2,175 research outputs found
Quantitative measurement by telemetry of ovulation and oviposition in the fowl
Radio telemetry used to determine body temperature cycles in fowl and its relation to ovulation and ovipositio
Estimating the Returns to Urban Boarding Schools: Evidence from SEED
The SEED schools, which combine a "No Excuses'' charter model with a five-day-a-week boarding program, are America's only urban public boarding schools for the poor. We provide the first causal estimate of the impact of attending SEED schools on academic achievement, with the goal of understanding whether changing a student's environment through boarding is a cost-effective strategy to increase achievement among the poor. Using admission lotteries, we show that attending a SEED school increases achievement by 0.198 standard deviations in reading and 0.230 standard deviations in math, per year of attendance. Despite these relatively large impacts, the return on investment in SEED is less than five percent due to the substantial costs of boarding. Similar "No Excuses'' charter schools -- without a boarding option -- have a return on investment of over eighteen percent.
Iron Opacity and the Pulsar of Supernova 1987A
Neutron stars formed in Type II supernovae are likely to be initially
obscured by late-time fallback. Although much of the late-time fallback is
quickly accreted via neutrino cooling, some material remains on the neutron
star, forming an atmosphere which slowly accretes through photon emission. In
this paper, we derive structure equations of the fallback atmosphere and
present results of one-dimensional simulations of that fallback. The atmosphere
remaining after neutrino cooling becomes unimportant (less than the Compton
Eddington limit) is only a fraction of the total mass accreted (10^-8 of the
accreted mass or 10^-9 solar masses.) Recombined iron dominates the opacity in
the outer regions leading to an opacity 1000-10,000 times higher than that of
electron scattering alone. The resultant photon emission of the remnant
atmosphere is limited to 1/1000th the Compton Eddington Luminosity. The
late-time evolution of this system leads to the formation of a photon-driven
wind from the accretion of the inner portion of the atmosphere, leaving, for
most cases, a bare neutron star on timescales shorter than a year. The
degenerate remnant of 1987a may not be a black hole. Instead, the fallback
material may have already accreted or blown off in the accretion-driven wind.
If the neutron star has either a low magnetic field or a low rotational spin
frequency, we would not expect to see the neutron star remnant of 1987a.Comment: 15 pages text + 8 figures, accepted by Ap
Theoretical Black Hole Mass Distributions
We derive the theoretical distribution function of black hole masses by
studying the formation processes of black holes. We use the results of recent
2D simulations of core-collapse to obtain the relation between remnant and
progenitor masses and fold it with an initial mass function for the
progenitors. We examine how the calculated black-hole mass distributions are
modified by (i) strong wind mass loss at different evolutionary stages of the
progenitors, and (ii) the presence of close binary companions to the black-hole
progenitors. Thus, we are able to derive the binary black hole mass
distribution. The compact remnant distribution is dominated by neutron stars in
the mass range 1.2-1.6Msun and falls off exponentially at higher remnant
masses. Our results are most sensitive to mass loss from winds which is even
more important in close binaries. Wind mass-loss causes the black hole
distribution to become flatter and limits the maximum possible black-hole mass
(<10-15Msun). We also study the effects of the uncertainties in the explosion
and unbinding energies for different progenitors. The distributions are
continuous and extend over a broad range. We find no evidence for a gap at low
values (3-5Msun) or for a peak at higher values (~7Msun) of black hole masses,
but we argue that our black hole mass distribution for binaries is consistent
with the current sample of measured black-hole masses in X-ray transients. We
discuss possible biases against the detection or formation of X-ray transients
with low-mass black holes. We also comment on the possibility of black-hole
kicks and their effect on binaries.Comment: 22 pages, submitted to Ap
Influence of the r-mode instability on hypercritically accreting neutron stars
We have investigated an influence of the r-mode instability on
hypercritically accreting () neutron stars in
close binary systems during their common envelope phases based on the scenario
proposed by Bethe et al. \shortcite{bethe-brown-lee}. On the one hand neutron
stars are heated by the accreted matter at the stellar surface, but on the
other hand they are also cooled down by the neutrino radiation. At the same
time, the accreted matter transports its angular momentum and mass to the star.
We have studied the evolution of the stellar mass, temperature and rotational
frequency.
The gravitational-wave-driven instability of the r-mode oscillation strongly
suppresses spinning-up of the star, whose final rotational frequency is well
below the mass-shedding limit, typically as small as 10% of that of the
mass-shedding state. On a very short time scale the rotational frequency tends
to approach a certain constant value and saturates there as far as the amount
of the accreted mass does not exceed a certain limit to collapse to a black
hole. This implies that the similar mechanism of gravitational radiation as the
so-called Wagoner star may work in this process. The star is spun up by
accretion until the angular momentum loss by gravitational radiation balances
the accretion torque. The time-integrated dimensionless strain of the radiated
gravitational wave may be large enough to be detectable by the gravitational
wave detectors such as LIGO II.Comment: 6 pages, 3 figure
What Can the Accretion Induced Collapse of White Dwarfs Really Explain?
The accretion induced collapse (AIC) of a white dwarf into a neutron star has
been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of
problematic neutron star populations and specific binary systems. The ejecta
from this collapse has also been claimed as a source of r-process
nucleosynthesis. So far, most AIC studies have focussed on determining the
event rates from binary evolution models and less attention has been directed
toward understanding the collapse itself. However, the collapse of a white
dwarf into a neutron star is followed by the ejection of rare neutron-rich
isotopes. The observed abundance of these chemical elements may set a more
reliable limit on the rate at which AICs have taken place over the history of
the galaxy.
In this paper, we present a thorough study of the collapse of a massive white
dwarf in 1- and 2-dimensions and determine the amount and composition of the
ejected material. We discuss the importance of the input physics (equation of
state, neutrino transport, rotation) in determining these quantities. These
simulations affirm that AICs are too baryon rich to produce gamm-ray bursts and
do not eject enough nickel to explain Type Ia supernovae (with the possible
exception of a small subclass of extremely low-luminosity Type Ias). Although
nucleosynthesis constraints limit the number of neutron stars formed via AICs
to <0.1% of the total galactic neutron star population, AICs remain a viable
scenario for forming systems of neutron stars which are difficult to explain
with Type II core-collapse supernovae.Comment: Latex File, aaspp4 style, 18 pages total (5 figures), accepted by Ap
The effect of 12C + 12C rate uncertainties on s-process yields
The slow neutron capture process in massive stars (the weak s-process)
produces most of the s-only isotopes in the mass region 60 < A < 90. The
nuclear reaction rates used in simulations of this process have a profound
effect on the final s-process yields. We generated 1D stellar models of a 25
solar mass star varying the 12C + 12C rate by a factor of 10 and calculated
full nucleosynthesis using the post-processing code PPN. Increasing or
decreasing the rate by a factor of 10 affects the convective history and
nucleosynthesis, and consequently the final yields.Comment: Conference proceedings for the Nuclear Physics in Astrophysics IV
conference, 8-12 June 2009. 4 pages, 3 figures. Accepted for publication to
the Journal of Physics: Conference Serie
The first accurate parallax distance to a black hole
Using astrometric VLBI observations, we have determined the parallax of the
black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds,
corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the
previously accepted value. This model-independent estimate is the most accurate
distance to a Galactic stellar-mass black hole measured to date. With this new
distance, we confirm that the source was not super-Eddington during its 1989
outburst. The fitted distance and proper motion imply that the black hole in
this system likely formed in a supernova, with the peculiar velocity being
consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred
to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations
of a larger sample of such systems would provide useful insights into the
formation and properties of accreting stellar-mass black holes.Comment: Accepted for publication in ApJ Letters. 6 pages, 2 figure
Skills, attitudes and uptake of evidence-based practice: a cross-sectional study of chiropractors in the Swedish Chiropractic Association.
BackgroundEvidence-based practice (EBP) is integral to the delivery of high-quality health care. Chiropractic has been a licensed health profession in Sweden since 1989, but little is known of the uptake of EBP in this professional group. This study explored the self-reported skills, attitudes and uptake of EBP, and the enablers and barriers of EBP uptake, among licensed chiropractors in Sweden.MethodsLicensed chiropractors (n = 172) of the Swedish Chiropractic Association (Legitimerade Kiropraktorers Riksorganisation) were invited to participate in an anonymous online questionnaire, using the Evidence-Based Practice Attitude and Utilisation Survey (EBASE) in February 2019.ResultsFifty-six (33%) chiropractors completed the survey. Participants were predominantly male, aged 30-49 years, held a Master's degree, and had received their highest qualification and practiced chiropractic for over a decade. Chiropractors rated their EBP skill-level mostly in the moderate to moderate-high range. The majority of chiropractors reported positive attitudes towards EBP, with most agreeing or strongly agreeing that EBP is necessary in the practice of chiropractic, and that EBP assists in making decisions about patient care. Chiropractors reported an average level of engagement in EBP activities. All participants indicated professional literature and research findings were useful in their day-to-day chiropractic practice. The main perceived enabler of EBP uptake was internet access in the workplace, whereas the main barrier to EBP uptake was lack of clinical evidence in chiropractic.ConclusionsParticipating chiropractors of the Swedish Chiropractic Association were generally favourable of EBP, though only reported modest levels of EBP-related skills and engagement in EBP activities. Our findings suggest future studies investigating interventions focussed on improving chiropractors' skills and uptake of EBP are warranted
Gravitational Waves from Gravitational Collapse
Gravitational wave emission from the gravitational collapse of massive stars
has been studied for more than three decades. Current state of the art
numerical investigations of collapse include those that use progenitors with
realistic angular momentum profiles, properly treat microphysics issues,
account for general relativity, and examine non--axisymmetric effects in three
dimensions. Such simulations predict that gravitational waves from various
phenomena associated with gravitational collapse could be detectable with
advanced ground--based and future space--based interferometric observatories.Comment: 68 pages including 13 figures; revised version accepted for
publication in Living Reviews in Relativity (http://www.livingreviews.org
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