74 research outputs found
On the photon polarization in radiative B -> phi K gamma decay
The photon polarization in radiative decays B -> Y gamma is known to be a
subtle probe of the effective Lagrangian structure and possible New Physics
effects. We discuss exclusive decay mode B -> phi K gamma where the
experimentally distinct final state makes analysis especially promising. The
possibility to extract information on the photon polarization out of the data
entirely depends on the partial waves interference pattern in the phi K system.Comment: RevTeX, 6 pages, 1 figure; the journal versio
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk
Binary and multiple star systems are a frequent outcome of the star formation
process, and as a result, almost half of all sun-like stars have at least one
companion star. Theoretical studies indicate that there are two main pathways
that can operate concurrently to form binary/multiple star systems: large scale
fragmentation of turbulent gas cores and filaments or smaller scale
fragmentation of a massive protostellar disk due to gravitational instability.
Observational evidence for turbulent fragmentation on scales of 1000~AU has
recently emerged. Previous evidence for disk fragmentation was limited to
inferences based on the separations of more-evolved pre-main sequence and
protostellar multiple systems. The triple protostar system L1448 IRS3B is an
ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in
an early phase of the star formation process, likely less than 150,000 years in
age, and all protostars in the system are separated by 200~AU. Here we
report observations of dust and molecular gas emission that reveal a disk with
spiral structure surrounding the three protostars. Two protostars near the
center of the disk are separated by 61 AU, and a tertiary protostar is
coincident with a spiral arm in the outer disk at a 183 AU separation. The
inferred mass of the central pair of protostellar objects is 1 M,
while the disk surrounding the three protostars has a total mass of 0.30
M_{\sun}. The tertiary protostar itself has a minimum mass of 0.085
M. We demonstrate that the disk around L1448 IRS3B appears susceptible
to disk fragmentation at radii between 150~AU and 320~AU, overlapping with the
location of the tertiary protostar. This is consistent with models for a
protostellar disk that has recently undergone gravitational instability,
spawning one or two companion stars.Comment: Published in Nature on Oct. 27th. 24 pages, 8 figure
Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole
Massive black holes are believed to reside at the centres of most galaxies.
They can be- come detectable by accretion of matter, either continuously from a
large gas reservoir or impulsively from the tidal disruption of a passing star,
and conversion of the gravitational energy of the infalling matter to light.
Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be
variable but have never been observed to turn on or off. Tidal disruption of
stars by dormant massive black holes has been inferred indirectly but the on-
set of a tidal disruption event has never been observed. Here we report the
first discovery of the onset of a relativistic accretion-powered jet in the new
extragalactic transient, Swift J164449.3+573451. The behaviour of this new
source differs from both theoretical models of tidal disruption events and
observations of the jet-dominated AGN known as blazars. These differences may
stem from transient effects associated with the onset of a powerful jet. Such
an event in the massive black hole at the centre of our Milky Way galaxy could
strongly ionize the upper atmosphere of the Earth, if beamed towards us.Comment: Submitted to Nature. 4 pages, 3 figures (main paper). 26 pages, 13
figures (supplementary information
The Dynamical Structure and Evolution of Giant Molecular Clouds
Giant molecular clouds (GMCs) are the sites of star formation in the Galaxy. Many of their properties can be understood in terms of a model in which the GMCs and the star-forming clumps within them are in approximate pressure equilibrium, with turbulent motions treated as a separate pressure component
Motivational Interviewing as Evidence-Based Practice? An Example from Sexual Risk Reduction Interventions Targeting Adolescents and Young Adults
This paper critically examines sexual risk reduction interventions, more specifically how they are evaluated and the implications that this has for sexual health policy. The focus is on motivational interviewing (MI) interventions which aim to promote protective behaviors related to sexual risk on the part of young people. MI has become increasingly popular, largely due to it being a highly flexible counseling approach that may, with adequate staff training, and fidelity in implementation, be tailored to many different settings (e.g., health care, schools and in community work). Following a scoping review that comprised 34 papers, of which 29 were unique studies, the range and type of existing research were examined. The results show a wide range of study designs and evaluation procedures, MI conceptualizations, modes of MI delivery, and the particular sub-populations of youth and sexual risk behaviors targeted. While this makes it difficult to draw any generalized conclusions about “what works” in prevention, it provides important insights about the complexity of sexual risk behavior as well as complex behavioral treatment approaches like MI. We therefore problematize the political drive to implement evidence-based methods without adequate resource allocation and contextual adaptation
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