546 research outputs found
Parallax of PSR J1744-1134 and the Local Interstellar Medium
We present the annual trigonometric parallax of PSR J1744-1134 derived from
an analysis of pulse times of arrival. The measured parallax, pi = 2.8+/-0.3
mas ranks among the most precisely determined distances to any pulsar. The
parallax distance of 357+/-39 pc is over twice that derived from the dispersion
measure using the Taylor & Cordes model for the Galactic electron distribution.
The mean electron density in the path to the pulsar, n_e = (0.0088 +/- 0.0009)
cm^{-3}, is the lowest for any disk pulsar. We have compared the n_e for PSR
J1744-1134 with those for another 11 nearby pulsars with independent distance
estimates. We conclude that there is a striking asymmetry in the distribution
of electrons in the local interstellar medium. The electron column densities
for pulsars in the third Galactic quadrant are found to be systematically
higher than for those in the first. The former correlate with the position of
the well known local HI cavity in quadrant three. The excess electrons within
the cavity may be in the form of HII clouds marking a region of interaction
between the local hot bubble and a nearby superbubble.Comment: revised version accepted for publication in ApJ Letters; reanalysis
of uncertainty in parallax measure and changes to fig
An analysis of the X-ray emission from the supernova remnant 3C397
The ASCA SIS and the ROSAT PSPC spectral data of the SNR 3C397 are analysed
with a two-component non-equilibrium ionization model. Besides, the ASCA SIS0
and SIS1 spectra are also fitted simultaneously in an equilibrium case. The
resulting values of the hydrogen column density yield a distance of \sim8\kpc
to 3C397. It is found that the hard X-ray emission, containing S and Fe
K lines, arises primarily from the hot component, while most of the
soft emission, composed mainly of Mg, Si, Fe L lines, and continuum, is
produced by the cool component. The emission measures suggest that the remnant
evolves in a cloudy medium and imply that the supernova progenitor might not be
a massive early-type star. The cool component is approaching ionization
equilibrium. The ages estimated from the ionization parameters and dynamics are
all much greater than the previous determination. We restore the X-ray maps
using the ASCA SIS data and compare them with the ROSAT HRI and the NRAO VLA
Sky Survey (NVSS) 20 cm maps. The morphology with two bright concentrations
suggests a bipolar remnant encountering a denser medium in the west.Comment: 20 pages, aasms4.sty, 3 figures To appear in ApJ (1999
ASCA Observations of the Thermal Composite Supernova Remnant 3C 391
We present the results from ASCA observations of the centrally enhanced
supernova remnant 3C 391 (G31.9+0.0). We use the ASCA SIS data to carry out an
investigation of the spatial and spectral properties of the X-ray emission from
this remnant. The collisional equilibrium ionization and non-equilibrium
ionization spectral fits indicate that the hot gas within the remnant has
basically reached ionization equilibrium. The variation of the hydrogen column
density across the remnant is in agreement with the presence of a molecular
cloud to the northwest. The comparisons of hydrogen column and X-ray hardness
between the NW and SE portions of the remnant support a scenario in which the
SNR has broken out of a dense region into an adjacent region of lower density.
The mean density within the SNR is observed to be much lower than the immediate
ambient cloud density. This and the centrally brightened X-ray morphology can
be explained either by the evaporation of engulfed cloudlets or by a radiative
stage of evolution for the remnant.Comment: 23 pages, 3 figures, accepted for the v563 ApJ 12/10/01 issu
Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first document
carefully the birth pattern within real clusters and associations. In this
study of Taurus-Auriga, we combine pre-main-sequence ages from our own
evolutionary tracks with stellar positions from observational surveys. Aided by
the extensive, millimeter data on the molecular clouds, we develop a picture of
the region's history. Star formation began, at a relatively low level and in a
spatially diffuse manner, at least 10 Myr in the past. Within the last few
million years, new stars have been produced at an accelerating rate, almost
exclusively within a confined group of striated cloud filaments. The gas both
inside and around the filaments appears to be in force balance. Thus, the
appearance of the filaments is due to global, quasi-static contraction of the
parent cloud material. Gravity drives this contraction and shock dissipation
mediates it, but the internal motion of the gas does not appear to be
turbulent. The accelerating nature of recent star formation means that the
condensation of cloud cores is a threshold phenomenon, requiring a minimum
background density. Other, nearby cloud regions, including Lupus and
Chamaeleon, contain some locales that have attained this density, and others
that have not. In the latter, we find extensive and sometimes massive molecular
gas that is still devoid of young stars.Comment: 19 pages, 7 figures, to be published in ApJ - December 20, 200
Invaded cluster algorithm for Potts models
The invaded cluster algorithm, a new method for simulating phase transitions,
is described in detail. Theoretical, albeit nonrigorous, justification of the
method is presented and the algorithm is applied to Potts models in two and
three dimensions. The algorithm is shown to be useful for both first-order and
continuous transitions and evidently provides an efficient way to distinguish
between these possibilities. The dynamic properties of the invaded cluster
algorithm are studied. Numerical evidence suggests that the algorithm has no
critical slowing for Ising models.Comment: 39 pages, revtex, 15 figures available on request from
[email protected], to appear in Phys. Rev.
Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N11
We present maps and a catalogue containing the J=1-0 12CO parameters of 29
individual molecular clouds in the second-brightest LMC star formation complex,
N11. In the southwestern part of N11, molecular clouds occur in a ring or shell
surrounding the major OB star association LH9. In the northeastern part, a
chain of molecular clouds delineates the rim of one of the so-called supergiant
shells in the LMC. The well-defined clouds have dimensions close to those of
the survey beam (diameters of 25 pc or less). Some of the clouds were also
observed in J=2-1 12CO, and in the lower two transitions of 13CO. Clouds mapped
with a twice higher angular resolution in J=2-1 12CO show substructure with
dimensions once again comparable to those of the mapping beam. The few clouds
for which we could model physical parameters have fairly warm (T(kin) = 60 -
150 K) and moderately dense (n(H2) = 3000 cm-3) gas. The northeastern chain of
CO clouds, although lacking in diffuse intercloud emission, is characteristic
of the more quiescent regions of the LMC and appears to have been subject to
relatively little photo-processing. The clouds forming part of the southwestern
shell or ring, however, are almost devoid of diffuse intercloud emission and
also exhibit other characteristics of an extreme photon-dominated region (PDR).Comment: 14 pages; accepted for publication in A&
Transcatheter Aortic Valve Implantation in Dialysis Patients
Background/Aims: Transcatheter aortic valve implantation (TAVI) has emerged as a new therapeutic option for high-risk patients. However, dialysis patients were excluded from all previous studies. The aim of this study is to compare the outcomes of TAVI for dialysis patients with those for patients with chronic kidney disease (CKD) stages 3 and 4 and to compare TAVI with open surgery in dialysis patients. Methods: Part I: comparison of 10 patients on chronic hemodialysis with 116 patients with non-dialysis-dependent CKD undergoing TAVI. Part II: comparison of transcatheter (n = 15) with open surgical (n = 24) aortic valve replacement in dialysis patients. Results: Part I: dialysis patients were significantly younger (72.3 vs. 82.0 years; p < 0.01). Hospital stay was significantly longer in dialysis patients (21.8 vs. 12.1 days; p = 0.01). Overall 30-day mortality was 3.17%, with no deaths among dialysis patients. Six-month survival rates were similar (log-rank p = 0.935). Part II: patient age was comparable (66.5 vs. 69.5 years; p = 0.42). Patients in the surgical group tended to stay longer in hospital than TAVI patients (29.5 vs. 22.5 days; p = 0.35). Conclusion: TAVI is a safe procedure in patients on chronic hemodialysis. Until new data become available, we find no compelling reason to refuse these patients TAVI. Copyright (C) 2012 S. Karger AG, Base
Pharmaceutical cognitive enhancement in Greek university students: differences between users and non-users in social cognitive variables, burnout and engagement
Pharmaceutical cognitive enhancement (PCE) represents the non-medical use of prescribed medication for the improvement of cognitive functioning and academic performance. Although there are some studies about PCE prevalence, it is less clear how users and non-users of PCE substances differ with respect to their positive and negative student experiences (e.g., academic burnout, engagement with studies) and in social cognitive variables that relate to decision-making and self-regulation of PCE use. The present study assessed whether students with different experiences of PCE substance use displayed differences in academic burnout, study engagement and social cognitive variables relevant to PCE use. Three hundred and forty seven University students (M age = 22.15, SD = 1.69; 54% females) completed a battery of anonymous questionnaires on academic burnout, engagement with studies, social cognitive variables relevant to PCE use, and self-reported use of PCE substances and non-prescribed nutritional supplements. Three user groups emerged, namely, non-users (51.9%, n = 180), single users of non-prescribed dietary supplements (25.4%, n = 88), and dual users of both non-prescribed dietary supplements and PCE (22.8%, n = 79). Multivariate analysis of variance indicated significant differences among the three user groups in intentions, attitudes, social norms, and anticipated regret towards PCE use. No significant differences were observed with respect to academic burnout and work engagement. The findings show that University students may engage in PCE use independently of their student experiences. Rather, a chemically-assisted performance enhancement mindset seems to differentiate users from non-users of PCE substances
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