Understanding the surface O3 response over a “receptor” region to emission changes over a foreign “source” region is key to evaluating the potential gains from an international approach to abate ...ozone (O3) pollution. We apply an ensemble of 21 global and hemispheric chemical transport models to estimate the spatial average surface O3 response over east Asia (EA), Europe (EU), North America (NA), and south Asia (SA) to 20% decreases in anthropogenic emissions of the O3 precursors, NOx, NMVOC, and CO (individually and combined), from each of these regions. We find that the ensemble mean surface O3 concentrations in the base case (year 2001) simulation matches available observations throughout the year over EU but overestimates them by >10 ppb during summer and early fall over the eastern United States and Japan. The sum of the O3 responses to NOx, CO, and NMVOC decreases separately is approximately equal to that from a simultaneous reduction of all precursors. We define a continental‐scale “import sensitivity” as the ratio of the O3 response to the 20% reductions in foreign versus “domestic” (i.e., over the source region itself) emissions. For example, the combined reduction of emissions from the three foreign regions produces an ensemble spatial mean decrease of 0.6 ppb over EU (0.4 ppb from NA), less than the 0.8 ppb from the reduction of EU emissions, leading to an import sensitivity ratio of 0.7. The ensemble mean surface O3 response to foreign emissions is largest in spring and late fall (0.7–0.9 ppb decrease in all regions from the combined precursor reductions in the three foreign regions), with import sensitivities ranging from 0.5 to 1.1 (responses to domestic emission reductions are 0.8–1.6 ppb). High O3 values are much more sensitive to domestic emissions than to foreign emissions, as indicated by lower import sensitivities of 0.2 to 0.3 during July in EA, EU, and NA when O3 levels are typically highest and by the weaker relative response of annual incidences of daily maximum 8‐h average O3 above 60 ppb to emission reductions in a foreign region (<10–20% of that to domestic) as compared to the annual mean response (up to 50% of that to domestic). Applying the ensemble annual mean results to changes in anthropogenic emissions from 1996 to 2002, we estimate a Northern Hemispheric increase in background surface O3 of about 0.1 ppb a−1, at the low end of the 0.1–0.5 ppb a−1 derived from observations. From an additional simulation in which global atmospheric methane was reduced, we infer that 20% reductions in anthropogenic methane emissions from a foreign source region would yield an O3 response in a receptor region that roughly equals that produced by combined 20% reductions of anthropogenic NOx, NMVOC, and CO emissions from the foreign source region.
Fuelling active galactic nuclei King, A. R.; Pringle, J. E.
Monthly notices of the Royal Astronomical Society. Letters,
05/2007, Letnik:
377, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We suggest that most nearby active galactic nuclei are fed by a series of small-scale, randomly oriented accretion events. Outside a certain radius these events promote rapid star formation, while ...within it they fuel the supermassive black hole. We show that the events have a characteristic time-evolution. This picture agrees with several observational facts. The expected luminosity function is broadly in agreement with that observed for moderate-mass black holes. The spin of the black hole is low, and aligns with the inner disc in each individual feeding event. This implies radio jets aligned with the axis of the obscuring torus, and uncorrelated with the large-scale structure of the host galaxy. The ring of young stars observed about the Galactic Centre are close to where our picture predicts that star formation should occur.
ABSTRACT
We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent ...with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass ${\sim }6\times 10^8\, \mathrm{M}_\odot$. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the H α and H β emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star ${\ge } 1 \, \mathrm{M}_{\odot }$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
We investigate the relationship between the star formation rate per unit area and the surface density of the interstellar medium (ISM; the local Kennicutt–Schmitt law) using a simplified model of the ...ISM and a simple estimate of the star formation rate based on the mass of gas in bound clumps, the local dynamical time–scales of the clumps and an efficiency parameter of around ε≈ 5 per cent. Despite the simplicity of the approach, we are able to reproduce the observed linear relation between star formation rate and surface density of dense (molecular) gas. We use a simple model for the dependence of H2 fraction on total surface density to argue why neither total surface density nor the H i surface density is a good local indicator of star formation rate. We also investigate the dependence of the star formation rate on the depth of the spiral potential. Our model indicates that the mean star formation rate does not depend significantly on the strength of the spiral potential, but that a stronger spiral potential, for a given mean surface density, does result in more of the star formation occurring close to the spiral arms. This agrees with the observation that grand design galaxies do not appear to show a larger degree of star formation compared to their flocculent counterparts.
ABSTRACT
We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in ...galaxy merger events involving supermassive black holes, and in the formation of stellar mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle θ between them obeys cos θ < −Jd/2Jb, on a time short compared with the mass gain time of the central accretor(s). The magnitude Jb remains unchanged in this process. Counteralignment can promote the rapid merger of supermassive black hole binaries, and possibly the formation of coplanar but retrograde planets around stars in binary systems.
We present numerical simulations of the passage of gas through a galactic spiral shock and the subsequent formation of giant molecular clouds (GMCs), and the triggering of star formation. In these ...simulations, we take account of the observed inhomogeneity, or clumpiness, of the pre-shock interstellar medium. As might be expected, the spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. But the effect of the clumpiness of gas as it passes through the shock is to generate chaotic internal motions in the gas. The kinematics of these motions are found to agree with the observed velocity dispersion/size relation found in star-forming regions. In contrast to the standard picture where continuously driven turbulence generates the density inhomogeneities in star-forming clouds, we find here that it is the clumpiness of the interstellar gas that produces the chaotic motions as it passes through the spiral shock and initiates the star formation process. The velocity dispersion can be understood as being due to the random mass loading of clumps as they converge in the spiral shock. Within these clouds both the time-scale for the decay of these motions, and the time-scale for forming stars, are comparable to the clouds' dynamical lifetimes. In this model there is no need for any internal or external continuous driving mechanism for the ‘turbulence’. In addition, the coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Indeed, while clearly some parts of the clouds are self-gravitating and able to form stars, most of the molecular material remains gravitationally unbound. This can provide a simple explanation for the low efficiency of star formation.
Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling ...the flow of mass and angular momentum in the radial direction. In a previous paper, we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper, we discuss the observational evidence for the magnitude of the dissipative effects which act in non-planar discs, both to transfer and to eliminate the non-planarity. Estimates based on the model by Ogilvie, which assumes a small-scale, isotropic viscosity, give alignment time-scales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the time-scales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate.
Abstract
The large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. However, the increasing importance of Faraday ...depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (∼0.3–1 kpc) this plane, from the vicinity of the warm interstellar medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. We show that this simple model accounts for the main observed properties of large-scale fields. We speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns.
Context.
In this Letter we aim to explore whether gas is also expected in the Kuiper belt (KB) in our Solar System.
Aims.
To quantify the gas release in our Solar System, we use models for gas ...release that have been applied to extrasolar planetary systems as well as a physical model that accounts for gas released due to the progressive internal warming of large planetesimals.
Methods.
We find that only bodies larger than about 4 km can still contain CO ice after 4.6 Gyr of evolution. This finding may provide a clue as to why Jupiter-family comets, thought to originate in the KB, are deficient in CO compared to Oort cloud comets. We predict that gas is still currently being produced in the KB at a rate of 2 × 10
−8
M
⊕
Myr
−1
for CO and that this rate was orders of magnitude higher when the Sun was younger. Once released, the gas is quickly pushed out by the solar wind. Therefore, we predict a gas wind in our Solar System starting at the KB location and extending far beyond with regards to the heliosphere, with a current total CO mass of ∼2 × 10
−12
M
⊕
(i.e., 20 times the CO quantity that was lost by the Hale-Bopp comet during its 1997 passage) and CO density in the belt of 3 × 10
−7
cm
−3
. We also predict the existence of a slightly more massive atomic gas wind made of carbon and oxygen (neutral and ionized), with a mass of ∼10
−11
M
⊕
.
Results.
We predict that gas is currently present in our Solar System beyond the KB and that, although it cannot be detected with current instrumentation, it could be observed in the future with an in situ mission using an instrument similar to Alice on New Horizons but with larger detectors. Our model of gas release due to slow heating may also work for exoplanetary systems and provide the first real physical mechanism for the gas observations. Lastly, our model shows that the amount of gas in the young Solar System should have been orders of magnitude greater and that it may have played an important role in, for example, planetary atmosphere formation.
Angular momentum is transported outwards through an accretion disc by magnetohydrodynamical (MHD) turbulence thus allowing material to accrete on to the central object. The magnetorotational ...instability (MRI) requires a minimum ionization fraction to drive turbulence in a disc. The inner parts of the disc around a young stellar object are sufficiently hot to be thermally ionized. Further out, cosmic rays ionize the surface layers and a dead zone forms at the mid-plane where the disc is too cool for the MRI to operate. The surface density in the turbulent active layer is often assumed to be constant with radius because the cosmic rays penetrate a constant layer. However, if a critical magnetic Reynolds number, Re
M, crit, is used to determine the extent of the dead zone, the surface density in the layer generally increases with radius. For small critical magnetic Reynolds number of the order of 1, the constant-layer approximation may be a reasonable fit. However, MHD simulations suggest that the critical magnetic Reynolds number may be much larger, of the order of 104. Analytical fits for the surface density in the magnetic active layer show that
, at temperature T and radius R, are a good fit for higher critical magnetic Reynolds number. For the metallicity variation between our Galaxy, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), there should be no significant difference in the extent of the dead zone. Observations suggest an increase in the lifetime of the disc with decreasing metallicity, which cannot be explained by the dead-zone structure (ignoring possible differences in dust abundances).
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