1,793 research outputs found
Simulated Tractor Chassis Suspension System
The development of a chassis suspension system for an agricultural tractor has been described to improve the operator ride comfort. Computer simulation tech-niques were used to formulate the tractor models and to compute the natural frequencies and frequency response of the models, as well as the RMS acceleration response, to evaluate the effect of the suspension system and cab position on operator ride comfort
Detectability of dirty dust grains in brown dwarf atmospheres
Dust clouds influence the atmospheric structure of brown dwarfs, and they
affect the heat transfer and change the gas-phase chemistry. However, the
physics of their formation and evolution is not well understood. In this
letter, we predict dust signatures and propose a potential observational test
of the physics of dust formation in brown dwarf atmosphere based on the
spectral features of the different solid components predicted by dust formation
theory. A momentum method for the formation of dirty dust grains (nucleation,
growth, evaporation, drift) is used in application to a static brown dwarf
atmosphere structure to compute the dust grain properties, in particular the
heterogeneous grain composition and the grain size. Effective medium and Mie
theory are used to compute the extinction of these spherical grains. Dust
formation results in grains whose composition differs from that of grains
formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models
because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong
broad absorption feature centered at 8.7mum, while amorphous
Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at
longer wavelength. Those features at lambda < 15mum are detectable in
absorption if grains are small (radius < 0.2mum) in the upper atmosphere as
suggested by our model. We suggest that the detection of a feature at 8.7mum in
deep infrared spectra could provide evidence for non-equilibrium dust formation
that yields grains composed of metastable solids in brown dwarf atmospheres.
This feature will shift towards 10mum and broaden if silicates (e.g. fosterite)
are much more abundant.Comment: A&A Letter, accepte
The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs
We identify the pressure--broadened red wings of the saturated potassium
resonance lines at 7700 \AA as the source of anomalous absorption seen in the
near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in
general. This conclusion is supported by the recent work of Tsuji {\it et al.}
1999, though unlike them we find that dust need not be invoked to explain the
spectra of methane dwarfs shortward of 1 micron. We find that a combination of
enhanced alkali abundances due to rainout and a more realistic non-Lorentzian
theory of resonant line shapes may be all that is needed to properly account
for these spectra from 0.5 to 1.0 microns. The WFPC2 measurement of Gliese
229B is also consistent with this theory. Furthermore, a combination of the
blue wings of this K I resonance doublet, the red wings of the Na D lines at
5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural way
the observed WFPC2 band flux of Gliese 229B. Hence, we conclude that the
neutral alkali metals play a central role in the near-infrared and optical
spectra of methane dwarfs and that their lines have the potential to provide
crucial diagnostics of brown dwarfs. We speculate on the systematics of the
near-infrared and optical spectra of methane dwarfs, for a given mass and
composition, that stems from the progressive burial with decreasing \teff of
the alkali metal atoms to larger pressures and depths.Comment: Revised and accepted to Ap.J. volume 531, March 1, 2000, also
available at http://jupiter.as.arizona.edu/~burrows/papers/BMS.p
Spitzer Mid-Infrared Photometry of 500 - 750 K Brown Dwarfs
Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is
critical for understanding the cold population of brown dwarfs now being found,
objects which have more in common with planets than stars. As effective
temperature (T_eff) drops from 800 K to 400 K, the fraction of flux emitted
beyond 3 microns increases rapidly, from about 40% to >75%. This rapid increase
makes a color like H-[4.5] a very sensitive temperature indicator, and it can
be combined with a gravity- and metallicity-sensitive color like H-K to
constrain all three of these fundamental properties, which in turn gives us
mass and age for these slowly cooling objects. Determination of mid-infrared
color trends also allows better exploitation of the WISE mission by the
community. We use new Spitzer Cycle 6 IRAC photometry, together with published
data, to present trends of color with type for L0 to T10 dwarfs. We also use
the atmospheric and evolutionary models of Saumon & Marley to investigate the
masses and ages of 13 very late-type T dwarfs, which have H-[4.5] > 3.2 and
T_eff ~ 500 K to 750 K.Comment: To be published in the on-line version of the Proceedings of Cool
Stars 16 (ASP Conference Series). This is an updated version of Leggett et
al. 2010 ApJ 710 1627; a photometry compilation is available at
http://www.gemini.edu/staff/slegget
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
Primary and secondary contributions to aerosol light scattering and absorption in Mexico City during the MILAGRO 2006 campaign
A photoacoustic spectrometer, a nephelometer, an aethalometer, and an aerosol mass spectrometer were used to measure at ground level real-time aerosol light absorption, scattering, and chemistry at an urban site located in North East Mexico City (Instituto Mexicano del Petroleo, Mexican Petroleum Institute, denoted by IMP), as part of the Megacity Impact on Regional and Global Environments field experiment, MILAGRO, in March 2006. Photoacoustic and reciprocal nephelometer measurements at 532 nm accomplished with a single instrument compare favorably with conventional measurements made with an aethalometer and a TSI nephelometer. The diurnally averaged single scattering albedo at 532 nm was found to vary from 0.60 to 0.85 with the peak value at midday and the minimum value at 07:00 a.m. local time, indicating that the Mexico City plume is likely to have a net warming effect on local climate. The peak value is associated with strong photochemical generation of secondary aerosol. It is estimated that the photochemical production of secondary aerosol (inorganic and organic) is approximately 75% of the aerosol mass concentration and light scattering in association with the peak single scattering albedo. A strong correlation of aerosol scattering at 532 nm and total aerosol mass concentration was found, and an average mass scattering efficiency factor of 3.8 m<sup>2</sup>/g was determined. Comparisons of photoacoustic and aethalometer light absorption with oxygenated organic aerosol concentration (OOA) indicate a very small systematic bias of the filter based measurement associated with OOA and the peak aerosol single scattering albedo
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
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: 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
Sorghum anthracnose and sustainable management strategies in West and Central Africa
Anthracnose caused by Colletotrichum graminicola remains the most important foliar disease of sorghum in West and Central Africa. This paper describes the advances made in sorghum anthracnose research that has led to a better understanding of pathogenic and genotypic diversity, epidemiology and importantly disease management strategies. We further highlight how understanding pathogen diversity interplays with the major sustainable anthracnose management strategies such as the use of host plant resistance and crop residue management within the region
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