316 research outputs found
A mechanical adapter for installing mission equipment on large space structures
A mechanical attachment adapter was designed, constructed, and tested. The adapter was was included in a simulation program that investigated techniques for assembling erectable structures under simulated zero-g conditions by pressure-suited subjects in a simulated EVA mode. The adapter was utilized as an interface attachment between a simulated equipment module and one node point of a tetrahedral structural cell. The mating performance of the adapter, a self-energized mechanism, was easily and quickly demonstrated and required little effort on the part of the test subjects
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The HERE project toolkit: a resource for programme teams interested in improving student engagement and retention
Space construction system analysis. Part 2: Construction analysis
The construction methods specific to the end to end construction process for building the ETVP in low Earth orbit, using the space shuttle orbiter as a construction base, are analyzed. The analyses concerned three missions required to build the basic platform. The first mission involved performing the fabrication of beams in space and assembling the beams into a basic structural framework. The second mission was to install the forward support structure and aft support structure, the forward assembly, and a TT&C antenna. The third mission plan was to complete the construction of the platform and activate it to begin operations in low Earth orbit. The integration of the activities for each mission is described along with the construction requirements and construction logic
The temperature dependency of Wolf-Rayet-type mass loss: An exploratory study for winds launched by the hot iron bump
CONTEXT: The mass loss of He-burning stars, which are partially or completely
stripped of their outer hydrogen envelope, is a catalyst of the cosmic matter
cycle and decisive ingredient of massive star evolution. Yet, its theoretical
fundament is only starting to emerge with major dependencies still to be
uncovered.
AIMS: A temperature or radius dependence is usually not included in
descriptions for the mass loss of classical Wolf-Rayet (cWR) stars, despite
being crucial for other hot star wind domains. We thus aim to determine whether
such a dependency will also be necessary for a comprehensive description of
mass loss in the cWR regime.
METHODS: Sequences of dynamically consistent atmosphere models were
calculated with the hydrodynamic branch of the PoWR code along the temperature
domain, using different choices for luminosity, mass, and surface abundances.
For the first time, we allowed nonmonotonic velocity fields when solving the
equation of motion. The resulting velocity structures were then interpolated
for the comoving-frame radiative transfer, ensuring that the main wind
characteristics were preserved.
RESULTS: We find a strong dependence of the mass-loss rate with the
temperature of the critical/sonic point which mainly reflects the different
radii and resulting gravitational accelerations. Moreover, we obtain a relation
between the observed effective temperature and the transformed mass-loss rate
which seems to be largely independent of the underlying stellar parameters. The
relation shifts for different clumping factors in the outer wind. Below a
characteristic value of -4.5, the slope of this relation changes and the winds
become transparent for He II ionizing photons.
CONCLUSIONS: The mass loss of cWR stars is a high-dimensional problem but
also shows inherent scalings which can be used to obtain an approximation of
the observed effective temperature. (...)Comment: 16 pages + 5 page appendix, 17+9 figures, 3+2 tables. Accepted for
publication in A&
Co-occurrence of resonant activation and noise-enhanced stability in a model of cancer growth in the presence of immune response
We investigate a stochastic version of a simple enzymatic reaction which
follows the generic Michaelis-Menten kinetics. At sufficiently high
concentrations of reacting species, the molecular fluctuations can be
approximated as a realization of a Brownian dynamics for which the model
reaction kinetics takes on the form of a stochastic differential equation.
After eliminating a fast kinetics, the model can be rephrased into a form of a
one-dimensional overdamped Langevin equation. We discuss physical aspects of
environmental noises acting in such a reduced system, pointing out the
possibility of coexistence of dynamical regimes where noise-enhanced stability
and resonant activation phenomena can be observed together.Comment: 18 pages, 11 figures, published in Physical Review E 74, 041904
(2006
Global existence for semilinear reaction-diffusion systems on evolving domains
We present global existence results for solutions of reaction-diffusion
systems on evolving domains. Global existence results for a class of
reaction-diffusion systems on fixed domains are extended to the same systems
posed on spatially linear isotropically evolving domains. The results hold
without any assumptions on the sign of the growth rate. The analysis is valid
for many systems that commonly arise in the theory of pattern formation. We
present numerical results illustrating our theoretical findings.Comment: 24 pages, 3 figure
The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
We present an examination of high resolution, ultraviolet spectroscopy from
Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the
massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based
optical spectra to determine the effective temperature and gravity of the O9.7
Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), line
blanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0
+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. The
optical spectrum is best fit with models that have enriched He and N
abundances. We fit the model spectral energy distribution for this temperature
and gravity to the UV, optical, and IR fluxes to determine the angular size of
and extinction towards the binary. The angular size then yields relations for
the stellar radius and luminosity as a function of distance. By assuming that
the supergiant rotates synchronously with the orbit, we can use the radius -
distance relation to find mass estimates for both the supergiant and black hole
as a function of the distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that limits the
solutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sun
for the supergiant and 11^{+5}_{-3} M_sun for the black hole.Comment: ApJ in pres
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910
As a result of the variability survey in Chi Persei and NGC6910, the number
of Beta Cep stars that are members of these two open clusters is increased to
twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational
properties, in particular the frequency spectra, of Beta Cep stars in both
clusters and explain the differences in terms of the global parameters of the
clusters. We also indicate that the more complicated pattern of the variability
among B type stars in Chi Persei is very likely caused by higher rotational
velocities of stars in this cluster. We conclude that the sample of pulsating
stars in the two open clusters constitutes a very good starting point for the
ensemble asteroseismology of Beta Cep-type stars and maybe also for other
B-type pulsators.Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife,
Lanzarote, Feb 2010, submitte
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