arXiv subjects

David Charbonneau

Publications and source records attributed to David Charbonneau.

299 records · Page 17Linked to original sources

arXiv2003

Current Challenges Facing Planet Transit Surveys

The initial task that confronted extrasolar-planet transit surveys was to monitor enough stars with sufficient photometric precision and complete phase coverage. Numerous searches have been pursued over the last few years. Among these projects are shallow, intermediate, and deep surveys of the Galactic plane, and monitoring of open clusters, and a globular cluster. These projects have all defeated the initial technical challenge, but a new obstacle has risen in its place: Single-color photometric time series are not sufficient to identify uniquely transiting planet systems, as eclipsing binary stars can mimic the signal. Multicolor photometric time series and multi-epoch spectroscopic monitoring are required to cull the list of candidates prior to high-precision radial velocity monitoring. I also discuss the prospects for detecting another transiting system among the planets found by the radial-velocity method, as well as review the recent announcement of OGLE-TR-56b, the first extrasolar planet detected by the transit method.

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arXiv2002

HD 209458 and the Power of the Dark Side

The rich wealth of observational data, and matching theoretical investigations, of the transiting planet of HD209458 stands in sharp contrast to systems for which only the radial velocity orbit is known. In this paper, I summarize the current status of these observations, and motivate a variety of projects that should be accessible with existing instruments. I describe observational estimates of the planetary radius, and discuss the relevant sources of uncertainty. I compare these estimates to those based on theoretical structural models. This discussion motivates the observational pursuit of three quantities that could be derived from measurements of the secondary eclipse: These are the albedo, the temperature, and the orbital eccentricity. I review the recent detection of the sodium D lines in the planetary atmosphere, and discuss ongoing work to search for molecular features in the near infrared. I also outline the use of the Rossiter effect to study the alignment of the orbit with the stellar equatorial plane, and transit timing to search for additional objects in the system.

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arXiv2002

A Search for CO Absorption in the Transmission Spectrum of HD 209458b

We observed one transit of the extrasolar planet HD 209458b with the NIRSPEC spectrograph on the Keck II telescope. Using time series of low-noise observations in the wavelength range 2.0 micron to 2.5 micron, we searched for extra absorption from the 1st overtone rotation-vibration band of CO near 2.3 micron. This was not detected, with a detection limit that fails to test simple models of the planetary atmosphere by a factor of about 3. Great improvements in the detectability of the CO spectrum features could be realized by observing a transit that is centered near stellar meridian passage, and in better weather. Since it appears that similar observations taken under better circumstances might succeed, we describe our analysis procedures in detail.

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arXiv2001

Detection of an Extrasolar Planet Atmosphere

We report high precision spectrophotometric observations of four planetary transits of HD 209458, in the region of the sodium resonance doublet at 589.3 nm. We find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to simultaneous observations of the transit in adjacent bands. We interpret this additional dimming as absorption from sodium in the planetary atmosphere, as recently predicted from several theoretical modeling efforts. Our model for a cloudless planetary atmosphere with a solar abundance of sodium in atomic form predicts more sodium absorption than we observe. There are several possibilities that may account for this reduced amplitude, including reaction of atomic sodium into molecular gases and/or condensates, photoionization of sodium by the stellar flux, a low primordial abundance of sodium, or the presence of clouds high in the atmosphere.

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arXiv2001

HST Time-Series Photometry of the Transiting Planet of HD 209458

We have observed 4 transits of the planet of HD 209458 using the STIS spectrograph on HST. Summing the recorded counts over wavelength between 582 nm and 638 nm yields a photometric time series with 80 s time sampling and relative precision of about 1.1E-4 per sample. The folded light curve can be fit within observational errors using a model consisting of an opaque circular planet transiting a limb-darkened stellar disk. In this way we estimate the planetary radius R_p = 1.347 +/- 0.060 R_Jup, the orbital inclination i = 86.68 +/- 0.14 degrees, the stellar radius R_* = 1.146 +/- 0.050 R_solar, and one parameter describing the stellar limb darkening. Our estimated radius is smaller than those from earlier studies, but is consistent within measurement errors, and is also consistent with theoretical estimates of the radii of irradiated Jupiter-like planets. Satellites or rings orbiting the planet would, if large enough, be apparent from distortions of the light curve or from irregularities in the transit timings. We find no evidence for either satellites or rings, with upper limits on satellite radius and mass of 1.2 Earth radii and 3 Earth masses, respectively. Opaque rings, if present, must be smaller than 1.8 planetary radii in radial extent. The high level of photometric precision attained in this experiment confirms the feasibility of photometric detection of Earth-sized planets circling Sun-like stars.

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arXiv2000

Multicolor Observations of a Planetary Transit of HD 209458

We present B, V, R, I and Z-band photometric observations of HD 209458 during the transit by its planetary companion on UT 1999 November 15 with the University of Hawaii 0.6m and 2.2m telescopes and the High Altitude Observatory STARE telescope. The detailed shape of the transit curve is predicted to vary with color due primarily to the color-dependent limb-darkening of the star, but potentially due as well to the effect of color-dependent opacity in the planetary atmosphere. We model the light curves and present refined values for the transit timing and orbital period, useful for planning future observations of the planetary transit. We also derive significantly improved measurements of the planetary radius, R_p = 1.55 +/- 0.10 R_Jup, stellar radius, R_s = 1.27 +/- 0.05 R_Sun, and orbital inclination, i = 85.9 +/- 0.5 deg. The derived planetary radius favors evolutionary models in which the planet has a low albedo.

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arXiv2000

The STARE Project: A Transit Search for Hot Jupiters

The STARE instrument is a small aperture, wide-field, CCD-based telescope that delivers high cadence time series photometry on roughly 40,000 stars in a typical field centered on the galactic plane. In a two-month observing run on a field, we obtain sufficient precision on roughly 4,000 stars to detect a close-in Jupiter-sized companion in an edge-on orbit. We also used this instrument to detect the planetary transits across the Sun-like star HD209458. The project is now in its third season, and we have acquired a large dataset on several fields. Given the frequency of close-in extrasolar planets found by the radial velocity surveys, and the recent confirmation that at least some of these are indeed gas giants, the STARE project should be able to detect roughly a dozen Jupiter-sized planets in its existing dataset.

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arXiv2000

Present and Near-Future Reflected Light Searches for Close-In Planets

Close-in extrasolar giant planets may be directly detectable by their reflected light, due to the proximity of the planet to the illuminating star. The spectrum of the system will contain a reflected light component that varies in amplitude and Doppler shift as the planet orbits the star. Intensive searches for this effect have been carried out for only one extrasolar planet system, tau Boo. There exist several other attractive targets, including the transiting planet system HD 209458.

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arXiv1999

Detection of Planetary Transits Across a Sun-like Star

We report high precision, high cadence photometric measurements of the star HD 209458, which is known from radial velocity measurements to have a planetary mass companion in a close orbit. We detect two separate transit events at times that are consistent with the radial velocity measurements. In both cases, the detailed shape of the transit curve due to both the limb darkening of the star and the finite size of the planet is clearly evident. Assuming stellar parameters of 1.1 R_Sun and 1.1 M_Sun, we find that the data are best interpreted as a gas giant with a radius of 1.27 +/- 0.02 R_Jup in an orbit with an inclination of 87.1 +/- 0.2 degrees. We present values for the planetary surface gravity, escape velocity, and average density, and discuss the numerous observations that are warranted now that a planet is known to transit the disk of its parent star.

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arXiv1999

An Upper Limit on the Reflected Light from the Planet Orbiting the Star tau Bootis

The planet orbiting tau Boo at a separation of 0.046 AU could produce a reflected light flux as bright as 1e-4 relative to that of the star. A spectrum of the system will contain a reflected light component which varies in amplitude and Doppler-shift as the planet orbits the star. Assuming the secondary spectrum is primarily the reflected stellar spectrum, we can limit the relative reflected light flux to be less than 5e-5. This implies an upper limit of 0.3 for the planetary geometric albedo near 480 nm, assuming a planetary radius of 1.2 R_Jup. This albedo is significantly less than that of any of the giant planets of the solar system, and is not consistent with certain published theoretical predictions.

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arXiv1998

Spectral Line Distortions in the Presence of a Close-In Planet

We discuss the interpretation of distortions to stellar spectral lines with particular attention to line bisectors in the presence of an orbiting planetary companion. We present a simple model whereby light reflected by the companion can cause temporal variations to the observed line profiles. These distortions have a characteristic signature depending on the inclination angle of the system. For the known close-in extrasolar giant planets, the expected amplitude of the effect might not be far from current detection capabilities. This method could be used to directly detect the presence of the companion, yielding the orbital inclination and hence the planetary mass. Futhermore, a detection would measure a combination of the planetary radius and albedo, from which a minimum radius may be deduced.

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