608 research outputs found
Blind prediction of broadband coherence time at basin scales
A blind comparison with data is made with a model for the coherence time of broadband sound (133 Hz, 17-Hz bandwidth) at 3709 km. Coherence time is limited by changes in the ocean because the acoustic instruments are fixed to the Earth on the bottom of the sea with time bases maintained by atomic clocks. Although the modeled coherence time depends a bit on the difficult problem of correctly modeling relative signal-to-noise ratios, normalized correlation coefficients of the broadband signals for the data (model) are 0.90 (0.83), 0.72 (0.59), and 0.51 (0.36) at lags of 2, 4.1, and 6.2 min, respectively. In all these cases, observed coherence times are a bit longer than modeled. The temporal evolution of the model is based on the linear dispersion relation for internal waves. Acoustic propagation is modeled with the parabolic approximation and the sound-speed insensitive operator
Deriving Stellar Effective Temperatures of Metal-Poor Stars with the Excitation Potential Method
It is well established that stellar effective temperatures determined from
photometry and spectroscopy yield systematically different results. We describe
a new, simple method to correct spectroscopically derived temperatures
("excitation temperatures") of metal-poor stars based on a literature sample
with -3.3<[Fe/H]<-2.5. Excitation temperatures were determined from FeI line
abundances in high-resolution optical spectra in the wavelength range of ~3700
to ~7000A, although shorter wavelength ranges, up to 4750 to 6800A, can also be
employed, and compared with photometric literature temperatures. Our adjustment
scheme increases the temperatures up to several hundred degrees for cool red
giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it
brings the excitation temperatures in good agreement with photometrically
derived values. The modified temperature also influences other stellar
parameters, as the FeI-FeII ionization balance is simultaneously used to
determine the surface gravity, while also forcing no abundance trend on the
absorption line strengths to obtain the microturbulent velocity. As a result of
increasing the temperature, the often too low gravities and too high
microturbulent velocities in red giants become higher and lower, respectively.
Our adjustment scheme thus continues to build on the advantage of deriving
temperatures from spectroscopy alone, independent of reddening, while at the
same time producing stellar chemical abundances that are more straightforwardly
comparable to studies based on photometrically derived temperatures. Hence, our
method may prove beneficial for comparing different studies in the literature
as well as the many high-resolution stellar spectroscopic surveys that are or
will be carried out in the next few years.Comment: 12 pages, emulateapj, accepted for publication in Ap
Saturn's Icy Moon Rhea: a Prediction for Bulk Chemical Composition and Physical Structure at the Time of the Cassini Spacecraft First Flyby
I report a model for the formation of Saturn's family of mid-sized icy moons
to coincide with the first flypast of Rhea by the Cassini Orbiter spacecraft on
26 November 2005. It is proposed that these moons had condensed from a
concentric family of orbiting gas rings that were cast off some 4600 Myr ago by
the contracting proto-Saturnian cloud. Numerical and structural models for Rhea
are constructed on the basis of a computed bulk chemical mix of hydrated rock
(mass fraction 0.385), H2O ice (0.395), and NH3 ice (0.220). The large
proportion of NH3 in the ice mass inhibits the formation of the dense
crystalline phase II of H2O ice at the satellite's centre. This may explain the
absence of compressional features on the surface. The favoured model of Rhea
has a chemically uniform interior and is very cold. The satellite is nearly
isodense and the predicted value of the axial moment-of-inertia factor is
C/MR^2 = 0.399 +/- 0.004. NH3 is unstable at Saturn's distance from the Sun,
except near the polar regions of the satellite. Perhaps the Cassini Orbiter
will discover indirect evidence for NH3 through the sublimative escape of this
ice from the outer layers, especially near the equatorial zones. Wasting of NH3
would weaken the residual soil, so making the edges of craters soft and prone
to landslides. It will be exciting to learn what Cassini discovers.Comment: This paper was submitted to the Publications of the Astronomical
Society of Australia (PASA) on 30 November 200
The Aquarius Co-Moving Group is Not a Disrupted Classical Globular Cluster
We present a detailed analysis of high-resolution, high S/N spectra for 5
Aquarius stream stars observed with the MIKE spectrograph on the Magellan Clay
telescope. Our sample represents one third of the 15 known members in the
stream. We find the stream is not mono-metallic: the metallicity ranges from
[Fe/H] = -0.63 to -1.58. No anti-correlation in Na-O abundances is present, and
we find a strong positive Mg-Al relationship, similar to that observed in the
thick disk. We find no evidence that the stream is a result of a disrupted
classical globular cluster, contrary to a previously published claim. High
[(Na, Ni, alpha)/Fe] and low [Ba/Y] abundance ratios in the stream suggests it
is not a tidal tail from a disrupted dwarf galaxy, either. The stream is
chemically indistinguishable from Milky Way field stars with the exception of
one candidate, C222531-145437. From its position, velocity, and detailed
chemical abundances, C222531-145437 is likely a star that was tidally disrupted
from omega-Centauri. We propose the Aquarius stream is Galactic in origin, and
could be the result from a disk-satellite perturbation in the Milky Way thick
disk on the order of a few Gyr ago: derived orbits, UVW velocities, and angular
momenta of the Aquarius members offer qualitative support for our hypothesis.
Assuming C222531-145437 is a tidally disrupted member of omega-Centauri, this
system is the most likely disk perturber. In the absence of compelling chemical
and/or dynamical evidence that the Aquarius stream is the tidal tail of a
disrupted satellite, we advocate the "Aquarius group" as a more appropriate
description. Like the Canis Major over-density, as well as the Hercules and
Monoceros groups, the Aquarius group joins the list of kinematically-identified
substructures that are not actually accreted material: they are simply part of
the rich complexity of the Milky Way structure.Comment: Accepted to MNRAS. Updated to journal versio
High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey
The SkyMapper Southern Sky Survey is carrying out a search for the most
metal-poor stars in the Galaxy. It identifies candidates by way of its unique
filter set that allows for estimation of stellar atmospheric parameters. The
set includes a narrow filter centered on the Ca II K 3933A line, enabling a
robust estimate of stellar metallicity. Promising candidates are then confirmed
with spectroscopy. We present the analysis of Magellan-MIKE high-resolution
spectroscopy of 122 metal-poor stars found by SkyMapper in the first two years
of commissioning observations. 41 stars have [Fe/H] <= -3.0. Nine have [Fe/H]
<= -3.5, with three at [Fe/H] ~ -4. A 1D LTE abundance analysis of the elements
Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, Ba and Eu shows
these stars have [X/Fe] ratios typical of other halo stars. One star with low
[X/Fe]
[X/Fe values appears to be "Fe-enhanced," while another star has an extremely
large [Sr/Ba] ratio: >2. Only one other star is known to have a comparable
value. Seven stars are "CEMP-no" stars ([C/Fe] > 0.7, [Ba/Fe] < 0). 21 stars
exhibit mild r-process element enhancements (0.3 <=[Eu/Fe] < 1.0), while four
stars have [Eu/Fe] >= 1.0. These results demonstrate the ability to identify
extremely metal-poor stars from SkyMapper photometry, pointing to increased
sample sizes and a better characterization of the metal-poor tail of the halo
metallicity distribution function in the future.Comment: Minor corrections to text, missing data added to Tables 3 and 4;
updated to match published version. Complete tables included in sourc
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The 2015-2016 carbon cycle as seen from OCO-2 and the global in situ network
The Orbiting Carbon Observatory-2 has been on orbit since 2014, and its global coverage holds the potential to reveal new information about the carbon cycle through the use of top-down atmospheric inversion methods combined with column average CO2 retrievals. We employ a large ensemble of atmospheric inversions utilizing different transport models, data assimilation techniques, and prior flux distributions in order to quantify the satellite-informed fluxes from OCO-2 Version 7r land observations and their uncertainties at continental scales. Additionally, we use in situ measurements to provide a baseline against which to compare the satellite-constrained results. We find that within the ensemble spread, in situ observations, and satellite retrievals constrain a similar global total carbon sink of 3.7±0.5 PgC yr−1, and 1.5±0.6 PgC yr−1 for global land, for the 2015–2016 annual mean. This agreement breaks down in smaller regions, and we discuss the differences between the experiments. Of particular interest is the difference between the different assimilation constraints in the tropics, with the largest differences occurring in tropical Africa, which could be an indication of the global perturbation from the 2015–2016 El Niño. Evaluation of posterior concentrations using TCCON and aircraft observations gives some limited insight into the quality of the different assimilation constraints, but the lack of such data in the tropics inhibits our ability to make strong conclusions there.
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Regional and temporal variability of melts during a Cordilleran magma pulse: Age and chemical evolution of the Jurassic arc, eastern Mojave Desert, California
Intrusive rock sequences in the central and eastern Mojave Desert segment of the Jurassic Cordilleran arc of the western United States record regional and temporal variations in magmas generated during the second prominent pulse of Mesozoic continental arc magmatism. U/Pb zircon ages provide temporal control for describing variations in rock and zircon geochemistry that reflect differences in magma source components. These source signatures are discernible through mixing and fractionation processes associated with magma ascent and emplacement. The oldest well-dated Jurassic rocks defining initiation of the Jurassic pulse are a 183 Ma monzodiorite and a 181 Ma ignimbrite. Early to Middle Jurassic intrusive rocks comprising the main stage of magmatism include two high-K calc-alkalic groups: to the north, the deformed 183–172 Ma Fort Irwin sequence and contemporaneous rocks in the Granite and Clipper Mountains, and to the south, the 167–164 Ma Bullion sequence. A Late Jurassic suite of shoshonitic, alkali-calcic intrusive rocks, the Bristol Mountains sequence, ranges in age from 164 to 161 Ma and was emplaced as the pulse began to wane. Whole-rock and zircon trace-element geochemistry defines a compositionally coherent Jurassic arc with regional and secular variations in melt compositions. The arc evolved through the magma pulse by progressively greater input of old cratonic crust and lithospheric mantle into the arc magma system, synchronous with progressive regional crustal thickening
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