309 research outputs found
The Detection of Cold Dust in Cas A: Evidence for the Formation of Metallic Needles in the Ejecta
Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of
CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the
remnant. Here we show that their interpretation of the observations faces
serious difficulties. Their inferred dust mass is larger than the mass of
refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model
faces even more difficulties if the 170 micron observations of the remnant are
included in the analysis, decreasing the cold dust temperature to ~ 8K, and
increasing its mass to > 20 M_sun. We offer here a more plausible
interpretation of their observation, in which the cold dust emission is
generated by conducting needles with properties that are completely determined
by the combined submillimeter and X-ray observations of the remnant. The
needles consist of metallic whiskers with <1% of embedded impurities that may
have condensed out of blobs of material that were expelled at high velocities
from the inner metal-rich layers of the star in an asymmetric explosion. The
needles are collisionally heated by the shocked gas to a temperature of 8K.
Taking the destruction of needles into account, a dust mass of only 1E-4 to
1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the
magnetic field, needles may give rise to observable polarized emission. The
detection of submillimeter polarization will therefore offer definitive proof
for a needle origin for the cold dust emission. Supernovae may still be proven
to be important sources of interstellar dust, but the evidence is still
inconclusive.Comment: 18 pages including 4 figures. Accepted for publication in the ApJ.
Missing reference adde
Novel room temperature ionic liquids of hexaalkyl substituted guanidinium salts for dye-sensitized solar cells
A novel family of room temperature ionic liquids, N,N-diethyl-N′,N′-dipropyl-N′′-hexyl-N′′-methylguanidinium iodide (SGI) and N,N,N′,N′-tetramethyl-N′′,N′′-dipentylguanidinium tricyanomethanide (SGTM) were designed and synthesized. Due to the strong charge delocalization on the tricyanomethanide anion and, thus, weaker ion-pairing, SGTM has a lower viscosity than SGI salt that has iodide as an anion. SGI was successfully used as an iodide resource for dye-sensitized nanocrystalline solar cells. The device with a solvent-free, SGI-based electrolyte achieved a 5.9% power conversion efficiency under an air mass 1.5 incident light of 9.47mW/cm
Episodic and long-lasting Paleozoic felsic magmatism in the pre-Alpine basement of the Suretta nappe (eastern Swiss Alps)
The Suretta nappe of eastern Switzerland contains a series of meta-igneous rocks, with the Rofna Porphyry Complex (RPC) being the most prominent member. We present LA-ICP-MS U-Pb zircon data from 12 samples representing a broad spectrum of meta-igneous rocks within the Suretta nappe, in order to unravel the pre-Alpine magmatic history of this basement unit. Fine-grained porphyries and coarse-grained augengneisses from the RPC give crystallization ages between 284 and 271Ma, which either represent distinct magma pulses or long-lasting magmatic activity in a complex magma chamber. There is also evidence for an earlier Variscan magmatic event at ~320-310Ma. Mylonites at the base of the Suretta nappe are probably derived from either the RPC augengneisses or another unknown Carboniferous-Permian magmatic protolith with a crystallization age between 320 and 290Ma. Two polymetamorphic orthogneisses from the southern Suretta nappe yield crystallization ages of ~490Ma. Inherited zircon cores are mainly of late Neoproterozoic age, with minor Neo- to Paleoproterozoic sources. We interpret the Suretta nappe as mainly representing a Gondwana-derived crustal unit, which was subsequently intruded by minor Cambrian-Ordovician and major Carboniferous-Permian magmatic rocks. Finally, the Suretta nappe was thrust into its present position during the Alpine orogeny, which hardly affected the U-Pb system in zirco
Decomposing Dusty Galaxies. I. Multi-Component Spectral Energy Distribution Fitting
We present a new multi-component spectral energy distribution (SED)
decomposition method and use it to analyze the ultraviolet to millimeter
wavelength SEDs of a sample of dusty infrared-luminous galaxies. SEDs are
constructed from spectroscopic and photometric data obtained with the Spitzer
Space Telescope, in conjunction with photometry from the literature. Each SED
is decomposed into emission from populations of stars, an AGN accretion disk,
PAHs, atomic and molecular lines, and distributions of graphite and silicate
grains. Decompositions of the SEDs of the template starburst galaxies NGC7714
and NGC2623 and the template AGNs PG0804+761 and Mrk463 provide baseline
properties to aid in quantifying the strength of star-formation and accretion
in the composite systems NGC6240 and Mrk1014. We find that obscured radiation
from stars is capable of powering the total dust emission from NGC6240,
although we cannot rule out a contribution from a deeply embedded AGN visible
only in X-rays. The decomposition of Mrk1014 is consistent with ~65% of its
power emerging from an AGN and ~35% from star-formation. We suggest that many
of the variations in our template starburst SEDs may be explained in terms of
the different mean optical depths through the clouds of dust surrounding the
young stars within each galaxy. Prompted by the divergent far-IR properties of
our template AGNs, we suggest that variations in the relative orientation of
their AGN accretion disks with respect to the disks of the galaxies hosting
them may result in different amounts of AGN-heated cold dust emission emerging
from their host galaxies. We estimate that 30-50% of the far-IR and PAH
emission from Mrk1014 may originate from such AGN-heated material in its host
galaxy disk.Comment: 27 pages, 12 figures. Accepted for publication in the Ap
An Investigation of Diffuse Interstellar Gas toward a Large, Low Extinction Window into the Inner Galaxy
Halpha and Hbeta spectroscopy with the Wisconsin H-Alpha Mapper (WHAM)
reveals a strong concentration of high velocity emission in a ~ 5 by 5 deg area
centered near (l.b) = (27,-3), known as the Scutum Cloud. The high velocities
imply that we are detecting optical emission from near the plane of the Galaxy
out to the tangent point at heliocentric distances of D \gtrsim 6 kpc, assuming
the gas participates in circular Galactic rotation. The ratio of the Halpha to
Hbeta emission as a function of velocity suggests that dust along these lines
of sight produces a total visual extinction of A_v ~ 3 at D ~ 6 kpc. This makes
it possible to use optical emission lines to explore the physical conditions of
ionized gas in the inner Galaxy. At a Galactocentric distance R_G ~ 4 kpc, for
example, we find that the H^+ has an rms midplane density of ~ 1 cm^-3 with a
vertical scale height of ~ 300 pc. We also find evidence for an increase in the
flux of Lyman continuum photons and an increase in the ratio of ionized to
neutral hydrogen toward the inner Galaxy. We have extended the measurements of
E(B-V) in this direction to distances far beyond what has been accessible
through stellar photometry and find E(B-V)/N_H to be near the local mean of 1.7
x 10^-22 cm^2 mag, with evidence for an increase in this ratio at R_G ~ 4 kpc.
Finally, our observations of [NII] 6583, [SII] 6716, and [OIII] 5007 toward the
window reveal that in the inner Galaxy the temperature of the gas and the
ionization state of oxygen increase with increasing height from the midplane.Comment: ApJ, accepted. 28 pages, 13 figures, 1 tabl
Hydration dynamics at fluorinated protein surfaces
Water-protein interactions dictate many processes crucial to protein function including folding, dynamics, interactions with other biomolecules, and enzymatic catalysis. Here we examine the effect of surface fluorination on water-protein interactions. Modification of designed coiled-coil proteins by incorporation of 5,5,5-trifluoroleucine or (4S)-2-amino-4-methylhexanoic acid enables systematic examination of the effects of side-chain volume and fluorination on solvation dynamics. Using ultrafast fluorescence spectroscopy, we find that fluorinated side chains exert electrostatic drag on neighboring water molecules, slowing water motion at the protein surface
Polarization of absorption lines as a diagnostics of circumstellar, interstellar and intergalactic magnetic fields: Fine structure atoms
The relative population of the fine structure sublevels of an atom's ground
state is affected by radiative transitions induced by an anisotropic radiation
flux. This causes the alignment of atomic angular momentum. In terms of
observational consequences for the interstellar and intergalactic medium, this
results in the polarization of the absorption lines. In the paper we consider
the conditions necessary for this effect and provide calculations of
polarization from a few astrophysically important atoms and ions with multiple
upper and lower levels for an arbitrary orientation of magnetic fields to the
a) source of optical pumping, b) direction of observation, c) absorbed source.
We also consider an astrophysically important ``degenerate'' case when the
source of optical pumping coincides with the source of the absorbed radiation.
We present analytical expressions that relate the degree of linear polarization
and the intensity of absorption to the 3D orientation of the magnetic field
with respect to the pumping source, the source of the absorbed radiation, and
the direction of observations. We discuss how all these parameters can be
determined via simultaneous observations of several absorption lines and
suggest graphical means that are helpful in practical data interpretation. We
prove that studies of absorption line polarization provide a unique tool to
study 3D magnetic field topology in various astrophysical conditions.Comment: 22 pages, 10 figures, ApJ, in pres
Discovery of Large-Scale Gravitational Infall in a Massive Protostellar Cluster
We report Mopra (ATNF), Anglo-Australian Telescope, and Atacama Submillimeter
Telescope Experiment observations of a molecular clump in Carina, BYF73 =
G286.21+0.17, which give evidence of large-scale gravitational infall in the
dense gas. From the millimetre and far-infrared data, the clump has mass ~ 2 x
10^4 Msun, luminosity ~ 2-3 x 10^4 Lsun, and diameter ~ 0.9 pc. From radiative
transfer modelling, we derive a mass infall rate ~ 3.4 x 10^-2 Msun yr-1. If
confirmed, this rate for gravitational infall in a molecular core or clump may
be the highest yet seen. The near-infrared K-band imaging shows an adjacent
compact HII region and IR cluster surrounded by a shell-like photodissociation
region showing H2 emission. At the molecular infall peak, the K imaging also
reveals a deeply embedded group of stars with associated H2 emission. The
combination of these features is very unusual and we suggest they indicate the
ongoing formation of a massive star cluster. We discuss the implications of
these data for competing theories of massive star formation.Comment: v1: 23 pages single-column, 6 figures (some multipart) at end v2: 14
pages 2-column, 6 figures interspersed v3: edited to referee's comments with
new sections and new figures; accepted to MNRAS, 20 pages 2-column, 8 figures
(some multipart) intersperse
Herschel observations of interstellar chloronium
Using the Herschel Space Observatory's Heterodyne Instrument for the
Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six
sources in the Galaxy. We detected interstellar chloronium absorption in
foreground molecular clouds along the sight-lines to the bright submillimeter
continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and
para-H2-37Cl+ isotopologues were detected, through observations of their
1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz,
respectively. For an assumed ortho-to-para ratio of 3, the observed optical
depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the
gas phase. We detected interstellar chloronium emission from two sources in the
Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion
South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the
observed emission line fluxes imply total beam-averaged column densities of ~
2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two
sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2
Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL
2591. The chloronium abundances inferred in this study are typically at least a
factor ~10 larger than the predictions of steady-state theoretical models for
the chemistry of interstellar molecules containing chlorine. Several
explanations for this discrepancy were investigated, but none has proven
satisfactory, and thus the large observed abundances of chloronium remain
puzzling.Comment: Accepted for publication in the Astrophysical Journa
Stellar Evolutionary Effects on the Abundances of PAH and SN-Condensed Dust in Galaxies
Spectral and photometric observations of nearby galaxies show a correlation
between the strength of their mid-IR aromatic features, attributed to PAH
molecules, and their metal abundance, leading to a deficiency of these features
in low-metallicity galaxies. In this paper, we suggest that the observed
correlation represents a trend of PAH abundance with galactic age, reflecting
the delayed injection of carbon dust into the ISM by AGB stars in the final
post-AGB phase of their evolution. AGB stars are the primary sources of PAHs
and carbon dust in galaxies, and recycle their ejecta back to the interstellar
medium only after a few hundred million years of evolution on the main
sequence. In contrast, more massive stars that explode as Type II supernovae
inject their metals and dust almost instantaneously after their formation. We
first determined the PAH abundance in galaxies by constructing detailed models
of UV-to-radio SED of galaxies that estimate the contribution of dust in
PAH-free HII regions, and PAHs and dust from photodissociation regions, to the
IR emission. All model components: the galaxies' stellar content, properties of
their HII regions, and their ionizing and non-ionizing radiation fields and
dust abundances, are constrained by their observed multiwavelength spectrum.
After determining the PAH and dust abundances in 35 nearby galaxies using our
SED model, we use a chemical evolution model to show that the delayed injection
of carbon dust by AGB stars provides a natural explanation to the dependence of
the PAH content in galaxies with metallicity. We also show that larger dust
particles giving rise to the far-IR emission follow a distinct evolutionary
trend closely related to the injection of dust by massive stars into the ISM.Comment: ApJ, 69 pages, 46 figures, Accepte
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