385 research outputs found
On the X-ray Emission from Massive Star Clusters and their Evolving Superbubbles
The X-ray emission properties from the hot thermalized plasma that results
from the collisions of individual stellar winds and supernovae ejecta within
rich and compact star clusters are discussed. We propose a simple analytical
way of estimating the X-ray emission generated by super star clusters and
derive an expression that indicates how this X-ray emission depends on the main
cluster parameters. Our model predicts that the X-ray luminosity from the star
cluster region is highly dependent on the star cluster wind terminal speed, a
quantity related to the temperature of the thermalized ejecta.We have also
compared the X-ray luminosity from the SSC plasma with the luminosity of the
interstellar bubbles generated from the mechanical interaction of the high
velocity star cluster winds with the ISM.We found that the hard (2.0 keV - 8.0
keV) X-ray emission is usually dominated by the hotter SSC plasma whereas the
soft (0.3 keV - 2.0 keV) component is dominated by the bubble plasma. This
implies that compact and massive star clusters should be detected as point-like
hard X-ray sources embedded into extended regions of soft diffuse X-ray
emission. We also compared our results with predictions from the population
synthesis models that take into consideration binary systems and found that in
the case of young,massive and compact super star clusters the X-ray emission
from the thermalized star cluster plasma may be comparable or even larger than
that expected from the HMXB population.Comment: 24 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
Modelling the radio to X-ray SED of galaxies
We present our model to interpret the SED of galaxies. The model for the UV
to sub-mm SED is already well established (Silva et al 1998). We remind here
its main features and show some applications. Recently we have extended the
model to the radio range (Bressan et al 2001), and we have started to include
the X-ray emission from the stellar component.Comment: 4 pages, to be published in "The link between stars and cosmology",
26-30 March, 2001, Puerto Vallarta, Mexico, by Kluwer, eds. M. Chavez, A.
Bressan, A. Buzzoni, and D. Mayy
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
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
Thermal Expansion in 3d-Metal Prussian Blue Analogs - A Survey Study
We present a comprehensive study of the structural properties and the thermal
expansion behavior of 17 different Prussian Blue Analogs (PBAs) with
compositions MII3[(M')III(CN)6]2.nH2O and MII2[FeII(CN)6].nH2O, where MII = Mn,
Fe, Co, Ni, Cu and Zn, (M')III = Co, Fe and n is the number of water molecules,
which range from 5 to 18 for these compounds. The PBAs were synthesized via
standard chemical precipitation methods, and temperature-dependent X-ray
diffraction studies were performed in the temperature range between -150oC (123
K) and room-temperature. The vast majority of the studied PBAs were found to
crystallize in cubic structures of space groups, and . The temperature
dependence of the lattice parameters was taken to compute an average
coefficient of linear thermal expansion in the studied temperature range. Of
the 17 compounds, 9 display negative values for the average coefficient of
linear thermal expansion, which can be as large as 39.7 x 10-6 K-1 for
Co3[Co(CN)6]2.12H2O. All of the MII3[CoIII(CN)6]2.nH2O compounds show negative
thermal expansion behavior, which correlates with the Irving-Williams series
for metal complex stability. The thermal expansion behavior for the PBAs of the
MII3[FeIII(CN)6]2.nH2O family are found to switch between positive (for M = Mn,
Co, Ni) and negative (M = Cu, Zn) behavior, depending on the choice of the
metal cation (M). On the other hand, all of the MII2[FeII(CN)6].nH2O compounds
show positive thermal expansion behavior.Comment: Submitted, 32 pages, 3 tables, 10 figure
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
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
On the metallicity dependence of HMXBs
It is commonly assumed that high mass X-ray binary (HMXB) populations are
little-affected by metallicity. However, the massive stars making up their
progenitor systems depend on metallicity in a number of ways, not least through
their winds. We present simulations, well-matched to the observed sample of
Galactic HMXBs, which demonstrate that both the number and the mean period of
HMXB progenitors can vary with metallicity, with the number increasing by about
a factor of three between solar and SMC metallicity. However, the SMC
population itself cannot be explained simply by metallicity effects; it
requires both that the HMXBs observed therein primarily sample the older end of
the HMXB population, and that the star formation rate at the time of their
formation was very large.Comment: 13 pages, 6 figures, MNRAS accepte
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