121 research outputs found
Projections for measuring the size of the solar core with neutrino-electron scattering
We quantify the amount of data needed in order to measure the size and
position of the B neutrino production region within the solar core, for
experiments looking at elastic scattering between electrons and solar
neutrinos. The directions of the electrons immediately after scattering are
strongly correlated with the incident directions of the neutrinos, however this
is degraded significantly by the subsequent scattering of these electrons in
the detector medium. We generate distributions of such electrons for different
neutrino production profiles, and use a maximum likelihood analysis to make
projections for future experimental sensitivity. We find that with
approximately 20 years worth of data the Super Kamiokande experiment could
constrain the central radius of the shell in which B neutrinos are produced
to be less than 0.22 of the total solar radius at 95% confidence.Comment: 5 pages, 2 figures. Matches version accepted to PRL. Improved 2D
analysis and results discussio
On the low-mass planethood criterion
We propose a quantitative concept for the lower planetary boundary, requiring
that a planet must keep its atmosphere in vacuum. The solution-set framework of
Pecnik and Wuchterl (2005) enabled a clear and quantitative criterion for the
discrimination of a planet and a minor body. Using a simple isothermal
core-envelope model, we apply the proposed planetary criterion to the large
bodies in the Solar System.Comment: 13 pages, 1 table, submitted to journal of Planetary and Space
Scienc
A critical review of the proposed definitions of fundamental chemical quantities and their impact on chemical communities (IUPAC Technical Report)
In the proposed new SI, the kilogram will be redefined in terms of the Planck constant and the mole will be redefined in terms of the Avogadro constant. These redefinitions will have some consequences for measurements in chemistry. The goal of the Mole Project (IUPAC Project Number 2013-048-1-100) was to compile published work related to the definition of the quantity \u2018amount of substance\u2019, its unit the \u2018mole\u2019, and the consequence of these definitions on the unit of the quantity mass, the kilogram. The published work has been reviewed critically with the aim of assembling all possible aspects in order to enable IUPAC to judge the adequateness of the existing definitions or new proposals. Compilation and critical review relies on the broadest spectrum of interested IUPAC members.Peer reviewed: YesNRC publication: Ye
Definition of the mole (IUPAC Recommendation 2017)
In 2011 the General Conference on Weights and Measures (CGPM) noted the intention of the International Committee for Weights and Measures (CIPM) to revise the entire International System of Units (SI) by linking all seven base units to seven fundamental physical constants. Of particular interest to chemists, new definitions for the kilogram and the mole have been proposed. A recent IUPAC Technical Report discussed these new definitions in relation to immediate consequences for the chemical community. This IUPAC Recommendation on the preferred definition of the mole follows from this Technical Report. It supports a definition of the mole based on a specified number of elementary entities, in contrast to the present 1971 definition.Peer reviewed: YesNRC publication: Ye
Solution pans and linear sand bedforms on the bare-rock limestone shelf of the Campeche Bank, Yucatán Peninsula, Mexico
A high-resolution, near-surface geophysical survey was conducted in 2013 on the Campeche Bank, a carbonate platform offshore of Yucatán, Mexico, to provide a hazard assessment for future scientific drilling into the Chicxulub impact crater. It also provided an opportunity to obtain detailed information on the seafloor morphology and shallow stratigraphy of this understudied region. The seafloor exhibited two morphologies: (1) small-scale (<2 m) bare-rock karstic features, and (2) thin (<1 m) linear sand accumulations overlying the bedrock. Solution pans, circular to oblong depressions featured flat bottoms and steep sides, were the dominant karstic features; they are known to form subaerially by the pooling of rainwater and dissolution of carbonate. Observed pans were 10–50 cm deep and generally 1–8 m wide, but occasionally reach 15 m, significantly larger than any solution pan observed on land (maximum 6 m). These features likely grew over the course of many 10's of thousands of years in an arid environment while subaerially exposed during lowered sea levels. Surface sands are organized into linear bedforms oriented NE-SW, 10's to 100's meters wide, and kilometers long. These features are identified as sand ribbons (longitudinal bedforms), and contained asymmetric secondary transverse bedforms that indicate NE-directed flow. This orientation is incompatible with the prevalent westward current direction; we hypothesize that these features are storm-generated
Tidal evolution of close-in giant planets : Evidence of Type II migration?
It is well accepted that 'hot Jupiters' did not form in situ, as the
temperature in the protoplanetary disc at the radius at which they now orbit
would have been too high for planet formation to have occurred. These planets,
instead, form at larger radii and then move into the region in which they now
orbit. The exact process that leads to the formation of these close-in planets
is, however, unclear and it seems that there may be more than one mechanism
that can produce these short-period systems. Dynamical interactions in
multiple-planet systems can scatter planets into highly eccentric orbits which,
if the pericentre is sufficiently close to the parent star, can be tidally
circularised by tidal interactions between the planet and star. Furthermore,
systems with distant planetary or stellar companions can undergo Kozai cycles
which can result in a planet orbiting very close to its parent star. However,
the most developed model for the origin of short period planets is one in which
the planet exchanges angular momentum with the surrounding protoplanetary disc
and spirals in towards the central star. In the case of 'hot Jupiters', the
planet is expected to open a gap in the disc and migrate through Type II
.migration. If this is the dominant mechanism for producing `hot Jupiters' then
we would expect the currect properties of observed close-in giant planets to be
consistent with an initial population resulting from Type II migration followed
by evolution due to tidal interactions with the central star. We consider
initial distributions that are consistent with Type II migration and find that
after tidal evolution, the final distributions can be consistent with that
observed. Our results suggest that a modest initial pile-up at a ~ 0.05 au is
required and that the initial eccentricity distribution must peak at e \sim 0.Comment: 10 pages, 15 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Stellar, Brown Dwarf, and Multiple Star Properties from Hydrodynamical Simulations of Star Cluster Formation
We report the statistical properties of stars, brown dwarfs and multiple
systems obtained from the largest hydrodynamical simulation of star cluster
formation to date that resolves masses down to the opacity limit for
fragmentation (a few Jupiter masses). The simulation is essentially identical
to that of Bate, Bonnell & Bromm except that the initial molecular cloud is
larger and more massive. It produces more than 1250 stars and brown dwarfs,
providing unprecedented statistical information that can be compared with
observational surveys.
We find that hydrodynamical/sink particle simulations can reproduce many of
the observed stellar properties very well. Binarity as a function of primary
mass, the frequency of very-low-mass (VLM) binaries, general trends for the
separation and mass ratio distributions of binaries, and the relative orbital
orientations of triples systems are all in reasonable agreement with
observations. We also examine the radial variations of binarity, velocity
dispersion, and mass function in the resulting stellar cluster and the
distributions of disc truncation radii due to dynamical interactions. For VLM
binaries, we find that their frequency when using small accretion radii and
gravitational softening is similar to that expected from observational surveys
(approximately 20 percent). We also find that VLM binaries evolve from wide,
unequal-mass systems towards close equal-mass systems as they form. The two
main deficiencies of the calculations are that they over produce brown dwarfs
relative to stars and that there are too few unequal mass binaries with K and
G-dwarf primaries. [Abridged]Comment: Accepted by MNRAS, 28 pages, 26 figures. Animations available at
http://www.astro.ex.ac.uk/people/mbate
Evaluation of a multifaceted "Resident-as-Teacher" educational intervention to improve morning report
BACKGROUND: Resident-led morning report is an integral part of most residency programs and is ranked among the most valuable of educational experiences. The objectives of this study were to evaluate the effect of a resident-as-teacher educational intervention on the educational and teaching experience of morning report. METHODS: All senior internal medicine residents were invited to participate in this study as teaching participants. All internal medicine residents and clerks were invited to participate as audience participants. The educational intervention included reading material, a small group session and feedback after teaching sessions. The educational and teaching experiences were rated prior to and three months after the intervention using questionnaires. RESULTS: Forty-six audience participants and 18 teaching participants completed the questionnaires. The degree to which morning report met the educational needs of the audience was higher after the educational intervention (effect size, d = 0.26, p = 0.01). The perceptions of the audience were that delivery had improved and that the sessions were less intimidating and more interactive. The perception of the teaching participants was that delivery was less stressful, but this group now reported greater difficulty in engaging the audience and less confidence in their medical knowledge. CONCLUSION: Following the educational intervention the audience's perception was that the educational experience had improved although there were mixed results for the teaching experience. When evaluating such interventions it is important to evaluate the impact on both the educational and teaching experiences as results may differ
The Diversity of Planetary Systems Architectures: Contrasting Theory with Observations
We develop a semi-analytical model for computing planetary system formation
with the aim of explaining the observed diversity of planetary systems
architectures and relate this primordial diversity with the initial properties
of the disc where they were born. We adopt different initial conditions based
on recent results in protoplanetary discs observations, to generate a variety
of planetary systems and analyze them statistically. We explore the relevance
of the mass and size of the disc, its metallicity, the mass of the central star
and the time-scale of gaseous disc dissipation, in defining the architecture of
the planetary system. We also test different values of some key parameters of
our model, to find out which factors best reproduce the diverse sample of
observed planetary systems. According to this, we predict which systems are the
most common in the solar neighbourhood. Our results show that planetary systems
with only terrestrial planets are the most common, being the only planetary
systems formed when considering low metallicity discs and which also represent
the best environment for the developing of rocky, potentially habitable
planets. We also found that planetary systems like our own are not rare in the
solar neighbourhood, being its formation favoured in massive discs where there
is not a large accumulation of solids in the inner region of the disc.
Regarding the planetary systems that harbor hot and warm Jupiter planets, we
found that this systems are born in very massive, metal-rich discs. Also a fast
migration rate is required in order to form these systems. According to our
results, most of the hot and warm Jupiter systems are composed by only one
giant planet, which is also a tendency of the current observational data.Comment: MNRAS in pres
The Kuiper Belt and Other Debris Disks
We discuss the current knowledge of the Solar system, focusing on bodies in
the outer regions, on the information they provide concerning Solar system
formation, and on the possible relationships that may exist between our system
and the debris disks of other stars. Beyond the domains of the Terrestrial and
giant planets, the comets in the Kuiper belt and the Oort cloud preserve some
of our most pristine materials. The Kuiper belt, in particular, is a
collisional dust source and a scientific bridge to the dusty "debris disks"
observed around many nearby main-sequence stars. Study of the Solar system
provides a level of detail that we cannot discern in the distant disks while
observations of the disks may help to set the Solar system in proper context.Comment: 50 pages, 25 Figures. To appear in conference proceedings book
"Astrophysics in the Next Decade
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