arXiv subjects

Joseph Harrington

Publications and source records attributed to Joseph Harrington.

66 records · Page 4Linked to original sources

arXiv2006

A Spitzer Infrared Radius for the Transiting Extrasolar Planet HD209458b

We have measured the infrared transit of the extrasolar planet HD209458b using the Spitzer Space Telescope. We observed two primary eclipse events (one partial and one complete transit) using the 24 micron array of the Multiband Imaging Photometer for Spitzer (MIPS). We analyzed a total of 2392 individual images (10-second integrations) of the planetary system, recorded before, during, and after transit. We perform optimal photometry on the images and use the local zodiacal light as a short-term flux reference. At this long wavelength, the transit curve has a simple box-like shape, allowing robust solutions for the stellar and planetary radii independent of stellar limb darkening, which is negligible at 24 microns. We derive a stellar radius of R$_*$ = 1.06 $\pm$ 0.07 R$_\sun$, a planetary radius of R$_p$ = 1.26 $\pm$ 0.08 R$_J$, and a stellar mass of 1.17 M$_\sun$. Within the errors, our results agree with the measurements at visible wavelengths. The 24-micron radius of the planet therefore does not differ significantly compared to the visible result. We point out the potential for deriving extrasolar transiting planet radii to high accuracy using transit photometry at slightly shorter IR wavelengths where greater photometric precision is possible.

astro-ph↗

arXiv2006

A Method To Remove Fringes From Images Using Wavelets

We have developed a new method that uses wavelet analysis to remove interference fringe patterns from images. This method is particularly useful for flat fields in the common case where fringes vary between the calibration and object data. We analyze the efficacy of this method by creating fake flats with fictitious fringes and removing the fringes. We find that the method removes 90% of the fringe pattern if its amplitude is equal to the random noise level and 60% if the fringe amplitude is $\approx 1/10$ of the noise level. We also present examples using real flat field frames. A routine written in the Interactive Data Language (IDL) that implements this algorithm is available from the authors and as an attachment to this paper.

astro-ph↗

arXiv2006

Shoemaker-Levy 9 Impact Modeling: I. High-Resolution 3D Bolides

We have run high-resolution, three-dimensional, hydrodynamic simulations of the impact of comet Shoemaker-Levy 9 into the atmosphere of Jupiter. We find that the energy deposition profile is largely similar to the previous two-dimensional calculations of Mac Low and Zahnle (1994), though perhaps somewhat broader in the range of height over which the energy is deposited. As with similar calculations for impacts into the Venusian atmosphere, there is considerable sensitivity in the results to small changes in the initial conditions, indicating dynamical chaos. We calculated the median depth of energy deposition (the height z at which 50% of the bolide's energy has been released) per run. The mean value among runs is approx 70 km below the 1-bar level, for a 1-km diameter impactor of porous ice of density rho=0.6 g cm^{-3}. The standard deviation among these runs is 14 km. We find little evidence of a trend in these results with the resolution of the calculations (up to 57 cells across the impactor radius, or 8.8-m resolution), suggesting that resolutions as low as 16 grid cells across the radius of the bolide may yield good results for this particular quantity.

astro-ph↗

arXiv2006

Strong Infrared Emission from the Extrasolar Planet HD189733b

We report detection of strong infrared thermal emission from the nearby (d=19 pc) transiting extrasolar planet HD189733b, by measuring the flux decrement during its prominent secondary eclipse. A 6-hour photometric sequence using Spitzer's infrared spectrograph in peak-up imaging mode at 16-microns shows the secondary eclipse depth to be 0.551 +/-0.030%, with accuracy limited by instrumental baseline uncertainties, but with 32-sigma precision (0.017%) on the detection. The 16-micron brightness temperature of this planet (1117+/-42K) is very similar to the Spitzer detections of TrES-1 and HD209458b, but the observed planetary flux (660 micro-Jy) is an order of magnitude greater. This large signal will allow a detailed characterization of this planet in the infrared. Our photometry has sufficient signal-to-noise (~400 per point) to motivate a search for structure in the ingress/egress portions of the eclipse curve, caused by putative thermal structure on the disk of the planet. We show that by binning our 6-second sampling down to 6-minute resolution, we detect the modulation in the intensity derivative during ingress/egress due to the overall shape of the planet, but our sensitivity is not yet sufficient to distinguish between realistic models of the temperature distribution across the planet's disk. We point out the potential for extending Spitzer secondary eclipse detections down to the regime of transiting hot Neptunes, if such systems are discovered among nearby lower main sequence stars.

astro-ph↗

arXiv2005

Infrared Radiation from Hot Jupiters

Recent Spitzer infrared (IR) observations of two transiting hot Jupiters during their secondary eclipses have provided the first direct detection of planets orbiting other stars (Charbonneau et al. 2005; Deming et al. 2005). We here elaborate on some aspects of our detection of HD209458b at 24 microns, and we compare to the detection of TrES-1 by Charbonneau et al. Spitzer will eventually determine the IR spectral energy distribution of these and similar hot Jupiters, opening the new field of comparative exoplanetology. For now, we have only three Spitzer data points, augmented by upper limits from the ground. We here interpret the available measurements from a purely observational perspective, and we point out that a blackbody spectrum having T=1100K can account for all current IR measurements, within the errors. This will surely not remain true for long, since ongoing Spitzer observations will be very sensitive to the IR characteristics of hot Jupiters.

astro-ph↗

arXiv2005

Infrared radiation from an extrasolar planet

A class of extrasolar giant planets - the so-called `hot Jupiters' - orbit within 0.05 AU of their primary stars. These planets should be hot and so emit detectable infrared radiation. The planet HD 209458b is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter), which may be the result of ongoing tidal dissipation, but this explanation requires a non-zero orbital eccentricity (~0.03), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 micron) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24 micron flux is 55 +/- 10 micro-Jy (1 sigma), with a brightness temperature of 1130 +/- 150 Kelvins, confirming the predicted heating by stellar irradiation. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within +/- 7 min, 1 sigma), which means that a dynamically significant orbital eccentricity is unlikely.

astro-ph↗

arXiv2004

A new Search for Carbon Monoxide Absorption in the Transmission Spectrum of the Extrasolar Planet HD 209458b

We have revisited the search for carbon monoxide absorption features in transmission during the transit of the extrasolar planet HD 209458b. We acquired 1077 high resolution spectra at 2 microns using NIRSPEC on Keck II during three transits. Our sensitivity is sufficient to test the degree of CO absorption in the first overtone bands during transit, based on plausible models of the planetary atmosphere. We compare to theoretical tangent geometry absorption spectra, computed by adding height-invariant ad hoc temperature pertubations to the model atmosphere of Sudarsky et al., and by treating cloud height as an adjustable parameter. We do not detect CO absorption. Our analysis indicates a weakening similar to the case of sodium, suggesting that a general masking mechanism is at work in the planetary atmosphere. If this masking is provided by high clouds, our analysis defines the maximum cloud top pressure (i.e., minimum height) as a function of the model atmospheric temperature. For the relatively hot model used by Charbonneau et al. to interpret their sodium detection, our CO limit requires cloud tops at or above 3.3 mbar, and these clouds must be opaque at a wavelength of 2 microns. High clouds comprised of submicron-sized particles are already present in some models, but may not provide sufficient opacity to account for our CO result. Cooler model atmospheres, having smaller atmospheric scale heights and lower CO mixing ratios, may alleviate this problem to some extent. However, even models 500K cooler that the Sudarsky et al. model require clouds above the 100 mbar level to be consistent with our observations. Our null result therefore requires that clouds exist at an observable level in the atmosphere of HD 209458b, unless this planet is dramatically colder than current belief.

astro-ph↗

arXiv2003

Spectroscopy of Molecular Hydrogen Emission from KH 15D

We report infrared spectroscopy of the unusual eclipsing pre-main sequence object KH 15D, obtained using NIRSPEC on Keck II. During eclipse, observations using low spectral resolution (1000) reveal the presence of prominent molecular hydrogen emission in 5 lines near 2 microns. The relative line strengths are consistent with thermal excitation at T = 2800 +/-300K. Observations out of eclipse, at both low and high spectral resolution (20,000), show reduced contrast with the stellar continuum. The change in contrast for the strongest line, 1-0 S(1), is consistent with an approximately constant emission line superposed on a variable stellar continuum. Emission in the 1-0 S(1) line is observed to extend by more than 4 arc-sec both east and west of the stellar point spread function (more than 3000 AU). Observed at high spectral resolution, the velocity and intensity structure of the 1-0 S(1) profile are both asymmetric. East of the stellar PSF (by 1.1 - 2.3 arc-sec) the emission is blueshifted (-63 km/sec), and has significantly greater intensity than the marginally redshifted component (+2 km/sec, consistent with zero) which dominates west of the stellar PSF. The spatial extent of the emission, and the excitation temperature, suggest shock-excitation of ambient gas by a bipolar outflow from the star and/or disk. However, it is difficult to account for the observed radial velocity unless the outflow axis is inclined significantly to the plane of the sky.

astro-ph↗

arXiv2002

Infrared Observations During the Secondary Eclipse of HD 209458b: I. 3.6-Micron Occultation Spectroscopy Using the VLT

We search for an infrared signature of the transiting extrasolar planet HD 209458b during secondary eclipse. Our method, which we call `occultation spectroscopy,' searches for the disappearance and reappearance of weak spectral features due to the exoplanet as it passes behind the star and later reappears. We argue that at the longest infrared wavelengths, this technique becomes preferable to conventional `transit spectroscopy'. We observed the system in the wing of the strong nu-3 band of methane near 3.6 microns during two secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution of 3300. Our analysis, which utilizes a model template spectrum, achieves sufficient precision to expect detection of the spectral structure predicted by an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium model for the exoplanet atmosphere. However, our observations show no evidence for the presence of this spectrum from the exoplanet, with the statistical significance of the non-detection depending on the timing of the secondary eclipse, which depends on the assumed value for the orbital eccentricity. Our results reject certain specific models of the atmosphere of HD 209458b as inconsistent with our observations at the 3-sigma level, given assumptions about the stellar and planetary parameters.

astro-ph↗

arXiv2001

Models of the SL9 Impacts II. Radiative-hydrodynamic Modeling of the Plume Splashback

We model the plume "splashback" phase of the SL9 collisions with Jupiter using the ZEUS-3D hydrodynamic code. We modified the Zeus code to include gray radiative transport, and we present validation tests. We couple the infalling mass and momentum fluxes of SL9 plume material (from paper I) to a jovian atmospheric model. A strong and complex shock structure results. The modeled shock temperatures agree well with observations, and the structure and evolution of the modeled shocks account for the appearance of high excitation molecular line emission after the peak of the continuum light curve. The splashback region cools by radial expansion as well as by radiation. The morphology of our synthetic continuum light curves agree with observations over a broad wavelength range (0.9 to 12 microns). A feature of our ballistic plume is a shell of mass at the highest velocities, which we term the "vanguard". Portions of the vanguard ejected on shallow trajectories produce a lateral shock front, whose initial expansion accounts for the "third precursors" seen in the 2-micron light curves of the larger impacts, and for hot methane emission at early times. Continued propagation of this lateral shock approximately reproduces the radii, propagation speed, and centroid positions of the large rings observed at 3-4 microns by McGregor et al. The portion of the vanguard ejected closer to the vertical falls back with high z-component velocities just after maximum light, producing CO emission and the "flare" seen at 0.9 microns. The model also produces secondary maxima ("bounces") whose amplitudes and periods are in agreement with observations.

astro-ph↗

arXiv2001

Models of the SL9 Impacts I. Ballistic Monte-Carlo Plume

We model the Comet Shoemaker-Levy 9 - Jupiter impact plumes to calculate synthetic plume views, atmospheric infall fluxes, and debris patterns. Our plume is a swarm of ballistic particles with one of several mass-velocity distributions (MVD). The swarm is ejected instantaneously and uniformly into a cone from its apex. Upon falling to the ejection altitude, particles slide with horizontal deceleration following one of several schemes. The model ignores hydrodynamic and Coriolis effects. We adjust plume tilt, opening angle, and minimum velocity, and choose MVD and sliding schemes, to create impact patterns that match observations. Our best match uses the power-law MVD from the numerical impact model of Zahnle and Mac Low, with velocity cutoffs at 4.5 and 11.8 km/sec, cone opening angle of 75 degrees, cone tilt of 30 degrees from vertical, and a sliding constant deceleration of 1.74 m/sec^2. A mathematically-derived feature of Zahnle and Mac Low's published cumulative MVD is a thin shell of mass at the maximum velocity, corresponding to the former atmospheric shock front. This vanguard contains 22% of the mass and 45% of the energy of the plume, and accounts for several previously-unexplained observations, including the large, expanding ring seen at 3.2 microns by McGregor et al. and the ``third precursors'' and ``flare'' seen near 300 and 1000 sec, respectively, in the infrared lightcurves. We present synthetic views of the plumes in flight and after landing and derive infall fluxes of mass, energy, and vertical momentum as a function of time and position on the surface. These fluxes initialize a radiative-hydrodynamic atmosphere model (Paper II of this series) that calculates the thermal and dynamical response of the atmosphere and produces synthetic lightcurves.

astro-ph↗

arXiv2001

The Optical/Infrared Astronomical Quality of High Atacama Sites. I. Preliminary Results of Optical Seeing

The region surrounding the Llano de Chajnantor, a high altitude plateau in the Atacama Desert in northern Chile, has caught the attention of the astronomical community for its potential as an observatory site. Combining high elevation and extremely low atmospheric water content, the Llano has been chosen as the future site of the Atacama Large Millimeter Array. We have initiated a campaign to investigate the astronomical potential of the region in the optical/infrared. Here, we report on an aspect of our campaign aimed at establishing a seeing benchmark to be used as a reference for future activities in the region. After a brief description of the region and its climate, we describe the results of an astronomical seeing campaign, carried out with a Differential Image Motion Monitor that operates at 0.5 micron wavelength. The seeing at the Llano level of 5000 m, measured over 7 nights in May 1998, yielded a median FWHM of 1.1". However, the seeing decreased to 0.7" at a modest 100 m gain above the plateau (Cerro Chico), as measured over 38 nights spread between July 1998 and October 2000. Neither of these represents the best seeing expected in the region; the set of measurements provides a reference base for simultaneous dual runs at Cerro Chico and at other sites of interest in the region, currently underway. A comparison between simultaneous measurements at Cerro Chico and Cerro Paranal indicates that seeing at Cerro Chico is about 12% better than at Paranal. The percentage of optically photometric nights in the Chajnantor region is about 60%, while that of nights useful for astronomical work is near 80%.

astro-ph↗