337 research outputs found
Some reflections on globalization, development and the less developed countries
The aim of the paper is to show that in reality globalization is an effectively global process
because it is of extreme importance for all the countries on earth, including the poorest and
least developed of them. Aside from the current impact of globalization on these latter
countries, the paper argues that it is not possible to find a solution to the problem of
development which does not involve globalization â which accordingly assumes (at least
potentially) a strongly positive connotation. On the other hand, it is precisely the existence of
problems of global scope â those of development, and in particular of its sustainability â
which shows, I believe, the validity and utility of the concept of globalization, distinguishing
it from mere internationalization and marking a sharp break with the past
Interstellar dust charging in dense molecular clouds: cosmic ray effects
The local cosmic-ray (CR) spectra are calculated for typical characteristic
regions of a cold dense molecular cloud, to investigate two so far neglected
mechanisms of dust charging: collection of suprathermal CR electrons and
protons by grains, and photoelectric emission from grains due to the UV
radiation generated by CRs. The two mechanisms add to the conventional charging
by ambient plasma, produced in the cloud by CRs. We show that the CR-induced
photoemission can dramatically modify the charge distribution function for
submicron grains. We demonstrate the importance of the obtained results for
dust coagulation: While the charging by ambient plasma alone leads to a strong
Coulomb repulsion between grains and inhibits their further coagulation, the
combination with the photoemission provides optimum conditions for the growth
of large dust aggregates in a certain region of the cloud, corresponding to the
densities between cm and
cm. The charging effect of CR is of generic nature, and therefore is
expected to operate not only in dense molecular clouds but also in the upper
layers and the outer parts of protoplanetary discs.Comment: accepted by Ap
Cosmic-ray ionisation in circumstellar discs
Galactic cosmic rays are a ubiquitous source of ionisation of the
interstellar gas, competing with UV and X-ray photons as well as natural
radioactivity in determining the fractional abundance of electrons, ions and
charged dust grains in molecular clouds and circumstellar discs. We model the
propagation of different components of Galactic cosmic rays versus the column
density of the gas. Our study is focussed on the propagation at high densities,
above a few g cm, especially relevant for the inner regions of
collapsing clouds and circumstellar discs. The propagation of primary and
secondary CR particles (protons and heavier nuclei, electrons, positrons, and
photons) is computed in the continuous slowing down approximation, diffusion
approximation, or catastrophic approximation, by adopting a matching procedure
for the different transport regimes. A choice of the proper regime depends on
the nature of the dominant loss process, modelled as continuous or
catastrophic. The CR ionisation rate is determined by CR protons and their
secondary electrons below g cm and by electron/positron
pairs created by photon decay above g cm. We show that a
proper description of the particle transport is essential to compute the
ionisation rate in the latter case, since the electron/positron differential
fluxes depend sensitively on the fluxes of both protons and photons. Our
results show that the CR ionisation rate in high-density environments, like,
e.g., the inner parts of collapsing molecular clouds or the mid-plane of
circumstellar discs, is larger than previously assumed. It does not decline
exponentially with increasing column density, but follows a more complex
behaviour due to the interplay of different processes governing the generation
and propagation of secondary particles.Comment: 19 pages, 11 figures, accepted by A&
Dust temperature and time-dependent effects in the chemistry of photodissociation regions
When studying the chemistry of PDRs, time dependence becomes important as
visual extinction increases, since certain chemical timescales are comparable
to the cloud lifetime. Dust temperature is also a key factor, since it
significantly influences gas temperature and mobility on dust grains,
determining the chemistry occurring on grain surfaces. We present a study of
the dust temperature impact and time effects on the chemistry of different
PDRs, using an updated version of the Meijerink PDR code and combining it with
the time-dependent code Nahoon. We find the largest temperature effects in the
inner regions of high PDRs, where high dust temperatures
favour the formation of simple oxygen-bearing molecules (especially that of
O), while the formation of complex organic molecules is much more efficient
at low dust temperatures. We also find that time-dependent effects strongly
depend on the PDR type, since long timescales promote the destruction of
oxygen-bearing molecules in the inner parts of low PDRs,
while favouring their formation and that of carbon-bearing molecules in high
PDRs. From the chemical evolution, we also conclude that, in
dense PDRs, CO is a late-forming ice compared to water ice, and confirm a
layered ice structure on dust grains, with HO in lower layers than CO.
Regarding steady state, the PDR edge reaches chemical equilibrium at early
times (10 yr). This time is even shorter (10 yr) for high
PDRs. By contrast, inner regions reach equilibrium much
later, especially low PDRs, where steady state is reached at
10-10 yr.Comment: 24 pages, 15 figures, 9 table
Production of atomic hydrogen by cosmic rays in dark clouds
The presence of small amounts of atomic hydrogen, detected as absorption dips
in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The
abundance of hydrogen atoms measured in the densest regions of molecular clouds
can be only explained by the dissociation of H due to cosmic rays. We want
to assess the role of Galactic cosmic rays in the formation of atomic hydrogen,
by using recent developments in the characterisation of the low-energy spectra
of cosmic rays and advances in the modelling of their propagation in molecular
clouds. We model the attenuation of the interstellar cosmic rays entering a
cloud and compute the dissociation rate of molecular hydrogen due to collisions
with cosmic-ray protons and electrons as well as fast hydrogen atoms. We
compare our results with the available observations. The cosmic-ray
dissociation rate is entirely determined by secondary electrons produced in
primary ionisation collisions. These secondary particles constitute the only
source of atomic hydrogen at column densities above cm. We
also find that the dissociation rate decreases with column density, while the
ratio between the dissociation and ionisation rates varies between about 0.6
and 0.7. From comparison with observations we conclude that a relatively flat
spectrum of interstellar cosmic-ray protons, as the one suggested by the most
recent Voyager 1 data, can only provide a lower bound for the observed atomic
hydrogen fraction. An enhanced spectrum of low-energy protons is needed to
explain most of the observations. Our findings show that a careful description
of molecular hydrogen dissociation by cosmic rays can explain the abundance of
atomic hydrogen in dark clouds. An accurate characterisation of this process at
high densities is crucial for understanding the chemical evolution of
star-forming regions.Comment: 7 pages, 7 figures, accepted by Astronomy and Astrophysic
Power Management Circuits for Low-Power RF Energy Harvesters
The paper describes the design and implementation of power management circuits for
RF energy harvesters suitable for integration in wireless sensor nodes. In particular, we report the
power management circuits used to provide the voltage supply of an integrated temperature sensor
with analog-to-digital converter. A DC-DC boost converter is used to transfer efficiently the energy
harvested from a generic radio-frequency rectifier into a charge reservoir, whereas a linear regulator
scales the voltage supply to a suitable value for a sensing and conversion circuit. Implemented in
a 65 nm CMOS technology, the power management system achieves a measured overall efficiency
of 20%, with an available power of 4.5 ÎŒW at the DC-DC converter input. The system can sustain
a temperature measurement rate of one sample/s with an RF input power of â28 dBm, making it
compatible with the power levels available in generic outdoor environments
Living through the Apocalypse(s): climate change and (non)human boundaries in Octavia Butlerâs Parable of the Sower and N.K. Jemisinâs The Broken Earth trilogy
Climate change is one of the most pressing issues of our time, and literature is one way of dealing with this seemingly incomprehensible phenomenon we are now called to face. Challenging the predominance of white and male writers in science fiction, cli-fi novels by African American authors N.K. Jemisin and Octavia Butler showcase new approaches to the issue of climate change and portray new possibilities for imagining a different future. Through the powers of the protagonists of their respective series and the journeys they go through, the two authors also problematize the relationship between humanity and life on/of the planet, in alignment with the posthuman strand of thought that has recently challenged the position of Man on Earth. Through a close reading of Jemisinâs The Broken Earth trilogy and Butlerâs Parable of the Sower, the aim of this thesis is then to analyse how the two aforementioned authors depict the issue of climate change and the consequent shift in human-nonhuman boundaries in their novels
The first frost in the Pipe Nebula
Spectroscopic studies of ices in nearby star-forming regions indicate that
ice mantles form on dust grains in two distinct steps, starting with polar ice
formation (H2O rich) and switching to apolar ice (CO rich). We test how well
the picture applies to more diffuse and quiescent clouds where the formation of
the first layers of ice mantles can be witnessed. Medium-resolution
near-infrared spectra are obtained toward background field stars behind the
Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in
seven lines of sight out of 21 sources for which observed spectra are
successfully reduced. The peak optical depth of the water ice is significantly
lower than those in Taurus with the same visual extinction. The source with the
highest water-ice optical depth shows CO ice absorption at 4.7 micron as well.
The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and
about half as much as the values typically seen in low-mass star-forming
regions. A small fractional abundance of CO ice is consistent with some of the
existing simulations. Observations of CO2 ice in the early diffuse phase of a
cloud play a decisive role in understanding the switching mechanism between
polar and apolar ice formation.Comment: 17 pages, 8 figures, accepted by A&
- âŠ