The temperature of interstellar dust particles is of great importance to astronomers. It plays a crucial role in the thermodynamics of interstellar clouds, because of the gas-dust collisional ...coupling. It is also a key parameter in astrochemical studies that governs the rate at which molecules form on dust. In 3D (magneto)hydrodynamic simulations often a simple expression for the dust temperature is adopted, because of computational constraints, while astrochemical modelers tend to keep the dust temperature constant over a large range of parameter space. Our aim is to provide an easy-to-use parametric expression for the dust temperature as a function of visual extinction (AV) and to shed light on the critical dependencies of the dust temperature on the grain composition. We obtain an expression for the dust temperature by semi-analytically solving the dust thermal balance for different types of grains and compare to a collection of recent observational measurements. We also explore the effect of ices on the dust temperature. Our results show that a mixed carbonaceous-silicate type dust with a high carbon volume fraction matches the observations best. We find that ice formation allows the dust to be warmer by up to 15% at high optical depths (AV> 20 mag) in the interstellar medium. Our parametric expression for the dust temperature is presented as Td = 11 + 5.7 × tanh(0.61 − log 10(AV) χuv1/5.9, where χuv is in units of the Draine (1978, ApJS, 36, 595) UV field.
We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is ...used and a careful treatment of PAHs and H2 formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between ~1020 and ~1025 cm-2. We show cumulative line intensities for a number of fine-structure lines (e.g., CII, OI, CI, SiII, FeII), as well as cumulative column densities and column density ratios for a number of species (e.g., CO/H2, CO/C, HCO+/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to $N_{\rm H}=10^{22}\ {\rm cm^{-2}}$ for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to $N_{\rm H}=10^{22}\ {\rm cm^{-2}}$. The CO/C and CO/H2 ratios are lower in XDRs at low column densities and rise at $N_{\rm H}>10^{23}\ {\rm cm^{-2}}$. At most column densities $N_{\rm H} > 10^{21.5}\ {\rm cm^{-2}}$, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher NH.
We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is ...used and a careful treatment of PAHs and H2 formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between 61020 and 61025 cm-2. We show cumulative line intensities for a number of fine-structure lines (e.g., CII, OI, CI, SiII, FeII), as well as cumulative column densities and column density ratios for a number of species (e.g., CO/H2, CO/C, HCO+/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to NH = 1022 cm-2 for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 1022 cm-2. The CO/C and CO/H2 ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 1021.5 cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher NH.
Observations of high-redshift quasars at z > 6 indicate that they harbour supermassive black holes (SMBHs) of a billion solar masses. The direct collapse scenario has emerged as the most plausible ...way to assemble SMBHs. The nurseries for the direct collapse black holes are massive primordial haloes illuminated with an intense UV flux emitted by Population II (Pop II) stars. In this study, we compute the critical value of such a flux (...) for realistic spectra of Pop II stars through three-dimensional cosmological simulations. We derive the dependence of ... on the radiation spectra, on variations from halo to halo, and on the impact of X-ray ionization. Our findings show that the value of ... is a few times ... and only weakly depends on the adopted radiation spectra in the range between ... For three simulated haloes of a few times ..., varies from ... The impact of X-ray ionization is almost negligible and within the expected scatter ... for background fluxes of ... The computed estimates of ... have profound implications for the quasar abundance at z = 10 as it lowers the number density of black holes forming through an isothermal direct collapse by a few orders of magnitude below the observed black hole density. However, the sites with moderate amounts of ... cooling may still form massive objects sufficient to be compatible with observations. (ProQuest: ... denotes formulae/symbols omitted.)
We present a multi-wavelength analysis of 52 submillimeter galaxies (SMGs), identified using ALMA 870 m continuum imaging in a pilot program to precisely locate bright SCUBA-2-selected submillimeter ...sources in the UKIDSS Ultra Deep Survey (UDS) field. Using the available deep (especially near-infrared) panoramic imaging of the UDS field at optical-to-radio wavelengths we characterize key properties of the SMG population. The median photometric redshift of the bright ALMA/SCUBA-2 UDS (AS2UDS) SMGs that are detected in a sufficient number of wavebands to derive a robust photometric redshift is z = 2.65 0.13. However, similar to previous studies, 27% of the SMGs are too faint at optical-to-near-infrared wavelengths to derive a reliable photometric redshift. Assuming that these SMGs lie at z 3 raises the median redshift of the full sample to z = 2.9 0.2. A subset of 23 unlensed, bright AS2UDS SMGs have sizes measured from resolved imaging of their rest-frame far-infrared emission. We show that the extent and luminosity of the far-infrared emission are consistent with the dust emission arising from regions that are, on average, optically thick at a wavelength of (1 dispersion of 55-90 m). Using the dust masses derived from our optically thick spectral energy distribution models, we determine that these galaxies have a median hydrogen column density of NH = 9.8 × 1023 cm−2, or a corresponding median V-band obscuration of Av = 540 mag, averaged along the line of sight to the source of their rest-frame ∼200 m emission. We discuss the implications of this extreme attenuation by dust for the multi-wavelength study of dusty starbursts and reddening-sensitive tracers of star formation.
In the earliest phases of star-forming clouds, stable molecular species, such as CO, are important coolants in the gas phase. Depletion of these molecules on dust surfaces affects the thermal balance ...of molecular clouds and with that their whole evolution. For the first time, we study the effect of grain surface chemistry (GSC) on star formation and its impact on the initial mass function (IMF). We follow a contracting translucent cloud in which we treat the gas–grain chemical interplay in detail, including the process of freeze-out. We perform 3D hydrodynamical simulations under three different conditions, a pure gas-phase model, a freeze-out model, and a complete chemistry model. The models display different thermal evolution during cloud collapse as also indicated in Hocuk, Cazaux & Spaans, but to a lesser degree because of a different dust temperature treatment, which is more accurate for cloud cores. The equation of state (EOS) of the gas becomes softer with CO freeze-out and the results show that at the onset of star formation, the cloud retains its evolution history such that the number of formed stars differ (by 7 per cent) between the three models. While the stellar mass distribution results in a different IMF when we consider pure freeze-out, with the complete treatment of the GSC, the divergence from a pure gas-phase model is minimal. We find that the impact of freeze-out is balanced by the non-thermal processes; chemical and photodesorption. We also find an average filament width of 0.12 pc (±0.03 pc), and speculate that this may be a result from the changes in the EOS caused by the gas-dust thermal coupling. We conclude that GSC plays a big role in the chemical composition of molecular clouds and that surface processes are needed to accurately interpret observations, however, that GSC does not have a significant impact as far as star formation and the IMF is concerned.
Context. The seeds of the supermassive black holes with masses of ~109M⊙ observed already at z ~ 6 may have formed through the direct collapse of primordial gas in Tvir ≳ 104 K halos, whereby the ...gas must stay hot (~104 K) in order to avoid fragmentation. Aims. The interplay between magnetic fields, turbulence, and a UV radiation background during the gravitational collapse of primordial gas in a halo is explored; in particular, the possibilities for avoiding fragmentation are examined. Methods. Using an analytical one-zone model, the evolution of a cloud of primordial gas is followed from its initial cosmic expansion through turnaround, virialization, and collapse up to a density of 107 cm-3. Results. It was found that in halos with no significant turbulence, the critical UV background intensity (J21crit) for keeping the gas hot is lower by a factor ~10 for an initial comoving magnetic field B0 ~ 2 nG than for the zero-field case, and even lower for stronger fields. In turbulent halos, J21crit is found to be a factor ~10 lower than for the zero-field-zero-turbulence case, and the stronger the turbulence (more massive halo and/or stronger turbulent heating), the lower J21crit. Conclusions. The reduction in J21crit is particularly important, since it exponentially increases the number of halos exposed to a supercritical radiation background.
Accelerated gain in fat mass (FM) in early life increases the risk for adult diseases. Longitudinal data on infant body composition are crucial for clinical and research use, but very difficult to ...obtain due to limited measurement tools and unsuccessful measurements between age 6-24 months. We compared FM% by dual-energy X-ray absorptiometry (DXA), with cushion to reduce movement artifacts, with FM% by air-displacement plethysmography (ADP) and evaluated the reliability of this cushion during DXA by comparing FM% with and without cushion. Subsequently, we constructed sex-specific longitudinal body composition charts from 1-24 months.
In 692 healthy, term-born infants (Sophia Pluto Cohort), FM% was measured by ADP from 1-6 months and DXA with cushion from 6-24 months. At 6 months, FM% was measured in triplicate by ADP and DXA with and without cushion(n = 278), later on in smaller numbers.
At 6 months, mean FM% by DXA with cushion was 24.1 and by ADP 25.0, mean difference of 0.9% (Bland-Altman p = 0.321, no proportional bias). Mean FM% by DXA without cushion was 12.5% higher compared to ADP (Bland-Altman p < 0.001). DXA without cushion showed higher mean FM% compared to DXA with cushion (+11.6%, p < 0.001) at 6 months. Longitudinally, FM% increased between 1-6 months and decreased from 6-24 months(both p < 0.001).
In infants, DXA scan with cushion limits movement artifacts and shows reliable FM%, comparable to ADP. This allowed us to construct longitudinal body composition charts until 24 months. Our study shows that FM% increases from 1-6 months and gradually declines until 24 months.
Aims. The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the ...sources of radiation, numerical models are constructed. Methods. These models iteratively determine the thermal and chemical balance of molecular gas that is exposed to X-rays (1-100 keV) and far-ultraviolet radiation (6-13.6 eV), as a function of depth. Results. We present a grid of XDR and PDR models that span ranges in density ( 10 super(2)-10 super(6.5) cm super(-3)), irradiation ( 10 super(0.5)-10 super(5) G sub(0) and F_{\rm X}=1.6\times 10-}160 erg cm super(-2) s super(-1)) and column density ( 3\times 10-}1\times 10cm super(-2)). Predictions are made for the most important atomic fine-structure lines, e.g., CII, OI, CI, SiII, and for molecular species like HCO super(+), HCN, HNC, CS and SiO up to J =4, CO and super(13) CO up to J =16, and column densities for CN, CH, CH super(+), HCO, HOC super(+), NO and N sub(2) H super(+). We find that surface temperatures are higher (lower) in PDRs compared to XDRs for densities >10 super(4) (<10 super(4)) cm super(-3). For the atomic lines, we find that, largely due to the different XDR ionization balance, the fine- structure line ratios of SiII 35 \mum/CII 158 \mum, OI 63 \mum/CII 158 \mum, FeII 26 \mum/CII 158 \mum and CI 369 \mum/CI 609 \mum are larger in XDRs than in PDRs, for a given density, column and irradiation strength. Similarly, for the molecular lines, we find that the line ratios HCN/HCO super(+) and HNC/HCN, as well as the column density ratio CN/HCN, discriminate between PDRs and XDRs. In particular, the HCN/HCO super(+) 1-0 ratio is <1 (>1) for XDRs (PDRs) if the density exceeds 10 super(5) cm super(-3) and if the column density is larger than 10 super(23) cm super(-2). For columns less than 10 super(22.5) cm super(-2) the XDR HCN/HCO super(+) 1-0 ratio becomes larger than one, although the individual HCN 1-0 and HCO super(+) 1-0 line intensities are weaker. For modest densities, n =10 super(4)-10 super(5) cm super(-3), and strong radiation fields (>100 erg s super(-1) cm super(-2)), HCN/HCO super(+) ratios can become larger in XDRs than PDRs as well. Also, the HCN/CO 1- 0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for high (column) density gas, with columns in excess of 10 super(23) cm super(-2) and densities larger than 10 super(4) cm super(-3). Furthermore, CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO J = N +1- N /CO 1-0, N\ge 1, line ratios in XDRs. In particular, lines with N\ge 10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. This is crucial since the XDR/AGN contribution will typically be of a much smaller (possibly beam diluted) angular scale and a 10-25% PDR contribution can already suppress XDR distinguishing features involving HCN/HCO+ and HNC/HCN. For possible future observations, column density ratios indicate that CH, CH super(+), NO, HOC super(+) and HCO are good PDR/XDR discriminators.
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