Slow Accretion Rates Research Articles

What drives the variability in AGN? Explaining the UV-Xray disconnect through propagating fluctuations

ABSTRACT Intensive broad-band reverberation mapping campaigns have shown that AGN variability is significantly more complex than expected from disc reverberation of the variable X-ray illumination. The UV/optical variability is highly correlated and lagged, with longer lags at longer wavelengths as predicted, but the observed time-scales are longer than expected. Worse, the UV/optical light curves are not well correlated with the X-rays, which should drive them. Instead, we consider an intrinsically variable accretion disc, where slow mass accretion rate fluctuations are generated in the optical-UV disc, propagating down to modulate intrinsically faster X-ray variability from the central regions. We match our model to Fairall 9, a well-studied AGN with L ∼ 0.1LEdd, where the spectrum is dominated by the UV/EUV. Our model produces light curves where the X-rays and UV have very different fast variability, yet are well correlated on longer time-scales, as observed. It predicts that the intrinsic variability has optical/UV leading the X-rays, but including reverberation of the variable EUV from an inner wind produces a lagged bound-free continuum that matches the observed UV-optical lags. We conclude that optical/UV AGN variability is likely driven by intrinsic fluctuations within the disc, not X-ray reprocessing: the observed longer than expected lags are produced by reverberation of the EUV illuminating a wind, not by X-ray illumination of the disc: the increasing lag with increasing wavelength is produced by the increased contribution of the (constant lag) bound-free continuum to the spectrum, rather than indicating intrinsically larger reverberation distances for longer wavelengths.

Slow Accretion of Helium Rich Matter onto C-O White Dwarf: Does Composition of WD Matter?

Background: In binary systems, the helium accretion onto carbon-oxygen (CO) white dwarfs (WDs) plays a vital role in many astrophysical scenarios, especially in supernovae type Ia. Moreover, ignition density for accretion rate M ≲ 10-9 M yr-1 in helium accreting CO white dwarfs decides the triggering mechanism of a supernova explosion which could be either off-centre helium flash or central carbon flash. Objectives: We aim to study the accretion of helium with a slow accretion rate 510-10 M yr-1 onto relatively cool and hot white dwarfs of different abundances of carbon and oxygen. Methods: The simulation code “Modules for Experiments in Stellar Astrophysics” (MESA) has been used for our study. We analyze the variation in several properties like surface gravity (g), helium luminosity (LHe), and effective temperature (Te f f ) during the accretion phase of the white dwarfs. We also calculate the ignition density (rHe) and ignition temperature (THe) of helium burning. Findings: As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Novelty: This is the first-ever study of the dependence of helium accreting WD evolution on its composition. We find that white dwarfs of the lower abundance of carbon accrete slightly longer before the onset of helium ignition. Keywords: Supernovae Ia; White Dwarfs; Accretion; Helium fusion; Stellar evolution

What galaxy masses perturb the local cosmic expansion?

AbstractWe use 12 cosmological N-body simulations of Local Group systems (the apostle models) to inspect the relation between the virial mass of the main haloes (Mvir,1 and Mvir,2), the mass derived from the relative motion of the halo pair (Mtim), and that inferred from the local Hubble flow (Mlhf). We show that within the spherical collapse model (SCM), the correspondence between the three mass estimates is exact, i.e. Mlhf = Mtim = Mvir,1 + Mvir,2. However, comparison with apostle simulations reveals that, contrary to what the SCM states, a relatively large fraction of the mass that perturbs the local Hubble flow and drives the relative trajectory of the main galaxies is not contained within Rvir, and that the amount of ‘extravirial’ mass tends to increase in galaxies with a slow accretion rate. In contrast, modelling the peculiar velocities around the Local Group returns an unbiased constraint on the virial mass ratio of the main galaxy pair. Adopting the outer halo profile found in N-body simulations, which scales as ρ ∼ R−4 at R ≳ Rvir, indicates that the galaxy masses perturbing the local Hubble flow roughly correspond to the asymptotically convergent (total) masses of the individual haloes. We show that estimates of Mvir based on the dynamics of tracers at R ≫ Rvir require a priori information on the internal matter distribution and the growth rate of the main galaxies, both of which are typically difficult to quantify.

The evolution of the Great Barrier Reef during the Last Interglacial Period

Reef response to Last Interglacial (LIG) sea level and palaeoenvironmental change has been well documented at a limited number of far-field sites remote from former ice sheets. However, the age and development of LIG reefs in the Great Barrier Reef (GBR) remain poorly understood due to their location beneath modern living reefs. Here we report thirty-nine new mass spectrometry U-Th ages from seven LIG platform reefs across the northern, central and southern GBR. Two distinct geochemical populations of corals were observed, displaying activity ratios consistent with either closed or open system evolution. Our closed-system ages (~129–126ka) provide the first reliable LIG ages for the entire GBR. Combined with our open-system model ages, we are able to constrain the interval of significant LIG reef growth in the southern GBR to between ~129–121ka. Using age-elevation data in conjunction with newly defined coralgal assemblages and sedimentary facies analysis we have defined three distinct phases of LIG reef development in response to major sea level and oceanographic changes. These phases include: Phase 1 (>129ka), a shallow-water coralgal colonisation phase following initial flooding of the older, likely Marine Isotope Stage 7 (MIS7) antecedent platform; Phase 2 (~129ka), a near drowning event in response to rapid sea level rise and greater nutrient-rich upwelling and; Phase 3 (~128–121ka), establishment of significant reef framework through catch-up reef growth, initially characterised by deeper, more turbid coralgal assemblages (Phase 3a) that transition to shallow-water assemblages following sea level stabilisation (Phase 3b). Coralgal assemblage analysis indicates that the palaeoenvironments during initial reef growth phases (1 and 2) of the LIG were significantly different than the initial reef growth phases in the Holocene. However, the similar composition of ultimate shallow-water coralgal assemblages and slow reef accretion rates following stabilisation of sea level (phase 3b) suggest that reefs of both ages developed in a similar way during the main phase of relatively stable sea level.

The problematic growth of dust in high-redshift galaxies

Abstract Dust growth via accretion of gas species has been proposed as the dominant process to increase the amount of dust in galaxies. We show here that this hypothesis encounters severe difficulties that make it unfit to explain the observed UV and IR properties of such systems, particularly at high redshifts. Dust growth in the diffuse ISM phases is hampered by (a) too slow accretion rates, (b) too high dust temperatures, and (c) the Coulomb barrier that effectively blocks accretion. In molecular clouds these problems are largely alleviated. Grains are cold (but not colder than the CMB temperature, TCMB ≈ 20 K at redshift z = 6). However, in dense environments accreted materials form icy water mantles, perhaps with impurities. Mantles are immediately (≲1 yr) photo-desorbed as grains return to the diffuse ISM at the end of the cloud lifetime, thus erasing any memory of the growth. We conclude that dust attenuating stellar light at high-z must be ready-made stardust largely produced in supernova ejecta.

Geologic history of seawater: A MAGic approach to carbon chemistry and ocean ventilation

We explore the relationship between atmospheric O2 and CO2 evolution and seawater chemistry, with particular focus on the CO2–carbonic acid system and ocean ventilation, over the Phanerozoic Eon using a coupled biogeochemical Earth system model (MAGic). This model describes the biogeochemical cycles involving the major components of seawater (Ca, Mg, Na, K, Cl, SO4, CO2HCO3CO3), as well as components (O2, Fe, P, organic C, reduced S) central to long-term ecosystem productivity. The MAGic calculations show that the first-order input fluxes from weathering of continental rocks of Ca, Mg, and dissolved inorganic carbon (DIC) to the ocean varied in a cyclical manner over the Phanerozoic. The cyclicity is mainly the result of the impact of changing atmospheric CO2 levels, and hence temperature and runoff, on these fluxes, reflecting the nature of hothouse (greenhouse, high CO2 and warm) versus icehouse (low CO2, cool, and continental glaciation) conditions during the Phanerozoic. Uptake of DIC by seafloor basalt–seawater reactions also varied in a corresponding fashion to the weathering fluxes. The fluxes of Ca, Mg, DIC and other seawater constituents removed in oceanic sinks were also calculated and hence with calculated inputs and outputs of seawater constituents, the changes in seawater chemistry through Phanerozoic time could be obtained. Seawater pH increased irregularly during the Phanerozoic from just above 7 in the Cambrian Period, approaching modern average values in the most recent several millions of years. Calcite saturation state also increased with decreasing age. Both pH and calcite saturation state trends exhibited a cyclic overprint of hothouse and icehouse environmental conditions. Dissolved sulfate changed in a cyclical manner reflecting mainly variations in weathering and accretion rates and redox conditions, whereas dissolved potassium exhibited little variation in concentration.Using our “standard” model results for the chemistry of seawater and changes in atmospheric CO2 and O2 as the basis for a series of sensitivity experiments, we vary the ventilation rate of the global ocean, and quantify the resulting changes in terms of processes such as net primary production, organic carbon burial and oxidation, pyrite weathering, and sulfate reduction. We use these preliminary results to discuss how changes in ocean ventilation affect atmospheric CO2 and O2, and in turn exert changes in the sulfur, organic carbon, and inorganic carbon systems. We postulate that periods of slow plate accretion rates, associated with lower atmospheric CO2, vigorous deep water formation, cooler, drier climatic conditions and greater poleward temperature gradients are more likely to be associated with a strong thermohaline circulation, and thus “enhanced” global ocean mixing. Conversely, periods of higher accretion rates, higher CO2, higher average global temperatures with more equable poleward gradients, and higher sea levels resulting in extensive continental inundation, would be more likely to be coincident with times of reduced mixing of the global ocean. It is important to recognize that the scale of these changes depends on how major tectonic cycles (controlling chemical weathering, CO2 and temperature) in turn affect nutrient supply, global ocean productivity, and global ocean thermohaline circulation. The key to elucidating these changes lies in an understanding of the relationship between long-term tectonic evolution, which leads to changes in climate, sea level, and the global distribution of continental landmasses and the sedimentary environments they host, and the circulation of the global ocean.

Long-term dispersal of heavy metals in a catchment affected by historic lead and zinc mining

The Matylda catchment, in southern Poland, was polluted by the discharge of mine waters from a lead and zinc mine that inundated parts of a valley floor and caused the accumulation of metal-polluted sediments. After a partial reclamation of the mine site in the early 1980s, polluted sediments continue to accumulate on downstream floodplains and in fishponds. The aim of this study was to reconstruct the changes in metal dispersal during 100 years of mining and during the 40-year post-mining period and to propose a strategy for pollution mitigation in the area. Analyses of Cu, Cd, Pb, Zn, Mn, Ca, Mg and Fe concentrations, speciation of heavy metals and mineralogical analyses were undertaken on overbank sediment cores and in stream sediments. Concentrations of the same elements and macro-ions soluble in stream waters were also determined. Concentrations of Zn, Cd and Pb in the sediment profiles vary between 40,000 and 55,000, 300 and 600 and 30,000 and 50,000 mg kg-1, respectively. Changes of metal concentrations and the stratigraphy of sediments from the floodplains, stream channels and fishponds suggest rapid changes of metal loads migrating downstream during both the mining and post-mining periods. Since the time of mine closure, fine-grained, mine-derived sediments (ca. 12 cm thick) have been the main source of pollution of post-mining sediments and surface waters. Closure of the mine was followed by a relatively short period of rapid redistribution of sediment-associated heavy metals in the stream channel. Since the 1980s, the floodplain and fishponds have received a constant supply of metals. It contrasts with the slow sediment accretion rate and a rapid decrease of metal concentrations in floodplain pools due to dilution by decomposed leaf litter. A fivefold increase of Cd content in waters over the 4.6 km reach of the Matylda stream indicates continuous leaching of this element from the contaminated valley floor. Unsuccessful mine site rehabilitation is due to leaching of mine-originated sediments dispersed over the valley bottom. However, the rate of metal remobilization over the last 40 years is low because of the small thickness and widespread anoxic conditions that prevail within both recent and mine-originated sediments and the alkaline pH of stream water, which reduces metal mobility. Distribution of the contaminated layer over a large area of the valley bottom precludes cost-efficient catchment rehabilitation.

Rhodoliths from deep fore-reef to shelf areas around Okinawa-jima, Ryukyu Islands, Japan

The distribution, abundance, composition, and growth history of rhodoliths were investigated based on 222 grab samples and 202 submarine photographs taken from 223 sites arranged at regular intervals and on 13 additional samples (5 dredge and 8 grab samples). These samples were collected at water depths ranging from 15 to 970 m around Okinawa-jima, Ryukyu Islands, Japan. The rhodoliths grow in deep fore-reef to shelf areas at water depths of 50 to 135 m. Where rhodoliths occur, they cover 45% of the sea bottom. The rhodoliths are primarily spheroidal to ellipsoidal in shape (with mean diameters usually less than 8 cm); internally they are primarily composed of nongeniculate coralline algae and an encrusting foraminifer Acervulina inhaerens. The rhodoliths have envelopes of well-preserved, concentric to irregular laminations or, much more commonly, are bored and display various degrees of bioerosion. Constructional voids (primary spaces between encrusters) and borings range from empty to completely filled with unlithified and lithified mixtures of micrites and bioclasts. The bioerosion is more extensive with increasing water depth and is progressively much more pervasive at water depths greater than 90 m. The rhodoliths are covered with nongeniculate coralline algae and A. inhaerens associated with other epilithic skeletal and nonskeletal organisms. The living biotic cover on rhodoliths is relatively great down to water depths ~ 100 m; below this, the cover decreases rapidly with increasing water depth. Rhodoliths with similar size, shape, and composing organisms to those in the Ryukyu Islands are commonly found on deep fore-reef to shelf areas or on the banks and seamounts of tropical reef regions, likely as the combined result of ecological degradation (=decreased number and coverage) of hermatypic corals and the relative predominance of nongeniculate coralline algae and encrusting foraminifers in such areas. The slow accretion rates of rhodoliths (< 0.1 mm/year) indicate that their formation is commonly to frequently intermittent, probably because of their burial in the surrounding sediment.

Effects of a New Triple- Reaction Rate on the Helium Ignition of Accreting White Dwarfs

Effects of a new triple-alpha reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary systems have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type Ia supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for the lower accretion rate compared to that of $dM/dt\simeq 4\times10^{-8} M_{\odot} \rm yr^{-1}$ which has been believed to the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates and the region of carbon deflagration for slow accretion rate is disappeared.

The effects of thermal conduction on the ADAF with a toroidal magnetic field

The observation of the hot gas surrounding Sgr $A^*$ and a few other nearby galactic nuclei imply that electron and proton mean free paths are comparable to the gas capture radius. So, the hot accretion flows is likely to proceed under week-collision conditions. Hence, thermal conduction has been suggested as a possible mechanism by which the sufficient extra heating is provided in hot ADAF accretion disks. We consider the effects of thermal conduction in the presence of a toroidal magnetic field in an advection-dominated accretion flow around a compact object. For a steady-state structure of such accretion flows a set of self-similar solutions are presented. We find two types solutions which represent high and slow accretion rate. They have different behaviors with saturated thermal conduction parameter, $\phi$.

Microscopic structure of soil Fe-Mn nodules: environmental implication

The internal structure, iron and manganese distributions of Fe-Mn nodules from imperfectly drained soils of central Greece were studied by scanning electron microscopy (SEM) and elemental analysis. Nodules exhibited a differentiated structure with discontinuous concentric rings as expression of their redox history. Fe and Mn have shown distinct repartition and were distributed mainly in concentric bands. The microstructure and the arrangement of iron—manganese compounds reflect the periodical development and the relative slow accretion rates of the studied nodules. These properties provide ample opportunity for adsorption and isolation of heavy metals from the soil system.

Imprints of accretion on gravitational waves from black holes

We explore the subtleties that may emerge if mass accretion events increase significantly the mass of a black hole horizon during its gravitational wave (GW) emission. We find that horizon growth extends the phenomenology of black hole ring-down in a well defined manner. The resulting GW signals are best described as amplitude and frequency (AM-FM) modulated versions of the simple damped sinusoidal wave forms. Any appreciable increase in the horizon mass during emission reflects on the instantaneous signal frequency, which shows a prominent negative branch in the $\mathrm{f\ifmmode \dot{}\else \.{}\fi{}}(f)$ evolution diagram. The features of the frequency evolution pattern reveal key properties of the accretion event, such as the total accreted mass and the accretion rate. For slow accretion rates the frequency evolution follows verbatim the accretion rate, as expected from dimensional arguments.

Slowly accreting neutron stars and the origin of gamma-ray bursts

Old, isolated neutron stars accrete interstellar gas at a rate of about 10 to the 10th g/s. At this slow accretion rate, the interior temperature is too low for thermonuclear reactions to proceed, and the hydrogen burns by pycnonuclear reactions. The resulting helium then burns through pycnonuclear triple-alpha and (alpha, gamma) channels until it is exhausted. As the pressure increases under the weight of the accreted gas, the electron Fermi energy becomes large enough for electron capture to increase the neutron fraction in the nuclei. The layer of accreted gas can then become denser than the underlying crust, and the interface is susceptible to elastic Rayleigh-Taylor instability. A crust supported by relativistic electron degeneracy pressure is unstable when the fractional density decrease at the interface exceeds a critical value between 3 and 8 percent, depending upon the composition of the two layers. If the star has a magnetosphere, then this will be excited as well, producing a gamma-ray burst. If a substantial amount of the energy released is converted into heat locally, then the resulting temperature, roughly one billion K, may be hot enough to trigger thermonuclear reactions and raise the total energy release by a factor of about 30.more » Energetic and statistical implications of the model are critically examined, and some observable consequences are described. The model's sensitive dependence on poorly known pycnonuclear and thermonuclear reaction rates is emphasized. 52 refs.« less

Magnetic stratigraphy observed in ferromanganese crust

Summary. A sample of ferromanganese crust deposit (SCHW-1D) was cut into two sets of 1 mm slices for palaeomagnetic study. Alternating field and thermal demagnetization, and isothermal remanent magnetization analyses were performed on each thin slice. The results reveal the presence of a stable magnetism and both normal and reversed polarity intervals in the specimens. The observed polarity intervals cannot be confidently correlated with the geomagnetic polarity time-scale of the last 10 Myr due to the polarity overlap inherent in the sampling technique. But the results confirm the slow accretion rate of ferromanganese deposit determined by 10Be method and suggest potential use of ferromanganese deposits in palaeomagnetic and tectonic studies.

Diagenetic mobilization of manganese in Peru Basin sediments

The Mn contents and sedimentation rates of two cores from different areas of the Peru Basin have been determined. The southern core is associated with Mn nodules of conventional slow accretion rates (~ mm/10 6 yr) while the northern one accompanies nodules with very high growth rates (~10 2 mm/10 6 yr). The depletion of Mn observed within the top 40 cm of the cores is interpreted as resulting from a diagenetic remobilization of Mn +2 due to the oxidative characteristics of the sedimentary column. In both cores, the calculated flux of Mn provided by the sediment is higher than the total Mn recovered in the overlying nodules. This indicates that the Mn content of the Peru Basin nodules can be supplied by diagenetic processes from the sediment, not only for the slowly growing nodules but also for the extremely fast growing concretions found in the north of the basin.

Accreting white dwarf models for type I supernovae. I - Presupernova evolution and triggering mechanisms

As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

Submarine Lithification on Windward Reef Slopes: Capricorn-Bunker Group, Southern Great Barrier Reef

ABSTRACT Windward reef slopes of the Capricorn-Bunker Group, southern Great Barrier reef, are the sites of extensive submarine lithification. Both crystalline (aragonite and bladed Mg-calcite spar) and microcrystalline (micrite and peloids) cements occur within skeletal pores and chambers, whereas interskeletal cements are mainly microcrystalline. These cements are considered to be contemporaneous with reef growth and have developed in an environment characterized by slow accretion rates. Continuous precipitation of aragonite cement has, in some places, been inhibited by the formation of Mg-calcite micrite. Apparent dissolution of some of the Mg-calcite cements was observed, but the processes involved are unknown.