arXiv subjects

Christopher J. Burke

Publications and source records attributed to Christopher J. Burke.

At least 73 records · Page 4Linked to original sources

arXiv2013

Planetary Candidates Observed by Kepler IV: Planet Sample From Q1-Q8 (22 Months)

We provide updates to the Kepler planet candidate sample based upon nearly two years of high-precision photometry (i.e., Q1-Q8). From an initial list of nearly 13,400 Threshold Crossing Events (TCEs), 480 new host stars are identified from their flux time series as consistent with hosting transiting planets. Potential transit signals are subjected to further analysis using the pixel-level data, which allows background eclipsing binaries to be identified through small image position shifts during transit. We also re-evaluate Kepler Objects of Interest (KOI) 1-1609, which were identified early in the mission, using substantially more data to test for background false positives and to find additional multiple systems. Combining the new and previous KOI samples, we provide updated parameters for 2,738 Kepler planet candidates distributed across 2,017 host stars. From the combined Kepler planet candidates, 472 are new from the Q1-Q8 data examined in this study. The new Kepler planet candidates represent ~40% of the sample with Rp~1 Rearth and represent ~40% of the low equilibrium temperature (Teq<300 K) sample. We review the known biases in the current sample of Kepler planet candidates relevant to evaluating planet population statistics with the current Kepler planet candidate sample.

astro-ph.EP↗

arXiv2013

A super-Earth-sized planet orbiting in or near the habitable zone around Sun-like star

We present the discovery of a super-earth-sized planet in or near the habitable zone of a sun-like star. The host is Kepler-69, a 13.7 mag G4V-type star. We detect two periodic sets of transit signals in the three-year flux time series of Kepler-69, obtained with the Kepler spacecraft. Using the very high precision Kepler photometry, and follow-up observations, our confidence that these signals represent planetary transits is >99.1%. The inner planet, Kepler-69b, has a radius of 2.24+/-0.4 Rearth and orbits the host star every 13.7 days. The outer planet, Kepler-69c, is a super-Earth-size object with a radius of 1.7+/-0.3 Rearth and an orbital period of 242.5 days. Assuming an Earth-like Bond albedo, Kepler-69c has an equilibrium temperature of 299 +/- 19 K, which places the planet close to the habitable zone around the host star. This is the smallest planet found by Kepler to be orbiting in or near habitable zone of a Sun-like star and represents an important step on the path to finding the first true Earth analog.

astro-ph.EP↗

arXiv2013

Detection of Potential Transit Signals in the First Twelve Quarters of Kepler Mission Data

We present the results of a search for potential transit signals in the first three years of photometry data acquired by the Kepler Mission. The targets of the search include 112,321 targets which were observed over the full interval and an additional 79,992 targets which were observed for a subset of the full interval. From this set of targets we find a total of 11,087 targets which contain at least one signal which meets the Kepler detection criteria: those criteria are periodicity of the signal, an acceptable signal-to-noise ratio, and three tests which reject false positives. Each target containing at least one detected signal is then searched repeatedly for additional signals, which represent multi-planet systems of transiting planets. When targets with multiple detections are considered, a total of 18,406 potential transiting planet signals are found in the Kepler Mission dataset. The detected signals are dominated by events with relatively low signal-to-noise ratios and by events with relatively short periods. The distribution of estimated transit depths appears to peak in the range between 20 and 30 parts per million, with a few detections down to fewer than 10 parts per million. The detections exhibit signal-to-noise ratios from 7.1 sigma, which is the lower cut-off for detections, to over 10,000 sigma, and periods ranging from 0.5 days, which is the shortest period searched, to 525 days, which is the upper limit of achievable periods given the length of the data set and the requirement that all detections include at least 3 transits. The detected signals are compared to a set of known transit events in the Kepler field of view, many of which were identified by alternative methods; the comparison shows that the current search recovery rate for targets with known transit events is 98.3%.

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arXiv2013

Fundamental Properties of Kepler Planet-Candidate Host Stars using Asteroseismology

We have used asteroseismology to determine fundamental properties for 66 Kepler planet-candidate host stars, with typical uncertainties of 3% and 7% in radius and mass, respectively. The results include new asteroseismic solutions for four host stars with confirmed planets (Kepler-4, Kepler-14, Kepler-23 and Kepler-25) and increase the total number of Kepler host stars with asteroseismic solutions to 77. A comparison with stellar properties in the planet-candidate catalog by Batalha et al. shows that radii for subgiants and giants obtained from spectroscopic follow-up are systematically too low by up to a factor of 1.5, while the properties for unevolved stars are in good agreement. We furthermore apply asteroseismology to confirm that a large majority of cool main-sequence hosts are indeed dwarfs and not misclassified giants. Using the revised stellar properties, we recalculate the radii for 107 planet candidates in our sample, and comment on candidates for which the radii change from a previously giant-planet/brown-dwarf/stellar regime to a sub-Jupiter size, or vice versa. A comparison of stellar densities from asteroseismology with densities derived from transit models in Batalha et al. assuming circular orbits shows significant disagreement for more than half of the sample due to systematics in the modeled impact parameters, or due to planet candidates which may be in eccentric orbits. Finally, we investigate tentative correlations between host-star masses and planet candidate radii, orbital periods, and multiplicity, but caution that these results may be influenced by the small sample size and detection biases.

astro-ph.SR↗

arXiv2013

Measuring Transit Signal Recovery in the Kepler Pipeline I: Individual Events

The Kepler Mission was designed to measure the frequency of Earth-size planets in the habitable zone of Sun-like stars. A crucial component for recovering the underlying planet population from a sample of detected planets is understanding the completeness of that sample - what fraction of the planets that could have been discovered in a given data set were actually detected. Here we outline the information required to determine the sample completeness, and describe an experiment to address a specific aspect of that question, which is the issue of transit signal recovery. We investigate the extent to which the Kepler pipeline preserves individual transit signals by injecting simulated transits into the pixel-level data, processing the modified pixels through the pipeline, and comparing the measured transit signal-to-noise ratio (SNR) to that expected without perturbation by the pipeline. We inject simulated transit signals across the full focal plane for a set of observations of length 89 days. On average, we find that the SNR of the injected signal is recovered at MS = 0.9973(+/-0.0012)xBS-0.0151(+/-0.0049), where MS is the measured SNR and BS is the baseline, or expected, SNR. The 1σ width of the distribution around this correlation is +/-2.64%. We discuss the pipeline processes that cause the measured SNR to deviate significantly from the baseline SNR; these are primarily the handling of data adjacent to spacecraft re-pointings and the removal of harmonics prior to the measurement of the SNR. Finally we outline the further work required to characterise the completeness of the Kepler pipeline.

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arXiv2013

Chi-Square Discriminators for Transiting Planet Detection in Kepler Data

The Kepler spacecraft observes a host of target stars to detect transiting planets. Requiring a 7.1 sigma detection in twelve quarters of data yields over 100,000 detections, many of which are false alarms. After a second cut is made on a robust detection statistic, some 50,000 or more targets still remain. These false alarms waste resources as they propagate through the remainder of the software pipeline and so a method to discriminate against them is crucial in maintaining the desired sensitivity to true events. This paper describes a chi-square test which represents a novel application of the formalism developed by Allen for false alarm mitigation in searches for gravitational waves. Using this technique, the false alarm rate can be lowered to ~5%.

astro-ph.EP↗

arXiv2012

Optimizing the second hyperpolarizability with minimally-parametrized potentials

The dimensionless zero-frequency intrinsic second hyperpolarizability γ_{int}=γ/4E_{10}^{-5}m^{-2}(e\hbar)^{4} was optimized for a single electron in a 1D well by adjusting the shape of the potential. Optimized potentials were found to have hyperpolarizabilities in the range -0.15\lessapproxγ_{int}\lessapprox0.60 ; potentials optimizing gamma were arbitrarily close to the lower bound and were within \sim0.5% of the upper bound. All optimal potentials posses parity symmetry. Analysis of the Hessian of γ_{int} around the maximum reveals that effectively only a single parameter, one of those chosen in the piecewise linear representation adopted, is important to obtaining an extremum. Prospects for designing new chromophores based on the design principle here elucidated are discussed.

physics.chem-ph↗

arXiv2012

Transmission spectroscopy of exoplanet XO-2b observed with HST NICMOS

Spectroscopy during planetary transits is a powerful tool to probe exoplanet atmospheres. We present the near-infrared transit spectroscopy of XO-2b obtained with HST NICMOS. Uniquely for NICMOS transit spectroscopy, a companion star of similar properties to XO-2 is present in the field of view. We derive improved star and planet parameters through a photometric white-light analysis. We show a clear correlation of the spectrum noise with instrumental parameters, in particular the angle of the spectral trace on the detector. An MCMC method using a decorrelation from instrumental parameters is used to extract the planetary spectrum. Spectra derived independently from each of the 3 visits have a RMS of 430, 510, and 1000 ppm respectively. The same analysis is performed on the companion star after numerical injection of a transit with a depth constant at all wavelengths. The extracted spectra exhibit residuals of similar amplitude as for XO-2, which represent the level of remaining NICMOS systematics. This shows that extracting planetary spectra is at the limit of NICMOS' capability. We derive a spectrum for the planet XO-2b using the companion star as a reference. The derived spectrum can be represented by a theoretical model including atmospheric water vapor or by a flat spectrum model. We derive a 3-sigma upper limit of 1570 ppm on the presence of water vapor absorption in the atmosphere of XO-2b. In an appendix, we perform a similar analysis for the gas giant planet XO-1b.

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arXiv2012

Transit Detection in the MEarth Survey of Nearby M Dwarfs: Bridging the Clean-First, Search-Later Divide

In the effort to characterize the masses, radii, and atmospheres of potentially habitable exoplanets, there is an urgent need to find examples of such planets transiting nearby M dwarfs. The MEarth Project is an ongoing effort to do so, as a ground-based photometric survey designed to detect exoplanets as small as 2 Earth radii transiting mid-to-late M dwarfs within 33 pc of the Sun. Unfortunately, identifying transits of such planets in photometric monitoring is complicated both by the intrinsic stellar variability that is common among these stars and by the nocturnal cadence, atmospheric variations, and instrumental systematics that often plague Earth-bound observatories. Here we summarize the properties of MEarth data gathered so far, and we present a new framework to detect shallow exoplanet transits in wiggly and irregularly-spaced light curves. In contrast to previous methods that clean trends from light curves before searching for transits, this framework assesses the significance of individual transits simultaneously while modeling variability, systematics, and the photometric quality of individual nights. Our Method for Including Starspots and Systematics in the Marginalized Probability of a Lone Eclipse (MISS MarPLE) uses a computationally efficient semi-Bayesian approach to explore the vast probability space spanned by the many parameters of this model, naturally incorporating the uncertainties in these parameters into its evaluation of candidate events. We show how to combine individual transits processed by MISS MarPLE into periodic transiting planet candidates and compare our results to the popular Box-fitting Least Squares (BLS) method with simulations. By applying MISS MarPLE to observations from the MEarth Project, we demonstrate the utility of this framework for robustly assessing the false alarm probability of transit signals in real data. [slightly abridged]

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arXiv2012

The Derivation, Properties and Value of Kepler's Combined Differential Photometric Precision

The Kepler Mission is searching for Earth-size planets orbiting solar-like stars by simultaneously observing >160,000 stars to detect sequences of transit events in the photometric light curves. The Combined Differential Photometric Precision (CDPP) is the metric that defines the ease with which these weak terrestrial transit signatures can be detected. An understanding of CDPP is invaluable for evaluating the completeness of the Kepler survey and inferring the underlying planet population. This paper describes how the Kepler CDPP is calculated, and introduces tables of rms CDPP on a per-target basis for 3-, 6-, and 12-hour transit durations, which are now available for all Kepler observations. Quarter 3 is the first typical set of observations at the nominal length and completeness for a quarter, from 2009 September 18 to 2009 December 16, and we examine the properties of the rms CDPP distribution for this data set. Finally, we describe how to employ CDPP to calculate target completeness, an important use case.

astro-ph.EP↗

arXiv2012

Calibrating Convective properties of Solar-like Stars in the Kepler Field of View

Stellar models generally use simple parametrizations to treat convection. The most widely used parametrization is the so-called "Mixing Length Theory" where the convective eddy sizes are described using a single number, α, the mixing-length parameter. This is a free parameter, and the general practice is to calibrate αusing the known properties of the Sun and apply that to all stars. Using data from NASA's Kepler mission we show that using the solar-calibrated αis not always appropriate, and that in many cases it would lead to estimates of initial helium abundances that are lower than the primordial helium abundance. Kepler data allow us to calibrate αfor many other stars and we show that for the sample of stars we have studied, the mixing-length parameter is generally lower than the solar value. We studied the correlation between αand stellar properties, and we find that αincreases with metallicity. We therefore conclude that results obtained by fitting stellar models or by using population-synthesis models constructed with solar values of αare likely to have large systematic errors. Our results also confirm theoretical expectations that the mixing-length parameter should vary with stellar properties.

astro-ph.SR↗

arXiv2012

Planetary Candidates Observed by Kepler, III: Analysis of the First 16 Months of Data

New transiting planet candidates are identified in sixteen months (May 2009 - September 2010) of data from the Kepler spacecraft. Nearly five thousand periodic transit-like signals are vetted against astrophysical and instrumental false positives yielding 1,091 viable new planet candidates, bringing the total count up to over 2,300. Improved vetting metrics are employed, contributing to higher catalog reliability. Most notable is the noise-weighted robust averaging of multi-quarter photo-center offsets derived from difference image analysis which identifies likely background eclipsing binaries. Twenty-two months of photometry are used for the purpose of characterizing each of the new candidates. Ephemerides (transit epoch, T_0, and orbital period, P) are tabulated as well as the products of light curve modeling: reduced radius (Rp/R*), reduced semi-major axis (d/R*), and impact parameter (b). The largest fractional increases are seen for the smallest planet candidates (197% for candidates smaller than 2Re compared to 52% for candidates larger than 2Re) and those at longer orbital periods (123% for candidates outside of 50-day orbits versus 85% for candidates inside of 50-day orbits). The gains are larger than expected from increasing the observing window from thirteen months (Quarter 1-- Quarter 5) to sixteen months (Quarter 1 -- Quarter 6). This demonstrates the benefit of continued development of pipeline analysis software. The fraction of all host stars with multiple candidates has grown from 17% to 20%, and the paucity of short-period giant planets in multiple systems is still evident. The progression toward smaller planets at longer orbital periods with each new catalog release suggests that Earth-size planets in the Habitable Zone are forthcoming if, indeed, such planets are abundant.

astro-ph.EP↗

arXiv2012

Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis

We present a new method for confirming transiting planets based on the combination of transit timingn variations (TTVs) and dynamical stability. Correlated TTVs provide evidence that the pair of bodies are in the same physical system. Orbital stability provides upper limits for the masses of the transiting companions that are in the planetary regime. This paper describes a non-parametric technique for quantifying the statistical significance of TTVs based on the correlation of two TTV data sets. We apply this method to an analysis of the transit timing variations of two stars with multiple transiting planet candidates identified by Kepler. We confirm four transiting planets in two multiple planet systems based on their TTVs and the constraints imposed by dynamical stability. An additional three candidates in these same systems are not confirmed as planets, but are likely to be validated as real planets once further observations and analyses are possible. If all were confirmed, these systems would be near 4:6:9 and 2:4:6:9 period commensurabilities. Our results demonstrate that TTVs provide a powerful tool for confirming transiting planets, including low-mass planets and planets around faint stars for which Doppler follow-up is not practical with existing facilities. Continued Kepler observations will dramatically improve the constraints on the planet masses and orbits and provide sensitivity for detecting additional non-transiting planets. If Kepler observations were extended to eight years, then a similar analysis could likely confirm systems with multiple closely spaced, small transiting planets in or near the habitable zone of solar-type stars.

astro-ph.EP↗

arXiv2011

The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope

Capitalizing on the observational advantage offered by its tiny M dwarf host, we present HST/WFC3 grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition atmosphere at 8.2 sigma, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. A dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 micron in diameter.

astro-ph.EP↗

arXiv2011

The GJ1214 Super-Earth System: Stellar Variability, New Transits, and a Search for Additional Planets

The super-Earth GJ1214b transits a nearby M dwarf that exhibits 1% intrinsic variability in the near-infrared. Here, we analyze new observations to refine the physical properties of both the star and planet. We present three years of out-of-transit photometric monitoring of the stellar host GJ1214 from the MEarth Observatory and find the rotation period to be long, mostly likely an integer multiple of 53 days, suggesting low levels of magnetic activity and an old age for the system. We show such variability will not pose significant problems to ongoing studies of the planet's atmosphere with transmission spectroscopy. We analyze 2 high-precision transit light curves from ESO's Very Large Telescope along with 7 others from the MEarth and FLWO 1.2 meter telescopes, finding physical parameters for the planet that are consistent with previous work. The VLT light curves show tentative evidence for spot occultations during transit. Using two years of MEarth light curves, we place limits on additional transiting planets around GJ1214 with periods out to the habitable zone of the system. We also improve upon the previous photographic V -band estimate for the star, finding V = 14.71 \pm 0.03.

astro-ph.EP↗

arXiv2011

LSPM J1112+7626: detection of a 41-day M-dwarf eclipsing binary from the MEarth transit survey

We report the detection of eclipses in LSPM J1112+7626, which we find to be a moderately bright (I_C = 12.14 +/- 0.05) very low-mass binary system with an orbital period of 41.03236 +/- 0.00002 days, and component masses M_1 = 0.395 +/- 0.002 Msol and M_2 = 0.275 +/- 0.001 Msol in an eccentric (e = 0.239 +/- 0.002) orbit. A 65 day out of eclipse modulation of approximately 2% peak-to-peak amplitude is seen in I-band, which is probably due to rotational modulation of photospheric spots on one of the binary components. This paper presents the discovery and characterization of the object, including radial velocities sufficient to determine both component masses to better than 1% precision, and a photometric solution. We find that the sum of the component radii, which is much better-determined than the individual radii, is inflated by 3.8 +0.9 -0.5 % compared to the theoretical model predictions, depending on the age and metallicity assumed. These results demonstrate that the difficulties in reproducing observed M-dwarf eclipsing binary radii with theoretical models are not confined to systems with very short orbital periods. This object promises to be a fruitful testing ground for the hypothesized link between inflated radii in M-dwarfs and activity.

astro-ph.SR↗

arXiv2011

The Kepler-19 System: A Transiting 2.2 R_Earth Planet and a Second Planet Detected via Transit Timing Variations

We present the discovery of the Kepler-19 planetary system, which we first identified from a 9.3-day periodic transit signal in the Kepler photometry. From high-resolution spectroscopy of the star, we find a stellar effective temperature Teff=5541 \pm 60 K, a metallicity [Fe/H]=-0.13 \pm 0.06, and a surface gravity log(g)=4.59 \pm 0.10. We combine the estimate of Teff and [Fe/H] with an estimate of the stellar density derived from the photometric light curve to deduce a stellar mass of M_star = 0.936 \pm 0.040 M_Sun and a stellar radius of R_star = 0.850 \pm 0.018 R_Sun. We rule out the possibility that the transits result from an astrophysical false positive by first identifying the subset of stellar blends that reproduce the precise shape of the light curve. We conclude that the planetary scenario is more than three orders of magnitude more likely than a blend. The blend scenario is independently disfavored by the achromaticity of the transit: we measure a transit depth with Spitzer at 4.5 μm of 547+113-110 ppm, consistent with the depth measured in the Kepler optical bandpass of 567\pm6 ppm. We determine a physical radius of the planet Kepler-19b of R_p = 2.209 \pm 0.048 R_Earth. From radial-velocity observations of the star, we find an upper limit on the planet mass of 20.3 M_Earth, corresponding to a maximum density of 10.4 g cm^-3. We report a significant sinusoidal deviation of the transit times from a predicted linear ephemeris, which we conclude is due to an additional perturbing body in the system. We cannot uniquely determine the orbital parameters of the perturber, as various dynamical mechanisms match the amplitude, period, and shape of the transit timing signal and satisfy the host star's radial velocity limits. However, the perturber in these mechanisms has period <160 days and mass <6 M_Jup, confirming its planetary nature as Kepler-19c. [Abridged]

astro-ph.EP↗

arXiv2011

Observational evidence for a metal rich atmosphere on the super-Earth GJ1214b

We report observations of two consecutive transits of the warm super-Earth exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera instrument on-board the Spitzer Space Telescope. The two transit light curves allow for the determination of the transit parameters for this system. We find these paremeters to be consistent with the previously determined values and no evidence for transit timing variations. The main investigation consists of measuring the transit depths in each bandpass to constrain the planet's transmission spectrum. Fixing the system scale and impact parameters, we measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6 and 4.5 microns, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical yields constraints on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free, solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma confidence. This independently confirms a recent finding that was based on a measurement of the planet's transmission spectrum using the VLT. The Spitzer, MEarth, and VLT observations together yield a remarkably flat transmission spectrum over the large wavelength domain spanned by the data. Consequently, cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H2O by volume to be consistent with all the observations.

astro-ph.EP↗