258 research outputs found
Centrosymmetric molecules as possible carriers of diffuse interstellar bands
In this paper, we present new data with interstellar C2 (Phillips bands A-X),
from observations made with the Ultraviolet-Visual Echelle Spectrograph of the
European Southern Observatory. We have determined the interstellar column
densities and excitation temperatures of C2 for nine Galactic lines. For seven
of these, C2 has never been observed before, so in this case the still small
sample of interstellar clouds (26 lines of sight), where a detailed analysis of
C2 excitation has been made, has increased significantly. This paper is a
continuation of previous works where interstellar molecules (C2 and diffuse
interstellar bands) have been analysed. Because the sample of interstellar
clouds with C2 has increased, we can show that the width and shape of the
profiles of some diffuse interstellar bands (6196 and 5797 A) apparently depend
on the gas kinetic and rotational temperatures of C2; the profiles are broader
because of the higher values of the gas kinetic and rotational temperatures of
C2. There are also diffuse interstellar bands (4964 and 5850 A) for which this
effect does not exist.Comment: 8 pages, 4 figures, accepted to MNRAS 201
Interstellar Matter and the Boundary Conditions of the Heliosphere
The interstellar cloud surrounding the solar system regulates the galactic
environment of the Sun, and determines the boundary conditions of the
heliosphere. Both the Sun and interstellar clouds move through space, so these
boundary conditions change with time. Data and theoretical models now support
densities in the cloud surrounding the solar system of n(HI)=0.22+/-0.06 cm^-3,
and n(e-)~0.1 cm-3, with larger values allowed for n(HI) by radiative transfer
considerations. Ulysses and Extreme Ultraviolet Explorer satellite HeI data
yield a cloud temperature of 6,400 K. Nearby interstellar gas appears to be
structured and inhomogeneous. The interstellar gas in the Local Fluff cloud
complex exhibits elemental abundance patterns in which refractory elements are
enhanced over the depleted abundances found in cold disk gas. Within a few
parsecs of the Sun, inconclusive evidence for factors of 2--5 variation in MgII
and FeII gas phase abundances is found, providing evidence for variable grain
destruction. Observations of the hydrogen pile-up at the nose of the
heliosphere are consistent with a barely subsonic motion of the heliosphere
with respect to the surrounding interstellar cloud. Uncertainties on the
velocity vector of the cloud that surrounds the solar system indicate that it
is uncertain as to whether the Sun and alpha Cen are or are not immersed in the
same interstellar cloud.Comment: 24 pages 3 figure
Abundances and rotational temperatures of the C2 interstellar molecule towards six reddened early-type stars
Using high-resolution (~85000) and high signal-to-noise ratio (~200) optical
spectra acquired with the European Southern Observatory Ultraviolet and Visual
Echelle Spectrograph, we have determined the interstellar column densities of
C2 for six Galactic lines of sight with E(B- V) ranging from 0.33 to 1.03. For
our purposes, we identified and measured absorption lines belonging to the (1,
0), (2, 0) and (3, 0) Phillips bands A1{\Pi}u-X1{\Sigma}+g. We report on the
identification of a few lines of the C2 (4, 0) Phillips system towards HD
147889. The curve-of-growth method is applied to the equivalent widths to
determine the column densities of the individual rotational levels of C2. The
excitation temperature is extracted from the rotational diagrams. The physical
parameters of the intervening molecular clouds (e.g. gas kinetic temperatures
and densities of collision partners) were estimated by comparison with the
theoretical model of excitation of C2.Comment: 11 pages, 3 figures, MNRAS 201
Electron density and carriers of the diffuse interstellar bands
We have used the ionisation equilibrium equation to derive the electron
density in interstellar clouds in the direction to 13 stars. A linear relation
was found, that allows the determination of the electron density from the Mg I
and Mg II column densities in diffuse clouds.
The comparison of normalised equivalent width of 12 DIBs with the electron
density shows that the DIBs equivalent width do not change with electron
density varying in the range ne=0.01-2.5 cm^-3. Therefore the DIBs carriers (1)
can be observed only in one ionisation stage, or (2) the DIBs are arising in
cloud regions (eg. cores or cloud coronas) for which we can not determine the
electron density
FUSE observations of molecular hydrogen on the line of sight towards HD141569A
We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE)
spectrum of HD141569A, a transitional object known to possess a circumstellar
disk. We observe two components of gas at widely different temperatures along
the line of sight. We detect cold H2, which is thermalized up to J=2 at a
kinetic temperature of 51K. Such low temperatures are typical of the diffuse
interstellar medium. Since the line of sight to HD141569A does not pass through
its disk, it appears that we are observing the cold H2 in a low extinction
envelope associated with the high Galactic latitude dark cloud complex L134N,
which is in the same direction and at nearly the same distance as HD141569A.
The column densities of the higher J-levels of H2 suggest the presence of warm
gas along the line of sight. The excitation conditions do not seem to be
consistent with what is generally observed in diffuse interstellar clouds. The
observed radial velocity of the gas implies that the UV spectral lines we
observe are likely interstellar in origin rather than circumstellar, although
our absorption line study does not definitely rule out the possibility that the
warm gas is close to the star. The discovery of such warm gas along the line of
sight may provide evidence for turbulent phenomena in the dark cloud L134N.Comment: accepted for publication in A&
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
The Interstellar Environment of our Galaxy
We review the current knowledge and understanding of the interstellar medium
of our galaxy. We first present each of the three basic constituents - ordinary
matter, cosmic rays, and magnetic fields - of the interstellar medium, laying
emphasis on their physical and chemical properties inferred from a broad range
of observations. We then position the different interstellar constituents, both
with respect to each other and with respect to stars, within the general
galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts
Evolutionary aspects of resurrection ecology: Progress, scope, and applications-an overview
This perspective provides an overview to the Special Issue on Resurrection Ecology (RE). It summarizes the contributions to this Special Issue, and provides background information and future prospects for the use of RE in both basic and applied evolutionary studies.Peer reviewedIntegrative Biolog
Pre-inverse-crowns : synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation
Two new alkali metal monoalkyl-bisamido magnesiates, the potassium compound [KMg(TMP)2nBu] and its sodium congener [NaMg(TMP) 2nBu] have been synthesised in crystalline form (TMP = 2,2,6,6-tetramethylpiperidide). Devoid of solvating ligands and possessing excellent solubility in hydrocarbon solvents, these compounds open up a new gateway for the synthesis of inverse crowns. X-ray crystallography established that [KMg(TMP)2nBu] exists in three polymorphic forms, namely a helical polymer with an infinite KNMgN chain, a hexamer with a 24-atom (KNMgN)6 ring having endo-disposed alkyl substituents, and a tetramer with a 16-atom (KNMgN)4 ring also having endo-disposed alkyl substituents. Proving their validity as pre-inverse-crowns, both magnesiates react with benzene and toluene to generate known inverse crowns in syntheses much improved from the original, supporting the idea that the metallations take place via a template effect. [KMg(TMP)2nBu] reacts with naphthalene to generate the new inverse crown [KMg(TMP)2(2-C 10H7)]6, the molecular structure of which shows a 24-atom (KNMgN)6 host ring with six naphthalene guest anions regioselectively magnesiated at the 2-position. An alternative unprecedented 1,4-dimagnesiation of naphthalene was accomplished via [NaMg(TMP) 2nBu] and its NaTMP co-complex "[NaMg(TMP) 2nBu]·NaTMP", manifested in [{Na 4Mg2(TMP)4(2,2,6-trimethyl-1,2,3,4- tetrahydropyridide)2}(1,4-C10H6)]. Adding to its novelty, this 12-atom (NaNNaNMgN)2 inverse crown structure contains two demethylated TMP ligands as well as four intact ones. Reactivity studies show that the naphthalen-ide and -di-ide inverse crowns can be regioselectively iodinated to 2-iodo and 1,4-diiodonaphthalene respectively
A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for 17
different elements sampled over 243 sight lines in the local part of our Galaxy
reveals that the depletions into solid form (dust grains) are extremely well
characterized by trends that employ only three kinds of parameters. One is an
index that describes the overall level of depletion applicable to the gas in
any particular sight line, and the other two represent linear coefficients that
describe how to derive each element's depletion from this sight-line parameter.
The information from this study reveals the relative proportions of different
elements that are incorporated into dust at different stages of grain growth.
An extremely simple scheme is proposed for deriving the dust contents and
metallicities of absorption-line systems that are seen in the spectra of
distant quasars or the optical afterglows of gamma-ray bursts. Contrary to
presently accepted thinking, the elements sulfur and krypton appear to show
measurable changes in their depletions as the general levels of depletions of
other elements increase, although more data are needed to ascertain whether or
not these findings truly compelling. Nitrogen appears to show no such increase.
The incorporation of oxygen into solid form in the densest gas regions far
exceeds the amounts that can take the form of silicates or metallic oxides;
this conclusion is based on differential measurements of depletion and thus is
unaffected by uncertainties in the solar abundance reference scale.Comment: 166 pages, 21 figures, pages 116-166 contain detailed tabulations
that may not be of interest to most readers. Accepted for publication in the
Astrophysical Journa
- …