774 research outputs found
Empirical Constraints on the Oblateness of an Exoplanet
No full textWe show that the gas giant exoplanet HD 189733b is less oblate than Saturn, based on Spitzer Space Telescope
photometry of seven transits. The observablemanifestations of oblatenesswould have been slight anomalies
during the ingress and egress phases, as well as variations in the transit depth due to spin precession. Our
nondetection of these effects gives the first empirical constraints on the shape of an exoplanet. The results are
consistent with the theoretical expectation that the planetary rotation period and orbital period are synchronized,
in which case the oblateness would be an order of magnitude smaller than our upper limits. Conversely,
if HD 189733b is assumed to be in a synchronous, zero-obliquity state, then the data give an upper bound
on the quadrupole moment of the planet (J[subscript 2] < 0.068 with 95% confidence) that is too weak to constrain the
interior structure of the planet. An Appendix describes a fast algorithm for computing the transit light curve of
an oblate planet, which was necessary for our analysis
The Detectability Of Transit Depth Variations Due To Exoplanetary Oblateness And Spin Precession
No full textKnowledge of an exoplanet's oblateness and obliquity would give clues about its formation and internal structure. In principle, a light curve of a transiting planet bears information about the planet's shape, but previous work has shown that the oblateness-induced signal will be extremely difficult to detect. Here, we investigate the potentially larger signals due to planetary spin precession. The most readily detectable effects are transit depth variations (T[delta]V's) in a sequence of light curves. For a planet as oblate as Jupiter or Saturn, the transit depth will undergo fractional variations of order 1%. The most promising systems are those with orbital periods of approximately 15-30 days, which are short enough for the precession period to be less than about 40 yr and long enough to avoid spin-down due to tidal friction. The detectability of the TδV signal would be enhanced by moons (which would decrease the precession period) or planetary rings (which would increase the amplitude). The Kepler mission should find several planets for which precession-induced T[delta]V signals will be detectable. Due to modeling degeneracies, Kepler photometry would yield only a lower bound on oblateness. The degeneracy could be lifted by observing the oblateness-induced asymmetry in at least one transit light curve or by making assumptions about the planetary interior
Analysis of exoplanetary transit light curves
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.Cataloged from PDF version of thesis.Includes bibliographical references.This Thesis considers the scenario in which an extra-solar planet (exoplanet) passes in front of its star relative to our observing perspective. In this event, the light curve measured for the host star features a systematic drop in flux occurring once every orbital period as the exoplanet covers a portion of the stellar disk. This exoplanetary transit light curve provides a wealth of information about both the planet and star. In this Thesis we consider the transit light curve as a tool for characterizing the exoplanet. The Thesis can divided into two parts. In the first part, comprised of the second and third chapters, I assess what observables describing the exoplanet (and host) may be measured, how well they can be measured, and what effect systematics in the light curve can have on our estimation of these parameters. In particular, we utilize a simplified transit light curve model to produce simple, analytic estimates of parameter values and uncertainties. Later, we suggest a transit parameter estimation technique that properly treats temporally correlated stochastic noise when determining a posteriori parameter distributions. In the second part, comprised of the fourth and fifth chapters, I direct my attention to real exoplanetary transit light curves, primarily for two exoplanets: HD 149026b and HD 189733b. We analyze four transits of the ultra-dense HD 149026b, as measured by an instrument on the Hubble Space Telescope, in an effort to properly constrain the stellar and exoplanetary radius. In addition, we assess a detection of strong, wavelength dependent absorption, possibly due to an unusual atmospheric composition. For HD 189733b, we utilize seven ultra-precise Spitzer Space Telescope transit light curves in an effort to make the first empirical measurement of asphericity in an exoplanet shape. In particular, we constrain the parameters describing an oblate spheriod shape for HD 189733b and, attributing oblateness to rigid-body rotation, we place lower bounds on the rotation period of the exoplanet.by Joshua Adam Carter.Ph.D
PARAMETER ESTIMATION FROM TIME-SERIES DATA WITH CORRELATED ERRORS: A WAVELET-BASED METHOD AND ITS APPLICATION TO TRANSIT LIGHT CURVES
No full textWe consider the problem of fitting a parametric model to time-series data that are afflicted by correlated noise. The noise is represented by a sum of two stationary Gaussian processes: one that is uncorrelated in time, and another that has a power spectral density varying as 1/f [superscript γ]. We present an accurate and fast [O(N)] algorithm for parameter estimation based on computing the likelihood in a wavelet basis. The method is illustrated and tested using simulated time-series photometry of exoplanetary transits, with particular attention to estimating the mid-transit time. We compare our method to two other methods that have been used in the literature, the time-averaging method and the residual-permutation method. For noise processes that obey our assumptions, the algorithm presented here gives more accurate results for mid-transit times and truer estimates of their uncertainties.National Aeronautics and Space Administration Origins progra
A SMALLER RADIUS FOR THE TRANSITING EXOPLANET WASP-10b
No full textWe present the photometry of WASP-10 during a transit of its short-period Jovian planet. We employed the novel point-spread function shaping capabilities of the Orthogonal Parallel Transfer Imaging Camera mounted on the UH 2.2 m telescope to achieve a photometric precision of 4.7 × 10[superscript –4] per 1.3 minute sample. With this new light curve, in conjunction with stellar evolutionary models, we improve on existing measurements of the planetary, stellar, and orbital parameters. We find a stellar radius R sstarf = 0.698 ± 0.012 R sun and a planetary radius RP = 1.080 ± 0.020 R Jup. The quoted errors do not include any possible systematic errors in the stellar evolutionary models. Our measurement improves the precision of the planet's radius by a factor of 4, and revises the previous estimate downward by 16% (2.5σ, where σ is the quadrature sum of the respective confidence limits). Our measured radius of WASP-10b is consistent with previously published theoretical radii for irradiated Jovian planets
ERRATUM: “A SMALLER RADIUS FOR THE TRANSITING EXOPLANET WASP-10b” (2009, ApJ, 692, L100)
No full textWe have identified an error in our Heliocentric Julian Dates (HJDs) of observation caused by incorrect input to the code used to convert from JD to HJD. The times in Table 1 have been corrected by adding 0.006382 day to each entry in the original Column 1. Similarly, the measured mid-transit time in Table 2 has been changed to Tc = 2454664.037295. We also note that the header in Column 1 of Table 1 is incorrect. The label should read HJD, rather than BJD. The updated Tables 1 and 2 have been included herein.
This error has no impact on our main conclusions. We thank Pedro Valdes Sada and Gracjan Maciejewski for pointing out the incorrect mid-transit time
NEAR-INFRARED TRANSIT PHOTOMETRY OF THE EXOPLANET HD 149026b
No full textThe transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure, for the planet's relatively small size has been interpreted as evidence for a highly metal-enriched composition. We present observations of four transits with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope within a wavelength range of 1.1-2.0 μm. Analysis of the light curve gives the most precise estimate yet of the stellar mean density, ρsstarf = 0.497[superscript +0.042] –0.057 g cm[superscript –3]. By requiring agreement between the observed stellar properties (including ρsstarf) and stellar evolutionary models, we refine the estimate of the stellar radius: R sstarf = 1.541[superscript +0.046] –0.042 R sun. We also find a deeper transit than has been measured at optical and mid-infrared wavelengths. Taken together, these findings imply a planetary radius of Rp = 0.813[superscript +0.027] –0.025 R Jup, which is larger than earlier estimates. Models of the planetary interior still require a metal-enriched composition, although the required degree of metal enrichment is reduced. It is also possible that the deeper NICMOS transit is caused by wavelength-dependent absorption by constituents in the planet's atmosphere, although simple model atmospheres do not predict this effect to be strong enough to account for the discrepancy. We use the four newly measured transit times to compute a refined transit ephemeris.Space Telescope Science Institut
Analytic approximations for transit light curve observables, uncertainties, and covariances
No full textThe light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.Space Telescope Science Institut
A THIRD HOT WHITE DWARF COMPANION DETECTED BY KEPLER
No full textWe have found a system listed in the Kepler Binary Catalog (P orb = 3.273 days) that we have determined is comprised of a low-mass, thermally bloated, hot white dwarf orbiting an A star of about 2.3 M ☉. In this work, we designate the object, KIC 10657664, simply as "KHWD3" (Kepler Hot White Dwarf 3). We use the transit depth of ~0.66%, the eclipse depth of ~1.9%, and regular smooth periodic variations at the orbital frequency and twice the orbital frequency to analyze the system parameters. The smooth periodic variations are identified with the classical ellipsoidal light variation (ELV) and illumination (ILL) effects, and the newly utilized Doppler boosting (DB) effect. Given the measured values of R/a and inclination angle of the binary, both the ELV and DB effects are mostly sensitive to the mass ratio, q = M [subscript 2]/M [subscript 1], of the binary. The two effects yield values of q which are somewhat inconsistent—presumably due to unidentified systematic effects—but which nonetheless provide a quite useful set of possibilities for the mass of the white dwarf (either 0.26 ± 0.04 M ☉ or 0.37 ± 0.08 M ☉). All of the other system parameters are determined fairly robustly. In particular, we show that the white dwarf has a radius of 0.15 ± 0.01 R ☉, which is extremely bloated over the radius it would have as a fully degenerate object, and an effective temperature T effsime14,500 K. Binary evolution scenarios and models for this system are discussed. We suggest that the progenitor binary was comprised of a primary of mass ~2.2 M ☉ (the progenitor of the current hot white dwarf) and a secondary of mass ~1.4 M ☉ (the progenitor of the current A star in the system). We compare this new system with three other white dwarfs in binaries that likely were formed via stable Roche-lobe overflow (KOI-74, KOI-81, and the inner Regulus binary).United States. National Aeronautics and Space Administration (NASA Origins of Solar Systems grant no. NNX09AD36G)NASA Exoplanet Science Institute (Michelson Fellowship)Space Telescope Science Institute (U.S.) (Hubble Fellowship grant HF-01210.01-A)Space Telescope Science Institute (U.S.) (Hubble Fellowship grant HF-51272.01-A)United States. National Aeronautics and Space Administration (Association of Universities for Research in Astronomy, Inc., NASA contract no. NAS5-26555
THE BANANA PROJECT. III. SPIN-ORBIT ALIGNMENT IN THE LONG-PERIOD ECLIPSING BINARY NY CEPHEI
No full textBinaries are not always neatly aligned. Previous observations of the DI Her system showed that the spin axes of both stars are highly inclined with respect to one another and the orbital axis. Here, we report on a measurement of the spin-axis orientation of the primary star of the NY Cep system, which is similar to DI Her in many respects: it features two young early-type stars (~6 Myr, B0.5V+B2V), in an eccentric and relatively long-period orbit (e = 0.48, P = 15fd3). The sky projections of the rotation vector and the spin vector are well aligned (beta [subscript p] = 2[degrees] ± 4[degrees]), in strong contrast to DI Her. Although no convincing explanation has yet been given for the misalignment of DI Her, our results show that the phenomenon is not universal, and that a successful theory will need to account for the different outcome in the case of NY Cep.United States. National Aeronautics and Space Administration (Origins grant (NNX09AD36G
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