Takashi Kozasa

Dust in the Early Universe: Dust Formation in the Ejecta of Population III Supernovae

Dust grains play a crucial role in the formation and evolution history of stars and galaxies in the early universe. We investigate the formation of dust grains in the ejecta of Population III supernovae, including pair-instability supernovae, which are expected to occur in the early universe, applying a theory of non-steady state nucleation and grain growth. Dust formation calculations are performed for core-collapse supernovae with progenitor mass Mpr ranging from 13 to 30 M? and for pair-instability supernovae with Mpr = 170 and 200 M?. In the calculations, the time evolution of gas temperature in the ejecta, which strongly affects the number density and size of newly formed grains, is calculated by solving the radiative transfer equation, taking account of the energy deposition of radioactive elements. Two extreme cases are considered for the elemental composition in the ejecta, unmixed and uniformly mixed cases within the He core, and formation of CO and SiO molecules is assumed to be complete. The results of calculations for core-collapse supernovae and pair-instability supernovae are summarized as follows: in the unmixed ejecta, a variety of grain species condense, reflecting the difference of the elemental composition at the formation site in the ejecta; otherwise only oxide grains condense in the uniformly mixed ejecta. The average size of newly formed grains spans a range of 3 orders of magnitude, depending on the grain species and the formation condition, and the maximum radius is limited to less than 1 ?m, which does not depend on the progenitor mass. The size distribution function of each grain species is approximately lognormal, except for Mg silicates, MgO, Si, and FeS in the unmixed case and Al2O3 in both cases. The size distribution function summed up over all grain species is approximated by a power-law formula whose index is -3.5 for the larger radius and -2.5 for the smaller one; the radius at the crossover point ranges from 0.004 to 0.1 ?m, depending on the model of supernovae. The fraction of mass locked into dust grains increases with increasing the progenitor mass: 2%-5% of the progenitor mass for core-collapse supernovae and 15%-30% for pair-instability supernovae whose progenitor mass ranges from 140 to 260 M?. Thus, if very massive stars populated the first generation of stars (Population III stars), a large amount of dust grains would be produced in the early universe. We also discuss the dependence of the explosion energy and the amount of 56Ni in the ejecta, as well as the efficiency of formation of CO and SiO molecules, on the formation of dust grains in the ejecta of supernovae.

Evolution of Dust in Primordial Supernova Remnants: Can Dust Grains Formed in the Ejecta Survive and Be Injected into the Early Interstellar Medium?

We investigate the evolution of dust that formed at Population III supernova (SN) explosions and its processing through the collisions with the reverse shocks resulting from the interaction of the SN ejecta with the ambient medium. In particular, we investigate the transport of the shocked dust within the SNR and its effect on the chemical composition, the size distribution, and the total mass of dust surviving in SNRs. We find that the evolution of the reverse shock, and hence its effect on the processing of the dust, depends on the thickness of the envelope retained by the progenitor star. Furthermore, the transport and survival of the dust grains depend on their initial radius, aini, and composition: for Type II SNRs expanding into the ISM with a density of nH,0 = 1 cm-3, small grains with aini 0.05 μm are completely destroyed by sputtering in the postshock flow, while grains with aini = 0.05-0.2 μm are trapped into the dense shell behind the forward shock. Very large grains of aini 0.2 μm are ejected into the ISM without decreasing their sizes significantly. We find that the total mass fraction of dust that is destroyed by the reverse shock ranges from 0.2 to 1.0, depending on the energy of the explosion and the density of the ambient ISM. The results of our calculations have significant impact on the abundance pattern of the second-generation stars that form in the dense shell of primordial SNRs.

Formation of dust grains in the ejecta of SN 1987A

Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolometric light curve around its maximum and the early emergence of X-rays and γ-rays, results in the sequential formation of Al 2 O 3 , MgSiO 3 and Fe 3 O 4 grains respectively in the ejecta at 1.0 M ⊙ ≤M r ≤4.4 M ⊙ as the gas cools down

Dust destruction in the high-velocity shocks driven by supernovae in the early universe

We investigate the destruction of dust grains by sputtering in the high-velocity interstellar shocks driven by supernovae(SNe)intheearlyuniversetorevealthedependenceofthetimescaleofdustdestructiononthegasdensity nH;0 in the interstellar medium (ISM), as well as on the progenitor mass Mpr and explosion energy E51 of SNe. The sputtering yields for the combinations of dust and ion species of interest to us are evaluated by applying the so-called universalrelationwithaslightmodification.Thedynamicsofdustgrainsandtheirdestructionbysputteringinshocks are calculated by taking into account the size distribution of each dust species, together with the time evolution of the temperature and density of the gas in spherically symmetric shocks. The results of the calculations show that the efficiency of dust destruction depends not only on the sputtering yield but also on the initial size distribution of each grain species. The efficiency of dust destruction increases with increasing E51 and/or increasing nH;0 but is almost independent of Mpr as long as E51 is the same. The mass of gas swept up by a shock is an increasing function of E51 andadecreasingfunctionof nH;0.Combiningtheseresults,wepresenttheapproximationformulaforthetimescaleof destruction for each grain species in the early universe as a function of E51 and nH;0. This formula is applicable for investigating the evolution of dust grains at the early epoch of the universe with the metallicity of Z P10 � 3 Z� . The effects of the cooling processes of gas on the destruction of dust are briefly discussed. Subject headingg dust, extinction — early universe — shock waves — supernova remnants — supernovae: general Online material: color figures

FORMATION AND EVOLUTION OF DUST IN TYPE IIb SUPERNOVAE WITH APPLICATION TO THE CASSIOPEIA A SUPERNOVA REMNANT

The amount and size of dust formed in the ejecta of core-collapse supernovae (CCSNe) and injected into the interstellar medium (ISM) depend on the type of CCSNe through the varying thicknesses of their outer envelopes. Recently Cas A was identified as a Type IIb SN (SN IIb) that is characterized by a small-mass hydrogen envelope. In order to clarify how the amount of dust formed in the ejecta and supplied into the ISM depends on the type of CCSNe, we investigate the formation of dust grains in the ejecta of an SN IIb and their evolution in the shocked gas in the SN remnant (SNR) by considering two sets of density structures (uniform and power-law profiles) for the circumstellar medium (CSM). Based on these calculations, we also simulate the time evolution of thermal emission from the shock-heated dust in the SNR and compare the results with the observations of Cas A SNR. We find that the total mass of dust formed in the ejecta of an SN IIb is as large as 0.167 M ☉ but the average radius of dust is smaller than 0.01 μm and is significantly different from those in SNe II-P with massive hydrogen envelopes; in the explosion with the small-mass hydrogen envelope, the expanding He core undergoes little deceleration, so that the gas density in the He core is too low for large-sized grains to form. In addition, the low-mass hydrogen envelope of the SN IIb leads to the early arrival of the reverse shock at the dust-forming region. If the CSM is more or less spherical, then the newly formed small grains would be completely destroyed in the relatively dense shocked gas for the CSM hydrogen density of n H>0.1 cm–3 without being injected into the ISM. However, the actual CSM is likely to be non-spherical, so a portion of the dust grains could be ejected into the ISM without being shocked. We demonstrate that the temporal evolution of the spectral energy distribution (SED) by thermal emission from dust is sensitive to the ambient gas density and structure that affects the passage of the reverse shock into the ejecta. Thus, the SED evolution reflects the evolution of dust through erosion by sputtering and stochastic heating. For Cas A, we consider the CSM produced by the steady mass loss of M ☉ yr–1 during the supergiant phase. Then we find that the observed infrared SED of Cas A is reasonably reproduced by thermal emission from the newly formed dust of 0.08 M ☉, which consists of the 0.008 M ☉ shock-heated warm dust and 0.072 M ☉ unshocked cold dust.

Early Formation of Dust in the Ejecta of Type Ib SN 2006jc and Temperature and Mass of the Dust

SN 2006jc is a peculiar supernova (SN), in which the formation of dust has been confirmed at an early epoch of ~50 days after the explosion. We investigate the possibility of such an early formation of dust grains in the expanding ejecta of SN 2006jc, applying the Type Ib SN model that is developed to reproduce the observed light curve. We find that the rapid decrease of the gas temperature in SN 2006jc enables the condensation of C grains in the C-rich layer at 40-60 days after the explosion, which is followed by the condensation of silicate and oxide grains until ~200 days. The average radius of each grain species is confined to be less than 0.01 -->μ m due to the low gas density at the condensation time. The calculated total dust mass reaches 1.5 -->M☉, of which C dust shares 0.7 -->M☉. On the other hand, based on the calculated dust temperature, we show that the dust species and mass evaluated to reproduce the spectral energy distribution observed by AKARI and MAGNUM at day 200 are different from those obtained by the dust formation calculations; the dust species contributing to the observed flux are hot C and FeS grains with masses of -->5.6 × 10−4 and -->2.0 × 10−3 -->M☉, respectively, although we cannot defy the presence of a large amount of cold dust such as silicate and oxide grains up to 0.5 -->M☉. One of the physical processes responsible for the difference between calculated and evaluated masses of C and FeS grains could be considered to be the destruction of small-sized clusters by energetic photons and electrons prevailing within the ejecta at the earlier epoch.

FORMATION OF DUST IN THE EJECTA OF TYPE Ia SUPERNOVAE

We investigate the formation of dust grains in the ejecta of Type Ia supernovae (SNe Ia), adopting the carbon-deflagration W7 model. In the calculations of dust formation, we apply the nucleation and grain growth theory and consider the two extreme cases of the formation of CO and SiO molecules: complete formation and no formation. The results of the calculations show that for the sticking probability of ? j = 1, C, silicate, Si, and FeS grains can condense at early times of ~100-300?days after the explosion, whereas Fe and SiC grains cannot form substantially. Due to the low gas density in SNe Ia with no H-envelope, the average radii of the newly formed grains are generally below 0.01 ?m, being much smaller than those in Type II-P SNe. This supports our previous conclusion that the radius of dust formed in the ejecta is smaller in SNe with less massive envelopes. The total dust mass ranges from 3 ? 10?4 M ? to 0.2 M ? for ? j = 0.1-1, depending on whether or not CO and SiO molecules are formed. We also estimate the optical depths and thermal emission by the newly formed dust and compare them to the relevant observations of SNe Ia. We find that the formation of C grains in SNe Ia must be suppressed to be consistent with observational constraints. This implies that energetic photons and electrons heavily depress the formation efficiency of C grains or that the outermost C-O layer of SNe Ia is almost fully burned. Finally, we calculate dust destruction in the SN remnants and find that dust grains formed in the ejecta of SNe Ia are almost completely destroyed in the shocked gas before being injected into the interstellar medium. This indicates that SNe Ia are unlikely to be the major sources of interstellar dust.

Spitzer Observations of the Young Core-Collapse Supernova Remnant 1E0102-72.3: Infrared Ejecta Emission and Dust Formation

We present Spitzer Infrared Spectrograph and Infrared Array Camera observations of the young supernova remnant E0102 (SNR 1E0102-7219) in the Small Magellanic Cloud. The infrared spectra show strong lines of Ne and O, with the [Ne II] line at 12.8 μm having a large velocity dispersion of 2000-4500 km s^(–1) indicative of fast-moving ejecta. Unlike the young Galactic SNR Cas A, E0102 lacks emission from Ar and Fe. Diagnostics of the observed [Ne III] line pairs imply that [Ne III] emitting ejecta have a low temperature of 650 K, while [Ne V] line pairs imply that the infrared [Ne V] emitting ejecta have a high density of ~10^4 cm^(–3). We have calculated radiative shock models for various velocity ranges including the effects of photoionization. The shock model indicates that the [Ne V] lines come mainly from the cooling zone, which is hot and dense, whereas [Ne II] and [Ne III] come mainly from the photoionization zone, which has a low temperature of 400-1000 K. We estimate an infrared-emitting Ne ejecta mass of 0.04 M_⊙ from the infrared observations, and discuss implications for the progenitor mass. The spectra also have a dust continuum feature peaking at 18 μm that coincides spatially with the ejecta, providing evidence that dust formed in the expanding ejecta. The 18 μm peak dust feature is fitted by a mixture of MgSiO_3 and Si dust grains, while the rest of the continuum requires either carbon or Al2O3 grains. We measure the total dust mass formed within the ejecta of E0102 to be ~0.014 M_⊙. The dust mass in E0102 is thus a factor of a few smaller than that in Cas A. The composition of the dust is also different, showing relatively less silicate and likely no Fe-bearing dust, as is suggested by the absence of Fe-emitting ejecta.

Spitzer spectral mapping of supernova remnant cassiopeia A

We present the global distribution of fine-structure infrared line emission in the Cassiopeia A supernova remnant using data from the Spitzer Space Telescope’s infrared spectrograph. We identify emission from ejecta materials in the interior, prior to their encounter with the reverse shock, as well as from the postshock bright ring. The global electron density increases by ≳ 100 at the shock to ~ 10^4 cm^(−3), providing evidence for strong radiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission from low-ionization interior species like [Si ii] giving way to [S iv] and, at even further distances, highenergy X-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon abundance are arranged symmetrically around the central neutron star. These neon crescents are very closely aligned with the “kick” direction of the compact object from the remnant’s expansion center, tracing a new axis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise from different compositional layers of ejecta now passing through the reverse shock along different directions.

THE ROLE OF DUST IN THE EARLY UNIVERSE. I. PROTOGALAXY EVOLUTION

We develop one-zone galaxy formation models in the early universe, taking into account dust formation and evolution by supernova (SN) explosions. We focus on the time evolution of dust size distribution, because H2 formation on the dust surface plays a critical role in the star formation process in the early universe. In the model, we assume that star formation rate (SFR) is proportional to the total amount of H2. We consistently treat (1) the formation and size evolution of dust, (2) the chemical reaction networks including H2 formation both on the surface of dust and in gas phase, and (3) the SFR in the model. First, we find that, because of dust destruction due to both reverse and forward shocks driven by SNe, H2 formation is more suppressed than in situations without such dust d ]