7,237 research outputs found
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
Optical properties of BiTeBr and BiTeCl
We present a comparative study of the optical properties - reflectance,
transmission and optical conductivity - and Raman spectra of two layered
bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized
in the a-b planes. Despite different space groups, the optical properties of
the two compounds are very similar. Both materials are doped semiconductors,
with the absorption edge above the optical gap which is lower in BiTeBr (0.62
eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two
materials. A non-Drude free carrier contribution in the optical conductivity,
as well as three Raman and two infrared phonon modes, are observed in each
compound. There is a dramatic difference in the highest infrared phonon
intensity for the two compounds, and a difference in the doping levels. Aspects
of the strong electron-phonon interaction are identified. Several interband
transitions are assigned, among them the low-lying absorption which has
the same value 0.25 eV in both compounds, and is caused by the Rashba spin
splitting of the conduction band. An additional weak transition is found in
BiTeCl, caused by the lower crystal symmetry.Comment: Accepted in PR
Meteoric water circulation in a rolling-hinge detachment system (northern snake range core complex, Nevada)
Combined petrofabric, microstructural, stable isotopic, and 40Ar/39Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40Ar/39Ar geochronology, we use hydrogen isotope ratios (δD) in synkinematic white mica to characterize fluid-rock interaction across the rolling-hinge detachment. Results indicate that the western flank of the range preserves mostly Eocene deformation (49-45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fluids, although the imprint of meteoric fluids increases structurally downward and culminates in a shear zone with a white mica 40Ar/39Ar plateau age of ca. 27 Ma. In contrast, the eastern flank of the range displays pervasive noncoaxial (top-tothe-east) fabrics defined by white mica that formed in the presence of meteoric fluids and yield Oligo cene-Miocene 40Ar/39Ar ages (27-21 Ma). Evolution of the Oligocene-Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fluids were able to circulate from the surface to the brittle-ductile transition. On the western flank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern flank of the detachment allowed surface fluids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fluid inter action reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment
BiTeCl and BiTeBr: a comparative high-pressure optical study
We here report a detailed high-pressure infrared transmission study of BiTeCl
and BiTeBr. We follow the evolution of two band transitions: the optical
excitation between two Rashba-split conduction bands, and the
absorption across the band gap. In the low pressure range, ~GPa,
for both compounds is approximately constant with pressure and
decreases, in agreement with band structure calculations. In BiTeCl, a clear
pressure-induced phase transition at 6~GPa leads to a different ground state.
For BiTeBr, the pressure evolution is more subtle, and we discuss the
possibility of closing and reopening of the band gap. Our data is consistent
with a Weyl phase in BiTeBr at 56~GPa, followed by the onset of a structural
phase transition at 7~GPa.Comment: are welcom
New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme
CONTEXT: Asymptotic giant branch (AGB) stars are in one of the latest
evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss
has a significant effect on the stellar evolution, and is a significant source
of heavy elements and dust grains for the interstellar medium. The mass-loss
rate can be well traced by carbon monoxide (CO) line emission.
AIMS: We present new Herschel HIFI and IRAM 30m telescope CO line data for a
sample of 53 galactic AGB stars. The lines cover a fairly large range of
excitation energy from the line to the line, and even the
line in a few cases. We perform radiative transfer modelling for 38
of these sources to estimate their mass-loss rates.
METHODS: We used a radiative transfer code based on the Monte Carlo method to
model the CO line emission. We assume spherically symmetric circumstellar
envelopes that are formed by a constant mass-loss rate through a smoothly
accelerating wind.
RESULTS: We find models that are consistent across a broad range of CO lines
for most of the stars in our sample, i.e., a large number of the circumstellar
envelopes can be described with a constant mass-loss rate. We also find that an
accelerating wind is required to fit, in particular, the higher-J lines and
that a velocity law will have a significant effect on the model line
intensities. The results cover a wide range of mass-loss rates (
to ) and gas expansion
velocities (2 to km s), and include M-, S-, and C-type AGB stars.
Our results generally agree with those of earlier studies, although we tend to
find slightly lower mass-loss rates by about 40%, on average. We also present
"bonus" lines detected during our CO observations.Comment: 36 page
Optical spectroscopy and the nature of the insulating state of rare-earth nickelates
Using a combination of spectroscopic ellipsometry and DC transport
measurements, we determine the temperature dependence of the optical
conductivity of NdNiO and SmNiO films. The optical spectra show the
appearance of a characteristic two-peak structure in the near-infrared when the
material passes from the metal to the insulator phase. Dynamical mean-field
theory calculations confirm this two-peak structure, and allow to identify
these spectral changes and the associated changes in the electronic structure.
We demonstrate that the insulating phase in these compounds and the associated
characteristic two-peak structure are due to the combined effect of
bond-disproportionation and Mott physics associated with half of the
disproportionated sites. We also provide insights into the structure of excited
states above the gap.Comment: 12 pages, 13 figure
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