6,745 research outputs found
Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
We have measured variations in scattering time scales in the Crab Pulsar over
a 30-year period, using observations made at 610 MHz with the 42-ft telescope
at Jodrell Bank Observatory. Over more recent years, where regular Lovell
Telescope observations at frequencies around 1400 MHz were available, we have
also determined the dispersion measure variations, after disentangling the
scattering delay from the dispersive delay. We demonstrate a relationship
between scattering and dispersion measure variations, with a correlation
coefficient of . The short time scales over which these quantities
vary, the size of the variations, and the close correlation between scattering
and dispersion measure all suggest that the effects are due to discrete
structures within the Crab Nebula, with size scales of AU
(corresponding to an angular size of mas at an assumed distance of 2200
pc). We mitigate the effects of scattering on the observed pulse shape by using
the measured scattering information to modify the template used for generating
the pulse arrival times, thus improving the precision to which the pulsar can
be timed. We test this on timing data taken during periods of high scattering,
and obtain a factor of two improvement in the root mean square of the timing
residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
What and how: doing good research with young people, digital intimacies, and relationships and sex education
© 2020, © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. As part of a project funded by the Wellcome Trust, we held a one-day symposium, bringing together researchers, practitioners, and policymakers, to discuss priorities for research on relationships and sex education (RSE) in a world where young people increasingly live, experience, and augment their relationships (whether sexual or not) within digital spaces. The introduction of statutory RSE in schools in England highlights the need to focus on improving understandings of young people and digital intimacies for its own sake, and to inform the development of learning resources. We call for more research that puts young people at its centre; foregrounds inclusivity; and allows a nuanced discussion of pleasures, harms, risks, and rewards, which can be used by those working with young people and those developing policy. Generating such research is likely to be facilitated by participation, collaboration, and communication with beneficiaries, between disciplines and across sectors. Taking such an approach, academic researchers, practitioners, and policymakers agree that we need a better understanding of RSE’s place in lifelong learning, which seeks to understand the needs of particular groups, is concerned with non-sexual relationships, and does not see digital intimacies as disconnected from offline everyday ‘reality’
The Propagation and Survival of Interstellar Grains
In this paper we discuss the propagation of dust through the interstellar
medium (ISM), and describe the destructive effects of stellar winds, jets, and
supernova shock waves on interstellar dust. We review the probability that
grains formed in stellar outflows or supernovae survive processing in and
propagation through the ISM, and incorporate themselves relatively unprocessed
into meteoritic bodies in the solar system. We show that very large (radii >= 5
micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the
pre-solar SiC and diamond grains extracted from meteorites, can survive the
passage through 100\kms shock waves relatively unscathed. High velocity (>= 250
km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the
stardust never encountered such fast shocks before incorporation into the solar
system. All grains should therefore retain traces of their passage through
interstellar shocks during their propagation through the ISM. The grain
surfaces should show evidence of processing due to sputtering and pitting due
to small grain cratering collisions on the micron-sized grains. This conclusion
seems to be in conflict with the evidence from the large grains recovered from
meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical
Implications of the Laboratory Study of Presolar Materials" AIP Conference
Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.
Low power laser generated ultrasound : signal processing for time domain data acquisition
The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound
Bipolar molecular outflows driven by hydromagnetic protostellar winds
We demonstrate that magnetically-collimated protostellar winds will sweep
ambient material into thin, radiative, momentum-conserving shells whose
features reproduce those commonly observed in bipolar molecular outflows. We
find the typical position-velocity and mass-velocity relations to occur in
outflows in a wide variety of ambient density distributions, regardless of the
time histories of their driving winds.Comment: 4 pages, 1 figure, submitted to ApJ
Low power laser generated ultrasound : signal processing for time domain data acquisition
The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound
Molecular Line Emission from Massive Protostellar Disks: Predictions for ALMA and the EVLA
We compute the molecular line emission of massive protostellar disks by
solving the equation of radiative transfer through the cores and disks produced
by the recent radiation-hydrodynamic simulations of Krumholz, Klein, & McKee.
We find that in several representative lines the disks show brightness
temperatures of hundreds of Kelvin over velocity channels ~10 km s^-1 wide,
extending over regions hundreds of AU in size. We process the computed
intensities to model the performance of next-generation radio and submillimeter
telescopes. Our calculations show that observations using facilities such as
the EVLA and ALMA should be able to detect massive protostellar disks and
measure their rotation curves, at least in the nearest massive star-forming
regions. They should also detect significant sub-structure and non-axisymmetry
in the disks, and in some cases may be able to detect star-disk velocity
offsets of a few km s^-1, both of which are the result of strong gravitational
instability in massive disks. We use our simulations to explore the strengths
and weaknesses of different observational techniques, and we also discuss how
observations of massive protostellar disks may be used to distinguish between
alternative models of massive star formation.Comment: 15 pages, 9 figures, emulateapj format, accepted for publication in
ApJ. Resolution of figures severely degraded to fit within size limits.
Download the full paper from
http://www.astro.princeton.edu/~krumholz/recent.htm
How Protostellar Outflows Help Massive Stars Form
We consider the effects of an outflow on radiation escaping from the
infalling envelope around a massive protostar. Using numerical radiative
transfer calculations, we show that outflows with properties comparable to
those observed around massive stars lead to significant anisotropy in the
stellar radiation field, which greatly reduces the radiation pressure
experienced by gas in the infalling envelope. This means that radiation
pressure is a much less significant barrier to massive star formation than has
previously been thought.Comment: 4 pages, 2 figures, emulateapj, accepted for publication in ApJ
Letter
Ethical issues in qualitative research addressing sensitive issues with children and young people
How do adults understand youth? How do their conceptions inform interventions into young lives or involve young people's experiences? This volume tackles these questions by exploring adults' ideas about youth
‘Generation rent’ and the ability to ‘settle down’: economic and geographical variation in young people’s housing transitions
© 2016 Informa UK Limited, trading as Taylor & Francis GroupThe term ‘Generation Rent’ denotes young people who are increasingly living in the private rented sector for longer periods of their lives because they are unable to access homeownership or social housing. Drawing on qualitative data from two studies with young people and key-actors, this paper considers the phenomenon of ‘Generation Rent’ from the perspective of youth transitions and the concept of ‘home’. These frameworks posit that young people leaving the parental home traverse housing and labour markets until they reach a point of ‘settling down’. However, our data indicate that many young people face difficulties in this ‘settling’ process as they have to contend with insecure housing, unstable employment and welfare cuts which often force them to be flexible and mobile. This leaves many feeling frustrated as they struggle to remain fixed in place in order to ‘settle down’ and benefit from the positive qualities of home. Taking a Scottish focus, this paper further highlights the geographical dimension to these challenges and argues that those living in expensive and/or rural areas may find it particularly difficult to settle down
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