2,802 research outputs found
The Gravitational Wave Signature of Core-Collapse Supernovae
We review the ensemble of anticipated gravitational-wave (GW) emission
processes in stellar core collapse and postbounce core-collapse supernova
evolution. We discuss recent progress in the modeling of these processes and
summarize most recent GW signal estimates. In addition, we present new results
on the GW emission from postbounce convective overturn and protoneutron star
g-mode pulsations based on axisymmetric radiation-hydrodynamic calculations.
Galactic core-collapse supernovae are very rare events, but within 3-5 Mpc from
Earth, the rate jumps to 1 in ~2 years. Using the set of currently available
theoretical gravitational waveforms, we compute upper-limit optimal
signal-to-noise ratios based on current and advanced LIGO/GEO600/VIRGO noise
curves for the recent SN 2008bk which exploded at ~3.9 Mpc. While initial LIGOs
cannot detect GWs emitted by core-collapse events at such a distance, we find
that advanced LIGO-class detectors could put significant upper limits on the GW
emission strength for such events. We study the potential occurrence of the
various GW emission processes in particular supernova explosion scenarios and
argue that the GW signatures of neutrino-driven, magneto-rotational, and
acoustically-driven core-collapse SNe may be mutually exclusive. We suggest
that even initial LIGOs could distinguish these explosion mechanisms based on
the detection (or non-detection) of GWs from a galactic core-collapse
supernova.Comment: Topical Review, accepted for publication in CQG. 51 pages, 13
figures, a version of the article with high-resolution figures is available
from http://stellarcollapse.org/papers/Ott_SN_GW_review2008.pdf. Update:
Added section on core collapse simulations and the treatment of general
relativit
Integration of vertical COM motion and angular momentum in an extended Capture Point tracking controller for bipedal walking
In this paper, we demonstrate methods for bipedal walking control based on the Capture Point (CP) methodology.
In particular, we introduce a method to intuitively derive a CP
reference trajectory from the next three steps and extend the
linear inverted pendulum (LIP) based CP tracking controller
introduced in [1], generalizing it to a model that contains
vertical CoM motions and changes in angular momentum.
Respecting the dynamics of general multibody systems, we
propose a measurement-based compensation of multi-body
effects, which leads to a stable closed-loop dynamics of bipedal walking robots. In addition we propose a ZMP projection method, which prevents the robots feet from tilting and ensures the best feasible CP tracking. The extended CP controllerâs performance is validated in OpenHRP3 [2] simulations and compared to the controller proposed in [1]
The Progenitor Dependence of the Preexplosion Neutrino Emission in Core-Collapse Supernovae
We perform spherically-symmetric general-relativistic simulations of core
collapse and the postbounce preexplosion phase in 32 presupernova stellar
models of solar metallicity with zero-age-main-sequence masses of 12 M_{sun} to
120 M_{sun}. Using energy-dependent three-species neutrino transport in the
two-moment approximation with an analytic closure, we show that the emitted
neutrino luminosities and spectra follow very systematic trends that are
correlated with the compactness (~M/R) of the progenitor star's inner regions
via the accretion rate in the preexplosion phase. We find that these
qualitative trends depend only weakly on the nuclear equation of state, but
quantitative observational statements will require independent constraints on
the equation of state and the rotation rate of the core as well as a more
complete understanding of neutrino oscillations. We investigate the simulated
response of water Cherenkov detectors to the electron antineutrino fluxes from
our models and find that the large statistics of a galactic core collapse event
may allow robust conclusions on the inner structure of the progenitor star.Comment: 16 emulateapj pages, 10 figures, 1 table. matches published versio
Convergence or mediation? Experts of vulnerability and the vulnerability of expertsâ discourses on nanotechnologies â a case study.
Recent discussions about the evolvement of nanotechnologies criticize that the notion âriskâ is too abstract and an all-inclusive category. Moreover, the concept of risk is not precise enough to describe the potential issues related to the development of nanotechnologies. Instead, experts of technological development speak more about risk communication. Within the field of nanotechnologies, they even redefined this expression in February 2005 and related it to the question of the societal acceptance of nanotechnologies. Risk communication is about to gain stakeholder acceptance of policy decisions, whereas public and stakeholders are encouraged to participate actively in the communication process through public consultations, hearings, etc. Thus on the one hand, the category of risk has been pragmatically nuanced in order to better highlight the vulnerability of the communication on nanotechnologies. On the other hand, this vulnerable communication is not the result of a deficit of information. It is based on the idea of participation, where the vulnerability relies on the social groups specialized in the design, the application, and the diffusion of nanotechnologies within society. How is this participation possible, and what does it mean? We develop this question in the framework of a comparative survey on experts that are involved in the deployment of nanotechnologies in Grenoble (France) and Hamburg (Germany).nanotechnologies, society, risks, experts, collaboration.
Converging institutions. Shaping the relationships between nanotechnologies, economy and society
This paper develops the concept of converging institutions and applies it to nanotechnologies. Starting point are economic and socialogical perspectives. We focus on the entire innovation process of nanotechnologies beginning with research and development over diffusion via downstream sectors until implementation in final goods. The concept is applied to the nano-cluster in the metropolitan region of Grenoble and a possible converging institution is identified.converging institutions, converging technologies, nanotechnologies, systemic risks
The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions
The neutrino-heated "gain layer" immediately behind the stalled shock in a
core-collapse supernova is unstable to high-Reynolds-number turbulent
convection. We carry out and analyze a new set of 19 high-resolution
three-dimensional (3D) simulations with a three-species neutrino
leakage/heating scheme and compare with spherically-symmetric (1D) and
axisymmetric (2D) simulations carried out with the same methods. We study the
postbounce supernova evolution in a - progenitor star and vary the
local neutrino heating rate, the magnitude and spatial dependence of
asphericity from convective burning in the Si/O shell, and spatial resolution.
Our simulations suggest that there is a direct correlation between the strength
of turbulence in the gain layer and the susceptability to explosion. 2D and 3D
simulations explode at much lower neutrino heating rates than 1D simulations.
This is commonly explained by the fact that nonradial dynamics allows accreting
material to stay longer in the gain layer. We show that this explanation is
incomplete. Our results indicate that the effective turbulent ram pressure
exerted on the shock plays a crucial role by allowing multi-D models to explode
at a lower postshock thermal pressure and thus with less neutrino heating than
1D models. We connect the turbulent ram pressure with turbulent energy at large
scales and in this way explain why 2D simulations are erroneously exploding
more easily than 3D simulations.Comment: 13 pages, 8 figures, accepted by Ap
Supernova Fallback onto Magnetars and Propeller-powered Supernovae
We explore fallback accretion onto newly born magnetars during the supernova of massive stars. Strong magnetic fields (~10^(15) G) and short spin periods (~1-10 ms) have an important influence on how the magnetar interacts with the infalling material. At long spin periods, weak magnetic fields, and high accretion rates, sufficient material is accreted to form a black hole, as is commonly found for massive progenitor stars. When B ⟠5 Ă 10^(14) G, accretion causes the magnetar to spin sufficiently rapidly to deform triaxially and produces gravitational waves, but only for â50-200 s until it collapses to a black hole. Conversely, at short spin periods, strong magnetic fields, and low accretion rates, the magnetar is in the "propeller regime" and avoids becoming a black hole by expelling incoming material. This process spins down the magnetar, so that gravitational waves are only expected if the initial protoneutron star is spinning rapidly. Even when the magnetar survives, it accretes at least â0.3 M_â, so we expect magnetars born within these types of environments to be more massive than the 1.4 M_â typically associated with neutron stars. The propeller mechanism converts the ~10^(52)erg of spin energy in the magnetar into the kinetic energy of an outflow, which shock heats the outgoing supernova ejecta during the first ~10-30 s. For a small ~5 M_â hydrogen-poor envelope, this energy creates a brighter, faster evolving supernova with high ejecta velocities ~(1-3) Ă 10^4 km s^(â1) and may appear as a broad-lined Type Ib/c supernova. For a large âł 10 M_â hydrogen-rich envelope, the result is a bright Type IIP supernova with a plateau luminosity of âł 10^(43)erg s^(â1) lasting for a timescale of ~60-80 days
A method for rough terrain locomotion based on Divergent Component of Motion
AbstractâFor humanoid robots to be used in real
world scenarios, there is a need of robust and simple
walking controllers. Limitation to flat terrain is a
drawback of many walking controllers. We overcome
this limitation by extending the concept of Divergent
Component of Motion (DCM, also called âCapture Pointâ)
to 3D. Therefor, we introduce the âEnhanced Centroidal
Moment Pivot pointâ (eCMP) and the âVirtual Repellent
Pointâ (VRP), which allow for a very intuitive understanding
of the robotâs CoM dynamics. Based on eCMP,
VRP and DCM, we present a method for real-time
planning and control of DCM trajectories in 3D
Low-mass X-ray binaries from black-hole retaining globular clusters
Recent studies suggest that globular clusters (GCs) may retain a substantial
population of stellar-mass black holes (BHs), in contrast to the long-held
belief of a few to zero BHs. We model the population of BH low-mass X-ray
binaries (BH-LMXBs), an ideal observable proxy for elusive single BHs, produced
from a representative group of Milky Way GCs with variable BH populations. We
simulate the formation of BH-binaries in GCs through exchange interactions
between binary and single stars in the company of tens to hundreds of BHs.
Additionally, we consider the impact of the BH population on the rate of
compact binaries undergoing gravitational wave driven mergers. The
characteristics of the BH-LMXB population and binary properties are sensitive
to the GCs structural parameters as well as its unobservable BH population. We
find that GCs retaining BHs produce a galactic population of ejected BH-LMXBs whereas GCs retaining only BHs produce zero
ejected BH-LMXBs. Moreover, we explore the possibility that some of the
presently known BH-LMXBs might have originated in GCs and identify five
candidate systems.Comment: 27 pages, 18 figures, 7 tables, submitted to MNRA
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