161 research outputs found
A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope's Wide Field Camera 3 Near-IR Detector and Applications to Transiting Exoplanets and Brown Dwarfs
The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-IR channel
is extensively used in time-resolved observations, especially for transiting
exoplanet spectroscopy and brown dwarf and directly imaged exoplanet rotational
phase mapping. The ramp effect is the dominant source of systematics in the
WFC3 for time-resolved observations, which limits its photometric precision.
Current mitigation strategies are based on empirical fits and require
additional orbits "to help the telescope reach a thermal equilibrium". We show
that the ramp effect profiles can be explained and corrected with high fidelity
using charge trapping theories. We also present a model for this process that
can be used to predict and to correct charge trap systematics. Our model is
based on a very small number of parameters that are intrinsic to the detector.
We find that these parameters are very stable between the different datasets,
and we provide best-fit values. Our model is tested with more than 120 orbits
( visits) of WFC3 observations and is proved to be able to provide near
photon noise limited corrections for observations made with both staring and
scanning modes of transiting exoplanets as well as for starting-mode
observations of brown dwarfs. After our model correction, the light curve of
the first orbit in each visit has the same photometric precision as subsequent
orbits, so data from the first orbit need no longer be discarded. Near IR
arrays with the same physical characteristics (e.g., JWST/NIRCam) may also
benefit from the extension of this model, if similar systematic profiles are
observed.Comment: 16 pages, 13 figures, accepted to Astronomical Journa
Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs
Among the greatest challenges in understanding ultracool brown dwarf and exoplanet atmospheres is the evolution of cloud structure as a function of temperature and gravity. In this study, we present the rotational modulations of GU Psc b—a rare mid-T spectral type planetary-mass companion at the end of the L/T spectral type transition. Based on the Hubble Space Telescope/WFC3 1.1–1.67 μm time-series spectra, we observe a quasi-sinusoidal light curve with a peak-to-trough flux variation of 2.7% and a minimum period of 8 h. The rotation-modulated spectral variations are weakly wavelength-dependent, or largely gray between 1.1 and 1.67 μm. The gray modulations indicate that heterogeneous clouds are present in the photosphere of this low-gravity mid-T dwarf. We place the color and brightness variations of GU Psc b in the context of rotational modulations reported for mid-L to late-T dwarfs. Based on these observations, we report a tentative trend: mid-to-late T dwarfs become slightly redder in J − H color with increasing J-band brightness, while L dwarfs become slightly bluer with increasing brightness. If this trend is verified with more T-dwarf samples, it suggests that in addition to the mostly gray modulations, there is a second-order spectral-type dependence on the nature of rotational modulations
Cloud Atlas: High-precision HST/WFC3/IR Time-resolved Observations of Directly Imaged Exoplanet HD 106906b
HD 106906b is an ~11M_(Jup), ~15 Myr old directly imaged exoplanet orbiting at an extremely large distance from its host star. The wide separation (7 11) between HD 106906b and its host star greatly reduces the difficulty in direct-imaging observations, making it one of the most favorable directly imaged exoplanets for detailed characterization. In this paper, we present HST/WFC3/IR time-resolved observations of HD 106906b in the F127M, F139M, and F153M bands. We have achieved ~1% precision in the lightcurves in all three bands. The F127M lightcurve demonstrates marginally detectable (2.7σ significance) variability with a best-fitting period of 4 hr, while the lightcurves in the other two bands are consistent with flat lines. We construct primary-subtracted deep images and use these images to exclude additional companions to HD 106906 that are more massive than 4 M_(Jup) and locate at projected distances of more than ~500 au. We measure the astrometry of HD 106906b in two HST/WFC3 epochs and achieve precisions better than 2.5 mas. The position angle and separation measurements do not deviate from those in the 2004 HST/ACS/HRC images for more than 1σ uncertainty. We provide the HST/WFC3 astrometric results for 25 background stars that can be used as reference sources in future precision astrometry studies. Our observations also provide the first 1.4 μm water band photometric measurement for HD 106906b. HD 106906b's spectral energy distribution and the best-fitting BT-Settl model have an inconsistency in the 1.4 μm water absorption band, which highlights the challenges in modeling atmospheres of young planetary-mass objects
Recommended from our members
Interim Report on Studies of the Mw ~7.9 Earthquake of 3 May 2006, Kingdom of Tonga
The very large and rare Mw ~7.9 Earthquake of 3 May 2006 in the Kingdom of Tonga aroused great interest among both Tongan scientists and their colleagues in Australia, New Zealand, and the United States. To investigate the earthquake we formed a collaborative research group of scientists from Australia, New Zealand, Tonga, and the United States. We brought in seven seismographs from Australia and the US to supplement the three-station network already in Tonga and eight GPS receivers primarily for the islands west of the earthquake epicenter. In addition, we made coastal observations to determine the regional pattern of subsidence associated with the earthquake. The GPS instruments can measure horizontal and vertical motion quite precisely, but only after the earthquake from the time of deployment onward, except for some sites on Tongatapu, Vava’u, and Lifuka that had been occupied by GPS receivers in the past. This report describes our efforts.Ministry of Lands and Survey, Kingdom of TongaGeoscience AustraliaGeological and Nuclear Sciences, New ZealandThe National Science Foundation, U.S.A.Institute for Geophysic
Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star
Observations of rotational modulations of brown dwarfs and giant exoplanets
allow the characterization of condensate cloud properties. As of now rotational
spectral modulations have only been seen in three L-type brown dwarfs. We
report here the discovery of rotational spectral modulations in LP261-75B, an
L6-type intermediate surface gravity companion to an M4.5 star. As a part of
the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3
grism spectroscopy (1.1--1.69~m) of LP261-75B. We find gray spectral
variations with the relative amplitude displaying only a weak wavelength
dependence and no evidence for lower-amplitude modulations in the 1.4~m
water band than in the adjacent continuum. The likely rotational modulation
period is 4.780.95 h, although the rotational phase is not well sampled.
The minimum relative amplitude in the white light curve measured over the whole
wavelength range is 2.410.14\%. We report an unusual light curve with
seemingly three peaks approximately evenly distributed in rotational phase. The
spectral modulations suggests that the upper atmosphere cloud properties in
{LP261-75B} are similar to two other mid-L dwarfs of typical infrared colors,
but differ from that of the extremely red L-dwarf WISE0047
Cloud Atlas: High-Contrast Time-Resolved Observations of Planetary-Mass Companions
Directly-imaged planetary-mass companions offer unique opportunities in
atmospheric studies of exoplanets. They share characteristics of both brown
dwarfs and transiting exoplanets, therefore, are critical for connecting
atmospheric characterizations for these objects. Rotational phase mapping is a
powerful technique to constrain the condensate cloud properties in ultra-cool
atmospheres. Applying this technique to directly-imaged planetary-mass
companions will be extremely valuable for constraining cloud models in low mass
and surface gravity atmospheres and for determining the rotation rate and
angular momentum of substellar companions. Here, we present Hubble Space
Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three
planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll
differential imaging and hybrid point spread function modeling, we achieve
sub-percent photometric precision for all three observations. We find tentative
modulations () for AB Pic B and 2M0122B but cannot reach
conclusive results on 2M1207b due to strong systematics. The relatively low
significance of the modulation measurements cannot rule out the hypothesis that
these planetary-mass companions have the same vertical cloud structures as
brown dwarfs. Our rotation rate measurements, combined with archival period
measurements of planetary-mass companions and brown dwarfs do not support a
universal mass-rotation relation. The high precision of our observations and
the high occurrence rates of variable low-surface gravity objects encourage
high-contrast time-resolved observations with the James Webb Space Telescope.Comment: Accepted for publication in AAS Journa
Cloud Atlas: Rotational Spectral Modulations and potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C
Measurements of photometric variability at different wavelengths provide
insights into the vertical cloud structure of brown dwarfs and planetary-mass
objects. In seven Hubble Space Telescope consecutive orbits, spanning 10
h of observing time}, we obtained time-resolved spectroscopy of the
planetary-mass T8-dwarf Ross 458C using the near-infrared Wide Field Camera 3.
We found spectrophotometric variability with a peak-to-peak signal of
2.620.02 % (in the 1.10-1.60~m white light curve). Using three
different methods, we estimated a rotational period of 6.751.58~h for the
white light curve, and similar periods for narrow - and - band light
curves. Sine wave fits to the narrow - and -band light curves suggest a
tentative phase shift between the light curves with wavelength when we allow
different periods between both light curves. If confirmed, this phase shift may
be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS
J22282889-310262. We find that, in contrast with 2M2228, the variability of
Ross~458C shows evidence for a {color trend} within the narrow -band, but
gray variations in the narrow -band. The spectral time-resolved variability
of Ross 458C might be potentially due to heterogeneous sulfide clouds in the
atmosphere of the object. Our discovery extends the study of spectral
modulations of condensate clouds to the coolest T dwarfs, planetary-mass
companions.Comment: Accepted in ApJ
High-resolution polarimetry of Parsamian 21: revealing the structure of an edge-on FU Ori disc
We present the first high spatial resolution near-infrared direct and
polarimetric observations of Parsamian 21, obtained with the VLT/NACO
instrument. We complemented these measurements with archival infrared
observations, such as HST/WFPC2 imaging, HST/NICMOS polarimetry, Spitzer IRAC
and MIPS photometry, Spitzer IRS spectroscopy as well as ISO photometry. Our
main conclusions are the following: (1) we argue that Parsamian 21 is probably
an FU Orionis-type object; (2) Parsamian 21 is not associated with any rich
cluster of young stars; (3) our measurements reveal a circumstellar envelope, a
polar cavity and an edge-on disc; the disc seems to be geometrically flat and
extends from approximately 48 to 360 AU from the star; (4) the SED can be
reproduced with a simple model of a circumstellar disc and an envelope; (5)
within the framework of an evolutionary sequence of FUors proposed by Green et
al. (2006) and Quanz et al. (2007), Parsamian 21 can be classified as an
intermediate-aged object.Comment: Accepted for publication in the MNRAS. 16 pages, 18 figures and 5
table
Supershells in Metal Clusters: Self-Consistent Calculations and their Semiclassical Interpretation
To understand the electronic shell- and supershell-structure in large metal
clusters we have performed self-consistent calculations in the homogeneous,
spherical jellium model for a variety of different materials. A scaling
analysis of the results reveals a surprisingly simple dependence of the
supershells on the jellium density. It is shown how this can be understood in
the framework of a periodic-orbit-expansion by analytically extending the
well-known semiclassical treatment of a spherical cavity to more realistic
potentials.Comment: 4 pages, revtex, 3 eps figures included, for additional information
see http://radix2.mpi-stuttgart.mpg.de/koch/Diss
- …