3,208 research outputs found
Initial mass function of intermediate mass black hole seeds
We study the Initial Mass Function (IMF) and host halo properties of
Intermediate Mass Black Holes (IMBH, 10^{4-6} Msun) formed inside metal-free,
UV illuminated atomic cooling haloes (virial temperature T_vir > 10^4 K) either
via the direct collapse of the gas or via an intermediate Super Massive Star
(SMS) stage. We achieve this goal in three steps: (a) we derive the gas
accretion rate for a proto-SMS to undergo General Relativity instability and
produce a direct collapse black hole (DCBH) or to enter the ZAMS and later
collapse into a IMBH; (b) we use merger-tree simulations to select atomic
cooling halos in which either a DCBH or SMS can form and grow, accounting for
metal enrichment and major mergers that halt the growth of the proto-SMS by gas
fragmentation. We derive the properties of the host halos and the mass
distribution of black holes at this stage, and dub it the "Birth Mass
Function"; (c) we follow the further growth of the DCBH due to accretion of
leftover gas in the parent halo and compute the final IMBH mass.We consider two
extreme cases in which minihalos (T_vir < 10^4 K) can (fertile) or cannot
(sterile) form stars and pollute their gas leading to a different IMBH IMF. In
the (fiducial) fertile case the IMF is bimodal extending over a broad range of
masses, M= (0.5-20)x10^5 Msun, and the DCBH accretion phase lasts from 10 to
100 Myr. If minihalos are sterile, the IMF spans the narrower mass range M=
(1-2.8)x10^6 Msun, and the DCBH accretion phase is more extended (70-120 Myr).
We conclude that a good seeding prescription is to populate halos (a) of mass
7.5 < log (M_h/Msun) < 8, (b) in the redshift range 8 < z < 17, (c) with IMBH
in the mass range 4.75 < log (M_BH/Msun) < 6.25.Comment: MNRAS, in press. Comments welcom
The Location Market
Individual location decisions are not given much respect by local governments. Governments frequently use zoning and other regulatory rules to spread development across a city, claiming that the whole city, and not just one favored or disfavored part, should get the benefits and bear the costs of new development. Local governments also create incentives to encourage certain types of development to locate in certain areas-using policy tools that range from non-cumulative zoning to outright subsidies-in order to create particular mixes of industrial, commercial, and residential development. However, the arguments in favor of these policies frequently rely upon a specious depiction of the incentives of governmental decision makers on the one hand and private citizens on the other. That is, they fail to see the wisdom behind the old saying that the three most important factors in real estate are location, location, location
The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
Runaway stellar collisions in dense star clusters are invoked to explain the
presence of very massive stars or blue stragglers in the center of those
systems. This process has also been explored for the first star clusters in the
Universe and shown to yield stars that may collapse at some points into an
intermediate mass black hole. Although the early evolution of star clusters
requires the explicit modeling of the gas out of which the stars form, these
calculations would be extremely time-consuming and often the effects of the gas
can be accurately treated by including a background potential to account for
the extra gravitational force. We apply this approximation to model the early
evolution of the first dense star clusters formed in the Universe by performing
-body simulations, our goal is to understand how the additional
gravitational force affects the growth of a very massive star through stellar
mergers in the central parts of the star cluster. Our results show that the
background potential increases the velocities of the stars, causing an overall
delay in the evolution of the clusters and in the runaway growth of a massive
star at the center. The population of binary stars is lower due to the
increased kinetic energy of the stars, initially reducing the number of stellar
collisions, and we show that relaxation processes are also affected. Despite
these effects, the external potential enhances the mass of the merger product
by a factor 2 if the collisions are maintained for long times.Comment: 16 pages. Accepted for publication in Astronomy and Astrophysic
Will the Sustainable Development Goals address the links between poverty and the natural environment?
The relationships between the natural environment and poverty have been a central theme in the sustainability and development literatures. However, they have been less influential in mainstream international development and conservation policies, which often neglect or fail to adequately address these relationships. This paper examines how the Sustainable Development Goals (SDGs) may influence the framing of environment-poverty relationships. We argue that the SDGsâ comprehensive nature could provide an opportunity for better environment-poverty integration. To realise this potential, SDG-related activities will need to challenge the institutional status quo; transform how we measure, understand and implement development; design interventions that reflect local visions of development; make trade-offs between SDGs explicit; and address ultimate drivers of environmental degradation and poverty.Cambridge Humanities Research Gran
Morphology and biomechanics of the nests of the Common Blackbird Turdus merula
Capsule Common blackbirds select different materials, with varying biomechanical properties, to construct different parts of their nest.
Aims This study tested the hypothesis that outer components of a nest have a more structural role and so are stronger than materials used to line the cup.
Methods Blackbird nests were measured prior to being dismantled to isolate structural components which were tested for mechanical strength and rigidity.
Results Outer nest wall materials were significantly thicker, stronger and more rigid than materials in the inner structural wall or the cup lining. In the vertical plane materials used in the structural wall did not differ. By contrast, lining materials from the bottom of the nest cup were significantly thicker, stronger and more rigid than materials from the top of the cup.
Conclusion Blackbirds use different materials in nest construction roles suited to their properties and so may be able to recognise the structural properties of these materials. Materials on the outside of the nest may have a key structural role during construction
A timeline for massive star-forming regions via combined observation of o-HD and ND
Context: In cold and dense gas prior to the formation of young stellar
objects, heavy molecular species (including CO) are accreted onto dust grains.
Under these conditions H and its deuterated isotopologues become more
abundant, enhancing the deuterium fraction of molecules such as NH that
are formed via ion-neutral reactions. Because this process is extremely
temperature sensitive, the abundance of these species is likely linked to the
evolutionary stage of the source.
Aims: We investigate how the abundances of o-HD and ND vary
with evolution in high-mass clumps.
Methods: We observed with APEX the ground-state transitions of o-HD
near 372 GHz, and ND(3-2) near 231 GHz for three massive clumps in
different evolutionary stages. The sources were selected within the
G351.77-0.51 complex to minimise the variation of initial chemical conditions,
and to remove distance effects. We modelled their dust continuum emission to
estimate their physical properties, and also modelled their spectra under the
assumption of local thermodynamic equilibrium to calculate beam-averaged
abundances.
Results: We find an anticorrelation between the abundance of o-HD and
that of ND, with the former decreasing and the latter increasing with
evolution. With the new observations we are also able to provide a qualitative
upper limit to the age of the youngest clump of about 10 yr, comparable to
its current free-fall time.
Conclusions: We can explain the evolution of the two tracers with simple
considerations on the chemical formation paths, depletion of heavy elements,
and evaporation from the grains. We therefore propose that the joint
observation and the relative abundance of o-HD and ND can act
as an efficient tracer of the evolutionary stages of the star-formation
process
The unusual volatile composition of the Halley-type comet 8P/Tuttle: Addressing the existence of an Inner Oort Cloud
We measured organic volatiles (CH4, CH3OH, C2H6, H2CO), CO, and water in
comet 8P/Tuttle, a comet from the Oort cloud reservoir now in a short-period
Halley-type orbit. We compare its composition with two other comets in
Halley-type orbits, and with comets of the "organics-normal" and
"organics-depleted" classes. Chemical gradients are expected in the
comet-forming region of the proto-planetary disk, and an individual comet
should reflect its specific heritage. If Halley-type comets came from the inner
Oort cloud as proposed, we see no common characteristics that could distinguish
such comets from those that were stored in the outer Oort cloud.Comment: 14 pages, including 1 figure and 2 Table
Prestack time-migration velocity analysis using remigration trajectories
We have developed a prestack time-migration tool for local improvement of the seismic migration-velocity model. The method is based on remigration trajectories that describe the position of an image point in the image domain for different source-receiver offsets as a function of the migration velocity. It determines kinematic migration parameters using local-slope information of migrated seismic reflection events. These parameters, in turn, are used to locally correct the velocity model. The main advantage of this technique is that it allows us to carry out a moveout correction not just at a fixed point in a zero-offset (poststack) time-migrated gather, but varying through all offsets of a common-image gather, taking into account the reflection-point displacement in the midpoint direction. In other words, it provides for time-migration velocity analysis from prestack data. We have tested the feasibility of the method on synthetic data from four simple models and the Sigsbee2B data. Our tests determined that the proposed tool increases the velocity-model resolution and provides a plausible time-migrated image. The quality of the initial model is not critical. The procedure is quite efficient. Significant effort was spent on manual event picking.We have developed a prestack time-migration tool for local improvement of the seismic migration-velocity model. The method is based on remigration trajectories that describe the position of an image point in the image domain for different source-receiver offsets as a function of the migration velocity. It determines kinematic migration parameters using local-slope information of migrated seismic reflection events. These parameters, in turn, are used to locally correct the velocity model. The main advantage of this technique is that it allows us to carry out a moveout correction not just at a fixed point in a zero-offset (poststack) time-migrated gather, but varying through all offsets of a common-image gather, taking into account the reflection-point displacement in the midpoint direction. In other words, it provides for time-migration velocity analysis from prestack data. We have tested the feasibility of the method on synthetic data from four simple models and the Sigsbee2B data. Our tests determined that the proposed tool increases the velocity-model resolution and provides a plausible time-migrated image. The quality of the initial model is not critical. The procedure is quite efficient. Significant effort was spent on manual event picking.804S151S16
Design optimization of high-speed proprotor aircraft
NASA's high-speed rotorcraft (HSRC) studies have the objective of investigating technology for vehicles that have both low downwash velocities and forward flight speed capability of up to 450 knots. This paper investigates a tilt rotor, a tilt wing, and a folding tilt rotor designed for a civil transport mission. Baseline aircraft models using current technology are developed for each configuration using a vertical/short takeoff and landing (V/STOL) aircraft design synthesis computer program to generate converged vehicle designs. Sensitivity studies and numerical optimization are used to illustrate each configuration's key design tradeoffs and constraints. Minimization of the gross takeoff weight is used as the optimization objective function. Several advanced technologies are chosen, and their relative impact on future configurational development is discussed. Finally, the impact of maximum cruise speed on vehicle figures of merit (gross weight, productivity, and direct operating cost) is analyzed. The three most important conclusions from the study are payload ratios for these aircraft will be commensurate with current fixed-wing commuter aircraft; future tilt rotors and tilt wings will be significantly lighter, more productive, and cheaper than competing folding tilt rotors; and the most promising technologies are an advanced-technology proprotor for both tilt rotor and tilt wing and advanced structural materials for the folding tilt rotor
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