592 research outputs found
A Semi-Analytic dynamical friction model that reproduces core stalling
We present a new semi-analytic model for dynamical friction based on
Chandrasekhar's formalism. The key novelty is the introduction of physically
motivated, radially varying, maximum and minimum impact parameters. With these,
our model gives an excellent match to full N-body simulations for isotropic
background density distributions, both cuspy and shallow, without any
fine-tuning of the model parameters. In particular, we are able to reproduce
the dramatic core-stalling effect that occurs in shallow/constant density
cores, for the first time. This gives us new physical insight into the
core-stalling phenomenon. We show that core stalling occurs in the limit in
which the product of the Coulomb logarithm and the local fraction of stars with
velocity lower than the infalling body tends to zero. For cuspy backgrounds,
this occurs when the infalling mass approaches the enclosed background mass.
For cored backgrounds, it occurs at larger distances from the centre, due to a
combination of a rapidly increasing minimum impact parameter and a lack of slow
moving stars in the core. This demonstrates that the physics of core-stalling
is likely the same for both massive infalling objects and low-mass objects
moving in shallow density backgrounds. We implement our prescription for
dynamical friction in the direct summation code NBODY6 as an analytic
correction for stars that remain within the Roche volume of the infalling
object. This approach is computationally efficient, since only stars in the
inspiralling system need to be evolved with direct summation. Our method can be
applied to study a variety of astrophysical systems, including young star
clusters orbiting near the Galactic Centre; globular clusters moving within the
Galaxy; and dwarf galaxies orbiting within dark matter halos.Comment: 16 pages, 21 figures, Accepted for publication in MNRA
A semi-analytic dynamical friction model for cored galaxies
We present a dynamical friction model based on Chandrasekhar's formula that
reproduces the fast inspiral and stalling experienced by satellites orbiting
galaxies with a large constant density core. We show that the fast inspiral
phase does not owe to resonance. Rather, it owes to the background velocity
distribution function for the constant density cores being dissimilar from the
usually-assumed Maxwellian distribution. Using the correct background velocity
distribution function and the semi-analytic model from Petts et al. (2015), we
are able to correctly reproduce the infall rate in both cored and cusped
potentials. However, in the case of large cores, our model is no longer able to
correctly capture core-stalling. We show that this stalling owes to the tidal
radius of the satellite approaching the size of the core. By switching off
dynamical friction when rt(r) = r (where rt is the tidal radius at the
satellite's position) we arrive at a model which reproduces the N-body results
remarkably well. Since the tidal radius can be very large for constant density
background distributions, our model recovers the result that stalling can occur
for Ms/Menc << 1, where Ms and Menc are the mass of the satellite and the
enclosed galaxy mass, respectively. Finally, we include the contribution to
dynamical friction that comes from stars moving faster than the satellite. This
next-to-leading order effect becomes the dominant driver of inspiral near the
core region, prior to stalling.Comment: 13 pages, 12 figures, resubmitted to MNRAS after responding to
feedback from the refere
Globular clusters as probes of dark matter cusp-core transformations
Bursty star formation in dwarf galaxies can slowly transform a steep dark
matter cusp into a constant density core. We explore the possibility that
globular clusters (GCs) retain a dynamical memory of this transformation. To
test this, we use the nbody6df code to simulate the dynamical evolution of GCs,
including stellar evolution, orbiting in static and time-varying potentials for
a Hubble time. We find that GCs orbiting within a cored dark matter halo, or
within a halo that has undergone a cusp-core transformation, grow to a size
that is substantially larger ( pc) than those in a static
cusped dark matter halo. They also produce much less tidal debris. We find that
the cleanest signal of an historic cusp-core transformation is the presence of
large GCs with tidal debris. However, the effect is small and will be
challenging to observe in real galaxies. Finally, we qualitatively compare our
simulated GCs with the observed GC populations in the Fornax, NGC 6822, IKN and
Sagittarius dwarf galaxies. We find that the GCs in these dwarf galaxies are
systematically larger ( pc), and have
substantially more scatter in their sizes, than in-situ metal rich GCs in the
Milky Way and young massive star clusters forming in M83 ( pc). We show that the size, scatter and survival of GCs in
dwarf galaxies are all consistent with them having evolved in a constant
density core, or a potential that has undergone a cusp-core transformation, but
not in a dark matter cusp.Comment: 14 pages, 10 figure
Scanned Imaging Techniques for Surface NDE
A phase sensitive laser probe in which the focussed spot is small, as compared with the acoustic wavelength, is capable of measuring the complex distributions of a SAW field along prescribed scan lines. Using the probe, it is possible, on a defect free sample, to measure the SAW velocity surface with an accuracy of a few parts in 105. Such accuracy suggests that the technique is sufficiently sensitive to detect small changes in surface characteristics; the presence of a defect is revealed by perturbations in the relationship between various scans. The scattered radiation patterns from a surface crack irradiated by acoustic surface waves can be utilised to determine the defect size and location with improved accuracy. Results on deliberate and real cracks are presented
Covid-19: Emerging needs for unemployed and economically inactive individuals
This Voices article presents emerging findings from research investigating the consequences of Covid-19 on individuals engaged on employability programmes in the United Kingdom. It outlines the challenges presented by âlockdownâ restrictions, as introduced to reduce the spread of Covid-19, on the delivery of employability programmes. Individuals engaged on such programmes experience a wealth of needs that have been compounded by Covid-19, resulting in new, emerging needs relating to personal wellbeing, social isolation, and confidence. Without effective and innovative support, individuals experiencing unemployment and economic inactivity will undoubtedly experience increased inequality; this inequality impacts on family, with individuals experiencing isolation from such, which in turn reduces wellbeing and stimuli, and indeed work, with Covid-19 compounding challenges in securing employment
Fathers stepping up? A cross-national comparison of fathersâ domestic labour and parentsâ satisfaction with the division of domestic labour during the COVID-19 pandemic
The COVID-19 pandemic disrupted work and family life around the world. For parents, this upending meant a potential re-negotiation of the âstatus quoâ in the gendered division of labour. A comparative lens provides extended understandings of changes in fathersâ domestic work based in socio-cultural contextâin assessing the size and consequences of change in domestic labour in relation to the type of work-care regime. Using novel harmonized data from four countries (the United States, Canada, the United Kingdom, and the Netherlands) and a work-care regime framework, this study examines cross-national changes in fathersâ shares of domestic labour during the early months of the pandemic and whether these changes are associated with parentsâ satisfaction with the division of labour. Results indicate that fathersâ shares of housework and childcare increased early in the pandemic in all countries, with fathersâ increased shares of housework being particularly pronounced in the US. Results also show an association between fathersâ increased shares of domestic labour and mothersâ increased satisfaction with the division of domestic labour in the US, Canada, and the UK. Such comparative work promises to be generative for understanding the pandemicâs imprint on gender relations far into the future
Introduction: A handbook on territorial impact assessment (TIA)
Territorial Impact Assessment (TIA) is a relatively ânew kid on the blockâ of policy evaluation. Resting upon the holistic notion of territory, which encompasses multiple analytic dimensions (economy, society, environment, governance, spatial planning), TIA is the most complex, yet with the policy evaluation procedure with the largest potential to assess projects, programmes and policies. Indeed, policy evaluation procedures are now deeply rooted in sub-national, national and transnational territorial development strategies and processes. However, unlike the plethora of published books on Environmental Impact Assessment (EIA) and other Impact Assessment (IA) methodologies, presently no TIA handbook has been published by any major international publisher. As such, this one intends to add a substantial contribution to available literature by presenting to the interested reader the most relevant TIA methodologies that have been produced so far. Furthermore, all the chapters, written by the authors of each TIA methodology presented, provide a detailed, updated and scientifically accurate explanation of their particular purpose and methodological operation. In the end, the reader is presented with a complete set of TIA methodologies to select from based on their advantages/disadvantages for a particular case-study. For a better understanding of how all the presented TIA methodologies work, concrete examples are presented in each chapter.info:eu-repo/semantics/acceptedVersio
Industrial constructions of publics and public knowledge: a qualitative investigation of practice in the UK chemicals industry
This is a post print version of the article. The official published version can be obtained from the link below - © 2007 by SAGE PublicationsWhile the rhetoric of public engagement is increasingly commonplace within industry, there has been little research that examines how lay knowledge is conceptualized and whether it is really used within companies. Using the chemicals sector as an example, this paper explores how companies conceive of publics and "public knowledge," and how this relates to modes of engagement/communication with them. Drawing on qualitative empirical research in four companies, we demonstrate that the public for industry are primarily conceived as "consumers" and "neighbours," having concerns that should be allayed rather than as groups with knowledge meriting engagement. We conclude by highlighting the dissonance between current advocacy of engagement and the discourses and practices prevalent within industry, and highlight the need for more realistic strategies for industry/public engagement.Funding was received from the ESRC Science in Society Programme
âWeâre in This Togetherâ: Bridging and Bonding Social Capital in Elementary School PTOs
Scholarship on the links between families and schools encompasses contradictory notions about social capital and its relation to inequality. One view holds that schools can narrow inequality by generating dense relationships among families, while others suggest that advantaged parents can use these networks to hoard opportunities. This multiple case study analyzes qualitative data from diverse North Carolina elementary schools to learn how parents build and deploy social capital. We distinguish between bonding social capital, built in dense, homogeneous networks, and bridging social capital, gained through relationships across a social distance. Our analyses suggest that bonding alone is associated with opportunity hoarding; however, when schools are committed to building both bridging and bonding social capital, they can produce more equitable and inclusive schools
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