8,468 research outputs found
Chemistry in Protoplanetary Disks
This comprehensive review summarizes our current understanding of the
evolution of gas, solids and molecular ices in protoplanetary disks. Key
findings related to disk physics and chemistry, both observationally and
theoretically, are highlighted. We discuss which molecular probes are used to
derive gas temperature, density, ionization state, kinematics, deuterium
fractionation, and study organic matter in protoplanetary disks.Comment: 83 pages, 8 figures, 5 tables, to be published in a Thematic Issue
"Astrochemistry" in Chem. Reviews (December 2013). This document is the
unedited Author's version of a Submitted Work that was subsequently accepted
for publication in Chemical Reviews, copyright (c) American Chemical Society
after peer revie
SO(9) supergravity in two dimensions
We present maximal supergravity in two dimensions with gauge group SO(9). The
construction is based on selecting the proper embedding of the gauge group into
the infinite-dimensional symmetry group of the ungauged theory. The bosonic
part of the Lagrangian is given by a (dilaton-)gravity coupled non-linear
gauged sigma-model with Wess-Zumino term. We give explicit expressions for the
fermionic sector, the Yukawa couplings and the scalar potential which supports
a half-supersymmetric domain wall solution. The theory is expected to describe
the low-energy effective action upon reduction on the D0-brane near-horizon
warped AdS_2 x S^8 geometry, dual to the supersymmetric (BFSS) matrix quantum
mechanics.Comment: 35 pages, 1 figur
Dust input from AGB stars in the Large Magellanic Cloud
The dust-forming population of AGB stars and their input to the interstellar
dust budget of the Large Magellanic Cloud (LMC) are studied with evolutionary
dust models with the main goals (1) to investigate how the amount and
composition of dust from AGB stars vary over galactic history; (2) to
characterise the mass and metallicity distribution of the present population of
AGB stars; (3) to quantify the contribution of AGB stars of different mass and
metallicity to the present stardust population in the interstellar medium
(ISM). We use models of the stardust lifecycle in the ISM developed and tested
for the Solar neighbourhood. The first global spatially resolved reconstruction
of the star formation history of the LMC from the Magellanic Clouds Photometric
Survey is employed to calculate the stellar populations in the LMC. The dust
input from AGB stars is dominated by carbon grains from stars with masses < 4
Msun almost over the entire history of the LMC. The production of silicate,
silicon carbide and iron dust is delayed until the ISM is enriched to about
half the present metallicity in the LMC. For the first time, theoretically
calculated dust production rates of AGB stars are compared to those derived
from IR observations of AGB stars for the entire galaxy. We find good agreement
within scatter of various observational estimates. We show that the majority of
silicate and iron grains in the present stardust population originate from a
small population of intermediate-mass stars consisting of only about 4% of the
total number of stars, whereas in the Solar neighbourhood they originate from
low-mass stars. With models of the lifecycle of stardust grains in the ISM we
confirm a large discrepancy between dust input from stars and the existing
interstellar dust mass in the LMC reported in Matsuura et al. 2009.Comment: Accepted to A&
Optical properties of cosmic dust analogs: A review
Nanometer- and micrometer-sized solid particles play an important role in the
evolutionary cycle of stars and interstellar matter. The optical properties of
cosmic grains determine the interaction of the radiation field with the solids,
thereby regulating the temperature structure and spectral appearance of dusty
regions. Radiation pressure on dust grains and their collisions with the gas
atoms and molecules can drive powerful winds. The analysis of observed spectral
features, especially in the infrared wavelength range, provides important
information on grain size, composition and structure as well as temperature and
spatial distribution of the material.
The relevant optical data for interstellar, circumstellar, and protoplanetary
grains can be obtained by measurements on cosmic dust analogs in the laboratory
or can be calculated from grain models based on optical constants. Both
approaches have made progress in the last years, triggered by the need to
interpret increasingly detailed high-quality astronomical observations. The
statistical theoretical approach, spectroscopic experiments at variable
temperature and absorption spectroscopy of aerosol particulates play an
important role for the successful application of the data in dust astrophysics.Comment: 18 pages, 6 figures, invited review for Journal of Nanophotonics,
Special Section to honour C.F. Bohre
Can star cluster environment affect dust input from massive AGB stars?
We examine the fraction of massive asymptotic giant branch (AGB) stars
remaining bound in their parent star clusters and the effect of irradiation of
these stars by intracluster ultraviolet (UV) field. We employ a set of N-body
models of dynamical evolution of star clusters rotating in a galactic potential
at the solar galactocentric radius. The cluster models are combined with
stellar evolution formulae, a library of stellar spectra, and simple models for
SiO photodissociation in circumstellar environment (CSE). The initial stellar
masses of clusters are varied from to .
Results derived for individual clusters are combined using a mass distribution
function for young star clusters. We find that about 30% of massive AGB stars
initially born in clusters become members of the field population, while the
rest evolves in star clusters. They are irradiated by strong intracluster UV
radiation resulting in the decrease of the photodissociation radius of SiO
molecules, in many stars down to the dust formation zone. In absence of dust
shielding, the UV photons penetrate in the CSE deeper than in 64% and
deeper than in 42% of all massive AGB stars. If this suppresses
following dust formation, the current injection rate of silicate dust from AGB
stars in the local Galaxy decreases from to at most. A lower revised value of 40% for the
expected fraction of presolar silicate grains from massive AGB stars is still
high to explain the non-detection of these grains in meteorites.Comment: accepted to ApJ, 14 pages, 9 figures, 5 table
Iron and silicate dust growth in the Galactic interstellar medium: clues from element depletions
The interstellar abundances of refractory elements indicate a substantial
depletion from the gas phase, that increases with gas density. Our recent model
of dust evolution, based on hydrodynamic simulations of the lifecycle of giant
molecular clouds (GMCs) proves that the observed trend for [Si/H] is
driven by a combination of dust growth by accretion in the cold diffuse
interstellar medium (ISM) and efficient destruction by supernova (SN) shocks
(Zhukovska et al. 2016). With an analytic model of dust evolution, we
demonstrate that even with optimistic assumptions for the dust input from stars
and without destruction of grains by SNe it is impossible to match the observed
[Si/H] relation without growth in the ISM. We extend the
framework developed in our previous work for silicates to include the evolution
of iron grains and address a long-standing conundrum: ``Where is the
interstellar iron?'. Much higher depletion of Fe in the warm neutral medium
compared to Si is reproduced by the models, in which a large fraction of
interstellar iron (70%) is locked as inclusions in silicate grains, where it is
protected from sputtering by SN shocks. The slope of the observed
[Fe/H] relation is reproduced if the remaining depleted iron
resides in a population of metallic iron nanoparticles with sizes in the range
of 1-10nm. Enhanced collision rates due to the Coulomb focusing are important
for both silicate and iron dust models to match the observed slopes of the
relations between depletion and density and the magnitudes of depletion at high
density.Comment: Accepted for publication in the ApJ, 15 pages, 9 figure
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