arXiv subjects

Daniel Fabrycky

Publications and source records attributed to Daniel Fabrycky.

48 records · Page 3Linked to original sources

arXiv2011

Determining Eccentricities of Transiting Planets: A Divide in the Mass-Period Plane

The two dominant features in the distribution of orbital parameters for close-in exoplanets are the prevalence of circular orbits for very short periods, and the observation that planets on closer orbits tend to be heavier. The first feature is interpreted as a signature of tidal evolution, while the origin of the second, a "mass-period relation" for hot Jupiters, is not understood. In this paper we re-consider the ensemble properties of transiting exoplanets with well-measured parameters, focussing on orbital eccentricity and the mass-period relation. We recalculate the constraints on eccentricity in a homogeneous way, using new radial-velocity data, with particular attention to statistical biases. We find that planets on circular orbits gather in a well-defined region of the mass-period plane, close to the minimum period for any given mass. Exceptions to this pattern reported in the Literature can be attributed to statistical biases. The ensemble data is compatible with classical tide theory with orbital circularisation caused by tides raised on the planet, and suggest that tidal circularisation and the stopping mechanisms for close-in planets are closely related to each other. The position mass-period relation is compatible with a relation between a planet's Hill radius and its present orbit.

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arXiv2011

The Transit Light Curve Project. XIII. Sixteen Transits of the Super-Earth GJ 1214b

We present optical photometry of 16 transits of the super-Earth GJ 1214b, allowing us to refine the system parameters and search for additional planets via transit timing. Starspot-crossing events are detected in two light curves, and the star is found to be variable by a few percent. Hence, in our analysis, special attention is given to systematic errors that result from star spots. The planet-to-star radius ratio is 0.11610+/-0.00048, subject to a possible upward bias by a few percent due to the unknown spot coverage. Even assuming this bias to be negligible, the mean density of planet can be either 3.03+/-0.50 g cm^{-3} or 1.89+/-0.33 g cm^{-3}, depending on whether the stellar radius is estimated from evolutionary models or from an empirical mass-luminosity relation combined with the light curve parameters. One possible resolution is that the orbit is eccentric (e approximately equal to 0.14), which would favor the higher density, and hence a much thinner atmosphere for the planet. The transit times were found to be periodic within about 15s, ruling out the existence of any other super-Earths with periods within a factor-of-two of the known planet.

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arXiv2010

A Third Hot White Dwarf Companion Detected by Kepler

We have found a system listed in the Kepler Binary Catalog (3.273 day period; Prsa et al. 2010) that we have determined is comprised of a low-mass, thermally-bloated, hot white dwarf orbiting an A star of about 2.3 solar masses. In this work we designate the object, KIC 10657664, simply as KHWD3. 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 and illumination effects, and the newly utilized Doppler boosting 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_2/M_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 0.18 +/- 0.03 M_Sun or 0.37 +/- 0.08 M_Sun). 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_Sun which is extremely bloated over the radius it would have as a fully degenerate object, and an effective temperature T_eff = 14,100 +/- 350 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_Sun (the progenitor of the current hot white dwarf) and a secondary of mass ~1.4 M_Sun (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 Regulus).

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arXiv2010

A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations

We discovered evidence for a possible additional 0.75 R_Earth transiting planet in the NASA EPOXI observations of the known M dwarf exoplanetary system GJ 436. Based on an ephemeris determined from the EPOXI data, we predicted a transit event in an extant Spitzer Space Telescope 8 micron data set of this star. Our subsequent analysis of those Spitzer data confirmed the signal of the predicted depth and at the predicted time, but we found that the transit depth was dependent on the aperture used to perform the photometry. Based on these suggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5 microns spanning a time of transit predicted from the EPOXI and Spitzer 8 micron candidate events. The 4.5 micron data permit us to rule out a transit at high confidence, and we conclude that the earlier candidate transit signals resulted from correlated noise in the EPOXI and Spitzer 8 micron observations. In the course of this investigation, we developed a novel method for correcting the intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of the Infrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of Warm Spitzer observations of M dwarfs to confirm sub-Earth sized planets. Our analysis will inform similar work that will be undertaken to use Warm Spitzer observations to confirm rocky planets discovered by the Kepler mission.

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arXiv2010

High-Contrast 3.8 Micron Imaging Of The Brown Dwarf/Planet-Mass Companion to GJ 758

We present L' band (3.8 $μm$) MMT/Clio high-contrast imaging data for the nearby star GJ 758, which was recently reported by Thalmann et al. (2009) to have one -- possibly two-- faint comoving companions (GJ 758B and ``C", respectively). GJ 758B is detected in two distinct datasets. Additionally, we report a \textit{possible} detection of the object identified by Thalmann et al as ``GJ 758C" in our more sensitive dataset, though it is likely a residual speckle. However, if it is the same object as that reported by Thalmann et al. it cannot be a companion in a bound orbit. GJ 758B has a H-L' color redder than nearly all known L--T8 dwarfs. Based on comparisons with the COND evolutionary models, GJ 758B has T$_{e}$ $\sim$ 560 K$^{^{+150 K}_{-90K}}$ and a mass ranging from $\sim$ 10--20 M$_{J}$ if it is $\sim$ 1 Gyr old to $\sim$ 25--40 M$_{J}$ if it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e $\sim$ 0.73$^{^{+0.12}_{-0.21}}$, with a semimajor axis of $\sim$ 44 AU$^{^{+32 AU}_{-14 AU}}$. Though GJ 758B is sometimes discussed within the context of exoplanet direct imaging, its mass is likely greater than the deuterium-burning limit and its formation may resemble that of binary stars rather than that of jovian-mass planets.

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arXiv2010

Hot Stars with Hot Jupiters Have High Obliquities

We show that stars with transiting planets for which the stellar obliquity is large are preferentially hot (T_eff > 6250 K). This could explain why small obliquities were observed in the earliest measurements, which focused on relatively cool stars drawn from Doppler surveys, as opposed to hotter stars that emerged later from transit surveys. The observed trend could be due to differences in planet formation and migration around stars of varying mass. Alternatively, we speculate that hot-Jupiter systems begin with a wide range of obliquities, but the photospheres of cool stars realign with the orbits due to tidal dissipation in their convective zones, while hot stars cannot realign because of their thinner convective zones. This in turn would suggest that hot Jupiters originate from few-body gravitational dynamics, and that disk migration plays at most a supporting role.

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arXiv2010

A Search for Additional Planets in the NASA EPOXI Observations of the Exoplanet System GJ 436

We present time series photometry of the M dwarf transiting exoplanet system GJ 436 obtained with the the EPOCh (Extrasolar Planet Observation and Characterization) component of the NASA EPOXI mission. We conduct a search of the high-precision time series for additional planets around GJ 436, which could be revealed either directly through their photometric transits, or indirectly through the variations these second planets induce on the transits of the previously known planet. In the case of GJ 436, the presence of a second planet is perhaps indicated by the residual orbital eccentricity of the known hot Neptune companion. We find no candidate transits with significance higher than our detection limit. From Monte Carlo tests of the time series, we rule out transiting planets larger than 1.5 R_Earth interior to GJ 436b with 95% confidence, and larger than 1.25 R_Earth with 80% confidence. Assuming coplanarity of additional planets with the orbit of GJ 436b, we cannot expect that putative planets with orbital periods longer than about 3.4 days will transit. However, if such a planet were to transit, we rule out planets larger than 2.0 R_Earth with orbital periods less than 8.5 days with 95% confidence. We also place dynamical constraints on additional bodies in the GJ 436 system. Our analysis should serve as a useful guide for similar analyses for which radial velocity measurements are not available, such as those discovered by the Kepler mission. These dynamical constraints on additional planets with periods from 0.5 to 9 days rule out coplanar secular perturbers as small as 10 M_Earth and non-coplanar secular perturbers as small as 1 M_Earth in orbits close in to GJ 436b. We present refined estimates of the system parameters for GJ 436. We also report a sinusoidal modulation in the GJ 436 light curve that we attribute to star spots. [Abridged]

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arXiv2009

On the Spin-Orbit Misalignment of the XO-3 Exoplanetary System

We present photometric and spectroscopic observations of the 2009 Feb. 2 transit of the exoplanet XO-3b. The new data show that the planetary orbital axis and stellar rotation axis are misaligned, as reported earlier by Hebrard and coworkers. We find the angle between the sky projections of the two axes to be 37.3 +/- 3.7 deg, as compared to the previously reported value of 70 +/- 15 deg. The significance of this discrepancy is unclear because there are indications of systematic effects. XO-3b is the first exoplanet known to have a highly inclined orbit relative to the equatorial plane of its parent star, and as such it may fulfill the predictions of some scenarios for the migration of massive planets into close-in orbits. We revisit the statistical analysis of spin-orbit alignment in hot-Jupiter systems. Assuming the stellar obliquities to be drawn from a single Rayleigh distribution, we find the mode of the distribution to be 13^{+5}_{-2} deg. However, it remains the case that a model representing two different migration channels--in which some planets are drawn from a perfectly-aligned distribution and the rest are drawn from an isotropic distribution--is favored over a single Rayleigh distribution.

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arXiv2008

Radiative Thrusters on Close-in Extrasolar Planets

The atmospheres of close-in extrasolar planets absorb most of the incident stellar radiation, advect this energy, then reradiate photons in preferential directions. Those photons carry away momentum, applying a force on the planet. Here we evaluate the resulting secular changes to the orbit, known as the Yarkovsky effect. For known transiting planets, typical fractional changes in semi-major axis are about 1% over their lifetime, but could be up to ~5% for close-in planets like OGLE-TR-56b or inflated planets like TrES-4. We discuss the origin of the correlation between semi-major axis and surface gravity of transiting planets in terms of various physical processes, finding that radiative thrusters are too weak by about a factor of 10 to establish the lower boundary that causes the correlation.

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arXiv2007

Shrinking binary and planetary orbits by Kozai cycles with tidal friction

At least two arguments suggest that the orbits of a large fraction of binary stars and extrasolar planets shrank by 1-2 orders of magnitude after formation: (i) the physical radius of a star shrinks by a large factor from birth to the main sequence, yet many main-sequence stars have companions orbiting only a few stellar radii away, and (ii) in current theories of planet formation, the region within ~0.1 AU of a protostar is too hot and rarefied for a Jupiter-mass planet to form, yet many "hot Jupiters" are observed at such distances. We investigate orbital shrinkage by the combined effects of secular perturbations from a distant companion star (Kozai oscillations) and tidal friction. We integrate the relevant equations of motion to predict the distribution of orbital elements produced by this process. Binary stars with orbital periods of 0.1 to 10 days, with a median of ~2 d, are produced from binaries with much longer periods (10 d to 10^5 d), consistent with observations indicating that most or all short-period binaries have distant companions (tertiaries). We also make two new testable predictions: (1) For periods between 3 and 10 d, the distribution of the mutual inclination between the inner binary and the tertiary orbit should peak strongly near 40 deg and 140 deg. (2) Extrasolar planets whose host stars have a distant binary companion may also undergo this process, in which case the orbit of the resulting hot Jupiter will typically be misaligned with the equator of its host star.

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arXiv2006

Catalog of fundamental mode RR Lyrae stars in the Galactic bulge from the Optical Gravitational Lensing Experiment

We present a catalog of 1888 fundamental-mode RR Lyrae stars detected in the Galactic bulge fields of the second phase of the Optical Gravitational Lensing Experiment (OGLE). The catalog includes basic parameters of the light curves, identifications of Blazhko frequencies, V-I colors at minimum light (for most stars), and other information for each star. We detect a high rate of incidence of the Blazhko phenomenon (at least 27.6%), including unprecedentedly many frequency triplets, which we attribute to our sensitive search method. We find that the minimum light V-I color (useful as a reddening indicator) grows slowly redder with increasing period and exhibits a star-to-star scatter of approximately 0.07 mag. We use this color to evaluate the zero-point accuracy of the reddening map of the Galactic bulge derived from OGLE data, and find that in addition to low-level random errors or resolution effects (responsible for much of the scatter), the map may systematically over-represent E(V-I) by approximately 0.05 mag in most fields. We present reasonably robust evidence that the RR Lyrae-to-red clump color separation is larger by 0.05--0.08 mag in the bulge than locally, which argues for caution in the use of these stars for reddening determinations. We consider the RR Lyrae constraint on the Galactocentric distance, but uncertainty about the absolute magnitude calibration leaves significant flexibility in the result. In contrast to previous results, we robustly detect the signature of the Galactic bar in the RR Lyrae population within the inner plus/minus 3 degrees of longitude, and we highlight the apparent differences between the structures traced by the red clump giants and the more metal-poor RR Lyrae stars. (abridged)

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arXiv2004

Rapid multiperiodic variability in a high-mass X-ray binary

Positions of High-Mass X-ray Binaries are often known precisely enough to unambiguously identify the optical component, and a number of those stars are monitored by the OGLE and MACHO collaborations. The light curves of two such candidates are examined for evidence of Be star behavior and for periodicity. One of the stars exhibits two periods of 6.833 and 15.586 hours, much shorter and more stable than periods of Be/X-Ray Binaries that are attributed to the Be star's disk, but consistent with short-term Be variability attributed to pulsations. The multiperiodicity is quantified with Fourier techniques and examined for phase stability; a combination of radial and non-radial pulsations is discussed.

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