5,206 research outputs found
Water ice in the Kuiper belt
We examine a large collection of low-resolution near-infrared spectra of Kuiper Belt objects (KBOs) and centaurs in an attempt to understand the presence of water ice in the Kuiper Belt. We find that water ice on the surface of these objects occurs in three separate manners: (1) Haumea family members uniquely show surfaces of nearly pure water ice, presumably a consequence of the fragmentation of the icy mantle of a larger differentiated proto-Haumea; (2) large objects with absolute magnitudes of H < 3 (and a limited number to H = 4.5) have surface coverings of water iceâperhaps mixed with ammoniaâthat appears to be related to possibly ancient cryovolcanism on these large objects; and (3) smaller KBOs and centaurs which are neither Haumea family members nor cold-classical KBOs appear to divide into two families (which we refer to as "neutral" and "red"), each of which is a mixture of a common nearly neutral component and either a slightly red or very red component that also includes water ice. A model suggesting that the difference between neutral and red objects due to formation in an early compact solar system either inside or outside, respectively, of the ~20 AU methanol evaporation line is supported by the observation that methanol is only detected on the reddest objects, which are those which would be expected to have the most of the methanol containing mixture
O stars effective temperature and HII regions ionization parameter gradients in the Galaxy
Extensive photoionization model grids are computed for single star HII
regions using stellar atmosphere models from the WM-basic code. Mid-IR emission
line intensities are predicted and diagnostic diagrams of [NeIII]/[NeII] and
[SIV]/[SIII] excitation ratio are build, taking into account the metallicities
of both the star and the HII region. The diagrams are used in conjunction with
galactic HII region observations obtained with the ISO Observatory to determine
the effective temperature Teff of the exciting O stars and the mean ionization
parameter U. Teff and U are found to increase and decrease, respectively, with
the metallicity of the HII region represented by the [Ne/Ne_sol] ratio. No
evidence is found for gradients of Teff or U with galactocentric distance Rgal.
The observed excitation sequence with Rgal is mainly due to the effect of the
metallicity gradient on the spectral ionizing shape, upon which the effect of
an increase in Teff with Z is superimposed. We show that not taking properly
into account the effect of metallicity on the ionizing shape of the stellar
atmosphere would lead to an apparent decrease of Teff with Z and an increase of
Teff with Rgal.Comment: Accepted in Ap
Classical and Quantum Gravity in 1+1 Dimensions, Part III: Solutions of Arbitrary Topology
All global solutions of arbitrary topology of the most general 1+1
dimensional dilaton gravity models are obtained. We show that for a generic
model there are globally smooth solutions on any non-compact 2-surface. The
solution space is parametrized explicitly and the geometrical significance of
continuous and discrete labels is elucidated. As a corollary we gain insight
into the (in general non-trivial) topology of the reduced phase space.
The classification covers basically all 2D metrics of Lorentzian signature
with a (local) Killing symmetry.Comment: 39 pages, 22 figures, uses AMSTeX, extended version of former chapter
7 (Gravitational Kinks) now available as gr-qc/9707053, problem with figure 6
fixe
Logical equivalence between generalized urn models and finite automata
To every generalized urn model there exists a finite (Mealy) automaton with
identical propositional calculus. The converse is true as well.Comment: 9 pages, minor change
Quantifying the impact of emission outbursts and non-stationary flow on eddy covariance CH<sub>4</sub> flux measurements using wavelet techniques
Methane flux measurements by the eddy-covariance technique are subject to large uncertainties, particularly linked to the partly highly intermittent nature of methane emissions. Outbursts of high methane emissions, termed event fluxes, hold the potential to introduce systematic biases into derived methane budgets, since under such conditions the assumption of stationarity of the flow is violated. In this study, we investigate the net impact of this effect by comparing eddy-covariance fluxes against a wavelet-derived reference that is not negatively influenced by non-stationarity. Our results demonstrate that methane emission events influenced 3â4â% of the flux measurements, and did not lead to systematic biases in methane budgets for the analyzed summer season; however, the presence of events substantially increased uncertainties in short-term flux rates. The wavelet results provided an excellent reference to evaluate the performance of three different gapfilling approaches for eddy-covariance methane fluxes, and we show that none of them could reproduce the range of observed flux rates. The integrated performance of the gapfilling methods for the longer-term dataset varied between the two eddy-covariance towers involved in this study, and we show that gapfilling remains a large source of uncertainty linked to limited insights into the mechanisms governing the short-term variability in methane emissions. With the capability to broaden our observational methane flux database to a wider range of conditions, including the direct resolution of short term variability at the order of minutes, wavelet-derived fluxes hold the potential to generate new insight into methane exchange processes with the atmosphere, and therefore also improve our understanding of the underlying processes
Discovery of Temperate Latitude Clouds on Titan
Until now, all the clouds imaged in Titan's troposphere have been found at far southern latitudes (60°-90° south). The occurrence and location of these clouds is thought to be the result of convection driven by the maximum annual solar heating of Titan's surface, which occurs at summer solstice (2002 October) in this south polar region. We report the first observations of a new recurring type of tropospheric cloud feature, confined narrowly to ~40° south latitude, which cannot be explained by this simple insolation hypothesis. We propose two classes of formation scenario, one linked to surface geography and the other to seasonally evolving circulation, which will be easily distinguished with continued observations over the next few years
Generalized 2d dilaton gravity with matter fields
We extend the classical integrability of the CGHS model of 2d dilaton gravity
[1] to a larger class of models, allowing the gravitational part of the action
to depend more generally on the dilaton field and, simultaneously, adding
fermion- and U(1)-gauge-fields to the scalar matter. On the other hand we
provide the complete solution of the most general dilaton-dependent 2d gravity
action coupled to chiral fermions. The latter analysis is generalized to a
chiral fermion multiplet with a non-abelian gauge symmetry as well as to the
(anti-)self-dual sector df = *df (df = -*df) of a scalar field f.Comment: 37 pages, Latex; typos and Eqs. (44,45) corrected; paragraph on p.
26, referring to a work of S. Solodukhin, reformulated; references adde
Habitability of Super-Earth Planets around Main-Sequence Stars including Red Giant Branch Evolution: Models based on the Integrated System Approach
In a previous study published in Astrobiology, we focused on the evolution of
habitability of a 10 M_E super-Earth planet orbiting a star akin to the Sun.
This study was based on a concept of planetary habitability in accordance to
the integrated system approach that describes the photosynthetic biomass
production taking into account a variety of climatological, biogeochemical, and
geodynamical processes. In the present study, we pursue a significant
augmentation of our previous work by considering stars with zero-age main
sequence masses between 0.5 and 2.0 M_sun with special emphasis on models of
0.8, 0.9, 1.2 and 1.5 M_sun. Our models of habitability consider again
geodynamical processes during the main-sequence stage of these stars as well as
during their red giant branch evolution. Pertaining to the different types of
stars, we identify so-called photosynthesis-sustaining habitable zones (pHZ)
determined by the limits of biological productivity on the planetary surface.
We obtain various sets of solutions consistent with the principal possibility
of life. Considering that stars of relatively high masses depart from the
main-sequence much earlier than low-mass stars, it is found that the biospheric
life-span of super-Earth planets of stars with masses above approximately 1.5
M_sun is always limited by the increase in stellar luminosity. However, for
stars with masses below 0.9 M_sun, the life-span of super-Earths is solely
determined by the geodynamic time-scale. For central star masses between 0.9
and 1.5 M_sun, the possibility of life in the framework of our models depends
on the relative continental area of the super-Earth planet.Comment: 25 pages, 6 figures, 2 tables; submitted to: International Journal of
Astrobiolog
Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope
We have used the Hubble Space Telescope to directly measure the angular size
of the large Kuiper belt object 2003 UB313. By carefully calibrating the point
spread function of a nearby field star, we measure the size of 2003 UB313 to be
34.31.4 milliarcseconds, corresponding to a diameter of 2400100 km or
a size % larger than Pluto. The V band geometric albedo of 2003 UB313 is
%. The extremely high albedo is consistent with the frosty methane
spectrum, the lack of red coloring, and the lack of observed photometric
variation on the surface of 2003 UB313. Methane photolysis should quickly
darken the surface of 2003 UB313, but continuous evaporation and redeposition
of surface ices appears capable of maintaining the extreme alebdo of this body
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