124 research outputs found
3.8-Micron Photometry During the Secondary Eclipse of the Extrasolar Planet HD 209458b
We report infrared photometry of the extrasolar planet HD 209458b during the
time of secondary eclipse (planet passing behind the star). Observations were
acquired during two secondary eclipses at the NASA Infrared Telescope Facility
(IRTF) in September 2003. We used a circular variable filter (1.5-percent
bandpass) centered at 3.8 microns to isolate the predicted flux peak of the
planet at this wavelength. Residual telluric absorption and instrument
variations were removed by offsetting the telescope to nearby bright comparison
stars at a high temporal cadence. Our results give a secondary eclipse depth of
0.0013 +/- 0.0011, not yet sufficient precision to detect the eclipse, whose
expected depth is approximately 0.002 - 0.003. We here elucidate the current
observational limitations to this technique, and discuss the approach needed to
achieve detections of hot Jupiter secondary eclipses at 3.8 microns from the
ground.Comment: 5 pages, 5 figures, in press for MNRA
A Ground-Based Search for Thermal Emission from the Exoplanet TrES-1
Eclipsing planetary systems give us an important window on extrasolar planet
atmospheres. By measuring the depth of the secondary eclipse, when the planet
moves behind the star, we can estimate the strength of the thermal emission
from the day side of the planet. Attaining a ground-based detection of one of
these eclipses has proven to be a significant challenge, as time-dependent
variations in instrument throughput and atmospheric seeing and absorption
overwhelm the small signal of the eclipse at infrared wavelengths. We gathered
a series of simultaneous L grism spectra of the transiting planet system TrES-1
and a nearby comparison star of comparable brightness, allowing us to correct
for these effects in principle. Combining the data from two eclipses, we
demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to
the stellar flux. This approaches the sensitivity required to detect the
planetary emission, which theoretical models predict should lie between
0.05-0.1% of the stellar flux in our 2.9-4.3 micron bandpass. We explore the
factors that ultimately limit the precision of this technique, and discuss
potential avenues for future improvements.Comment: 10 pages, 1 table, four figures, accepted for publication in PAS
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.Comment: 14 pages, 3 figures, accepted for Ap
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 0.07 R_\sun, a planetary radius of R = 1.26 0.08
R, 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.Comment: 18 pages, 3 figures, Accepted to Astrophysical Journa
Non-detection of L-band Line Emission from the Exoplanet HD189733b
We attempt to confirm bright non-local thermodynamic equilibrium (non-LTE) emission from the exoplanet HD 189733b at 3.25 μm, as recently reported by Swain et al. based on observations at low spectral resolving power (λ/δλ ≈ 30). Non-LTE emission lines from gas in an exoplanet atmosphere will not be significantly broadened by collisions, so the measured emission intensity per resolution element must be substantially brighter when observed at high spectral resolving power. We observed the planet before, during, and after a secondary eclipse event at a resolving power λ/δλ = 27, 000 using the NIRSPEC spectrometer on the Keck II telescope. Our spectra cover a spectral window near the peak found by Swain et al., and we compare emission cases that could account for the magnitude and wavelength dependence of the Swain et al. result with our final spectral residuals. To model the expected line emission, we use a general non-equilibrium formulation to synthesize emission features from all plausible molecules that emit in this spectral region. In every case, we detect no line emission to a high degree of confidence. After considering possible explanations for the Swain et al. results and the disparity with our own data, we conclude that an astrophysical source for the putative non-LTE emission is unlikely. We note that the wavelength dependence of the signal seen by Swain et al. closely matches the 2ν_2 band of water vapor at 300 K, and we suggest that an imperfect correction for telluric water is the source of the feature claimed by Swain et al
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