1,007 research outputs found
Formulation of Non-steady-state Dust Formation Process in Astrophysical Environments
The non-steady-state formation of small clusters and the growth of grains
accompanied by chemical reactions are formulated under the consideration that
the collision of key gas species (key molecule) controls the kinetics of dust
formation process. The formula allows us to evaluate the size distribution and
condensation efficiency of dust formed in astrophysical environments. We apply
the formulation to the formation of C and MgSiO3 grains in the ejecta of
supernovae, as an example, to investigate how the non-steady effect influences
the formation process, condensation efficiency f_{con}, and average radius
a_{ave} of newly formed grains in comparison with the results calculated with
the steady-state nucleation rate. We show that the steady-state nucleation rate
is a good approximation if the collision timescale of key molecule tau_{coll}
is much smaller than the timescale tau_{sat} with which the supersaturation
ratio increases; otherwise the effect of the non-steady state becomes
remarkable, leading to a lower f_{con} and a larger a_{ave}. Examining the
results of calculations, we reveal that the steady-state nucleation rate is
applicable if the cooling gas satisfies Lambda = tau_{sat}/tau_{coll} > 30
during the formation of dust, and find that f_{con} and a_{ave} are uniquely
determined by Lambda_{on} at the onset time t_{on} of dust formation. The
approximation formulae for f_{con} and a_{ave} as a function of Lambda_{on}
could be useful in estimating the mass and typical size of newly formed grains
from observed or model-predicted physical properties not only in supernova
ejecta but also in mass-loss winds from evolved stars.Comment: 44 pages, 10 figures, 1 table, accepted for publication in Ap
Dust formation and mass loss around intermediate-mass AGB stars with initial metallicity in the early Universe I: Effect of surface opacity on the stellar evolution and dust-driven wind
Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB)
stars with initial metallicity in the range of and initial mass are explored by
the hydrodynamical calculations of dust-driven wind (DDW) along the AGB
evolutionary tracks. We employ the MESA code to simulate the evolution of
stars, assuming an empirical mass-loss rate in the post-main sequence phase,
and considering the three types of low-temperature opacities (scaled-solar,
CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar
evolution and the DDW. We find that the treatment of low-temperature opacity
strongly affects the dust formation and resulting DDW; in the carbon-rich AGB
phase, the maximum of 3 star with the
CO-enhanced opacity is at least one order of magnitude smaller than that with
the CNO-enhanced opacity. A wide range of stellar parameters being covered, a
necessary condition for driving efficient DDW with
yr is expressed as the effective temperature K and with the carbon excess defined as
and the Rosseland mean opacity
in units of cmg in the surface layer, and the
stellar mass (luminosity) in solar units. The derived fitting
formulae of gas and dust mass-loss rates in terms of input stellar parameters
could be useful for investigating the dust yield from AGB stars in the early
Universe being consistent with the stellar evolution calculations.Comment: 26 pages, 7 figures, 4 tables, accepted for publication in MNRA
Alfv\'en wave-driven wind from RGB and AGB stars
We develop a magnetohydrodynamical model of Alfv\'en wave-driven wind in open
magnetic flux tubes piercing the stellar surface of Red Giant Branch (RGB) and
Asymptotic Giant Branch (AGB) stars, and investigate the physical properties of
the winds. The model simulations are carried out along the evolutionary tracks
of stars with initial mass in the range of 1.5 to 3.0 and initial
metallicity =0.02. The surface magnetic field strength being set
to be 1G, we find that the wind during the evolution of star can be classified
into the following four types; the first is the wind with the velocity higher
than 80 km s in the RGB and early AGB (E-AGB) phases; the second is the
wind with outflow velocity less than 10 km s seen around the tip of RGB
or in the E-AGB phase; the third is the unstable wind in the E-AGB and
thermally pulsing AGB (TP-AGB) phases; the fourth is the stable massive and
slow wind with the mass-loss rate higher than 10 yr and
the outflow velocity lower than 20 km s in the TP-AGB phase. The
mass-loss rates in the first and second types of wind are two or three orders
of magnitude lower than the values evaluated by an empirical formula. The
presence of massive and slow wind of the fourth type suggests the possibility
that the massive outflow observed in TP-AGB stars could be attributed to the
Alfv\'en wave-driven wind.Comment: 17 pages, 15 figures, accepted for publication in Ap
Modeling Porous Dust Grains with Ballistic Aggregates. II. Light Scattering Properties
We study the light scattering properties of random ballistic aggregates
constructed in Shen et al. (Paper I). Using the discrete-dipole-approximation,
we compute the scattering phase function and linear polarization for random
aggregates with various sizes and porosities, and with two different
compositions: 100% silicate and 50% silicate-50% graphite. We investigate the
dependence of light scattering properties on wavelength, cluster size and
porosity using these aggregate models. We find that while the shape of the
phase function depends mainly on the size parameter of the aggregates, the
linear polarization depends on both the size parameter and the porosity of the
aggregates, with increasing degree of polarization as the porosity increases.
Contrary to previous studies, we argue that monomer size has negligible effects
on the light scattering properties of ballistic aggregates, as long as the
constituent monomer is smaller than the incident wavelength up to
2*pi*a_0/lambda\sim 1.6 where a_0 is the monomer radius. Previous claims for
such monomer size effects are in fact the combined effects of size parameter
and porosity. Finally, we present aggregate models that can reproduce the phase
function and polarization of scattered light from the AU Mic debris disk and
from cometary dust, including the negative polarization observed for comets at
scattering angles 160<theta<180 deg. These aggregates have moderate porosities,
P\sim 0.6, and are of sub-micron-size for the debris disk case, or micron-size
for the comet case.Comment: Submitted to ApJ. Scattering properties can be downloaded at
http://www.astro.princeton.edu/~draine/SDJ2009.html Target geometries are at
http://www.astro.princeton.edu/~draine/agglom.htm
On the formation and processing of carbon and nitrogen compounds in carbonaceous chondrites
On the basis of chemical kinetic consideration, we examine processing of carbon and nitrogen compounds that leads to the linear relation between the logarithmic contents of C and N in carbonaceous chondrites : log N=a log C-b, found by A. SHIMOYAMA et al. (Chem. Lett., 10,2013,1987), where a>1 and b are constants. It is shown that the linear relation results from dissociation of organic polymer in the grains before accretion to parent body-sized objects by radiation that penetrates through the grains such as cosmic ray. Condensation of volatile molecules composed of C and N is also examined as a possible process to form precursors of organic compounds in carbonaceous chondrites. From an analysis of a model thermodynamic system, it is conjectured that this process can also realize the linear relation under certain conditions
Dust Production Factories in the Early Universe: Formation of Carbon Grains in Red-supergiant Winds of Very Massive Population III Stars
We investigate the formation of dust in a stellar wind during the
red-supergiant (RSG) phase of a very massive Population III star with the
zero-age main sequence mass of 500 M_sun. We show that, in a carbon-rich wind
with a constant velocity, carbon grains can form with a lognormal-like size
distribution, and that all of the carbon available for dust formation finally
condense into dust for wide ranges of the mass-loss rate ((0.1-3)x10^{-3} M_sun
yr^{-1}) and wind velocity (1-100 km s^{-1}). We also find that the
acceleration of the wind driven by newly formed dust suppresses the grain
growth but still allows more than half of gas-phase carbon to be finally locked
up in dust grains. These results indicate that at most 1.7 M_sun of carbon
grains can form in total during the RSG phase of 500 M_sun Population III
stars. Such a high dust yield could place very massive primordial stars as
important sources of dust at the very early epoch of the universe if the
initial mass function of Population III stars was top-heavy. We also briefly
discuss a new formation scenario of carbon-rich ultra-metal-poor stars
considering the feedback from very massive Population III stars.Comment: 1 table, 4 figures, accepted for publication in the ApJ Letter
A model for the infrared dust emission from forming galaxies
In the early epoch of galaxy evolution, dust is only supplied by supernovae
(SNe). With the aid of a new physical model of dust production by SNe developed
by Nozawa et al. (2003) (N03), we constructed a model of dust emission from
forming galaxies on the basis of the theoretical framework of Takeuchi et al.
(2003) (T03). N03 showed that the produced dust species depends strongly on the
mixing within SNe. We treated both unmixed and mixed cases and calculated the
infrared (IR) spectral energy distribution (SED) of forming galaxies for both
cases. Our model SED is less luminous than the SED of T03 model by a factor of
2-3. The difference is due to our improved treatment of UV photon absorption
cross section, as well as different grain size and species newly adopted in
this work. The SED for the unmixed case is found to have an enhanced near to
mid-IR (N-MIR) continuum radiation in its early phase of the evolution (age <
10^{7.25} yr) compared with that for the mixed case. The strong N--MIR
continuum is due to the emission from Si grains, which only exist in the
species of the unmixed dust production. We also calculated the IR extinction
curves for forming galaxies. Then we calculated the SED of a local starbursting
dwarf galaxy SBS 0335-052. Our present model SED naturally reproduced the
strong N--MIR continuum and the lack of cold FIR emission of SBS 0335-052. We
found that only the SED of unmixed case can reproduce the NIR continuum of this
galaxy. We then made a prediction for the SED of another typical star-forming
dwarf, I Zw 18. We also presented the evolution of the SED of LBGs. Finally, we
discussed the possibility of observing forming galaxies at z > 5.Comment: MNRAS, in press. 18 pages, 15 figures. Abstract abridge
Supernova dust for the extinction law in a young infrared galaxy at z = 1
We apply the supernova(SN) extinction curves to reproduce the observed
properties of SST J1604+4304 which is a young infrared (IR) galaxy at z = 1.
The SN extinction curves used in this work were obtained from models of unmixed
ejecta of type II supernovae(SNe II) for the Salpeter initial mass function
(IMF) with a mass range from 8 to 30 M_sun or 8 to 40 M_sun.
The effect of dust distributions on the attenuation of starlight is
investigated by performing the chi-square fitting method against various dust
distributions. These are the commonly used uniform dust screen, the clumpy dust
screen, and the internal dust geometry. We add to these geometries three
scattering properties, namely, no-scattering, isotropic scattering, and
forward-only scattering. Judging from the chi-square values, we find that the
uniform screen models with any scattering property provide good approximations
to the real dust geometry. Internal dust is inefficient to attenuate starlight
and thus cannot be the dominant source of the extinction.
We show that the SN extinction curves reproduce the data of SST J1604+4304
comparable to or better than the Calzetti extinction curve. The Milky Way
extinction curve is not in satisfactory agreement with the data unless several
dusty clumps are in the line of sight. This trend may be explained by the
abundance of SN-origin dust in these galaxies; SN dust is the most abundant in
the young IR galaxy at z = 1, abundant in local starbursts, and less abundant
in the Galaxy. If dust in SST J1604+4304 is dominated by SN dust, the dust
production rate is about 0.1 M_sun per SN.Comment: 12 pages, 8 figures, 1 tabl
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