18 research outputs found
Dust origin in late-type dwarf galaxies: ISM growth vs. type II supernovae
We re-evaluate the roles of different dust sources in dust production as a
function of metallicity in late-type dwarf galaxies, with the goal of
understanding the relation between dust content and metallicity. The dust
content of late-type dwarf galaxies with episodic star formation is studied
with a multicomponent model of dust evolution, which includes dust input from
AGB stars, type II SNe and dust mass growth by accretion of gas species in the
ISM. Dust growth in the ISM becomes an important dust source in dwarf galaxies,
on the timescale of 0.1 - few Gyrs. It increases the dust-to-gas ratio (DGR)
during post-burst evolution, unlike type II SNe, which eject grains into the
ISM only during starbursts. Before the dust growth in the ISM overtakes the
dust production, AGB stars can be major sources of dust in metal-poor dwarf
galaxies. Our models reproduce the relation between the DGR and oxygen
abundance, derived from observations of a large sample of dwarf galaxies. The
steep decrease in the DGR at low O values is explained by the relatively low
efficiency of dust condensation in stars. The scatter observed at higher O
values is determined mainly by different critical metallicities for the
transition from stardust- to ISM-growth dominated dust production, depending on
the star formation history. In galaxies with episodic star formation,
additional dispersion in the DGR is introduced by grain destruction during
starbursts, followed by an increase of the dust mass due to dust growth in the
ISM during post-burst evolution. We find that the carbon-to-silicate ratio
changes dramatically, when the ISM growth becomes the dominant dust source,
therefore this ratio can be used as an indicator of the transition. The
observed DGR-O relation in dwarf galaxies favours low condensation efficiencies
in type II SNe, together with an increase in the total dust mass by means of
dust growth in the ISM.Comment: 11 pages, 10 figures, language and minor errors correcte
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&
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
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
Dust formation by stars and evolution in the interstellar medium
The main goal of this thesis is the study of the origin and evolution of interstellar dust in the Milky Way. We develop a model for the chemical evolution of the galactic disk as a basis for our new model of dust evolution, which considers for the first time the individual evolutions of stardust and of dust condensed in molecular clouds of the Galactic disk. We include dust production by AGB stars in detail, using the results of synthetic AGB models combined with models of dust condensation in stellar outflows, and estimate the efficiency of dust condensation in supernovae by matching model results for the Solar neighbourhood with observed abundances of presolar dust grains of supernova origin. Our results indicate that supernovae produce mainly carbon dust, with only small amounts of silicates, iron and silicon carbonate. We show that the interstellar dust population is dominated by dust grown in the interstellar medium across the Galactic history; moreover, dust formed in AGB stars and supernovae is a dominant source of dust only at metallicities lower than the minimal value for efficient dust growth in molecular clouds
Modelling Dust Evolution in Galaxies with a Multiphase, Inhomogeneous ISM
We develop a model of dust evolution in a multiphase, inhomogeneous ISM
including dust growth and destruction processes. The physical conditions for
grain evolution are taken from hydrodynamical simulations of giant molecular
clouds in a Milky Way-like spiral galaxy. We improve the treatment of dust
growth by accretion in the ISM to investigate the role of the
temperature-dependent sticking coefficient and ion-grain interactions. From
detailed observational data on the gas-phase Si abundances [Si/H]_{gas}
measured in the local Galaxy, we derive a relation between the average
[Si/H]_{gas} and the local gas density n(H) which we use as a critical
constraint for the models. This relation requires a sticking coefficient that
decreases with the gas temperature. The synthetic relation constructed from the
spatial dust distribution reproduces the slope of -0.5 of the observed relation
in cold clouds. This slope is steeper than that for the warm medium and is
explained by the dust growth. We find that it occurs for all adopted values of
the minimum grain size a_{min} from 1 to 5nm. For the classical cut-off of
a_{min}=5 nm, the ion-grain interactions result in longer growth timescales and
higher [Si/H]_{gas} than the observed values. For a_{min} below 3 nm, the
ion-grain interactions enhance the growth rates, steepen the slope of
[Si/H]_{gas}-n(H) relation and provide a better match to observations. The
rates of dust re-formation in the ISM by far exceed the rates of dust
production by stellar sources as expected from simple evolution models. After
the cycle of matter in and out of dust reaches a steady state, the dust growth
balances the destruction operating on similar timescales of 350 Myr.Comment: 17 pages, 11 figures, accepted to Ap
Thermal and nonthermal dust sputtering in hydrodynamical simulations of the multiphase interstellar medium
We study the destruction of interstellar dust via sputtering in supernova
(SN) shocks using three-dimensional hydrodynamical simulations. With a novel
numerical framework, we follow both sputtering and dust dynamics governed by
direct collisions, plasma drag and betatron acceleration. Grain-grain
collisions are not included and the grain-size distribution is assumed to be
fixed. The amount of dust destroyed per SN is quantified for a broad range of
ambient densities and fitting formulae are provided. Integrated over the
grain-size distribution, nonthermal (inertial) sputtering dominates over
thermal sputtering for typical ambient densities. We present the first
simulations that explicitly follow dust sputtering within a turbulent
multiphase interstellar medium. We find that the dust destruction timescales
are 0.35 Gyr for silicate dust and 0.44 Gyr for carbon dust in solar
neighborhood conditions. The SN environment has an important impact on .
SNe that occur in preexisting bubbles destroy less dust as the destruction is
limited by the amount of dust in the shocked gas. This makes about 2.5
times longer than the estimate based on results from a single SN explosion. We
investigate the evolution of the dust-to-gas mass ratio (DGR), and find that a
spatial inhomogeneity of 14\% develops for scales below 10 pc. It
locally correlates positively with gas density but negatively with gas
temperature even in the exterior of the bubbles due to incomplete gas mixing.
This leads to a 30\% lower DGR in the volume filling warm gas compared
to that in the dense clouds.Comment: 20 pages, 16 figures, accepted versio
Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-Dust Ratio Variations across ISM Phases
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on
the chemical evolution and lifecycle of dust in galaxies. We examine the
relation between dust and gas at 10-50 pc resolution in the Large and Small
Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21
cm, CO, and Halpha observations. In the diffuse atomic ISM, we derive the
gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust
ratios of 380+250-130 in the LMC, and 1200+1600-420 in the SMC, not including
helium. The atomic-to-molecular transition is located at dust surface densities
of 0.05 Mo pc-2 in the LMC and 0.03 Mo pc-2 in the SMC, corresponding to AV ~
0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion
factor to best account for all the molecular gas in the beam of the
observations, and find upper limits on XCO to be 6x1020 cm-2 K-1 km-1 s in the
LMC (Z=0.5Zo) at 15 pc resolution, and 4x 1021 cm-2 K-1 km-1 s in the SMC
(Z=0.2Zo) at 45 pc resolution. In the LMC, the slope of the dust-gas relation
in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after
accounting for the effects of CO-dark H2 in the translucent envelopes of
molecular clouds. Coagulation of dust grains and the subsequent dust emissivity
increase in molecular clouds, and/or accretion of gas-phase metals onto dust
grains, and the subsequent dust abundance (dust-to-gas ratio) increase in
molecular clouds could explain the observations. In the SMC, variations in the
dust-gas slope caused by coagulation or accretion are degenerate with the
effects of CO-dark H2. Within the expected 5--20 times Galactic XCO range, the
dust-gas slope can be either constant or decrease by a factor of several across
ISM phases. Further modeling and observations are required to break the
degeneracy between dust grain coagulation, accretion, and CO-dark H2