88 research outputs found
Dark mammoth trunks in the merging galaxy NGC 1316 and a mechanism of cosmic double helices
NGC 1316 is a giant, elliptical galaxy containing a complex network of dark,
dust features. The morphology of these features has been examined in some
detail using a Hubble Space Telescope, Advanced Camera for Surveys image. It is
found that most of the features are constituted of long filaments. There also
exist a great number of dark structures protruding inwards from the filaments.
Many of these structures are strikingly similar to elephant trunks in H II
regions in the Milky Way Galaxy, although much larger. The structures, termed
mammoth trunks, generally are filamentary and often have shapes resembling the
letters V or Y. In some of the mammoth trunks the stem of the Y can be resolved
into two or more filaments, many of which showing signs of being intertwined. A
model of the mammoth trunks, related to a recent theory of elephant trunks, is
proposed. Based on magnetized filaments, the model is capable of giving an
account of the various shapes of the mammoth trunks observed, including the
twined structures.Comment: Accepted for publication in Astrophysics & Space Scienc
Electrodynamics of Magnetars IV: Self-Consistent Model of the Inner Accelerator, with Implications for Pulsed Radio Emission
We consider the voltage structure in the open-field circuit and outer
magnetosphere of a magnetar. The standard polar-cap model for radio pulsars is
modified significantly when the polar magnetic field exceeds 1.8x10^{14} G.
Pairs are created by accelerated particles via resonant scattering of thermal
X-rays, followed by the nearly instantaneous conversion of the scattered photon
to a pair. A surface gap is then efficiently screened by e+- creation, which
regulates the voltage in the inner part of the circuit to ~10^9 V. We also
examine the electrostatic gap structure that can form when the magnetic field
is somewhat weaker, and deduce a voltage 10-30 times larger over a range of
surface temperatures. We examine carefully how the flow of charge back to the
star above the gap depends on the magnitude of the current that is extracted
from the surface of the star, on the curvature of the magnetic field lines, and
on resonant drag. The rates of different channels of pair creation are
determined self-consistently, including the non-resonant scattering of X-rays,
and collisions between gamma rays and X-rays. We find that the electrostatic
gap solution has too small a voltage to sustain the observed pulsed radio
output of magnetars unless i) the magnetic axis is nearly aligned with the
rotation axis and the light of sight; or ii) the gap is present on the closed
as well as the open magnetic field lines. Several properties of the radio
magnetars -- their rapid variability, broad pulses, and unusually hard radio
spectra -- are consistent with a third possibility, that the current in the
outer magnetosphere is strongly variable, and a very high rate of pair creation
is sustained by a turbulent cascade.Comment: 32 pages, submitted to the Astrophysical Journa
Structure of the interstellar medium around Cas A
We present a three-year series of observations at 24 microns with the Spitzer
Space Telescope of the interstellar material in a 200 x 200 arcmin square area
centered on Cassiopeia A. Interstellar dust heated by the outward light pulse
from the supernova explosion emits in the form of compact, moving features.
Their sequential outward movements allow us to study the complicated
three-dimensional structure of the interstellar medium (ISM) behind and near
Cassiopeia A. The ISM consists of sheets and filaments, with many structures on
a scale of a parsec or less. The spatial power spectrum of the ISM appears to
be similar to that of fractals with a spectral index of 3.5. The filling factor
for the small structures above the spatial wavenumber k ~ 0.5 cycles/pc is only
~ 0.4%.Comment: 29 pages including 10 figures; accepted by The Astrophysical Journa
The magnetar emission in the IR band: the role of magnetospheric currents
There is a general consensus about the fact that the magnetar scenario
provides a convincing explanation for several of the observed properties of the
Anomalous X-ray Pulsars and the Soft Gamma Repeaters. However, the origin of
the emission observed at low energies is still an open issue. We present a
quantitative model for the emission in the optical/infrared band produced by
curvature radiation from magnetospheric charges, and compare results with
current magnetars observations.Comment: 6 Pages, 2 Figures. Astrophysics and Space Science, in press.
Proceedings of the ICREA Workshop on The High-Energy Emission from Pulsars
and their Systems, Sant Cugat, April 12-16 201
Light Echoes of Transients and Variables in the Local Universe
Astronomical light echoes, the time-dependent light scattered by dust in the
vicinity of varying objects, have been recognized for over a century.
Initially, their utility was thought to be confined to mapping out the
three-dimensional distribution of interstellar dust. Recently, the discovery of
spectroscopically-useful light echoes around centuries-old supernovae in the
Milky Way and the Large Magellanic Cloud has opened up new scientific
opportunities to exploit light echoes.
In this review, we describe the history of light echoes in the local Universe
and cover the many new developments in both the observation of light echoes and
the interpretation of the light scattered from them. Among other benefits, we
highlight our new ability to spectroscopically classify outbursting objects, to
view them from multiple perspectives, to obtain a spectroscopic time series of
the outburst, and to establish accurate distances to the source event. We also
describe the broader range of variable objects whose properties may be better
understood from light echo observations. Finally, we discuss the prospects of
new light echo techniques not yet realized in practice.Comment: 18 pages, 7 figures, 1 table. Accepted for publication in PAS
Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
We present new high resolution (100 arcsec) neutral hydrogen (H I)
self-absorption images of the Riegel-Crutcher cloud obtained with the Australia
Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher
cloud lies in the direction of the Galactic center at a distance of 125 +/- 25
pc. Our observations resolve the very large, nearby sheet of cold hydrogen into
a spectacular network of dozens of hair-like filaments. Individual filaments
are remarkably elongated, being up to 17 pc long with widths of less than ~0.1
pc. The strands are reasonably cold, with spin temperatures of 40 K and in many
places appearing to have optical depths larger than one. Comparing the H I
images with observations of stellar polarization we show that the filaments are
very well aligned with the ambient magnetic field. We argue that the structure
of the cloud has been determined by its magnetic field. In order for the cloud
to be magnetically dominated the magnetic field strength must be > 30
microGauss.Comment: To appear in the Astrophysical Journal. 26 pages, 6 figures. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/rc_cloud.pd
Improving lives by accelerating progress towards the UN Sustainable Development Goals for adolescents living with HIV: a prospective cohort study
BACKGROUND: Low-income and middle-income countries (LMICs) face major challenges in achieving the UN's Sustainable Development Goals (SDGs) for vulnerable adolescents. We aimed to test the UN Development Programme's proposed approach of development accelerators-provisions that lead to progress across multiple SDGs-and synergies between accelerators on achieving SDG-aligned targets in a highly vulnerable group of adolescents in South Africa. METHODS: We did standardised interviews and extracted longitudinal data from clinical records at baseline (2014-15) and 18-month follow-up (2016-17) for adolescents aged 10-19 years living with HIV in the Eastern Cape province of South Africa. We used standardised tools to measure 11 SDG-aligned targets-antiretroviral therapy adherence, good mental health, no substance use, HIV care retention, school enrolment, school progression, no sexual abuse, no high-risk sex, no violence perpetration, no community violence, and no emotional or physical abuse. We also assessed receipt at both baseline and follow-up of six hypothesised development accelerators-government cash transfers to households, safe schools (ie, without teacher or student violence), free schools, parenting support, free school meals, and support groups. Associations of all provisions with SDG-aligned targets were assessed jointly in a multivariate path model, controlling for baseline outcomes and sociodemographic and HIV-related covariates, and adjusted for multiple outcome testing. Cumulative effects were tested by marginal effects modelling. FINDINGS: 1063 (90%) of 1176 eligible adolescents were interviewed. Three provisions were shown to be development accelerators. Parenting support was associated with good mental health (odds ratio 2·13, 95% CI 1·43-3·15, p<0·0001), no high-risk sex (2·44, 1·45-5·03, p=0·005), no violence perpetration (2·59, 1·63-4·59, p<0·0001), no community violence (2·43, 1·65-3·86, p<0·0001), and no emotional or physical abuse (2·38, 1·65-3·76; p<0·0001). Cash transfers were associated with HIV care retention (1·87, 1·15-3·02, p=0·010), school progression (2·05, 1·33-3·24, p=0·003), and no emotional or physical abuse (1·76, 1·12-3·02, p=0·025). Safe schools were associated with good mental health (1·74, 1·30-2·34, p<0·0001), school progression (1·57, 1·17-2·13, p=0·004), no violence perpetration (2·02, 1·45-2·91, p<0·0001), no community violence (1·81, 1·30-2·55, p<0·0001), and no emotional or physical abuse (2·20, 1·58-3·17, p<0·0001). For five of 11 SDG-aligned targets, a combination of two or more accelerators showed cumulative positive associations, suggesting accelerator synergies of combination provisions. For example, the fitted probability of adolescents reporting no emotional or physical abuse (SDG 16.2) with no safe schools, cash transfers, or parenting support was 0·25 (0·16-0·34). With cash transfer alone it was 0·37 (0·33-0·42), with safe school alone 0·42 (0·30-0·55), and with parenting support alone 0·44 (0·30-0·59). With all three development accelerators combined, the probability of adolescents reporting no emotional or physical abuse was 0·76 (0·67-0·84). After correcting for multiple tests, four of the SDG-aligned targets (antiretroviral therapy adherence, no substance use, school enrolment, and no sexual abuse) were not associated with any hypothesised accelerators. INTERPRETATION: The findings suggest the UN's accelerator approach for this high-risk adolescent population has policy and potential financing usefulness. Services that simultaneously promote several SDG targets, or combine to support particular targets, might be important to meet not only health-related targets, but also to ensure that adolescents in LMICs thrive within a new development framework. FUNDING: Nuffield Foundation, UK Research and Innovation Global Challenges Research Fund, UKAID, Janssen Pharmaceutica, International AIDS Society, John Fell Fund, European Research Council, Economic and Social Research Council, Philip Leverhulme Trust, and UNICEF
H2 Temperatures in the Crab Nebula
We used K-band spectra to measure the H2 excitation temperatures in six
molecular knots associated with the filaments in the Crab Nebula. The
temperatures are quite high - in the range T ~ 2000-3000K, just below the H2
dissociation temperature. This is the temperature range over which the H2 1-0
S(1) line at 2.121\mum has its maximum emissivity per unit mass, so there may
be many additional H2 cores with lower temperatures that are too faint to
detect. We also measured the electron density in adjacent ionized gas, which on
the assumption of gas pressure balance indicates densities in the molecular
region n_mol ~ 20,000 H baryons cm-3, although this really is just a lower
limit since the H2 gas may be confined by other means. The excited region may
be just a thin skin on a much more extensive blob of molecular gas that does
not have the correct temperature and density to be as easily detectable. At the
opposite extreme, the observed knots could consist of a fine mist of molecular
gas in which we are detecting essentially all of the H2. Future CO observations
could distinguish between these two cases. The Crab filaments serve as the
nearby laboratories for understanding the very much larger filamentary
structures that have formed in the intracluster medium of cool-core galaxy
clusters.Comment: 16 pages, 5 figure
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