999 research outputs found
CGIAR citations in IPCC reports: a summary report
In March 2015, Science-Metrix was contracted by CGIAR to evaluate the presence of CGIAR publications in the IPCC assessment reports (ARs). The study was carried through the analysis of the IPCC ARs' references
Interstellar Polarization in the Taurus Dark Clouds, Wavelength Dependent Position Angles and Cloud Structure Near TMC-1
We use polarimetric observations of two stars (HD29647, HD283809) in the
general direction of TMC-1 in the Taurus Dark Cloud to investigate grain
properties and cloud structure in this region. We show the data to be
consistent with a simple two-component model, in which general interstellar
polarization in the Taurus Cloud is produced by a widely distributed cloud
component with relatively uniform magnetic field orientation; the light from
stars close to TMC-1 suffers additional polarization arising in one (or more)
subcloud(s) with larger average grain size and different magnetic field
directions compared with the general trend. Towards HD29647, in particular, we
show that the unusually low degree of visual polarization relative to
extinction is due to the presence of distinct cloud components in the line of
sight with markedly different magnetic field orientations. Stokes parameter
calculations allow us to separate out the polarization characteristics of the
individual components. Results are fit with the Serkowski empirical formula to
determine the degree and wavelength of maximum polarization. Whereas lambda_max
values in the widely distributed material are similar to the average (0.55um)
for the diffuse interstellar medium, the subcloud in line of sight to
HD~283809, the most heavily reddened star in our study, has lambda_max approx.
0.73um, indicating the presence of grains about 30% larger than this average.
Our model also predicts detectable levels of circular polarization toward both
HD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical
Journal, vol 48
Probing the role of Nd3+ ions in the weak multiferroic character of NdMn2O5 by optical spectroscopies
Raman and infrared spectroscopies are used as local probes to study the
dynamics of the Nd-O bonds in the weakly multiferroic NdMn2O5 system. The
temperature dependence of selected Raman excitations reveals the splitting of
the Nd-O bonds in NdMn2O5. The Nd3+ ion crystal field (CF) excitations in
NdMn2O5 single crystals are studied by infrared transmission as a function of
temperature, in the 1800-8000 cm-1 range, and under an applied magnetic field
up to 11 T. The frequencies of all 4Ij crystal-field levels of Nd3+ are
determined. We find that the degeneracy of the ground-state Kramers doublet is
lifted ({\Delta}0 ~7.5 cm-1) due to the Nd3+-Mn3+ interaction in the
ferroelectric phase, below TC ~ 28 K. The Nd3+ magnetic moment mNd(T) and its
contribution to the magnetic susceptibility and the specific heat are evaluated
from {\Delta}0(T) indicating that the Nd3+ ions are involved in the magnetic
and the ferroelectric ordering observed below ~ 28 K. The Zeeman splitting of
the excited crystal field levels of the Nd3+ ions at low temperature is also
analyzed.Comment: This paper is accepted for publication as a Regular Article in
Physical Review
Barnett Relaxation in Thermally-Rotating Grains
We present an exact formulation of the physics of Barnett relaxation. Our
formulation is based on a realistic kinetic model of the relaxation mechanism
which includes the alignment of the grain angular momentum in body coordinates
by Barnett dissipation, disalignment by thermal fluctuations, and coupling of
the angular momentum to the gas via gas damping. We solve the Fokker-Planck
equation for the measure of internal alignment using numerical integration of
the equivalent Langevin equation for Brownian rotation. The accuracy of our
results is calibrated by comparing our numerical solutions with exact analytic
results obtained for special cases.We describe an analytic approximation for
the measure of alignment which fits our numerical results for cases of
practical interest.Comment: 17 pages, 5 figures. Accepted to ApJ
The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
The edge-on disk surrounding the nearby young star Beta Pictoris is the
archetype of the "debris disks", which are composed of dust and gas produced by
collisions and evaporation of planetesimals, analogues of Solar System comets
and asteroids. These disks provide a window on the formation and early
evolution of terrestrial planets. Previous observations of Beta Pic concluded
that the disk gas has roughly solar abundances of elements [1], but this poses
a problem because such gas should be rapidly blown away from the star, contrary
to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we
report the detection of singly and doubly ionized carbon (CII, CIII) and
neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these
abundant volatile species permits a much more complete gas inventory. Carbon is
extremely overabundant relative to every other measured element. This appears
to solve the problem of the stable gas disk, since the carbon overabundance
should keep the gas disk in Keplerian rotation [3]. New questions arise,
however, since the overabundance may indicate the gas is produced from material
more carbon-rich than the expected Solar System analogues.Comment: Accepted for publication in Nature. PDF document, 12 pages.
Supplementary information may be found at
http://www.dtm.ciw.edu/akir/Documents/roberge_supp.pdf *** Version 2 :
Removed extraneous publication information, per instructions from the Nature
editor. No other changes mad
The Transiting Exocomets in the HD 172555 System
The Earth is thought to have formed dry, in a part of the Solar Nebula deficient in organic material, and to have acquired its organics and water through bombardment by minor bodies. Observations of this process in well-dated systems can provide insight into the probable origin and composition of the bombarding parent bodies. Transiting cometary activity has previously been reported in Ca II for the late-A member of the 241 Myr old Pictoris Moving Group member, HD 172555(Kiefer et al. 2014). We present HST STIS and COS spectra of HD 172555 demonstrating that the star has chromospheric emission and variable in falling gas features in transitions of silicon and carbon ions at times when no Fe II absorption is seen in the UV data, and no Ca II absorption is seen in contemporary optical spectra. The lack of CO absorption and stable gas absorption at the system velocity is consistent with the absence of a cold Kuiper belt analog (Riviere-Marichalar et al. 2012) in this system. The presence of infall in some species at one epoch and others at different epochs suggests that, like Pictoris, there may be more than one family of exocomets. If perturbed into star-grazing orbits by the same mechanism as for Pic, these data suggest that the wide planet frequency among A-early F stars in the PMG is at least 37.5, well above the frequency estimated for young moving groups independent of host star spectral type
FUSE and HST STIS Observations of Hot and Cold Gas in the AB Aurigae System
We present the first observations of a Herbig Ae star with a circumstellar
disk by the Far Ultraviolet Spectroscopic Explorer (FUSE), as well as a
simultaneous observation of the star obtained with the Hubble Space Telescope
Space Telescope Imaging Spectrograph (STIS). The spectra of AB Aurigae show
emission and absorption features arising from gasses that have a wide range in
temperature, from hot OVI emission to cold molecular hydrogen and CO
absorption. Emissions from the highly ionized species OVI and CIII present in
the FUSE spectrum are redshifted, while absorption features arising from
low-ionization species like OI, NI, and SiII are blueshifted and show
characteristic stellar wind line-profiles. We find the total column density of
molecular hydrogen toward AB Aur from the FUSE apectrum, N(H_2) = (6.8 +/- 0.5)
x 10^19 cm^-2. The gas kinetic temperature of the molecular hydrogen derived
from the ratio N(J=1)/N(J=0) is 65 +/- 4 K. The column density of the CO
observed in the STIS spectrum is N(CO) = (7.1 +/- 0.5) x 10^13 cm^-2, giving a
CO/H_2 ratio of (1.04 +/- 0.11) x 10^-6. We also use the STIS spectrum to find
the column density of HI, permitting us to calculate the total column density
of hydrogen atoms, the fractional abundance of H_2, and the gas-to-dust ratio.Comment: 5 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted for
publication in ApJ Letter
Tensor interaction constraints from beta decay recoil spin asymmetry of trapped atoms
We have measured the angular distribution of recoiling daughter nuclei
emitted from the Gamow-Teller decay of spin-polarized Rb. The
asymmetry of this distribution vanishes to lowest order in the Standard Model
(SM) in pure Gamow-Teller decays, producing an observable very sensitive to new
interactions. We measure the non-SM contribution to the asymmetry to be
= 0.015 0.029 (stat) 0.019 (syst), consistent with the SM
prediction. We constrain higher-order SM corrections using the measured
momentum dependence of the asymmetry, and their remaining uncertainty dominates
the systematic error. Future progress in determining the weak magnetism term
theoretically or experimentally would reduce the final errors. We describe the
resulting constraints on fundamental 4-Fermi tensor interactions.Comment: 11 pages, 13 figures; v2 published in Phys. Rev. C, with referee
clarifications and figures improved for black-and-whit
A 40 Myr Old Gaseous Circumstellar Disk at 49 Ceti: Massive CO-Rich Comet Clouds at Young A-Type Stars
The gaseous molecular disk that orbits the main sequence A-type star 49 Ceti
has been known since 1995, but the stellar age and the origin of the observed
carbon monoxide molecules have been unknown. We now identify 49 Ceti as a
member of the 40 Myr old Argus Association and present a colliding comet model
to explain the high CO concentrations seen at 49 Ceti and the 30 Myr old A-type
star HD 21997. The model suggests that massive -- 400 Earth mass -- analogs of
the Sun's Kuiper Belt are in orbit about some A-type stars, that these large
masses are composed primarily of comet-like objects, and that these objects are
rich in CO and perhaps also CO2. We identify additional early-type members of
the Argus Association and the Tucana/Horologium and Columba Associations; some
of these stars display excess mid-infrared emission as measured with the
Widefield Infrared Survey Explorer (WISE).Comment: Accepted to Ap
Numerical Study of the Two Color Attoworld
We consider QCD at very low temperatures and non-zero quark chemical
potential from lattice Monte Carlo simulations of the two-color theory in a
very small spatial volume (the attoscale). In this regime the quark number
rises in discrete levels in qualitative agreement with what is found
analytically at one loop on S3xS1 with radius R_S3 << 1/{\Lambda}_QCD. The
detailed level degeneracy, however, cannot be accounted for using weak coupling
arguments. At each rise in the quark number there is a corresponding spike in
the Polyakov line, also in agreement with the perturbative results. In addition
the quark number susceptibility shows a similar behaviour to the Polyakov line
and appears to be a good indicator of a confinement-deconfinement type of
transition.Comment: 18 pages, 10 figure
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