33 research outputs found
Thermal and Magnetorotational Instability in the ISM: Two-Dimensional Numerical Simulations
The structure and dynamics of diffuse gas in the Milky Way and other disk
galaxies may be strongly influenced by thermal and magnetorotational
instabilities (TI and MRI) on scales of about 1-100 pc. We initiate a study of
these processes, using two-dimensional numerical hydrodynamic and
magnetohydrodynamic (MHD) simulations with conditions appropriate for the
atomic interstellar medium (ISM). We demonstrate, consistent with previous
work, that nonlinear development of ``pure TI'' produces a network of filaments
that condense into cold clouds at their intersections, yielding a distinct
two-phase warm/cold medium within about 20 Myr. TI-driven turbulent motions of
the clouds saturate at subsonic amplitudes for uniform initial P/k=2000 K
cm^-3. MRI has previously been studied in near-uniform media; our simulations
include both TI+MRI models, which begin from uniform-density conditions, and
cloud+MRI models, which begin with a two-phase cloudy medium. Both the TI+MRI
and cloud+MRI models show that MRI develops within a few galactic orbital
times, just as for a uniform medium. The mean separation between clouds can
affect which MRI mode dominates the evolution. Provided intercloud separations
do not exceed half the MRI wavelength, we find the MRI growth rates are similar
to those for the corresponding uniform medium. This opens the possibility, if
low cloud volume filling factors increase MRI dissipation times compared to
those in a uniform medium, that MRI-driven motions in the ISM could reach
amplitudes comparable to observed HI turbulent linewidths.Comment: 42 pages, 15 figures, accepted for publication in ApJ; For better
postscript figures and mpeg animations, see
http://www.astro.umd.edu/~rpiontek/papers/ti_mri_2D.htm
Vertical structure of a supernova-driven turbulent magnetized ISM
Stellar feedback drives the circulation of matter from the disk to the halo
of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a
vertical column of the interstellar medium with initial conditions typical of
the solar circle in which supernovae drive turbulence and determine the
vertical stratification of the medium. The simulations were run using a stable,
positivity-preserving scheme for ideal MHD implemented in the FLASH code. We
find that the majority (\approx 90 %) of the mass is contained in
thermally-stable temperature regimes of cold molecular and atomic gas at T <
200 K or warm atomic and ionized gas at 5000 K < T < 10^{4.2} K, with strong
peaks in probability distribution functions of temperature in both the cold and
warm regimes. The 200 - 10^{4.2} K gas fills 50-60 % of the volume near the
plane, with hotter gas associated with supernova remnants (30-40 %) and cold
clouds (< 10 %) embedded within. At |z| ~ 1-2 kpc, transition-temperature (10^5
K) gas accounts for most of the mass and volume, while hot gas dominates at |z|
> 3 kpc. The magnetic field in our models has no significant impact on the
scale heights of gas in each temperature regime; the magnetic tension force is
approximately equal to and opposite the magnetic pressure, so the addition of
the field does not significantly affect the vertical support of the gas. The
addition of a magnetic field does reduce the fraction of gas in the cold (< 200
K) regime with a corresponding increase in the fraction of warm (~ 10^4 K) gas.
However, our models lack rotational shear and thus have no large-scale dynamo,
which reduces the role of the field in the models compared to reality. The
supernovae drive oscillations in the vertical distribution of halo gas, with
the period of the oscillations ranging from ~ 30 Myr in the T < 200 K gas to ~
100 Myr in the 10^6 K gas, in line with predictions by Walters & Cox.Comment: Accepted for publication in ApJ. Replacement corrects an error in the
observed CNM pressure distribution in Figure 15 and associated discussio
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Assessing the impacts of 1.5 °C global warming – simulation protocol of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP2b)
In Paris, France, December 2015, the Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change (UNFCCC) invited the Intergovernmental Panel on Climate Change (IPCC) to provide a "special report in 2018 on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways". In Nairobi, Kenya, April 2016, the IPCC panel accepted the invitation. Here we describe the response devised within the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) to provide tailored, cross-sectorally consistent impact projections to broaden the scientific basis for the report. The simulation protocol is designed to allow for (1) separation of the impacts of historical warming starting from pre-industrial conditions from impacts of other drivers such as historical land-use changes (based on pre-industrial and historical impact model simulations); (2) quantification of the impacts of additional warming up to 1.5°C, including a potential overshoot and long-term impacts up to 2299, and comparison to higher levels of global mean temperature change (based on the low-emissions Representative Concentration Pathway RCP2.6 and a no-mitigation pathway RCP6.0) with socio-economic conditions fixed at 2005 levels; and (3) assessment of the climate effects based on the same climate scenarios while accounting for simultaneous changes in socio-economic conditions following the middle-of-the-road Shared Socioeconomic Pathway (SSP2, Fricko et al., 2016) and in particular differential bioenergy requirements associated with the transformation of the energy system to comply with RCP2.6 compared to RCP6.0. With the aim of providing the scientific basis for an aggregation of impacts across sectors and analysis of cross-sectoral interactions that may dampen or amplify sectoral impacts, the protocol is designed to facilitate consistent impact projections from a range of impact models across different sectors (global and regional hydrology, lakes, global crops, global vegetation, regional forests, global and regional marine ecosystems and fisheries, global and regional coastal infrastructure, energy supply and demand, temperature-related mortality, and global terrestrial biodiversity)
The dynamic cilium in human diseases
Cilia are specialized organelles protruding from the cell surface of almost all mammalian cells. They consist of a basal body, composed of two centrioles, and a protruding body, named the axoneme. Although the basic structure of all cilia is the same, numerous differences emerge in different cell types, suggesting diverse functions. In recent years many studies have elucidated the function of 9+0 primary cilia. The primary cilium acts as an antenna for the cell, and several important pathways such as Hedgehog, Wnt and planar cell polarity (PCP) are transduced through it. Many studies on animal models have revealed that during embryogenesis the primary cilium has an essential role in defining the correct patterning of the body. Cilia are composed of hundreds of proteins and the impairment or dysfunction of one protein alone can cause complete loss of cilia or the formation of abnormal cilia. Mutations in ciliary proteins cause ciliopathies which can affect many organs at different levels of severity and are characterized by a wide spectrum of phenotypes. Ciliary proteins can be mutated in more than one ciliopathy, suggesting an interaction between proteins. To date, little is known about the role of primary cilia in adult life and it is tempting to speculate about their role in the maintenance of adult organs. The state of the art in primary cilia studies reveals a very intricate role. Analysis of cilia-related pathways and of the different clinical phenotypes of ciliopathies helps to shed light on the function of these sophisticated organelles. The aim of this review is to evaluate the recent advances in cilia function and the molecular mechanisms at the basis of their activity
Polycystic kidney diseases: From molecular discoveries to targeted therapeutic strategies
Polycystic kidney diseases (PKDs) represent a large group of progressive renal disorders characterized by the development of renal cysts leading to end-stage renal disease. Enormous strides have been made in understanding the pathogenesis of PKDs and the development of new therapies. Studies of autosomal dominant and recessive polycystic kidney diseases converge on molecular mechanisms of cystogenesis, including ciliary abnormalities and intracellular calcium dysregulation, ultimately leading to increased proliferation, apoptosis and dedifferentiation. Here we review the pathobiology of PKD, highlighting recent progress in elucidating common molecular pathways of cystogenesis. We discuss available models and challenges for therapeutic discovery as well as summarize the results from preclinical experimental treatments targeting key disease-specific pathways