arXiv subjects

Hiroyuki Hirashita

Publications and source records attributed to Hiroyuki Hirashita.

At least 73 records · Page 4Linked to original sources

arXiv2015

Dust Cooling in Supernova Remnants in the Large Magellanic Cloud

The infrared-to-X-ray (IRX) flux ratio traces the relative importance of dust cooling to gas cooling in astrophysical plasma such as supernova remnants (SNRs). We derive IRX ratios of SNRs in the LMC using Spitzer and Chandra SNR survey data and compare them with those of Galactic SNRs. IRX ratios of all the SNRs in the sample are found to be moderately greater than unity, indicating that dust grains are a more efficient coolant than gas although gas cooling may not be negligible. The IRX ratios of the LMC SNRs are systematically lower than those of the Galactic SNRs. As both dust cooling and gas cooling pertain to the properties of the interstellar medium, the lower IRX ratios of the LMC SNRs may reflect the characteristics of the LMC, and the lower dust-to- gas ratio (a quarter of the Galactic value) is likely to be the most significant factor. The observed IRX ratios are compared with theoretical predictions that yield IRX ratios an order of magnitude larger. This discrepancy may originate from the dearth of dust in the remnants due to either the local variation of the dust abundance in the preshock medium with respect to the canonical abundance or the dust destruction in the postshock medium. The non-equilibrium ionization cooling of hot gas, in particular for young SNRs, may also cause the discrepancy. Finally, we discuss implications for the dominant cooling mechanism of SNRs in low-metallicity galaxies.

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arXiv2015

A new era of sub-millimeter GRB afterglow follow-ups with the Greenland Telescope

A planned rapid submillimeter (submm) Gamma Ray Burst (GRBs) follow-up observations conducted using the Greenland Telescope (GLT) is presented. The GLT is a 12-m submm telescope to be located at the top of the Greenland ice sheet, where the high-altitude and dry weather porvides excellent conditions for observations at submm wavelengths. With its combination of wavelength window and rapid responding system, the GLT will explore new insights on GRBs. Summarizing the current achievements of submm GRB follow-ups, we identify the following three scientific goals regarding GRBs: (1) systematic detection of bright submm emissions originating from reverse shock (RS) in the early afterglow phase, (2) characterization of forward shock and RS emissions by capturing their peak flux and frequencies and performing continuous monitoring, and (3) detections of GRBs as a result of the explosion of first-generation stars result of GRBs at a high redshift through systematic rapid follow ups. The light curves and spectra calculated by available theoretical models clearly show that the GLT could play a crucial role in these studies.

astro-ph.HE↗

arXiv2014

Two-size approximation: a simple way of treating the evolution of grain size distribution in galaxies

Full calculations of the evolution of grain size distribution in galaxies are in general computationally heavy. In this paper, we propose a simple model of dust enrichment in a galaxy with a simplified treatment of grain size distribution by imposing a `two-size approximation'; that is, all the grain population is represented by small (grain radius a < 0.03 micron) and large (a > 0.03 micron) grains. We include in the model dust supply from stellar ejecta, destruction in supernova shocks, dust growth by accretion, grain growth by coagulation and grain disruption by shattering, considering how these processes work on the small and large grains. We show that this simple framework reproduces the main features found in full calculations of grain size distributions as follows. The dust enrichment starts with the supply of large grains from stars. At a metallicity level referred to as the critical metallicity of accretion, the abundance of the small grains formed by shattering becomes large enough to rapidly increase the grain abundance by accretion. Associated with this epoch, the mass ratio of the small grains to the large grains reaches the maximum. After that, this ratio converges to the value determined by the balance between shattering and coagulation, and the dust-to-metal ratio is determined by the balance between accretion and shock destruction. With a Monte Carlo simulation, we demonstrate that the simplicity of our model has an advantage in predicting statistical properties. We also show some applications for predicting observational dust properties such as extinction curves.

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arXiv2014

Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

We present the detection and analysis of molecular hydrogen emission toward ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of twelve regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0--0 transitions of H$_2$ at 28.2 and 17.1${\,\rm μm}$ are detected in the IRS spectra for ten regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3$σ$ detection threshold is achieved for lines at 12.2 and 8.6${\,\rm μm}$. The excitation diagrams of the detected H$_2$ transitions are used to determine the warm H$_2$ gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range $86-137\,{\rm K}$. The bulk of the excited H$_2$ gas is found at these temperatures and contributes $\sim$5-17% to the total gas mass. We find a tight correlation of the H$_2$ surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photo-electric heating in photodissociation regions. We find the excitation of H$_2$ by this process is equally efficient in both atomic and molecular dominated regions. We also present the correlation of the warm H$_2$ physical conditions with dust properties. The warm H$_2$ mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H$_2$ excitation in photodissociation regions.

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arXiv2014

Evolution of grain size distribution in high-redshift dusty quasars: Integrating large amounts of dust and unusual extinction curves

The discoveries of huge amounts of dust and unusual extinction curves in high-redshift quasars (z > 4) cast challenging issues on the origin and properties of dust in the early universe. In this Letter, we investigate the evolutions of dust content and extinction curve in a high-z quasar, based on the dust evolution model taking account of grain size distribution. First, we show that the Milky-Way extinction curve is reproduced by introducing a moderate fraction (~0.2) of dense molecular-cloud phases in the interstellar medium for a graphite-silicate dust model. Then we show that the peculier extinction curves in high-z quasars can be explained by taking a much higher molecular-cloud fraction (>0.5), which leads to more efficient grain growth and coagulation, and by assuming amorphous carbon instead of graphite. The large dust content in high-z quasar hosts is also found to be a natural consequence of the enhanced dust growth. These results indicate that grain growth and coagulation in molecular clouds are key processes that can increase the dust mass and change the size distribution of dust in galaxies, and that, along with a different dust composition, can contribute to shape the extinction curve.

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arXiv2014

Effects of grain growth on the interstellar polarization curve

We apply the time evolution of grain size distributions by accretion and coagulation found in our previous work to the modelling of the wavelength dependence of interstellar linear polarization. We especially focus on the parameters of the Serkowski curve $K$ and $λ_{\max}$ characterizing the width and the maximum wavelength of this curve, respectively. We use aligned silicate and non-aligned carbonaceous spheroidal particles with different aspect ratios $a/b$. The imperfect alignment of grains with sizes larger than a cut-off size $r_{V,\rm cut}$ is considered. We find that the evolutionary effects on the polarization curve are negligible in the original model with commonly used material parameters (hydrogen number density $n_\mathrm{H}=10^3$ cm$^{-3}$, gas temperature $T_\mathrm{gas}=10$~K, and the sticking probability for accretion $S_\mathrm{acc}=0.3$). Therefore, we apply the tuned model where the coagulation threshold of silicate is removed. In this model, $λ_{\max}$ displaces to the longer wavelengths and the polarization curve becomes wider ($K$ reduces) on time-scales $\sim (30 - 50) (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr. The tuned models at $T < 30 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1} $ Myr and different values of the parameters $r_{V,\rm cut}$ can also explain the observed trend between $K$ and $λ_{\max}$. It is significant that the evolutionary effect appears in the perpendicular direction to the effect of $r_{V,\rm cut}$ on the $K$ - $λ_{\max}$ diagram. Very narrow polarization curves can be reproduced if we change the type of particles (prolate/oblate) and/or vary $a/b$.

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arXiv2014

Constraining dust formation in high-redshift young galaxies

Core-collapse supernovae (SNe) are believed to be the first significant source of dust in the Universe. Such SNe are expected to be the main dust producers in young high-redshift Lyman $α$ emitters (LAEs) given their young ages, providing an excellent testbed of SN dust formation models during the early stages of galaxy evolution. We focus on the dust enrichment of a specific, luminous LAE (Himiko, $z\simeq 6.6$) for which a stringent upper limit of $52.1~μ$Jy ($3σ$) has recently been obtained from ALMA continuum observations at 1.2 mm. We predict its submillimetre dust emission using detailed models that follow SN dust enrichment and destruction and the equilibrium dust temperature, and obtain a plausible upper limit to the dust mass produced by a single SN: $m_\mathrm{d,SN} < 0.15$--0.45 M$_\odot$, depending on the adopted dust optical properties. These upper limits are smaller than the dust mass deduced for SN 1987A and that predicted by dust condensation theories, implying that dust produced in SNe are likely to be subject to reverse shock destruction before being injected into the interstellar medium. Finally, we provide a recipe for deriving $m_\mathrm{d,SN}$ from submillimetre observations of young, metal poor objects wherein condensation in SN ejecta is the dominant dust formation channel.

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arXiv2014

The dust production rate of AGB stars in the Magellanic Clouds

We compare theoretical dust yields for stars with mass 1 Msun < mstar < 8 Msun, and metallicities 0.001 < Z < 0.008 with observed dust production rates (DPR) by carbon- rich and oxygen-rich Asymptotic Giant Branch (C-AGB and O-AGB) stars in the Large and Small Magellanic Clouds (LMC, SMC). The measured DPR of C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very efficient during the carbon-star stage. The same yields over-predict the observed DPR in the SMC, suggesting a stronger metallicity dependence of the mass-loss rates during the carbon- star stage. DPR of O-AGB stars suggest that rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if mstar > 3 Msun and Z > 0.001. When compared to the most recent observations, our models support a stellar origin for the existing dust mass, if no significant destruction in the ISM occurs, with a contribution from AGB stars of 70% in the LMC and 15% in the SMC.

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arXiv2014

Evolution of dust content in galaxies probed by gamma-ray burst afterglows

Because of their brightness, gamma-ray burst (GRB) afterglows are viable targets for investigating the dust content in their host galaxies. Simple intrinsic spectral shapes of GRB afterglows allow us to derive the dust extinction. Recently, the extinction data of GRB afterglows are compiled up to redshift $z=6.3$, in combination with hydrogen column densities and metallicities. This data set enables us to investigate the relation between dust-to-gas ratio and metallicity out to high redshift for a wide metallicity range. By applying our evolution models of dust content in galaxies, we find that the dust-to-gas ratio derived from GRB afterglow extinction data are excessively high such that they can be explained with a fraction of gas-phase metals condensed into dust ($f_\mathrm{in}$) $\sim 1$, while theoretical calculations on dust formation in the wind of asymptotic giant branch stars and in the ejecta of Type II supernovae suggest a much more moderate condensation efficiency ($f_\mathrm{in}\sim 0.1$). Efficient dust growth in dense clouds has difficulty in explaining the excessive dust-to-gas ratio at metallicities $Z/\mathrm{Z}_\odot <ε$, where $ε$ is the star formation efficiency of the dense clouds. However, if $ε$ is as small as 0.01, the dust-to-gas ratio at $Z\sim 10^{-2}$ Z$_\odot$ can be explained with $n_\mathrm{H}\gtrsim 10^6$ cm$^{-3}$. Therefore, a dense environment hosting dust growth is required to explain the large fraction of metals condensed into dust, but such clouds should have low star formation efficiencies to avoid rapid metal enrichment by stars.

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arXiv2014

Planet Traps and First Planets: the Critical Metallicity for Gas Giant Formation

The ubiquity of planets poses an interesting question: when first planets are formed in galaxies. We investigate this problem by adopting a theoretical model developed for understanding the statistical properties of exoplanets. Our model is constructed as the combination of planet traps with the standard core accretion scenario in which the efficiency of forming planetary cores directly relates to the dust density in disks or the metallicity ([Fe/H]). We statistically compute planet formation frequencies (PFFs) as well as the orbital radius ($ $) within which gas accretion becomes efficient enough to form Jovian planets. The three characteristic exoplanetary populations are considered: hot Jupiters, exo-Jupiters densely populated around 1 AU, and low-mass planets such as super-Earths. We explore the behavior of the PFFs as well as $ $ for the three different populations as a function of metallicity ($-2 \leq$[Fe/H]$\leq -0.6$). We show that the total PFFs increase steadily with metallicity, which is the direct outcome of the core accretion picture. For the entire range of the metallicity considered here, the population of the low-mass planets dominates over the Jovian planets. The Jovian planets contribute to the PFFs above [Fe/H]-1. We find that the hot Jupiters form at lower metallcities than the exo-Jupiters. This arises from the radially inward transport of planetary cores by their host traps, which is more effective for lower metallicity disks due to the slower growth of the cores. The PFFs for the exo-Jupiters exceed those for the hot Jupiters around [Fe/H]-0.7. Finally, we show that the critical metallicity for forming Jovian planets is [Fe/H]-1.2, which is evaluated by comparing the values of $ $ between the hot Jupiters and the low-mass planets. The comparison intrinsically links to the different gas accretion efficiency between them.

astro-ph.EP↗

arXiv2014

Discovery of an Outstanding Disk in the cD Galaxy of the Hydra A Cluster

The central cD galaxy of the Hydra A cluster has one of the most powerful active galactic nuclei (AGNs) in the nearby Universe (z <0.2). We report on the discovery of a dust lane in the cD galaxy using Subaru telescope. The i'-band image shows the existence of a dark band of the size of 3.6"x 0.7" (4 kpc x 0.8 kpc), which appears to be quite similar to the dust lane observed in Centaurus A. The morphology indicates that the cold disk that seen as the dust lane is almost edge-on and rotates around the AGN. Since the minor axis of the dust lane is nearly parallel to the radio jets emerging from the AGN, the disk is probably feeding its gas into the central black hole. From the absorption, we estimate the hydrogen column density of the lane is N_H=2.0 x 10^21 cm^-2, and the mass of the disk is ~8 x 10^7 M_sun. The column density is consistent with constraints obtained from Chandra X-ray observations. The age of the disk is >~ 4 x 10^7 yr. The position angle of the disk and the galaxy's photometric axis are misaligned, which may imply that the cold gas in the disk is brought via galaxy mergers. Our observations may indicate that the supply of cold gas by galaxy mergers is required for the most intensive feedback from AGNs.

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arXiv2014

Evolution of the extinction curves in galaxies

We investigate the evolution of extinction curves in galaxies based on our evolution model of grain size distribution. In this model, we considered various processes: dust formation by SNe II and AGB stars, dust destruction by SN shocks in the ISM, metal accretion onto the surface of grains (referred to as grain growth), shattering and coagulation. We find that the extinction curve is flat in the earliest stage of galaxy evolution. As the galaxy is enriched with dust, shattering becomes effective to produce a large abundance of small grains ($a \la 0.01\;μ$m). Then, grain growth becomes effective at small grain radii, forming a bump at $a \sim 10^{-3}${--}$10^{-2}\;μ$m on the grain size distribution. Consequently, the extinction curve at ultraviolet (UV) wavelengths becomes steep, and a bump at $1/λ\sim 4.5\;μ{\rm m}^{-1}\;(λ: \mbox{wavelength})$ on the extinction curve becomes prominent. Once coagulation becomes effective, the extinction curves become flatter, but the UV extinction remains overproduced when compared with the Milky Way extinction curve. This discrepancy can be resolved by introducing a stronger contribution of coagulation. Consequently, an interplay between shattering and coagulation could be important to reproduce the Milky Way extinction curve.

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arXiv2014

Dense molecular cloud cores as a source of micrometer-sized grains in galaxies

Coreshine in dense molecular cloud cores (dense cores) is interpreted as evidence for micrometer-sized grains (referred to as very large grains, VLGs). VLGs may have a significant influence on the total dust amount and the extinction curve. We estimate the total abundance of VLGs in the Galaxy, assuming that dense cores are the site of VLG formation. We find that the VLG abundance relative to the total dust mass is roughly $ϕ_\mathrm{VLG}\sim 0.01(1-ε)/ε(τ_\mathrm{SF}/5\times 10^9~\mathrm{yr})^{-1} (f_\mathrm{VLG}/0.5)(t_\mathrm{shat}/10^8~\mathrm{yr})$, where $ε$ is the star formation efficiency in dense cores, $τ_\mathrm{SF}$ the timescale of gas consumption by star formation, $f_\mathrm{VLG}$ the fraction of dust mass eventually coagulated into VLGs in dense cores, and $t_\mathrm{shat}$ the lifetime of VLGs (determined by shattering). Adopting their typical values for the Galaxy, we obtain $ϕ_\mathrm{VLG}\sim 0.02$--0.09. This abundance is well below the value detected in the heliosphere by Ulysses and Galileo, which means that local enhancement of VLG abundance in the solar neighborhood is required if the VLGs originate from dense cores. We also show that the effects of VLGs on the extinction curve are negligible even with the upper value of the above range, $ϕ_\mathrm{VLG}\sim 0.09$. If we adopt an extreme value, $ϕ_\mathrm{VLG}\sim 0.5$, close to that inferred from the above spacecraft data, the extinction curve is still in the range of the variation in Galactic extinction curves, but is not typical of the diffuse ISM.

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arXiv2014

Formation History of Polycyclic Aromatic Hydrocarbons in Galaxies

Polycyclic aromatic hydrocarbons (PAHs) are some of the major dust components in the interstellar medium (ISM). We present our evolution models for the abundance of PAHs in the ISM on a galaxy-evolution timescale. We consider shattering of carbonaceous dust grains in interstellar turbulence as the formation mechanism of PAHs while the PAH abundance can be reduced by coagulation onto dust grains, destruction by supernova shocks, and incorporation into stars. We implement these processes in a one-zone chemical evolution model to obtain the evolution of the PAH abundance in a galaxy. We find that PAH formation becomes accelerated above certain metallicity where shattering becomes efficient. For PAH destruction, while supernova shock is the primary mechanism in the metal-poor environment, coagulation is dominant in the metal-rich environment. We compare the evolution of the PAH abundances in our models with observed abundances in galaxies with a wide metallicity range. Our models reproduce both the paucity of PAH detection in low metallicity galaxies and the metallicity-dependence of the PAH abundance in high-metallicity galaxies. The strong metallicity dependence of PAH abundance appears as a result of the strong metallicity dependence of the dust mass increase by the accretion of metals onto dust grains, which are eventually shattered into PAHs. We conclude that the observational trend of the PAH abundance can be a natural consequence of shattering of carbonaceous grains being the source of PAHs. To establish our scenario of PAH formation, observational evidence of PAH formation by shattering would be crucial.

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arXiv2013

Effects of grain growth mechanisms on the extinction curve and the metal depletion in the interstellar medium

Dust grains grow their sizes in the interstellar clouds (especially in molecular clouds) by accretion and coagulation. Here we model and test these processes by examining the consistency with the observed variation of the extinction curves in the Milky Way. We find that, if we simply use the parameters used in previous studies, the model fails to explain the flattening of far-UV extinction curve for large $R_V$ (flatness of optical extinction curve) and the existence of carbon bump even in flat extinction curves. This discrepancy is resolved by adopting a `tuned' model, in which coagulation of carbonaceous dust is less efficient (by a factor of 2) and that of silicate is more efficient with the coagulation threshold removed. The tuned model is also consistent with the relation between silicon depletion (indicator of accretion) and $R_V$ if the duration of accretion and coagulation is >100(n_H/10^3 cm^{-3})^{-1} Myr, where n_H is the number density of hydrogen nuclei in the cloud. We also examine the relations between each of the extinction curve features (UV slope, far-UV curvature, and carbon bump strength) and $R_V$. The correlation between UV slope and $R_V$, which is the strongest among the three correlations, is well reproduced by the tuned model. For far-UV curvature and carbon bump strength, the observational data are located between the tuned model and the original model without tuning, implying that the large scatters in the observational data can be explained by the sensitive response to the coagulation efficiency. The overall success of the tuned model indicates that accretion and coagulation are promising mechanisms of producing the variation of extinction curves in the Milky Way, although we do not exclude possibilities of other dust-processing mechanisms changing extinction curves.

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arXiv2013

Condition for the formation of micron-sized dust grains in dense molecular cloud cores

We investigate the condition for the formation of micron-sized grains in dense cores of molecular clouds. This is motivated by the detection of the mid-infrared emission from deep inside a number of dense cores, the so-called `coreshine,' which is thought to come from scattering by micron-sized grains. Based on numerical calculations of coagulation starting from the typical grain size distribution in the diffuse interstellar medium, we obtain a conservative lower limit to the time $t$ to form micron-sized grains: $t/t_\mathrm{ff}>3 (5/S) (n_\mathrm{H}/10^5 \mathrm{cm}^{-3})^{-1/4}$ (where $t_\mathrm{ff}$ is the free-fall time at hydrogen number density $n_\mathrm{H}$ in the core, and $S$ the enhancement factor to the grain-grain collision cross-section to account for non-compact aggregates). At the typical core density $n_\mathrm{H}=10^5 \mathrm{cm}^{-3}$, it takes at least a few free-fall times to form the micron-sized grains responsible for coreshine. The implication is that those dense cores observed in coreshine are relatively long-lived entities in molecular clouds, rather than dynamically transient objects that last for one free-fall time or less.

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arXiv2013

What determines the grain size distribution in galaxies?

We construct a dust evolution model taking into account the grain size distribution, and investigate what kind of dust processes determine the grain size distribution at each stage of galaxy evolution. In addition to the dust production by type II supernovae (SNeII) and asymptotic giant branch (AGB) stars, we consider three processes in the ISM: (i) dust destruction by SN shocks, (ii) metal accretion onto the surface of preexisting grains in the cold neutral medium (CNM) (called grain growth), and (iii) grain-grain collisions (shattering and coagulation) in the warm neutral medium (WNM) and CNM. We found that the grain size distribution in galaxies is controlled by stellar sources in the early stage of galaxy evolution, and that afterwards the main processes that govern the size distribution changes to those in the ISM. Since shattering produces a large abundance of small grains (consequently, the surface-to-volume ratio of grains increases), it enhances the efficiency of grain growth, contributing to the significant increase of the total dust mass. Grain growth creates a large bump in the grain size distribution around a ~ 0.01 μm. Coagulation occurs effectively after the number of small grains is enhanced by shattering, and the grain size distribution is deformed to have a bump at a ~ 0.03 - 0.05 μm at t ~ 10 Gyr. We conclude that the evolutions of the total dust mass and the grain size distribution in galaxies are closely related to each other, and the grain size distribution changes considerably through the galaxy evolution because the dominant dust processes which regulate the grain size distribution change.

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arXiv2013

Evolution of dust grain size distribution by shattering in the interstellar medium: robustness and uncertainty

Shattering of dust grains in the interstellar medium is a viable mechanism of small grain production in galaxies. We examine the robustness or uncertainty in the theoretical predictions of shattering. We identify $P_1$ (the critical pressure above which the deformation destroys the original lattice structures) as the most important quantity in determining the timescale of small grain production, and confirm that the same $P_1/t$ ($t$ is the duration of shattering) gives the same grain size distribution [$n(a)$, where $a$ is the grain radius] after shattering within a factor of 3. The uncertainty in the fraction of shocked material that is eventually ejected as fragments causes uncertainties in $n(a)$ by a factor of 1.3 and 1.6 for silicate and carbonaceous dust, respectively. The size distribution of shattered fragments have minor effects as long as the power index of the fragment size distribution is less than ~ 3.5, since the slope of grain size distribution $n(a)$ continuously change by shattering and becomes consistent with $n(a)\propto a^{-3.5}$. The grain velocities as a function of grain radius can have an imprint in the grain size distribution especially for carbonaceous dust. We also show that the formulation of shattering can be simplified without losing sufficient precision.

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