1,818 research outputs found
Dynamics of stratospheric wave reflection over the North Pacific
Stratospheric wave reflection events involve the upward propagation of planetary waves, which are subsequently reflected downward by the stratospheric polar vortex. This phenomenon establishes a connection between the large-scale circulations in the troposphere and in the stratosphere. Here, we investigate wave reflection events characterised by an enhanced difference between poleward eddy heat flux over the Northwest Pacific and equatorward eddy heat flux over Canada. Previous research has pointed to a link between these events and anomalies in the tropospheric circulation over North America, with an associated abrupt continental-scale surface temperature decrease over the same region. In this study, we aim to elucidate the dynamical mechanisms governing this chain of events.
We find that anomalies of meridional eddy heat flux over the Northwest Pacific and Canada change sign before and after reflection events. A westward-propagating ridge, associated with a positive geopotential height anomaly, and the development of a trough downstream can explain this sign change. The trough advects colder-than-average air southwards in the lower troposphere over North America, leading to an abrupt temperature decrease close to the surface. This corresponds in the upper troposphere to negative and in the lower troposphere to positive anomalies of meridional eddy heat flux. The evolution of this large-scale pattern resembles the shift from a Pacific Trough to an Alaskan Ridge weather regime. Furthermore, stratospheric wave reflection events exert a far-reaching influence beyond North America on the tropospheric circulation across the Northern mid- and high latitudes. One example is the zonalisation and intensification of the North Atlantic eddy-driven jet stream resulting in more frequent occurrences of windy extremes over Europe a few days after the temperature decrease across North America
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Notations and conventions in molecular spectroscopy: part 1. General spectroscopic notation
The field of Molecular Spectroscopy was surveyed in order to determine a set of
conventions and symbols which are in common use in the spectroscopic literature. This
document, which is Part I in a series, establishes the notations and conventions used for
general spectroscopic notations and deals with quantum mechanics, quantum numbers
(vibrational states, angular momentum and energy levels), spectroscopic transitions, and
miscellaneous notations (e.g. spectroscopic terms). Further parts will follow, dealing inter
alia with symmetry notation, permutation and permutation-inversion symmetry notation,
vibration-rotation spectroscopy and electronic spectroscopy
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Notations and conventions in molecular spectroscopy: part 2. Symmetry notation
The field of Molecular Spectroscopy was surveyed in order to determine a set of
conventions and symbols which are in common use in the spectroscopic literature. This
document, which is Part 2 in a series, establishes the notations and conventions used for the
description of symmetry in rigid molecules, using the Schoenflies notation. It deals firstly
with the symmetry operators of the molecular point groups (also drawing attention to the
difference between symmetry operators and elements). The conventions and notations of the
molecular point groups are then established, followed by those of the representations of these
groups as used in molecular spectroscopy. Further parts will follow, dealing inter alia with
permutation and permutation-inversion symmetry notation, vibration-rotation spectroscopy
and electronic spectroscopy
Blackbody-radiation-assisted molecular laser cooling
The translational motion of molecular ions can be effectively cooled
sympathetically to temperatures below 100 mK in ion traps through Coulomb
interactions with laser-cooled atomic ions. The distribution of internal
rovibrational states, however, gets in thermal equilibrium with the typically
much higher temperature of the environment within tens of seconds. We consider
a concept for rotational cooling of such internally hot, but translationally
cold heteronuclear diatomic molecular ions. The scheme relies on a combination
of optical pumping from a few specific rotational levels into a ``dark state''
with redistribution of rotational populations mediated by blackbody radiation.Comment: 4 pages, 5 figure
Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118
WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by
a permanent dust shell absorbing and re-emitting in the infrared a considerable
fraction of the stellar luminosity. We present the first diffraction-limited
2.13micron speckle interferometric observations of WR 118 with 73 mas
resolution. The speckle interferograms were obtained with the 6m telescope at
the Special Astrophysical Observatory. The two-dimensional visibility function
of the object does not show any significant deviation from circular symmetry.
The visibility curve declines towards the diffraction cut-off frequency to 0.66
and can be approximated by a linear function. Radiative transfer calculations
have been carried out to model the spectral energy distribution, given in the
range of 0.5-25micron, and our 2.13micron visibility function, assuming
spherical symmetry of the dust shell. Both can be fitted with a model
containing double-sized grains (``small'' and ``large'') with the radii of a =
0.05micron and 0.38micron, and a mass fraction of the large grains greater than
65%. Alternatively, a good match can be obtained with the grain size
distribution function n(a)~a^-3, with a ranging between 0.005micron and
0.6micron. At the inner boundary of the modelled dust shell (angular diameter
(17 +/- 1)mas), the temperature of the smallest grains and the dust shell
density are 1750K +/- 100K and (1 +/- 0.2)x10^-19 g/cm^3, respectively. The
dust formation rate is found to be (1.3 +/- 0.5)x10^-7 Msol/yr assuming Vwind =
1200 km/s.Comment: 6 pages including 4 PostScript figures, also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html;
accepted for publication in Astronomy & Astrophysic
Dust Dynamics in Compressible MHD Turbulence
We calculate the relative grain-grain motions arising from interstellar
magnetohydrodynamic (MHD) turbulence. The MHD turbulence includes both fluid
motions and magnetic fluctuations. While the fluid motions accelerate grains
through hydro-drag, the electromagnetic fluctuations accelerate grains through
resonant interactions. We consider both incompressive (Alfv\'{e}n) and
compressive (fast and slow) MHD modes and use descriptions of MHD turbulence
obtained in Cho & Lazarian (2002). Calculations of grain relative motion are
made for realistic grain charging and interstellar turbulence that is
consistent with the velocity dispersions observed in diffuse gas, including
cutoff of the turbulence from various damping processes. We show that fast
modes dominate grain acceleration, and can drive grains to supersonic
velocities. Grains are also scattered by gyroresonance interactions, but the
scattering is less important than acceleration for grains moving with
sub-Alfv\'{e}nic velocities. Since the grains are preferentially accelerated
with large pitch angles, the supersonic grains will be aligned with long axes
perpendicular to the magnetic field. We compare grain velocities arising from
MHD turbulence with those arising from photoelectric emission, radiation
pressure and H thrust. We show that for typical interstellar conditions
turbulence should prevent these mechanisms from segregating small and large
grains. Finally, gyroresonant acceleration is bound to preaccelerate grains
that are further accelerated in shocks. Grain-grain collisions in the shock may
then contribute to the overabundance of refractory elements in the composition
of galactic cosmic rays.Comment: 15 pages, 17 figure
VLT-ISAAC 3-5 micron spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in Vela
We performed a spectroscopic survey toward five intermediate-mass class I
YSOs located in the Southern Vela molecular cloud in the L and M bands at
resolving powers 600-800 up to 10,000, using the Infrared Spectrometer and
Array Camera mounted on the VLT-ANTU. Lower mass companion objects were
observed simultaneously in both bands. Solid H2O at 3 micron is detected in all
sources, including the companion objects. CO ice at 4.67 micron is detected in
a few main targets and one companion object. One object (LLN 19) shows little
CO ice but strong gas-phase CO ro-vibrational lines in absorption. The CO ice
profiles are different from source to source. The amount of water ice and CO
ice trapped in a water-rich mantle may correlate with the flux ratio at 12 and
25 micron. The abundance of H2O-rich CO likely correlates with that of water
ice. A weak feature at 3.54 mu attributed to solid CH3OH and a broad feature
near 4.62 mu are observed toward LLN17, but not toward the other sources. The
derived abundances of solid CH3OH and OCN- are ~10% and ~1% of the H2O ice
abundance respectively. The H2O optical depths do not show an increase with
envelope mass, nor do they show lower values for the companion objects compared
with the main protostar. The line-of-sight CO ice abundance does not correlate
with the source bolometric luminosity. Comparison of the solid CO profile
toward LLN17, which shows an extremely broad CO ice feature, and that of its
lower mass companion at a few thousand AU, which exhibits a narrow profile,
together with the detection of OCN- toward LLN17 provide direct evidences for
local thermal processing of the ice.Comment: Replace wrong files. Accepted by A&A, 22 pages, 18 figure
Effects of CO2 on H2O band profiles and band strengths in mixed H2O:CO2 ices
H2O is the most abundant component of astrophysical ices. In most lines of
sight it is not possible to fit both the H2O 3 um stretching, the 6 um bending
and the 13 um libration band intensities with a single pure H2O spectrum.
Recent Spitzer observations have revealed CO2 ice in high abundances and it has
been suggested that CO2 mixed into H2O ice can affect relative strengths of the
3 um and 6 um bands. We used laboratory infrared transmission spectroscopy of
H2O:CO2 ice mixtures to investigate the effects of CO2 on H2O ice spectral
features at 15-135 K. We find that the H2O peak profiles and band strengths are
significantly different in H2O:CO2 ice mixtures compared to pure H2O ice. In
all H2O:CO2 mixtures, a strong free-OH stretching band appears around 2.73 um,
which can be used to put an upper limit on the CO2 concentration in the H2O
ice. The H2O bending mode profile also changes drastically with CO2
concentration; the broad pure H2O band gives way to two narrow bands as the CO2
concentration is increased. This makes it crucial to constrain the environment
of H2O ice to enable correct assignments of other species contributing to the
interstellar 6 um absorption band. The amount of CO2 present in the H2O ice of
B5:IRS1 is estimated by simultaneously comparing the H2O stretching and bending
regions and the CO2 bending mode to laboratory spectra of H2O, CO2, H2O:CO2 and
HCOOH.Comment: 12 pages, 11 figures, accepted by A&
New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints
We present new interstellar dust models which have been derived by
simultaneously fitting the far-ultraviolet to near-infrared extinction, the
diffuse infrared (IR) emission and, unlike previous models, the elemental
abundance constraints on the dust for different interstellar medium abundances,
including solar, F and G star, and B star abundances. The fitting problem is a
typical ill-posed inversion problem, in which the grain size distribution is
the unknown, which we solve by using the method of regularization. The dust
model contains various components: PAHs, bare silicate, graphite, and amorphous
carbon particles, as well as composite particles containing silicate, organic
refractory material, water ice, and voids. The optical properties of these
components were calculated using physical optical constants. As a special case,
we reproduce the Li & Draine (2001) results, however their model requires an
excessive amount of silicon, magnesium, and iron to be locked up in dust: about
50 ppm (atoms per million of H atoms), significantly more than the upper limit
imposed by solar abundances of these elements, about 34, 35, and 28 ppm,
respectively. A major conclusion of this paper is that there is no unique
interstellar dust model that simultaneously fits the observed extinction,
diffuse IR emission, and abundances constraints.Comment: 70 pages, 23 figures, accepted for publication in the Astrophysical
Journal Supplemen
Understanding the Spectral Energy Distributions of the Galactic Star Forming Regions IRAS 18314-0720, 18355-0532 & 18316-0602
Embedded Young Stellar Objects (YSO) in dense interstellar clouds is treated
self-consistently to understand their spectral energy distributions (SED).
Radiative transfer calculations in spherical geometry involving the dust as
well as the gas component, have been carried out to explain observations
covering a wide spectral range encompassing near-infrared to radio continuum
wavelengths. Various geometric and physical details of the YSOs are determined
from this modelling scheme. In order to assess the effectiveness of this
self-consistent scheme, three young Galactic star forming regions associated
with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test
cases. They cover a large range of luminosity ( 40). The modelling of
their SEDs has led to information about various details of these sources, e.g.
embedded energy source, cloud structure & size, density distribution,
composition & abundance of dust grains etc. In all three cases, the best fit
model corresponds to the uniform density distribution.Comment: AAMS style manuscript with 3 tables (in a separate file) and 4
figures. To appear in Journal of Astronophysics & Astronom
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