5,327 research outputs found
On the Fast Magnetic Rotator Regime of Stellar Winds
Aims: We study the acceleration of the stellar winds of rapidly rotating low
mass stars and the transition between the slow magnetic rotator and fast
magnetic rotator regimes. We aim to understand the properties of stellar winds
in the fast magnetic rotator regime and the effects of magneto-centrifugal
forces on wind speeds and mass loss rates.
Methods: We extend the solar wind model of Johnstone et al. (2015b) to 1D
magnetohydrodynamic (MHD) simulations of the winds of rotating stars. We test
two assumptions for how to scale the wind temperature to other stars and assume
the mass loss rate scales as Mdot ~ Rstar^2 OmegaStar^1.33 Mstar^-3.36, in the
unsaturated regime, as estimated by Johnstone et al. (2015a).
Results: For 1.0 Msun stars, the winds can be accelerated to several thousand
km/s, and the effects of magneto-centrifugal forces are much weaker for lower
mass stars. We find that the different assumptions for how to scale the wind
temperature to other stars lead to significantly different mass loss rates for
the rapid rotators. If we assume a constant temperature, the mass loss rates of
solar mass stars do not saturate at rapid rotation, which we show to be
inconsistent with observed rotational evolution. If we assume the wind
temperatures scale positively with rotation, the mass loss rates are only
influenced significantly at rotation rates above 75 OmegaSun. We suggest that
models with increasing wind speed for more rapid rotators are preferable to
those that assume a constant wind speed. If this conclusion is confirmed by
more sophisticated wind modelling. it might provide an interesting
observational constraint on the properties of stellar winds.Comment: Accepted for publication in A&A. All data and codes from the paper
can be downloaded from https://goo.gl/hTuEV
Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds
Aims: We study the evolution of stellar rotation and wind properties for
low-mass main-sequence stars. Our aim is to use rotational evolution models to
constrain the mass loss rates in stellar winds and to predict how their
properties evolve with time on the main-sequence.
Methods: We construct a rotational evolution model that is driven by observed
rotational distributions of young stellar clusters. Fitting the free parameters
in our model allows us to predict how wind mass loss rate depends on stellar
mass, radius, and rotation. We couple the results to the wind model developed
in Paper I of this series to predict how wind properties evolve on the
main-sequence.
Results: We estimate that wind mass loss rate scales with stellar parameters
as . We
estimate that at young ages, the solar wind likely had a mass loss rate that is
an order of magnitude higher than that of the current solar wind. This leads to
the wind having a higher density at younger ages; however, the magnitude of
this change depends strongly on how we scale wind temperature. Due to the
spread in rotation rates, young stars show a large range of wind properties at
a given age. This spread in wind properties disappears as the stars age.
Conclusions: There is a large uncertainty in our knowledge of the evolution
of stellar winds on the main-sequence, due both to our lack of knowledge of
stellar winds and the large spread in rotation rates at young ages. Given the
sensitivity of planetary atmospheres to stellar wind and radiation conditions,
these uncertainties can be significant for our understanding of the evolution
of planetary environments.Comment: 26 pages, 14 figures, 2 tables, to be published in A&
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
The role of built environment energy efficiency in a sustainable UK energy economy
Energy efficiency in the built environment can make significant contributions to a sustainable energy economy. In order to achieve this, greater public awareness of the importance of energy efficiency is required. In the short term, new efficient domestic appliances, building technologies, legislation quantifying building plant performance, and improved building regulations to include installed plant will be required. Continuing these improvements in the longer term is likely to see the adoption of small-scale renewable technologies embedded in the building fabric. Internet-based energy services will see low-cost building energy management and control delivered to the mass market in order that plant can be operated and maintained at optimum performance levels and energy savings quantified. There are many technology options for improved energy performance of the building fabric and energy systems and it's not yet clear which will prove to be the most economic. Therefore, flexibility is needed in legislation and energy-efficiency initiatives
Stellar activity and planetary atmosphere evolution in tight binary star systems
Context. In tight binary star systems, tidal interactions can significantly
influence the rotational and orbital evolution of both stars, and therefore
their activity evolution. This can have strong effects on the atmospheric
evolution of planets that are orbiting the two stars.
Aims. In this paper, we aim to study the evolution of stellar rotation and of
X-ray and ultraviolet (XUV) radiation in tight binary systems consisting of two
solar mass stars and use our results to study planetary atmosphere evolution in
the habitable zones of these systems.
Methods. We have applied a rotation model developed for single stars to
binary systems, taking into account the effects of tidal interactions on the
rotational and orbital evolution of both stars. We used empirical
rotation-activity relations to predict XUV evolution tracks for the stars,
which we used to model hydrodynamic escape of hydrogen dominated atmospheres.
Results. When significant, tidal interactions increase the total amount of
XUV energy emitted, and in the most extreme cases by up to factor of 50.
We find that in the systems that we study, habitable zone planets with masses
of 1~M can lose huge hydrogen atmospheres due to the extended high
levels of XUV emission, and the time that is needed to lose these atmospheres
depends on the binary orbital separation.For some orbital separations, and when
the stars are born as rapid rotators, it is also possible for tidal
interactions to protect atmospheres from erosion by quickly spinning down the
stars. For very small orbital separations, the loss of orbital angular momentum
by stellar winds causes the two stars to merge. We suggest that the merging of
the two stars could cause previously frozen planets to become habitable due to
the habitable zone boundaries moving outwards.Comment: Accepted for publication by A&
The EDEM methodology for housing upgrade analysis, carbon and energy labelling and national policy development
The ESRU Domestic Energy Model (EDEM) has been developed in response to demand from policy makers for a tool to assist in analysis of options for improving carbon and energy performance of housing across a range of possible future technologies, behaviours and environmental factors. A major challenge is to comprehend the large variation in fabric, systems (heating, hot water, lighting and appliances) and behaviours across the housing stock as well as uncertainty over future trends. Existing static models have limited ability to represent dynamic behaviour while use of detailed simulation has been based on modelling only a small number of representative designs. To address these challenges, EDEM has been developed as an easy to use, Web based tool, built on detailed simulation models aligned with national house survey data. From pragmatic inputs, EDEM can determine energy use and carbon emissions at any scale, from individual dwelling to national housing stock. EDEM was used at the behest of the Scottish Building Standards Agency and South Ayrshire Council to quantify the impact of upgrades including new and renewable energy systems. EDEM was also used to rate energy/carbon performance of dwellings as required by the EU Directive (EU, 2002). This paper describes the evolving EDEM methodology, its structure and operation then presents findings from applications. While initial EDEM projects have been for the Scottish housing stock the methodology is structured to facilitate project development and application to other countries
Colliding Winds in Low-Mass Binary Star Systems: wind interactions and implications for habitable planets
Context. In binary star systems, the winds from the two components impact
each other, leading to strong shocks and regions of enhanced density and
temperature. Potentially habitable circumbinary planets must continually be
exposed to these interactions regions.
Aims. We study, for the first time, the interactions between winds from
low-mass stars in a binary system, to show the wind conditions seen by
potentially habitable circumbinary planets.
Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model
the wind interactions of two identical winds from two solar mass stars with
circular orbits and a binary separation of 0.5 AU. As input into this model, we
use a 1D hydrodynamic simulation of the solar wind, run using the Versatile
Advection Code. We derive the locations of stable and habitable orbits in this
system to explore what wind conditions potentially habitable planets will be
exposed to during their orbits.
Results. Our wind interaction simulations result in the formation of two
strong shock waves separated by a region of enhanced density and temperature.
The wind-wind interaction region has a spiral shape due to Coriolis forces
generated by the orbital motions of the two stars. The stable and habitable
zone in this system extends from approximately 1.4 AU to 2.4 AU. (TRUNCATED)Comment: 15 pages, 11 figures, to be published in A&
CW high intensity non-scaling FFAG proton drivers
Accelerators are playing increasingly important roles in basic science,
technology, and medicine including nuclear power, industrial irradiation,
material science, and neutrino production. Proton and light-ion accelerators in
particular have many research, energy and medical applications, providing one
of the most effective treatments for many types of cancer. Ultra high-intensity
and high-energy (GeV) proton drivers are a critical technology for
accelerator-driven sub-critical reactors (ADS) and many HEP programs (Muon
Collider). These high-intensity GeV-range proton drivers are particularly
challenging, encountering duty cycle and space-charge limits in the synchrotron
and machine size concerns in the weaker-focusing cyclotrons; a 10-20 MW proton
driver is not presently considered technically achievable with conventional
re-circulating accelerators. One, as-yet, unexplored re-circulating
accelerator, the Fixed-field Alternating Gradient, or FFAG, is an attractive
alternative to the cyclotron. Its strong focusing optics are expected to
mitigate space charge effects, and a recent innovation in design has coupled
stable tunes with isochronous orbits, making the FFAG capable of
fixed-frequency, CW acceleration, as in the classical cyclotron. This paper
reports on these new advances in FFAG accelerator technology and references
advanced modeling tools for fixed-field accelerators developed for and unique
to the code COSY INFINITY.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
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