433 research outputs found
Young Crab-like pulsars and luminous X-ray sources in starbursts and optically dull galaxies
Recent Chandra observations of nearby galaxies have revealed a number of
ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from
the central regions of non-active galaxies. The nature of these sources is
still debated. We argue that a fraction of them could be young, Crab-like
pulsars, the X-ray luminosity of which is powered by rotation. We use the
pulsar birth parameters estimated from radio pulsar data to compute the
steady-state pulsar X-ray luminosity distribution as a function of the star
formation rate (SFR) in the galaxy. We find that ~10% of optically dull
galaxies are expected to have a source with L_x >~ 10^{39} erg/s, while
starbursts galaxies should each have several of these sources. We estimate that
the X-ray luminosity of a few percents of galaxies is dominated by a single
bright pulsar with L_x >~10^{39} erg/s, roughly independently of its SFR. We
discuss observational diagnostics that can help distinguish the young pulsar
population in ULXs.Comment: 17 pages, 4 figures, accepted to Ap
Molecular diversity of arbuscular mycorrhizal fungi and patterns of host association over time and space in a tropical forest
We have used molecular techniques to investigate the diversity and distribution of the arbuscular mycorrhizal (AM) fungi colonizing tree seedling roots in the tropical forest on Barro Colorado Island (BCI), Republic of Panama. In the first year, we sampled newly emergent seedlings of the understory treelet Faramea occidentalis and the canopy emergent Tetragastris panamensis, from mixed seedling carpets at each of two sites. The following year we sampled surviving seedlings from these cohorts. The roots of 48 plants were analysed using AM fungal-specific primers to amplify and clone partial small subunit (SSU) ribosomal RNA gene sequences. Over 1300 clones were screened for random fragment length polymorphism (RFLP) variation and 7% of these were sequenced. Compared with AM fungal communities sampled from temperate habitats using the same method, the overall diversity was high, with a total of 30 AM fungal types identified. Seventeen of these types have not been recorded previously, with the remainder being similar to types reported from temperate habitats. The tropical mycorrhizal population showed significant spatial heterogeneity and nonrandom associations with the different hosts. Moreover there was a strong shift in the mycorrhizal communities over time. AM fungal types that were dominant in the newly germinated seedlings were almost entirely replaced by previously rare types in the surviving seedlings the following year. The high diversity and huge variation detected across time points, sites and hosts, implies that the AM fungal types are ecologically distinct and thus may have the potential to influence recruitment and host composition in tropical forests
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High Angular Resolution NIR Imaging and Spectroscopy
NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.Comment: accepted for publication in the Astrophysical Journal, 46 pages, 15
figure
Geochemistry and weathering history of the Balfour sandstone formation, Karoo basin, South Africa: Insight to provenance and tectonic setting
Geochemical analysis on sandstones from clastic sediments was carried out to understand the tectonic setting and subsequent post-depositional change in the Karoo basinal fill of the fluvial deposits of the Balfour Formation during the Late Permian to Early Triassic period. The major and trace element analysis reveal a relatively homogeneous provenance for the sandstones. The geochemical analysis shows that these rocks are first order mature sediments, derived from igneous and/or meta-igneous rocks of a felsic composition. The results show that the sandstone consists of SiO2 (71.58 wt. %), followed by Al2O3 (14.48 wt. %), but with low contents of Fe2O3+MgO (4.09 wt. %) and TiO2 (0.47 wt. %). These sandstones are classified as litharenites and arkoses based on the elements constituent ratio of various schemed adopted. The sandstone in the provenance discrimination diagram plots in the dissected and transitional arc fields suggesting an active margin and continental island arc provenance, preserving the signature of a recycled provenance. The Chemical Index of Alteration (CIA) ranging from 63.56 to 67.10% suggests recycling processes, and that the source area has undergone a moderate degree of chemical weathering. The geochemical characteristics of the sediment suggest the source area of uplifted terrane of folded and faulted strata with detritus of sedimentary and metasedimentary origin
VACTERL association etiology: The impact of de novo and rare copy number variations
Copy number variations (CNVs), either DNA gains or losses, have been found at common regions throughout the human genome. Most CNVs neither have a pathogenic significance nor result in disease-related phenotypes but, instead, reflect the normal population variance. However, larger CNVs, which often arise de novo, are frequently associated with human disease. A genetic contribution has long been suspected in VACTERL (Vertebral, Anal, Cardiac, TracheoEsophageal fistula, Renal and Limb anomalies) association. The anomalies observed in this association overlap with several monogenetic conditions associated with mutations in specific genes, e.g. Townes Brocks (SALL1), Feingold syndrome (MYCN) or Fanconi anemia. So far VACTERL association has typically been considered a diagnosis of exclusion. Identifying recurrent or de novo genomic variations in individuals with VACTERL association could make it easier to distinguish VACTERL association from other syndromes and could provide insight into disease mechanisms. Sporadically, de novo CNVs associated with VACTERL are described in literature. In addition to this literature review of genomic variation in published VACTERL association patients, we describe CNVs present in 68 VACTERL association patients collected in our institution. De novo variations (>30 kb) are absent in our VACTERL association cohort. However, we identified recurrent rare CNVs which, although inherited, could point to mechanisms or biological processes contributing to this constellation of developmental defects
Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei
We present an investigation of H-alpha emission line variations observed in
the massive Algol binary, RY Per. We give new radial velocity data for the
secondary based upon our optical spectra and for the primary based upon high
dispersion UV spectra. We present revised orbital elements and an estimate of
the primary's projected rotational velocity (which indicates that the primary
is rotating 7 times faster than synchronous). We use a Doppler tomography
algorithm to reconstruct the individual primary and secondary spectra in the
region of H-alpha, and we subtract the latter from each of our observations to
obtain profiles of the primary and its disk alone. Our H-alpha observations of
RY Per show that the mass gaining primary is surrounded by a persistent but
time variable accretion disk. The profile that is observed outside-of-eclipse
has weak, double-peaked emission flanking a deep central absorption, and we
find that these properties can be reproduced by a disk model that includes the
absorption of photospheric light by the band of the disk seen in projection
against the face of the star. We developed a new method to reconstruct the disk
surface density distribution from the ensemble of H-alpha profiles observed
around the orbit, and this method accounts for the effects of disk occultation
by the stellar components, the obscuration of the primary by the disk, and flux
contributions from optically thick disk elements. The resulting surface density
distribution is elongated along the axis joining the stars, in the same way as
seen in hydrodynamical simulations of gas flows that strike the mass gainer
near trailing edge of the star. This type of gas stream configuration is
optimal for the transfer of angular momentum, and we show that rapid rotation
is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu
Modelling shoreline evolution in the vicinity of a groyne and a river
Analytical solutions to the equations governing shoreline evolution are well-known and have value both as pedagogical tools and for conceptual design. Nevertheless, solutions have been restricted to a fairly narrow class of conditions with limited applicability to real-life situations. We present a new analytical solution for a widely encountered situation where a groyne is constructed close to a river to control sediment movement. The solution, which employs Laplace transforms, has the advantage that a solution for time-varying conditions may be constructed from the solution for constant conditions by means of the Heaviside procedure. Solutions are presented for various combinations of wave conditions and sediment supply/removal by the river. An innovation introduced in this work is the capability to provide an analytical assessment of the accretion or erosion caused near the groyne due to its proximity to the river which may act either as a source or a sink of sediment material
Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in samples with
well-defined ages and initial chemical composition. Here we present a survey of
projected rotational velocities for a large sample of mainly B-type stars in
young clusters to study the time evolution of the rotational properties of
massive stars. The survey is based upon moderate resolution spectra made with
the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,
and the target stars are members of 19 young open clusters with an age range of
approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,
4471 and Mg II 4481 using model theoretical profiles to find projected
rotational velocities for a total of 496 OB stars. We find that there are fewer
slow rotators among the cluster B-type stars relative to nearby B stars in the
field. We present evidence consistent with the idea that the more massive B
stars (M > 9 solar masses) spin down during their main sequence phase. However,
we also find that the rotational velocity distribution appears to show an
increase in the numbers of rapid rotators among clusters with ages of 10 Myr
and higher. These rapid rotators appear to be distributed between the zero age
and terminal age main sequence locations in the Hertzsprung-Russell diagram,
and thus only a minority of them can be explained as the result of a spin up at
the terminal age main sequence due to core contraction. We suggest instead that
some of these rapid rotators may have been spun up through mass transfer in
close binary systems.Comment: 33 pages, 11 figures, accepted by Ap
Luminosity function of binary X-ray sources calculated using the Scenario Machine
Using the ``Scenario Machine'' we have carried out a population synthesis of
X-ray binaries for the purpose of modelling of X-ray luminosity functions
(XLFs) in different types of galaxies: star burst, spiral, and elliptical. This
computer code allows to calculate, by using Monte Carlo simulations, the
evolution of a large ensemble of binary systems, with proper accounting for the
spin evolution of magnetized neutron stars.
We show that the XLF has no universal type. It depends on the star formation
rate in the galaxy. Also it is of importance to take into account the evolution
of binary systems and life times of X-ray stages in theoretical models of such
functions. We have calculated cumulative and differential XLFs for the galaxy
with the constant star formation rate. Also we have calculated cumulative
luminosity functions for different intervals of time after the star formation
burst in the galaxy and curves depicting the evolution of the X-ray luminosity
after the star formation burst in the galaxy.Comment: 22 pages, 13 figure
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