22 research outputs found
NEOWISE: Observations of the Irregular Satellites of Jupiter and Saturn
We present thermal model fits for 11 Jovian and 3 Saturnian irregular
satellites based on measurements from the WISE/NEOWISE dataset. Our fits
confirm spacecraft-measured diameters for the objects with in situ observations
(Himalia and Phoebe) and provide diameters and albedo for 12 previously
unmeasured objects, 10 Jovian and 2 Saturnian irregular satellites. The
best-fit thermal model beaming parameters are comparable to what is observed
for other small bodies in the outer Solar System, while the visible, W1, and W2
albedos trace the taxonomic classifications previously established in the
literature. Reflectance properties for the irregular satellites measured are
similar to the Jovian Trojan and Hilda Populations, implying common origins.Comment: 17 pages, 3 figures, accepted for publication in Astrophysical
Journa
Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 2. Determination of the Mean Composition of the Upper Troposphere and Stratosphere
Mid-infrared spectral observations Uranus acquired with the Infrared
Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the
abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at
the time of the 2007 equinox. For vertically uniform eddy diffusion
coefficients in the range 2200-2600 cm2 s-1, photochemical models that
reproduce the observed methane emission also predict C2H6 profiles that compare
well with emission in the 11.6-12.5 micron wavelength region, where the nu9
band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion
coefficient Kzz = 2430 cm2 sec-1 and a CH4 tropopause mole fraction of 1.6x10-5
provides a good fit to other hydrocarbon emission features, such as those of
C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of
0.43, suggesting that improvements are needed in the chemical reaction
mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4
ratio of 3.0+-0.2x10-4. From the best-fit scaling of these photochemical-model
profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/-0.8
x 10+19 molecule-cm-2 for CH4, 6.2 +- 1.0 x 10+16 molecule-cm-2 for C2H2 (with
a value 24% higher from a different longitudinal sampling), 3.1 +- 0.3 x 10+16
molecule-cm-2 for C2H6, 8.6 +- 2.6 x 10+13 molecule-cm-2 for CH3C2H, 1.8 +- 0.3
x 10+13 molecule-cm-2 for C4H2, and 1.7 +- 0.4 x 10+13 molecule-cm-2 for CO2 on
Uranus. Our results have implications with respect to the influx rate of
exogenic oxygen species and the production rate of stratospheric hazes on
Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures
The NEO Surveyor Near Earth Asteroid Known Object Model
The known near-Earth object (NEO) population consists of over 32,000 objects,
with a yearly discovery rate of over 3000 NEOs per year. An essential component
of the next generation of NEO surveys is an understanding of the population of
known objects, including an accounting of the discovery rate per year as a
function of size. Using a near-Earth asteroid (NEA) reference model developed
for NASA's NEO Surveyor (NEOS) mission and a model of the major current and
historical ground-based surveys, an estimate of the current NEA survey
completeness as a function of size and absolute magnitude has been determined
(termed the Known Object Model; KOM). This allows for understanding of the
intersection of the known catalog of NEAs and the objects expected to be
observed by NEOS. The current NEA population is found to be complete
for objects larger than 140m, consistent with estimates by Harris & Chodas
(2021). NEOS is expected to catalog more than two thirds of the NEAs larger
than 140m, resulting in of NEAs cataloged at the end of its 5 year
nominal survey (Mainzer et al, 2023}, making significant progress towards the
US Congressional mandate. The KOM estimates that of the currently
cataloged objects will be detected by NEOS, with those not detected
contributing to the final completeness at the end its 5 year mission.
This model allows for placing the NEO Surveyor mission in the context of
current surveys to more completely assess the progress toward the goal of
cataloging the population of hazardous asteroids.Comment: 27 pages, 18 figures, 3 tables. Accepted for publication in Planetary
Science Journal (PSJ
Validation of the Survey Simulator tool for the NEO Surveyor mission using NEOWISE data
The Near Earth Object Surveyor mission has a requirement to find two-thirds
of the potentially hazardous asteroids larger than 140 meters in size. In order
to determine the mission's expected progress toward this goal during design and
testing, as well as the actual progress during the survey, a simulation tool
has been developed to act as a consistent and quantifiable yardstick. We test
that the survey simulation software is correctly predicting on-sky positions
and thermal infrared fluxes by using it to reproduce the published measurements
of asteroids from the NEOWISE mission. We then extended this work to find
previously unreported detections of known near Earth asteroids in the NEOWISE
data archive, a search that resulted in 21,661 recovery detections, including
1,166 objects that had no previously reported NEOWISE observations. These
efforts demonstrate the reliability of the NEOS Survey Simulator tool, and the
perennial value of searchable image and source catalog archives for extending
our knowledge of the small bodies of the Solar System.Comment: 19 pages, 6 figures, accepted for publication in PS
Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs
We present the first mid-infrared spectra of brown dwarfs, together with
observations of a low-mass star. Our targets are the M3.5 dwarf GJ 1001A, the
L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary system epsilon Indi Ba/Bb. As
expected, the mid-infrared spectral morphology of these objects changes rapidly
with spectral class due to the changes in atmospheric chemistry resulting from
their differing effective temperatures and atmospheric structures. By taking
advantage of the unprecedented sensitivity of the Infrared Spectrograph on the
Spitzer Space Telescope we have detected the 7.8 micron methane and 10 micron
ammonia bands for the first time in brown dwarf spectra.Comment: 4 pages, 2 figure
The Discovery of Infrared Rings in the Planetary Nebula NGC 1514 During the WISE All-Sky Survey
We report the discovery of a pair of infrared, axisymmetric rings in the
planetary nebula NGC 1514 during the course of the WISE all-sky mid-infrared
survey. Similar structures are seen at visible wavelengths in objects such as
the "Engraved Hourglass Nebula" (MyCn 18) and the "Southern Crab Nebula" (Hen
2-104). However, in NGC 1514 we see only a single pair of rings and they are
easily observed only in the mid-infrared. These rings are roughly 0.2 pc in
diameter, are separated by 0.05 pc, and are dominated by dust emission with a
characteristic temperature of 160 K. We compare the morphology and color of the
rings to the other nebular structures seen at visible, far-infrared, and radio
wavelengths, and close with a discussion of a physical model and formation
scenario for NGC 1514.Comment: 16 pages, 10 figures, final version published in 2010 December
Astronomical Journa
Size and Albedo Constraints for (152830) Dinkinesh Using WISE Data
Probing small main-belt asteroids provides insight into their formation and
evolution through multiple dynamical and collisional processes. These asteroids
also overlap in size with the potentially hazardous near-earth object
population and supply the majority of these objects. The Lucy mission will
provide an opportunity for study of a small main-belt asteroid, (152830)
Dinkinesh. The spacecraft will perform a flyby of this object on November 1,
2023, in preparation for its mission to the Jupiter Trojan asteroids. We
employed aperture photometry on stacked frames of Dinkinesh obtained by the
Wide-field-Infrared Survey Explorer and performed thermal modeling on a
detection at 12 m to compute diameter and albedo values. Through this
method, we determined Dinkinesh has an effective spherical diameter of
km and a visual geometric albedo of
at the 16th and 84th percentiles. This albedo is
consistent with typical stony (S-type) asteroids.Comment: Submitted to Astrophysical Journal Letter
A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs
We present a low-resolution (R ≡ λ/Δλ ≈ 90), 5.5-38 μm spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27 μm, CH_4 at 7.65 μm, and NH_3 at 10.5 μm and are relatively featureless at λ ≳ 15 μm. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence, while the CH_4 and NH_3 bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 μm. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5 to 7.5 μm relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 μm spectra and find good agreement with the values of Golimowski et al., who use L'- and M'-band photometry to account for the flux emitted at λ > 2.5 μm