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 Zini≤10−4Z_{\rm ini} \leq 10^{-4} 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 $0 \leq Z_{\rm ini} \leq 10^{-4}$ and initial mass $2\leq M_{\rm ini}/M_{\odot} \leq 5$ 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 $\dot{M}$ of $M_{\rm ini} \geq$ 3 $M_{\odot}$ 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 $\dot{M} \ge 10^{-6}$ $M_{\odot}$ yr$^{-1}$ is expressed as the effective temperature $T_{\rm eff} \lesssim 3850$ K and $\log(\delta_{\rm C}L/\kappa_{\rm R} M) \gtrsim 10.43\log T_{\rm eff}-32.33$ with the carbon excess $\delta_{\rm C}$ defined as $\epsilon_{\rm C} - \epsilon_{\rm O}$ and the Rosseland mean opacity $\kappa_{\rm R}$ in units of cm$^2$g$^{-1}$ in the surface layer, and the stellar mass (luminosity) $M$ $(L)$ 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

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

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

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 $a_{\rm ini}$. For Type II SNRs expanding into the ambient medium with density of $n_{\rm H,0} = 1$ cm$^{-3}$, grains of $a_{\rm ini} < 0.05$ $\mu$m are detained in the shocked hot gas and are completely destroyed, while grains of $a_{\rm ini} > 0.2$ $\mu$m are injected into the surrounding medium without being destroyed significantly. Grains with $a_{\rm ini}$ = 0.05-0.2 $\mu$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$^{-6}-10^{-4}$ $Z_\odot$, high enough to form low-mass stars with 0.1-1 $M_\odot$. 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

Extinction curves flattened by reverse shocks in supernovae

We investigate the extinction curves of young galaxies in which dust is supplied from Type II supernovae (SNe II) and/or pair instability supernovae (PISNe). Since at high redshift (z>5), low-mass stars cannot be dominant sources for dust grains, SNe II and PISNe, whose progenitors are massive stars with short lifetimes, should govern the dust production. Here, we theoretically investigate the extinction curves of dust produced by SNe II and PISNe, taking into account reverse shock destruction induced by collision with ambient interstellar medium. We find that the extinction curve is sensitive to the ambient gas density around a SN, since the efficiency of reverse shock destruction strongly depends on it. The destruction is particularly efficient for small-sized grains, leading to a flat extinction curve in the optical and ultraviolet wavelengths. Such a large ambient density as n_H > 1 cm^{-3} produces too flat an extinction curve to be consistent with the observed extinction curve for SDSS J104845.05+463718.3 at z=6.2. Although the extinction curve is highly sensitive to the ambient density, the hypothesis that the dust is predominantly formed by SNe at z~6 is still allowed by the current observational constraints. For further quantification, the ambient density should be obtained by some other methods. Finally we also discuss the importance of our results for observations of high-z galaxies, stressing a possibility of flat extinction curves.Comment: 8 pages, 5 figures, Accepted for publication in MNRA

The Three-Dimensional Structure of Interior Ejecta in Cassiopeia A at High Spectral Resolution

We used the Spitzer Space Telescope's Infrared Spectrograph to create a high resolution spectral map of the central region of the Cassiopeia A supernova remnant, allowing us to make a Doppler reconstruction of its 3D structure. The ejecta responsible for this emission have not yet encountered the remnant's reverse shock or the circumstellar medium, making it an ideal laboratory for exploring the dynamics of the supernova explosion itself. We observe that the O, Si, and S ejecta can form both sheet-like structures as well as filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in velocity space in some regions, and separated by 500 km/s or more in others. Ejecta traveling toward us are, on average, ~900 km/s slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features. However, models of different supernova explosions can collectively produce the observed geometries and structures of the interior emission. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cassiopeia A will begin brightening in ~30 years, and the front surface in ~100 years.Comment: 35 pages, 16 figures, accepted to Ap