurve by manipulation of Kepler's Third Law. In an analogous way, the planetary density is calculable for a planet-moon system which transits a star, and thus the ratio-of-densities is known. By combining this ratio with the observed ratio-of-radii and the radial velocity measurements of the system, we show that the absolute dimensions of the star and planet are determinable. This means such systems could be used as calibrators of stellar evolution. The detection of dynamical effects, such as transit timing variations, allows the absolute mass of the moon to be determined as well, which may be combined with the radius to infer the satellite's composition.

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arXiv2010

Nightside Pollution of Exoplanet Transit Depths

Out of the known transiting extrasolar planets, the majority are gas giants orbiting their host star at close proximity. Both theoretical and observational studies support the hypothesis that such bodies emit significant amounts of flux relative to the host star, increasing towards infrared wavelengths. For the dayside of the exoplanet, this phenomenon typically permits detectable secondary eclipses at such wavelengths, which may be used to infer atmospheric composition. In this paper, we explore the effects of emission from the nightside of the exoplanet on the primary transit lightcurve, which is essentially a self-blend. Allowing for nightside emission, an exoplanet's transit depth is no longer exclusively a function of the ratio-of-radii. The nightside of an exoplanet is emitting flux and the contrast to the star's emission is of the order of ~10^(-3) for hot-Jupiters. Consequently, we show that the transit depth in the mid-infrared will be attenuated due to flux contribution from the nightside emission by ~10^(-4). We show how this effect can be compensated for in the case where exoplanet phase curves have been measured, in particular for HD 189733b. For other systems, it may be possible to make a first-order correction by using temperature estimates of the planet. Unless the effect is accounted for, transmission spectra will also be polluted by nightside emission and we estimate that a Spitzer broadband spectrum on a bright target is altered at the 1-sigma level. Using archived Spitzer measurements, we show that the effect respectively increases the 8.0um and 24.0um transit depths by 1-sigma and 0.5-sigma per transit for HD 189733b. Consequently, we estimate that this would be 5-10 sigma effect for near-future JWST observations.

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arXiv2010

Investigations of approximate expressions for the transit duration

In this work, we investigate the accuracy of various approximate expressions for the transit duration of a detached binary against the exact solution, found through solving a quartic equation. Additionally, a new concise approximation is derived, which offers more accurate results than those currently in the literature. Numerical simulations are performed to test the accuracy of the various expressions. We find that our proposed expression show yields a >200% improvement in accuracy relative to the most previously employed expression. We derive a new set of equations for retrieving the lightcurve parameters and consider the effect of falsely using circular expressions for eccentric orbits, with particularly important consequences for transit surveys. The new expression also allows us to propose a new lightcurve fitting parameter set, which minimizes the mutual correlations and thus improves computational efficiency. The equation is also readily differentiated to provide analytic expressions for the transit duration variation (TDV) due to secular variations in the system parameters, for example due to apsidal precession induced by perturbing planets.

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arXiv2009

Pathways Towards Habitable Moons

The search for life outside of the Solar System should not be restricted to exclusively planetary bodies; large moons of extrasolar planets may also be common habitable environments throughout the Galaxy. Extrasolar moons, or exomoons, may be detected through transit timing effects induced onto the host planet as a result of mutual gravitational interaction. In particular, transit timing variations (TTV) and transit duration variations (TDV) are predicted to produce a unique exomoon signature, which is not only easily distinguished from other gravitational perturbations, but also provides both the period and mass of an exomoon. Using these timing effects, photometry greater or equal to that of the Kepler Mission is readily able to detect habitable-zone exomoons down to 0.2 Earth masses and could survey up to 25,000 stars for 1 Earth-mass satellites. We discuss future possibilities for spectral retrieval of such bodies and show that transmission spectroscopy with JWST should be able to detect molecular species with ~30 transit events, in the best cases.

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arXiv2009

On the detectability of habitable exomoons with Kepler-class photometry

In this paper we investigate the detectability of a habitable-zone exomoon around various configurations of exoplanetary systems with the Kepler Mission or photometry of approximately equal quality. We calculate both the predicted transit timing signal amplitudes and the estimated uncertainty on such measurements in order to calculate the confidence in detecting such bodies across a broad spectrum of orbital arrangements. The effects of stellar variability, instrument noise and photon noise are all accounted for in the analysis. We validate our methodology by simulating synthetic lightcurves and performing a Monte Carlo analysis for several cases of interest. We find that habitable-zone exomoons down to 0.2 Earth masses may be detected and ~25,000 stars could be surveyed for habitable-zone exomoons within Kepler's field-of-view. A Galactic Plane survey with Kepler-class photometry could potentially survey over one million stars for habitable-zone exomoons. In conclusion, we propose that habitable exomoons will be detectable should they exist in the local part of the galaxy.

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arXiv2009

Transit timing effects due to an exomoon II

In our previous paper, we evaluated the transit duration variation (TDV) effect for a co-aligned planet-moon system at an orbital inclination of i=90 degrees. Here, we will consider the effect for the more general case of i <= 90 degrees and an exomoon inclined from the planet-star plane by Euler rotation angles $α$, $β$ and $γ$. We find that the TDV signal has two major components, one due to the velocity variation effect described in our first paper and one new component due to transit impact parameter variation. By evaluating the dominant terms, we find the two effects are additive for prograde exomoon orbits, and deductive for retrograde orbits. This asymmetry could allow for future determination of the orbital sense of motion. We re-evaluate the ratio of TDV and TTV effects, $η$, in the more general case of an inclined planetary orbit with a circular orbiting moon and find that it is still possible to directly determine the moon's orbital separation from just the ratio of the two amplitudes, as first proposed in our previous paper.

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arXiv2009

Detection of a transit by the planetary companion of HD 80606

We report the detection of a transit egress by the ~ 3.9-Jupiter-mass planet HD 80606b, an object in a highly-eccentric orbit (e ~ 0.93) about its parent star of approximately solar type. The astrophysical reality of the signal of variability in HD 80606 is confirmed by observation with two independent telescope systems, and checks against several reference stars in the field. Differential photometry with respect to the nearby comparison star HD 80607 provides a precise light curve. Modelling of the light curve with a full eccentric-orbit model indicates a planet/star-radius ratio of 0.1057 +/- 0.0018, corresponding to a planet radius of 1.029 R_J for a solar-radius parent star; and a precise orbital inclination of 89.285 +/- 0.023 degrees, giving a total transit duration of 12.1 +/- 0.4 hours. The planet hence joins HD 17156b in a class of highly eccentric transiting planets, in which HD 80606b has both the longest period and most eccentric orbit. The recently reported discovery of a secondary eclipse of HD 80606b by the Spitzer Space Observatory permits a combined analysis with the mid-time of primary transit in which the orbital parameters of the system can be tightly constrained. We derive a transit ephemeris of T_tr = HJD (2454876.344 +/- 0.011) + (111.4277 +/- 0.0032) E.

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