We use a suite of high-resolution molecular cloud simulations carried out with the moving mesh code arepo to explore the nature of star-forming filaments. The simulated filaments are identified and ...categorized from column density maps in the same manner as for recent Herschel observations. When fitted with a Plummer-like profile, the filaments are in excellent agreement with observations, and have shallow power-law profiles of p ∼ 2.2 without the need for magnetic support. When data within 1 pc of the filament centre are fitted with a Gaussian function, the average full width at half-maximum (FWHM) is ∼0.3 pc, in agreement with predictions for accreting filaments. However, if the fit is constructed using only the inner regions, as in Herschel observations, the resulting FWHM is only ∼0.2 pc. This value is larger than that measured in IC 5146 and Taurus, but is similar to that found in the Planck Galactic cold cores and in Cygnus X. The simulated filaments have a range of widths rather than a constant value. When the column density maps are compared to the 3D gas densities, the filaments seen in column density do not belong to a single structure. Instead, they are made up of a network of short ribbon-like sub-filaments reminiscent of those seen in Taurus. The sub-filaments are pre-existing within the simulated clouds, have radii similar to their Jeans radius, and are not primarily formed through fragmentation of the larger filament seen in column density. Instead, small filamentary clumps are swept together into a single column density structure by the large-scale collapse of the cloud.
Mid-infrared spectroscopic measurements from the Infrared Spectrometer (IRS) on Spitzer are given for 125 hard X-ray active galactic nuclei (AGNs; 14-195 keV) from the Swift Burst Alert Telescope ...(BAT) sample and for 32 AGNs with black hole masses (BHMs) from reverberation mapping. The 9.7 mum silicate feature in emission or absorption defines an infrared AGN classification describing whether AGNs are observed through dust clouds, indicating that 55% of the BAT AGNs are observed through dust. Dust luminosity scales closely with BHM, log vL sub(v)(7.8 mu) = (37.2 + or - 0.5) + 0.87 log BHM for luminosity in erg s super(-1) and BHM in M sub(middot in circle). Comparing with dust-obscured galaxies from Spitzer and WISE surveys, we find no evidence of hyperluminous obscured quasars whose maximum infrared luminosities exceed the maximum infrared luminosities of optically discovered quasars. For the SDSS/WISE quasars, the median log L sub(IR)/L sub(bol) = 0.1, with extremes indicating that ultraviolet-derived L sub(bol) can be seriously underestimated even for type 1 quasars.
We study the star formation efficiency (SFE) in simulations and observations of turbulent, magnetized, molecular clouds. We find that the probability density functions (PDFs) of the density and the ...column density in our simulations with solenoidal, mixed, and compressive forcing of turbulence, sonic Mach numbers of 3-50, and magnetic fields in the super- to the trans-Alfvenic regime all develop power-law tails of flattening slope with increasing SFE. The high-density tails of the PDFs are consistent with equivalent radial density profiles, {rho}{proportional_to}r {sup -{kappa}} with {kappa} {approx} 1.5-2.5, in agreement with observations. Studying velocity-size scalings, we find that all the simulations are consistent with the observed v{proportional_to}l{sup 1/2} scaling of supersonic turbulence and seem to approach Kolmogorov turbulence with v{proportional_to}l{sup 1/3} below the sonic scale. The velocity-size scaling is, however, largely independent of the SFE. In contrast, the density-size and column density-size scalings are highly sensitive to star formation. We find that the power-law slope {alpha} of the density power spectrum, P {sub 3D}({rho}, k){proportional_to}k {sup {alpha}}, or equivalently the {Delta}-variance spectrum of the column density, {sigma}{sup 2} {sub {Delta}}({Sigma}, l) {proportional_to} l{sup -{alpha}}, switches sign from {alpha} {approx}< 0 for SFE {approx} 0 to {alpha} {approx}> 0 when star formation proceeds (SFE > 0). We provide a relation to compute the SFE from a measurement of {alpha}. Studying the literature, we find values ranging from {alpha} = -1.6 to +1.6 in observations covering scales from the large-scale atomic medium, over cold molecular clouds, down to dense star-forming cores. From those {alpha} values, we infer SFEs and find good agreement with independent measurements based on young stellar object (YSO) counts, where available. Our SFE-{alpha} relation provides an independent estimate of the SFE based on the column density map of a cloud alone, without requiring a priori knowledge of star formation activity or YSO counts.
The evolution of supermassive Population III stars Haemmerlé, Lionel; Woods, T E; Klessen, Ralf S ...
Monthly notices of the Royal Astronomical Society,
02/2018, Letnik:
474, Številka:
2
Journal Article
Recenzirano
Abstract
Supermassive primordial stars forming in atomically cooled haloes at z ∼ 15–20 are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star ...evolves under accretion rates of 0.1–1 M⊙ yr−1 until the general relativistic instability triggers its collapse to a black hole at masses of ∼105 M⊙. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionizing radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates 0.001–10 M⊙ yr−1, computed with the geneva stellar evolution code including general relativistic corrections to the internal structure. We compute for the first time evolutionary tracks in the mass range M > 105 M⊙. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than 0.01 M⊙ yr−1, the stars evolve as red, cool supergiants with surface temperatures below 104 K towards masses >105 M⊙. Moreover, even with the lower rates 0.001 ${\mathrm{M}_{{\odot }}}\, {\rm yr}{^{-1}}<\dot{M}<$ 0.01 M⊙ yr−1, the surface temperature is substantially reduced from 105 to 104 K for M ≳ 600 M⊙. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionizing feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars. We provide numerical tables for the surface properties of our models.
ABSTRACT We develop a model of dust evolution in a multiphase, inhomogeneous interstellar medium (ISM) using hydrodynamical simulations of giant molecular clouds in a Milky Way-like spiral galaxy. We ...improve the treatment of dust growth by accretion in the ISM to investigate the role of the temperature-dependent sticking coefficient and ion-grain interactions. From detailed observational data on the gas-phase Si abundances measured in the local Galaxy, we derive a relation between the average and the local gas density that we use as a critical constraint for the models. This relation requires a sticking coefficient that decreases with the gas temperature. The relation predicted by the models reproduces the slope of −0.5 for the observed relation in cold clouds, which is steeper than that for the warm medium and is explained by dust growth. We find that growth occurs in the cold medium for all adopted values of the minimum grain size amin from 1 to 5 nm. For the classical cutoff of , the Coulomb repulsion results in slower accretion and higher than the observed values. For , the Coulomb interactions enhance the growth rate, steepen the slope of the - relation, and provide a better match to observations. The rates of dust re-formation in the ISM by far exceed the rates of dust production by stellar sources. After the initial 140 Myr, the cycle of matter in and out of dust reaches a steady state, in which the dust growth balances the destruction on a similar timescale of 350 Myr.
Abstract
Star clusters interact with the interstellar medium (ISM) in various ways, most importantly in the destruction of molecular star-forming clouds, resulting in inefficient star formation on ...galactic scales. On cloud scales, ionizing radiation creates H ii regions, while stellar winds and supernovae (SNe) drive the ISM into thin shells. These shells are accelerated by the combined effect of winds, radiation pressure, and SN explosions, and slowed down by gravity. Since radiative and mechanical feedback is highly interconnected, they must be taken into account in a self-consistent and combined manner, including the coupling of radiation and matter. We present a new semi-analytic 1D feedback model for isolated massive clouds (≥105 M⊙) to calculate shell dynamics and shell structure simultaneously. It allows us to scan a large range of physical parameters (gas density, star formation efficiency, and metallicity) and to estimate escape fractions of ionizing radiation f
esc, i, the minimum star formation efficiency εmin required to drive an outflow, and recollapse time-scales for clouds that are not destroyed by feedback. Our results show that there is no simple answer to the question of what dominates cloud dynamics, and that each feedback process significantly influences the efficiency of the others. We find that variations in natal cloud density can very easily explain differences between dense-bound and diffuse-open star clusters. We also predict, as a consequence of feedback, a 4–6 Myr age difference for massive clusters with multiple generations.
Gravitational waves (GWs) provide a revolutionary tool to investigate yet unobserved astrophysical objects. Especially the first stars, which are believed to be more massive than present-day stars, ...might be indirectly observable via the merger of their compact remnants. We develop a self-consistent, cosmologically representative, semi-analytical model to simulate the formation of the first stars. By extrapolating binary stellar-evolution models at 10 per cent solar metallicity to metal-free stars, we track the individual systems until the coalescence of the compact remnants. We estimate the contribution of primordial stars to the merger rate density and to the detection rate of the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO). Owing to their higher masses, the remnants of primordial stars produce strong GW signals, even if their contribution in number is relatively small. We find a probability of ≳1 per cent that the current detection GW150914 is of primordial origin. We estimate that aLIGO will detect roughly 1 primordial BH–BH merger per year for the final design sensitivity, although this rate depends sensitively on the primordial initial mass function (IMF). Turning this around, the detection of black hole mergers with a total binary mass of ∼ 300 M⊙ would enable us to constrain the primordial IMF.
The probability density function (PDF) of the gas density in turbulent supersonic flows is investigated with high-resolution numerical simulations. In a systematic study, we compare the density ...statistics of compressible turbulence driven by the usually adopted solenoidal forcing (divergence-free) and by compressive forcing (curl-free). Our results are in agreement with studies using solenoidal forcing. However, compressive forcing yields a significantly broader density distribution with standard deviation image3 times larger at the same rms Mach number. The standard deviation-Mach number relation used in analytical models of star formation is reviewed and a modification of the existing expression is proposed, which takes into account the ratio of solenoidal and compressive modes of the turbulence forcing.
Abstract
We introduce the FirstLight project, which aims to generate a large data base of high-resolution, zoom-in simulations of galaxy formation around the epoch of reionization (z ≥ 6). The first ...results of this programme agree well with recent observational constraints at z = 6–8, including the ultraviolet (UV) luminosity function and galaxy stellar mass function, as well as the scaling relationships between halo mass, stellar mass and UV magnitude. The UV luminosity function starts to flatten below M
UV > −14 due to stellar feedback in haloes with maximum circular velocities of V = 30–40 km s−1. The power-law slope of the luminosity function evolves rapidly with redshift, reaching a value of α ≃ −2.5 at z = 10. On the other hand, the galaxy stellar mass function evolves slowly with time between z = 8 and 10, in particular, at the low-mass end.
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