7,862 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
Rebuilding health care in Iraq
The effects of three wars within 25 years, a decade of international sanctions, and a brutal regime have had tragic consequences on Iraq’s health system and on the health of the Iraqi people. While the scale of these problems is becoming clearer, it has been difficult in the current security situation to know how best to respond to requests for help.
A workshop organised by the International Committee of the Faculty of Public Health (FPH) in November 2003 has now addressed this very issue. This paper describes the health service needs presented at the workshop by representatives from the Department for International Development (DFID), World Health Organisation (WHO), International Non-Governmental Organisations (INGOs) and, most importantly, Iraq’s Ministry of Health. We will also consider current responses and how professional public health bodies from around the world might contribute to the development of Iraq’s health sector
The Coronal Temperatures of Low-Mass Main-Sequence Stars
Aims. We study the X-ray emission of low-mass main-sequence stars to derive a
reliable general scaling law between coronal temperature and the level of X-ray
activity.
Methods. We collect ROSAT measurements of hardness ratios and X-ray
luminosities for a large sample of stars to derive which stellar X-ray emission
parameter is most closely correlated with coronal temperature. We calculate
average coronal temperatures for a sample of 24 low-mass main-sequence stars
with measured emission measure distributions (EMDs) collected from the
literature. These EMDs are based on high-resolution X-ray spectra measured by
XMM-Newton and Chandra.
Results. We confirm that there is one universal scaling relation between
coronal average temperature and surface X-ray flux, Fx, that applies to all
low-mass main-sequence stars. We find that coronal temperature is related to Fx
by Tcor=0.11 Fx^0.26, where Tcor is in MK and Fx is in erg/s/cm^2.Comment: 5 pages, 3 figures, 1 table; accepted for publication in A&
The cuticle
The nematode cuticle is an extremely flexible and resilient exoskeleton that permits locomotion via
attachment to muscle, confers environmental protection and allows growth by molting. It is synthesised five
times, once in the embryo and subsequently at the end of each larval stage prior to molting. It is a highly
structured extra-cellular matrix (ECM), composed predominantly of cross-linked collagens, additional
insoluble proteins termed cuticlins, associated glycoproteins and lipids. The cuticle collagens are encoded by a large gene family that are subject to strict patterns of temporal regulation. Cuticle collagen biosynthesis
involves numerous co- and post-translational modification, processing, secretion and cross-linking steps that
in turn are catalysed by specific enzymes and chaperones. Mutations in individual collagen genes and their
biosynthetic pathway components can result in a range of defects from abnormal morphology (dumpy and
blister) to embryonic and larval death, confirming an essential role for this structure and highlighting its
potential as an ECM experimental model system
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&
The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
Aims. We aim to describe the pre-main sequence and main-sequence evolution of
X-ray and extreme-ultaviolet radiation of a solar mass star based on its
rotational evolution starting with a realistic range of initial rotation rates.
Methods. We derive evolutionary tracks of X-ray radiation based on a
rotational evolution model for solar mass stars and the rotation-activity
relation. We compare these tracks to X-ray luminosity distributions of stars in
clusters with different ages.
Results. We find agreement between the evolutionary tracks derived from
rotation and the X-ray luminosity distributions from observations. Depending on
the initial rotation rate, a star might remain at the X-ray saturation level
for very different time periods, approximately from 10 Myr to 300 Myr for slow
and fast rotators, respectively.
Conclusions. Rotational evolution with a spread of initial conditions leads
to a particularly wide distribution of possible X-ray luminosities in the age
range of 20 to 500 Myrs, before rotational convergence and therefore X-ray
luminosity convergence sets in. This age range is crucial for the evolution of
young planetary atmospheres and may thus lead to very different planetary
evolution histories.Comment: 4 pages, 4 figures, accepted for publication in A&
The Gelfand spectrum of a noncommutative C*-algebra: a topos-theoretic approach
We compare two influential ways of defining a generalized notion of space.
The first, inspired by Gelfand duality, states that the category of
'noncommutative spaces' is the opposite of the category of C*-algebras. The
second, loosely generalizing Stone duality, maintains that the category of
'pointfree spaces' is the opposite of the category of frames (i.e., complete
lattices in which the meet distributes over arbitrary joins). One possible
relationship between these two notions of space was unearthed by Banaschewski
and Mulvey, who proved a constructive version of Gelfand duality in which the
Gelfand spectrum of a commutative C*-algebra comes out as a pointfree space.
Being constructive, this result applies in arbitrary toposes (with natural
numbers objects, so that internal C*-algebras can be defined). Earlier work by
the first three authors, shows how a noncommutative C*-algebra gives rise to a
commutative one internal to a certain sheaf topos. The latter, then, has a
constructive Gelfand spectrum, also internal to the topos in question. After a
brief review of this work, we compute the so-called external description of
this internal spectrum, which in principle is a fibered pointfree space in the
familiar topos Sets of sets and functions. However, we obtain the external
spectrum as a fibered topological space in the usual sense. This leads to an
explicit Gelfand transform, as well as to a topological reinterpretation of the
Kochen-Specker Theorem of quantum mechanics, which supplements the remarkable
topos-theoretic version of this theorem due to Butterfield and Isham.Comment: 12 page
- …