7,582 research outputs found
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
On the penetration of meridional circulation below the solar convection zone
Meridional flows with velocities of a few meters per second are observed in
the uppermost regions of the solar convection zone. The amplitude and pattern
of the flows deeper in the solar interior, in particular near the top of the
radiative region, are of crucial importance to a wide range of solar
magnetohydrodynamical processes. In this paper, we provide a systematic study
of the penetration of large-scale meridional flows from the convection zone
into the radiative zone. In particular, we study the effects of the assumed
boundary conditions applied at the convective-radiative interface on the deeper
flows. Using simplified analytical models in conjunction with more complete
numerical methods, we show that penetration of the convectively-driven
meridional flows into the deeper interior is not necessarily limited to a
shallow Ekman depth but can penetrate much deeper, depending on how the
convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap
Kinematics of Gas Near the Galactic Center
The number of similarities between the nuclei of some ordinary galaxies and the quasars, radio galaxies, and Seyfert nuclei has become impressively large. Ambartsumian's recent account of the resemblances makes it hard not to believe that these energetic objects are galaxies passing through a difficult age, or perhaps suffering a not-so-rare galactic malady. And very few would deny that to understand this exceptional behavior it would be well to know how normal galaxies function in their innermost regions
Imaging the Solar Tachocline by Time-Distance Helioseismology
The solar tachocline at the bottom of the convection zone is an important
region for the dynamics of the Sun and the solar dynamo. In this region, the
sound speed inferred by global helioseismology exhibits a bump of approximately
0.4% relative to the standard solar model. Global helioseismology does not
provide any information on possible latitudinal variations or asymmetries
between the Northern and Southern hemisphere. Here, we develop a time-distance
helioseismology technique, including surface- and deep-focusing measurement
schemes and a combination of both, for two-dimensional tomographic imaging of
the solar tachocline that infers radial and latitudinal variations in the sound
speed. We test the technique using artificial solar oscillation data obtained
from numerical simulations. The technique successfully recovers major features
of the simplified tachocline models. The technique is then applied to SOHO/MDI
medium-l data and provides for the first time a full two-dimensional
sound-speed perturbation image of the solar tachocline. The one-dimensional
radial profile obtained by latitudinal averaging of the image is in good
agreement with the previous global helioseismology result. It is found that the
amplitude of the sound-speed perturbation at the tachocline varies with
latitude, but it is not clear whether this is in part or fully an effect of
instrumental distortion. Our initial results demonstrate that time-distance
helioseismology can be used to probe the deep interior structure of the Sun,
including the solar tachocline.Comment: accepted for publication by Ap
Particle Aggregation in a turbulent Keplerian flow
In the problem of planetary formation one seeks a mechanism to gather small
solid particles together into larger accumulations of solid matter. Here we
describe a scenario in which turbulence mediates this process by aggregating
particles into anticyclonic regions. If, as our simulations suggest,
anticyclonic vortices form as long-lived coherent structures, the process
becomes more powerful because such vortices trap particles effectively. Even if
the turbulence is decaying, following the upheaval that formed the disk, there
is enough time to make the dust distribution quite lumpy.Comment: 16 pages, 9 figure
The effect of the tachocline on differential rotation in the Sun
In this paper, we present a model for the effects of the tachocline on the
differential rotation in the solar convection zone. The mathematical technique
relies on the assumption that entropy is nearly constant ("well-mixed") in
isorotation surfaces both outside and within the tachocline. The resulting
solutions exhibit nontrivial features that strikingly resemble the true
tachocline isorotation contours in unexpected detail. This strengthens the
mathematical premises of the theory. The observed rotation pattern in the
tachocline shows strong quadrupolar structure, an important feature that is
explicitly used in constructing our solutions. The tachocline is treated
locally as an interior boundary layer of small but finite thickness, and an
explicit global solution is then constructed. A dynamical link can thus be
established between the internal jump in the angular velocity at the tachocline
and the spread of angular velocities observed near the solar surface. In
general, our results suggest that the bulk of the solar convection zone is in
thermal wind balance, and that simple quadrupolar stresses, local in radius,
mediate the tachocline transition from differential rotation to uniform
rotation in the radiative interior.Comment: 20 Pages, 4 figures, to appear in MNRA
Instabilities in the Envelopes and Winds of Very Massive Stars
The high luminosity of Very Massive Stars (VMS) means that radiative forces
play an important, dynamical role both in the structure and stability of their
stellar envelope, and in driving strong stellar-wind mass loss. Focusing on the
interplay of radiative flux and opacity, with emphasis on key distinctions
between continuum vs. line opacity, this chapter reviews instabilities in the
envelopes and winds of VMS. Specifically, we discuss how: 1) the iron opacity
bump can induce an extensive inflation of the stellar envelope; 2) the density
dependence of mean opacity leads to strange mode instabilities in the outer
envelope; 3) desaturation of line-opacity by acceleration of near-surface
layers initiates and sustains a line-driven stellar wind outflow; 4) an
associated line-deshadowing instability leads to extensive small-scale
structure in the outer regions of such line-driven winds; 5) a star with
super-Eddington luminosity can develop extensive atmospheric structure from
photon bubble instabilities, or from stagnation of flow that exceeds the
"photon tiring" limit; 6) the associated porosity leads to a reduction in
opacity that can regulate the extreme mass loss of such continuum-driven winds.
Two overall themes are the potential links of such instabilities to Luminous
Blue Variable (LBV) stars, and the potential role of radiation forces in
establishing the upper mass limit of VMS.Comment: 44 pages, 13 figures. Chapter to appear in the book "Very Massive
Stars in the Local Universe", Springer, J.S. Vink, e
Approaching community-level greywater managementin non-sewered settlements in South Africa
This study investigates sustainable options for community-level management of greywater in low-income
settlements without on-site waterborne sanitation in South Africa. As a consequence of the limited resource
base of both the local authorities and the inhabitants of the settlements, these options need to be inexpensive,
technologically simple and socially acceptable. A variety of social and technological options are thus being
implemented in collaboration with the inhabitants and the local authorities, and their efficacy evaluated.
The hypothesis is that involving the inhabitants in the development of solutions is likely to be more effective
in the management of greywater in these settlements than the traditional approach where the local authorities
provide rudimentary engineered services after minimal consultation. Following an initial scoping study
that covered six of the nine provinces of South Africa, four settlements in the Western Cape Province were
selected for a second, more detailed study. The study employs the Participatory Action Research (PAR)
method which emphasises participation, collaboration and consensual decision-making with the goal of
ensuring long term sustainability of social and technological interventions
Uncertainties in stellar evolution models: convective overshoot
In spite of the great effort made in the last decades to improve our
understanding of stellar evolution, significant uncertainties remain due to our
poor knowledge of some complex physical processes that require an empirical
calibration, such as the efficiency of the interior mixing related to
convective overshoot. Here we review the impact of convective overshoot on the
evolution of stars during the main Hydrogen and Helium burning phases.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
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