380 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
Robinson-Trautman solutions with scalar hair and Ricci flow
The vacuum Robinson-Trautman solution admits a shear-free and twist-free null
geodesic congruence with a nonvanishing expansion. We perform a comprehensive
classification of solutions exhibiting this property in Einstein's gravity with
a massless scalar field, assuming that the solution belongs at least to
Petrov-type II and some of the components of Ricci tensor identically vanish.
We find that these solutions can be grouped into three distinct classes: (I-a)
a natural extension of the Robinson-Trautman family incorporating a scalar hair
satisfying the time derivative of the Ricci flow equation, (I-b) a novel
non-asymptotically flat solution characterized by two functions satisfying
Perelman's pair of the Ricci flow equations, and (II) a dynamical solution
possessing , or symmetry. We
provide a complete list of all explicit solutions falling into Petrov type D
for classes (I-a) and (I-b). Moreover, leveraging the massless solution in
class (I-a), we derive the neutral Robinson-Trautman solution to the gauged supergravity with the prepotential . By flipping
the sign of the kinetic term of the scalar field, the Petrov-D class (I-a)
solution leads to a time-dependent wormhole with an instantaneous spacetime
singularity. Although the general solution is unavailable for class (II), we
find a new dynamical solution with spherical symmetry from the AdS-Roberts
solution via AdS/Ricci-flat correspondence.Comment: v2: 42 pages, 2 figures; clarifications amended, references added, to
appear in CQ
Wormhole C-metric
The C-metric in vacuum general relativity describes a pair of accelerated
black holes supported by conical singularity. In this paper, we present a new
family of exact solutions to the Einstein-phantom scalar system that describes
accelerated wormholes in AdS. In the zero acceleration limit with a vanishing
potential, the present solution recovers the asymptotically flat wormhole
originally constructed by Ellis and Bronnikov. The scalar potential of the
phantom field has an infinite number of critical points and is expressed in
terms of the superpotential, which is obtained by suitable analytic
continuation of one parameter family of the gauged supergravity.
As one traverses two asymptotic regions connected by throat, the scalar field
evolves from AdS, corresponding to the origin of the potential, towards the
neighboring AdS local minimum of the potential. We find that the flipping
transformation, which interchanges the role of ``radial'' and "angular''
coordinates at the expense of double Wick rotation, is an immediate cause for
the existence of two branches of static AdS wormholes discovered previously.
Contrary to the ordinary C-metric, the conical singularity along the symmetry
axis can be completely resolved, when the (super)potential is periodic or zero.
We explore the global causal structure in detail.Comment: 17 pages, 5 figure
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
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
Constraining the amount of circumstellar matter and dust around Type Ia supernovae through near-infrared echoes
The circumstellar (CS) environment is key to understanding progenitors of
type Ia supernovae (SNe Ia), as well as the origin of a peculiar extinction
property toward SNe Ia for cosmological application. It has been suggested that
multiple scatterings of SN photons by CS dust may explain the non-standard
reddening law. In this paper, we examine the effect of re-emission of SN
photons by CS dust in the infrared (IR) wavelength regime. This effect allows
the observed IR light curves to be used as a constraint on the position/size
and the amount of CS dust. The method was applied to observed near-infrared
(NIR) SN Ia samples; meaningful upper limits on the CS dust mass were derived
even under conservative assumptions. We thereby clarify a difficulty associated
with the CS dust scattering model as a general explanation for the peculiar
reddening law, while it may still apply to a sub-sample of highly reddened SNe
Ia. For SNe Ia in general, the environment at the interstellar scale appears to
be responsible for the non-standard extinction law. Furthermore, deeper limits
can be obtained using the standard nature of SN Ia NIR light curves. In this
application, an upper limit of Mdot ~10^{-8}-10^{-7} Msun/yr (for the wind
velocity of ~10 km/s) is obtained for a mass loss rate from a progenitor up to
~0.01 pc, and Mdot ~10^{-7}-10^{-6} Msun/yr up to ~0.1 pc.Comment: 13 pages, 12 figures. Accepted for publication in MNRA
Evolution of newly formed dust in Population III supernova remnants and its impact on the elemental composition of Population II.5 stars
We investigate the evolution of dust formed in Population III supernovae
(SNe) by considering its transport and processing by sputtering within the SN
remnants (SNRs). We find that the fates of dust grains within SNRs heavily
depend on their initial radii . For Type II SNRs expanding into
the ambient medium with density of cm, grains of
m are detained in the shocked hot gas and are
completely destroyed, while grains of m are injected
into the surrounding medium without being destroyed significantly. Grains with
= 0.05-0.2 m are finally trapped in the dense shell behind
the forward shock. We show that the grains piled up in the dense shell enrich
the gas up to 10 , high enough to form low-mass stars
with 0.1-1 . In addition, [Fe/H] in the dense shell ranges from -6 to
-4.5, which is in good agreement with the ultra-metal-poor stars with [Fe/H] <
-4. We suggest that newly formed dust in a Population III SN can have great
impacts on the stellar mass and elemental composition of Population II.5 stars
formed in the shell of the SNR.Comment: 5 pages, 3 figures and 1 table. To appear in the proceedings of IAU
Symposium 255 "Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxies", Rapallo, June 2008, eds. L.K. Hunt, S. Madden, & R. Schneider
(Cambridge Univ. Press
- …