796 research outputs found
Millisecond Pulsar Ages: Implications of Binary Evolution and a Maximum Spin Limit
In the absence of constraints from the binary companion or supernova remnant,
the standard method for estimating pulsar ages is to infer an age from the rate
of spin-down. While the generic spin-down age may give realistic estimates for
normal pulsars, it can fail for pulsars with very short periods. Details of the
spin-up process during the low mass X-ray binary phase pose additional
constraints on the period (P) and spin-down rates (Pdot) that may consequently
affect the age estimate. Here, we propose a new recipe to estimate millisecond
pulsar (MSP) ages that parametrically incorporates constraints arising from
binary evolution and limiting physics. We show that the standard method can be
improved by this approach to achieve age estimates closer to the true age
whilst the standard spin-down age may over- or under-estimate the age of the
pulsar by more than a factor of ~10 in the millisecond regime. We use this
approach to analyze the population on a broader scale. For instance, in order
to understand the dominant energy loss mechanism after the onset of radio
emission, we test for a range of plausible braking indices. We find that a
braking index of n=3 is consistent with the observed MSP population. We
demonstrate the existence and quantify the potential contributions of two main
sources of age corruption: the previously known "age bias" due to secular
acceleration and "age contamination" driven by sub-Eddington progenitor
accretion rates. We explicitly show that descendants of LMXBs that have
accreted at very low rates will exhibit ages that appear older than the age of
the Galaxy. We further elaborate on this technique, the implications and
potential solutions it offers regarding MSP evolution, the underlying age
distribution and the post-accretion energy loss mechanism.Comment: Replaced with version published by ApJ. Tables reformatted and minor
changes to the text. Full resolution color figures and movies available at
http://www.kiziltan.org/research.html#age
X-ray Evidence of the Common Envelope Phase of V471 Tauri
Chandra Low Energy Transmission Grating Spectrograph observations of the
pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance
ratio of the K dwarf component for the first time. While the white dwarf
component dominates the spectrum longward of 50 AA, at shorter wavelengths the
observed X-ray emission is entirely due to coronal emission from the K dwarf.
The H-like resonance lines of C and N yield an estimate of their logarithmic
abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the
currently accepted solar value. We interpret this result as the first clear
observational evidence for the presumed common envelope phase of this system,
during which the surface of the K dwarf was contaminated by CN-cycle processed
material dredged up into the red giant envelope. We use the measured C/N ratio
to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and
show that this is consistent with a recent tentative detection of 13C in the K
dwarf photosphere, and with the measured Li abundance in the scenario where the
red giant companion was Li-rich during the common envelope phase.Comment: 6 pages, 2 figures, ApJL accepte
Looking back, moving forward: Access to antiretroviral therapy for HIV infected adults and children in developing countries: Horizons Studies, 2002 to 2008
In 1997, the Population Council initiated the Horizons Program—a decade-long USAID-funded collaboration with the International Center for Research on Women, the International HIV/AIDS Alliance, PATH, Tulane University, Family Health International, and Johns Hopkins University—designing, implementing, evaluating, and expanding innovative strategies for HIV prevention and care. Horizons developed and tested ways to optimize HIV prevention, care, and treatment programs; worked to reduce stigma and improve gender-biased behaviors; and greatly expanded knowledge about the best ways to support, protect, and treat children affected by HIV and AIDS. In all its projects, Horizons strengthened the capacity of local institutions by providing support and training to colleagues. This synthesis paper presents lessons learned and best practices on one of the key topics that Horizons investigated: access to antiretroviral therapy
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
On the use of 12CO/13CO as a test of common-envelope evolution
We present K-band echelle spectra of the cataclysmic variable SS Cyg and the pre-cataclysmic variable V471 Tau in order to measure the strengths of the 12CO and 13CO bands at 2.3525 and 2.3448 microns, respectively, and so perform the observational test of the common-envelope model of close binary star evolution proposed by Sarna et al. (1995). Although we find evidence of an absorption feature coincident with the expected wavelength of 13CO in both objects, we attribute it instead to a cluster of neutral atomic absorption features (primarily due to TiI) possibly arising from star-spots on the surfaces of the rapidly rotating secondary stars in these systems, thereby rendering the test inconclusive. We present a modified observational test of common-envelope evolution, based on the observation of the 13CO bands at 2.3739 and 2.4037 microns, which is insensitive to spectral contamination by star-spots
The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09
We report on Keck and {\em Hubble Space Telescope} observations of the binary
millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and
measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the
reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature
\Teff=4800\pm800 K. The white-dwarf mass is known accurately from
measurements of the Shapiro delay of the pulsar signal,
\Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can
use the measured temperature to determine the cooling age. The main uncertainty
in the cooling models for such low-mass white dwarfs is the amount of residual
nuclear burning, which is set by the thickness of the hydrogen layer
surrounding the helium core. From the properties of similar systems, it has
been inferred that helium white dwarfs form with thick hydrogen layers, with
mass \simgt3\times10^{-3} \Msun, which leads to significant additional
heating. This is consistent with expectations from simple evolutionary models
of the preceding binary evolution. For PSR B1855+09, though, such models lead
to a cooling age of Gyr, which is twice the spin-down age of the
pulsar. It could be that the spin-down age were incorrect, which would call the
standard vacuum dipole braking model into question. For two other pulsar
companions, however, ages well over 10 Gyr are inferred, indicating that the
problem may lie with the cooling models. There is no age discrepancy for models
in which the white dwarfs are formed with thinner hydrogen layers
(\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ
Mid-Infrared Observations of the White Dwarf Brown Dwarf Binary GD 1400
Fluxes are measured for the DA white dwarf plus brown dwarf pair GD 1400 with
the Infrared Array Camera on the {\em Spitzer Space Telescope}. GD 1400
displays an infrared excess over the entire m region consistent with
the presence of a mid- to late-type L dwarf companion. A discussion is given
regarding current knowledge of this unique system.Comment: 14 pages, 1 figure, 3 tables, accepted to A
Directly observed antiretroviral therapy: a systematic review and meta-analysis of randomised clinical trials.
BACKGROUND: Directly observed therapy has been recommended to improve adherence for patients with HIV infection who are on highly active antiretroviral therapy, but the benefit and cost-effectiveness of this approach has not been established conclusively. We did a systematic review and meta-analysis of randomised trials of directly observed versus self-administered antiretroviral treatment. METHODS: We did duplicate searches of databases (from inception to July 27, 2009), searchable websites of major HIV conferences (up to July, 2009), and lay publications and websites (March-July, 2009) to identify randomised trials assessing directly observed therapy to promote adherence to antiretroviral therapy in adults. Our primary outcome was virological suppression at study completion. We calculated relative risks (95% CIs), and pooled estimates using a random-effects method. FINDINGS: 12 studies met our inclusion criteria; four of these were done in groups that were judged to be at high risk of poor adherence (drug users and homeless people). Ten studies reported on the primary outcome (n=1862 participants); we calculated a pooled relative risk of 1.04 (95% CI 0.91-1.20, p=0.55), and noted moderate heterogeneity between the studies (I(2)= 53.8%, 95% CI 0-75.7, p=0.0247) for directly observed versus self-administered treatment. INTERPRETATION: Directly observed antiretroviral therapy seems to offer no benefit over self-administered treatment, which calls into question the use of such an approach to support adherence in the general patient population. FUNDING: None
Seminar Users in the Arabic Twitter Sphere
We introduce the notion of "seminar users", who are social media users
engaged in propaganda in support of a political entity. We develop a framework
that can identify such users with 84.4% precision and 76.1% recall. While our
dataset is from the Arab region, omitting language-specific features has only a
minor impact on classification performance, and thus, our approach could work
for detecting seminar users in other parts of the world and in other languages.
We further explored a controversial political topic to observe the prevalence
and potential potency of such users. In our case study, we found that 25% of
the users engaged in the topic are in fact seminar users and their tweets make
nearly a third of the on-topic tweets. Moreover, they are often successful in
affecting mainstream discourse with coordinated hashtag campaigns.Comment: to appear in SocInfo 201
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