57 research outputs found
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?
Subdwarf B stars show chemical peculiarities that cannot be explained by
diffusion theory alone. Both mass loss and turbulence have been invoked to slow
down atomic diffusion in order to match observed abundances. The fact that some
sdB stars show pulsations gives upper limits on the amount of mass loss and
turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the
potential to decide which process is responsible for the abundance anomalies.
We compute for the first time seismic properties of sdB models with atomic
diffusion included consistently during the stellar evolution. The diffusion
equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and
Ni. We examine the effects of various mass-loss rates and mixed surface masses
on the abundances and mode stability. It is shown that the mass-loss rates
needed to simulate the observed He abundances (10^{-14}<=Mdot
[Msun/yr]<=10^{-13}) are not consistent with observed pulsations. We find that
for pulsations to be driven the rates should be Mdot<=10^{-15} Msun/yr. On the
other hand, weak turbulent mixing of the outer 10^{-6} Msun can explain the He
abundance anomalies while still allowing pulsations to be driven. The origin of
the turbulence remains unknown but the presence of pulsations gives tight
constraints on the underlying turbulence model.Comment: 12 pages, 8 figures, 1 table, accepted for publication in MNRA
The Blue Stragglers of the Old Open Cluster NGC 188
The old (7 Gyr) open cluster NGC 188 has yielded a wealth of astrophysical
insight into its rich blue straggler population. Specifically, the NGC 188 blue
stragglers are characterized by: A binary frequency of 80% for orbital periods
less than days;Typical orbital periods around 1000 days;Typical
secondary star masses of 0.5 M; At least some white dwarf companion
stars; Modestly rapid rotation; A bimodal radial spatial distribution;
Dynamical masses greater than standard stellar evolution masses (based on
short-period binaries); Under-luminosity for dynamical masses (short-period
binaries). Extensive -body modeling of NGC 188 with empirical initial
conditions reproduces the properties of the cluster, and in particular the
main-sequence solar-type binary population. The current models also reproduce
well the binary orbital properties of the blue stragglers, but fall well short
of producing the observed number of blue stragglers. This deficit could be
resolved by reducing the frequency of common-envelope evolution during Roche
lobe overflow. Both the observations and the -body models strongly indicate
that the long-period blue-straggler binaries - which dominate the NGC 188 blue
straggler population - are formed by asymptotic-giant (primarily) and red-giant
mass transfer onto main sequence stars. The models suggest that the few
non-velocity-variable blue stragglers formed from mergers or collisions.
Several remarkable short-period double-lined binaries point to the importance
of subsequent dynamical exchange encounters, and provide at least one example
of a likely collisional origin for a blue straggler.Comment: Chapter 3, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Gravitational settling in pulsating subdwarf B stars and their progenitors
Diffusion of atoms can be important during quiescent phases of stellar
evolution. Particularly in the very thin inert envelopes of subdwarf B stars,
diffusive movements will considerably change the envelope structure and the
surface abundances on a short timescale. Also, the subdwarfs will inherit the
effects of diffusion in their direct progenitors, namely giants near the tip of
the red giant branch. This will influence the global evolution and the
pulsational properties of subdwarf B stars. We investigate the impact of
gravitational settling, thermal diffusion and concentration diffusion on the
evolution and pulsations of subdwarf B stars. Our diffusive stellar models are
compared with models evolved without diffusion. We constructed subdwarf B
models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low
mass star ignited helium in an energetic flash, while the intermediate mass
star started helium fusion gently. For each progenitor type we computed series
with and without atomic diffusion. Atomic diffusion in red giants causes the
helium core mass at the onset of helium ignition to be larger. We find an
increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun
model. The effects on the red giant surface abundances are small after the
first dredge up. The evolutionary tracks of the diffusive subdwarf B models are
shifted to lower surface gravities and effective temperatures due to outward
diffusion of hydrogen. This affects both the frequencies of the excited modes
and the overall frequency spectrum. Especially the structure and pulsations of
the post-non-degenerate sdB star are drastically altered, proving that atomic
diffusion cannot be ignored in these stars.Comment: 10 pages, 6 figures, accepted for publication in A&
Fear of crime on the rail networks: Perceptions of the UK public and British Transport Police
Counter-terrorism on the rail network is vital to the security of the United Kingdom. The British Transport Police (BTP) employ covert and overt security measures to prevent crime, which includes: closed circuit television, armed police, unarmed polisce, police community support officers, police dogs, stops and searches and awareness campaigns. All security measures aim to deter crime while importantly reassuring the public. We surveyed both members of the public and BTP officers about the perceived effectiveness of current security measures, specifically with regards to fear of terrorism. Feelings of reassurance and the perceived effectiveness of security measures were positively related. The most effective and reassuring security measure was the use of armed police; whereas the least effective and reassuring was the use of awareness campaigns. However, interestingly, qualitative analyses suggested that an increase in armed police without informed awareness campaigns would have a negative impact on public reassurance by increasing fear
Gravitational Dynamics of Large Stellar Systems
Internal dynamical evolution can drive stellar systems into states of high
central density. For many star clusters and galactic nuclei, the time scale on
which this occurs is significantly less than the age of the universe. As a
result, such systems are expected to be sites of frequent interactions among
stars, binary systems, and stellar remnants, making them efficient factories
for the production of compact binaries, intermediate-mass black holes, and
other interesting and eminently observable astrophysical exotica. We describe
some elements of the competition among stellar dynamics, stellar evolution, and
other mechanisms to control the dynamics of stellar systems, and discuss
briefly the techniques by which these systems are modeled and studied.
Particular emphasis is placed on pathways leading to massive black holes in
present-day globular clusters and other potentially detectable sources of
gravitational radiation.Comment: 21 pages, invited talk presented at the 18th International Conference
on General Relativity and Gravitation (GRG18), Sydney, July 2007. To appear
in Classical and Quantum Gravit
Cycles of Police Reform in Latin America.
yesOver the last quarter century post-conflict and post-authoritarian transitions in Latin America have been accompanied by a surge in social violence, acquisitive crime, and insecurity. These phenomena have been driven by an expanding international narcotics trade, by the long-term effects of civil war and counter-insurgency (resulting in, inter alia, an increased availability of small arms and a pervasive grammar of violence), and by structural stresses on society (unemployment, hyper-inflation, widening income inequality). Local police forces proved to be generally ineffective in preventing, resolving, or detecting such crime and forms of “new violence”3 due to corruption, frequent complicity in criminal networks, poor training and low pay, and the routine use of excessive force without due sanction. Why, then, have governments been slow to prioritize police reform and why have reform efforts borne largely “limited or nonexistent” long-term results?
This chapter highlights a number of lessons suggested by various efforts to reform the police in Latin America over the period 1995-2010 . It focuses on two clusters of countries in Latin America. One is Brazil and the Southern Cone countries (Chile, Argentina, and Uruguay), which made the transition to democracy from prolonged military authoritarian rule in the mid- to late 1980s. The other is Central America and the Andean region (principally El Salvador, Guatemala, Honduras, Peru, and Colombia), which emerged/have been emerging from armed conflict since the mid- 1990s.
The chapter examines first the long history of international involvement in police and security sector reform in order to identify long-run tropes and path dependencies. It then focuses on a number of recurring themes: cycles of de- and re-militarization of the policing function; the “security gap” and “democratization dilemmas” involved in structural reforms; the opportunities offered by decentralization for more community-oriented police; and police capacity to resist reform and undermine accountability mechanisms
The effect of autonomy, training opportunities, age and salaries on job satisfaction in the South East Asian retail petroleum industry
South East Asian petroleum retailers are under considerable pressure to improve service quality by reducing turnover. An empirical methodology from this industry determined the extent to which job characteristics, training opportunities, age and salary influenced the level of job satisfaction, an indicator of turnover. Responses are reported on a random sample of 165 site employees (a 68% response rate) of a Singaporean retail petroleum firm. A restricted multivariate regression model of autonomy and training opportunities explained the majority (35.4%) of the variability of job satisfaction. Age did not moderate these relationships, except for employees >21 years of age, who reported enhanced job satisfaction with additional salary. Human Capital theory, Life Cycle theory and Job Enrichment theory are invoked and explored in the context of these findings in the South East Asian retail petroleum industry. In the South East Asian retail petroleum industry, jobs providing employees with the opportunity to undertake a variety of tasks that enhanced the experienced meaningfulness of work are likely to promote job satisfaction, reduce turnover and increase the quality of service
The VLT-FLAMES Tarantula Survey: XXV. Surface nitrogen abundances of O-type giants and supergiants
Context. Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the
sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics.
Aims. Using a sample of 72 presumably single O-type giants to supergiants observed in the context of the VLT-FLAMES Tarantula
Survey (VFTS), we aim to investigate rotational mixing in evolved core-hydrogen burning stars initially more massive than 15 M� by
analysing their surface nitrogen abundances.
Methods. Using stellar and wind properties derived in a previous VFTS study we computed synthetic spectra for a set of up to
21 N ii-v lines in the optical spectral range, using the non-LTE atmosphere code FASTWIND. We constrained the nitrogen abundance
by fitting the equivalent widths of relatively strong lines that are sensitive to changes in the abundance of this element. Given the quality
of the data, we constrained the nitrogen abundance in 38 cases; for 34 stars only upper limits could be derived, which includes almost
all stars rotating at 3e sin i > 200 km s−1
.
Results. We analysed the nitrogen abundance as a function of projected rotation rate 3e sin i and confronted it with predictions of rotational
mixing. We found a group of N-enhanced slowly-spinning stars that is not in accordance with predictions of rotational mixing
in single stars. Among O-type stars with (rotation-corrected) gravities less than log gc = 3.75 this group constitutes 30−40 percent
of the population. We found a correlation between nitrogen and helium abundance which is consistent with expectations, suggesting
that, whatever the mechanism that brings N to the surface, it displays CNO-processed material. For the rapidly-spinning O-type stars
we can only provide upper limits on the nitrogen abundance, which are not in violation with theoretical expectations. Hence, the data
cannot be used to test the physics of rotation induced mixing in the regime of high spin rates.
Conclusions. While the surface abundances of 60−70 percent of presumed single O-type giants to supergiants behave in conformity
with expectations, at least 30−40 percent of our sample can not be understood in the current framework of rotational mixing for single
stars. Even though we have excluded stars showing radial velocity variations, of our sample may have remained contaminated by postinteraction
binary products. Hence, it is plausible that effects of binary interaction need to be considered to understand their surface
properties. Alternatively, or in conjunction, the effects of magnetic fields or alternative mass-loss recipes may need to be invoked
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