Phase Of Rapid Accretion Research Articles

Rapid Growth of Seed Black Holes during Early Bulge Formation

Abstract We study the early growth of massive seed black holes (BHs) via accretion in protogalactic nuclei where the stellar bulge component is assembled, performing axisymmetric two-dimensional radiation hydrodynamical simulations. We find that when a seed BH with M • ∼ 105 M ⊙ is embedded in dense metal-poor gas (Z = 0.01 Z ⊙) with a density of ≳ 100 cm−3 and bulge stars with a total mass of M ⋆ ≳ 100 M •, a massive gaseous disk feeds the BH efficiently at rates of ≳ 0.3–1 M ⊙ yr−1, and the BH mass increases nearly tenfold within ∼2 Myr. This rapid accretion phase lasts until a good fraction of the gas bounded within the bulge accretes onto the BH, although the feeding rate is regulated owing to strong outflows driven by ionizing radiation emitted from the accreting BH. The transient growing mode can be triggered for seed BHs formed in massive dark-matter halos with masses of ≳ 109 M ⊙ at z ∼ 15–20 (the virial temperature is T vir ≃ 105 K). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by z ≃ 6. This mechanism naturally yields a mass ratio of M •/M ⋆ > 0.01 higher than the value seen in the local universe. The existence of such overmassive BHs provides us with a unique opportunity to detect highly accreting seed BHs at z ∼ 15 with AB magnitude of m AB ∼ 26–29 mag at 2 μm (rest frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.

Embayment-scale coastal evolution and shoreline progradation in southeast Tasmania, Australia

Two barrier systems in southeastern Tasmania afford the opportunity to contrast Holocene barrier evolution and explore large-scale coastal morphodynamics under consistent regional-scale boundary conditions on an embayed wave-dominated coastline. New Optically Stimulated Luminescence (OSL) ages for the previously undated seaward ridges of Seven Mile Beach constrain the deposition of the foredune ridges over the past ~1000 years indicating continued shoreline progradation at ~0.4 m/yr. A further 13 OSL ages collected from the foredune ridges of Nine Mile Beach barrier detail the evolution of this barrier system and indicate progradation commenced approximately ~8500 years ago. Following this, a steady rate of shoreline advance at 0.14 m/yr was evident to near present-day with one phase of rapid accretion ~5000 years ago. This occurred as the barrier system extended eastward to span the full embayment width which resulted in the redirection of the estuarine channel and the subsequent erosion of previously deposited ridges due to channel migration likely over several thousand years. The eroded sediment may have contributed to ongoing positive sand budget in the embayment and continued shoreline progradation. Barrier ‘recycling’ was also suggested as a potential driver of progradation at Seven Mile Beach based on the truncated margin of the landward ridges. However, at both sites, the dynamics of possible sediment exchange between the back-barrier and open shoreline requires further detailed modelling. The relatively small river systems in this region supply minimal sand to the coast at these sites, however, there is some minor alongshore sediment supply along the western margin of these deep embayment's. In light of this, these two barrier systems demonstrate the importance of internal morphodynamic thresholds and cross-shore sediment exchange from the shoreface as key drivers of barrier evolution and large-scale coastal change in embayed coastal settings.

The Mass Function of Supermassive Black Holes in the Direct Collapse Scenario

Abstract One of the ideas to explain the existence of supermassive black holes (SMBH) in place by z ∼ 7 is a phase of rapid accretion onto direct collapse black holes (DCBH) that started their lives with masses ∼104–5 M ⊙ . Considering the DCBH scenario, we show that the SMBH mass function has two characteristic features: 1. a power-law at intermediate masses whose index is the dimensionless ratio α ≡ λ/γ, where λ is the growth rate of the number of DCBH and γ is the growth rate of DCBH masses, both during the DCBH growth era; 2. a break in the power law profile set by β ≡ γ T, where T is the duration of the DCBH growth era. The observed quasar luminosity function can be described with this tapered power-law function.

Mass Accretion toward Black Holes in the Final Phase of Galaxy Mergers

Abstract We studied the final phases of galactic mergers, focusing on interactions between supermassive black holes (SMBHs) and the interstellar medium in a central subkiloparsec region, using an N-body/hydrodynamics code. We observed that the mass accretion rate to one SMBH (107 M ⊙) exceeds the Eddington accretion rate when the distance between two black holes (BHs) rapidly decreases. However, this rapid accretion phase does not last for more than 107 yr, and it drops to ∼10% of the Eddington rate in the quasi-steady accretion phase. The second merger event enhances the mass accretion to the BHs; however, this phase takes place on a similar timescale to the first merger event. We also found that the active galactic nucleus (AGN) feedback and the mass accretion to BHs can coexist in the central region of merged galaxies, if the amount of feedback energy is given as , where is the accretion rate to r = 1 pc. The accretion rate is suppressed by ∼1/50 in the quasi-steady accretion phase for . The fraction of the gas that finally falls to each BH is approximately 5%–7% of the supplied total gas mass (108 M ⊙), and 15%–20% of the gas forms a circumnuclear gas inside 100 pc. This remnant gas heavily obscures the luminous phase of the AGNs during merger events, and the moderate AGN feedback does not alter this property.

Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes

Abstract A remarkable discovery of NASA’s Kepler mission is the wide diversity in the average densities of planets of similar mass. After gas disk dissipation, fully formed planets could interact with nearby planetesimals from a remnant planetesimal disk. These interactions would often lead to planetesimal accretion due to the relatively high ratio between the planet size and the hill radius for typical planets. We present calculations using the open-source stellar evolution toolkit mesa (Modules for Experiments in Stellar Astrophysics) modified to include the deposition of planetesimals into the H/He envelopes of sub-Neptunes (∼1–20 M ⊕). We show that planetesimal accretion can alter the mass–radius isochrones for these planets. The same initial planet, as a result of the same total accreted planetesimal mass, can have up to ≈5% difference in mean densities approximately several gigayears after the last accretion due to the inherent stochasticity of the accretion process. During the phase of rapid accretion, these differences are more dramatic. The additional energy deposition from the accreted planetesimals increase the ratio between the planet’s radius to that of the core during rapid accretion, which in turn leads to enhanced loss of atmospheric mass. As a result, the same initial planet can end up with very different envelope mass fractions. These differences manifest as differences in mean densities long after accretion stops. These effects are particularly important for planets that are initially less massive than ∼10 M ⊕ and with envelope mass fractions less than ∼10%, thought to be the most common type of planets discovered by Kepler.

Rapid Cycles of Episodic Adjustment: Understanding the Holocene fluvial archive of the Daintree River of Northeastern Australia

The dramatic decline in the quality of coral reef cover of the Great Barrier Reef (GBR) over recent decades has resulted in targeted research to better understand the dynamics of sedimentary sources within river systems of Northeastern Australia. European land-use practices are thought to have increased sediment yields to the GBR lagoon by 5–10 times, yet there is a poor understanding of the pre-1850 sediment dynamics. This study presents the first detailed alluvial chronology of the spatial and temporal responses of fluvial systems in the Wet Tropics of Northeastern Australia over the late Holocene. Valley fill sequences of the Daintree River, one of the least disturbed catchments draining to the GBR lagoon, are examined to investigate the significance of floodplain dynamics in sedimentary process. An optically stimulated luminescence chronology combined with a detailed sedimentary analysis suggests that floodplain stripping is a major, and hitherto unrecognised, source of sediment. Furthermore, rates of floodplain accretion are far greater than has been previously estimated from sediment modelling for wet tropical catchments. Spatial analysis of the topographical relationships between floodplain morphological units suggests that a total of 178,516 ton/ha of sediment has been stripped from three small confined floodplain reaches between 1038 ± 215 and 99 ± 10 years. Evidence suggests that these erosion events are followed by phases of rapid accretion with an average depositional rate of 3.87 ± 0.92 cm/yr between 572 ± 74 and 51 ± 12 years across the study area. The floodplain appears to be in a constant state of disequilibrium, experiencing spatially discontinuous phases of erosion and aggradation resulting in much higher volumes of sediment being redistributed within the catchment than previously considered. The unpredictable nature of these regimes and the shear volume of sediment mobilised poses significant challenges in managing sediment sources to the GBR lagoon.

Accretion on the long-lived continental margin of northeastern Australia

The northern extremity of the late Neoproterozoic-Paleozoic Tasman Orogenic zone exposed in north Queensland forms a narrow belt of tectonised rock assemblages abutting Paleoproterozoic - Mesoproterozoic rocks of the North Australian craton. The craton-orogen contact (Tasman Line) is extensively exposed, a unique circumstance for Australia. Sedimentary protoliths of the cratonic rocks were mainly deposited between 1700-1600 Ma and multiply deformed between 1600-1500 Ma. The Lynd Mylonite Zone, one expression of the Tasman Line, separates rocks of the late Neoproterozoic - Ordovician Thomson Orogen from those of the craton. The succeeding Silurian - Devonian Mossman Orogen is generally faulted against the Thomson Orogen, but in its northern extent it may directly abut the craton along the Palmerville Fault, also an expression of the Tasman Line. These two orogenic systems are dominantly of active margin association and E-stepping but deep seismic imaging indicates that they are extensively underlain by crust of Archean or Paleoproterozoic age. The Tasman Orogenic Zone in its southern part represents a broad tract of crust c. 1,000 km across, added to the cratonic core of Australia in a phase of rapid accretion. In contrast, for its north Queensland development a much smaller volume of new crust was generated, expressing slow accretion. For this region the orogenic system laps extensively onto cratonic crust, a geometry which at least in part reflects overthusting during episodes of Paleozoic contractional orogenesis. As a consequence of little orogenic accretionary outgrowth of the north Queensland continental margin, three large-scale, successive igneous assemblages of active margin association generated throughout the Paleozoic form largely co-located and overprinting belts with plutonic suites stitching the Tasman Line and extending into the craton.

Protostellar collapse of rotating cloud cores

Aims. We investigate the influence of turbulent viscosity on the collapse of a rotating molecular cloud core with axial symmetry, in particular, on the first and second collapse phase, as well as the evolution of the second (protostellar) core during its first accretion period. By using extensive numerical calculations, we monitor the intricate interactions between the newly formed protostar and the surrounding accretion disk (the first core) in which the star is embedded.Methods. We use a grid-based radiation-hydrodynamics code with a spatial grid designed to meet the high resolution required to study the second core. The radiative transfer is treated in the flux-limited diffusion approximation. A slightly supercritical Bonnor-Ebert sphere of 1 M ⊙ and uniform rotation according to a fixed centrifugal radius of 100 AU serves as the initial condition without exception. In a parameter study, we vary the β -viscosity driving the angular momentum transport.Results. Without viscosity (β = 0), a highly flattened accretion disk forms that fragments into several “cold” rings. For β = 10-4 , a single “warm” ring forms that undergoes collapse due to hydrogen dissociation. For β = 10-3 , ring formation is suppressed completely. The second collapse proceeds on the local thermal timescale, which is in contrast to the current view of a generally dynamical second collapse. During the first accretion period of the second core, the first core heats up globally and, as a consequence, a nearly spherical outflow occurs, destroying the structure of the former accretion disk completely. Finally, for β = 10-2 , we see the classical dynamical second collapse and a shorter but more rapid accretion phase. The impact on the surrounding accretion disk is even more pronounced. We follow the resulting massive outflow up to several kyr after the second collapse, where the central parts (R < 0.7 AU) are now cut out and replaced with an appropriate inner boundary condition. Matter is found to turn back to the center at a radius of 500 AU after about 4 kyr and to reach the protostar again after approximately 7 kyr. The results suggest that the star formation process consists of short and rapid accretion phases (lasting on the order of 100 yr) between long and quiet outflow periods (lasting several kyr).

Changes in lipid concentrations and fatty acid composition in rat cerebrum during maturation.

Abstract— Rat cerebrum was analysed at 20 different ages from birth to 45 days of age, for its concentration of protein, cholesterol, cerebrosides, phospholipids and gangliosides, and for the concentration of fatty acids of the linoleic and linolenic acid series. The fatty acid patterns of choline phosphoglycerides and ethanolamine phosphoglycerides were determined at the same ages. Phases of rapid accretion were found for protein, phospholipids, gangliosides and cholesterol. The accretion of the fatty acids of the linoleic acid series ceased at 20 days of age, while that of the fatty acids of the linolenic acid series continued. The fatty acid composition of the phosphoglycerides changed during the maturation of rat cerebrum and these changes consisted of chain elongation, increased unsaturation and variation in the pattern of the polyenoic acids. These changes varied irregularly with age and each developmental stage had characteristic fatty acid patterns of choline and ethanolamine phosphoglycerides.