2,081 research outputs found
An infrared imaging search for low-mass companions to members of the young nearby beta Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby stars in
the Tucana/Horologium and beta Pic associations, all ~ 10 to 35 Myrs young and
at ~10 to 60 pc distance. Such young nearby stars are well-suited for direct
imaging searches for brown dwarf and even planetary companions, because young
sub-stellar objects are still self-luminous due to contraction and accretion.
We performed our observations at the ESO 3.5m NTT with the normal infrared
imaging detector SofI and the MPE speckle camera Sharp-I. Three arc sec north
of GSC 8047-0232 in Horologium a promising brown dwarf companion candidate is
detected, which needs to be confirmed by proper motion and/or spectroscopy.
Several other faint companion candidates are already rejected by second epoch
imaging. Among 21 stars observed in Tucana/Horologium, there are not more than
one to five brown dwarf companions outside of 75 AU (1.5" at 50 pc); most
certainly only < 5 % of the Tuc/HorA stars have brown dwarf companions (13 to
78 Jupiter masses) outside of 75 AU. For the first time, we can report an upper
limit for the frequency of massive planets (~ 10 M_jup) at wide separations (~
100 AU) using a meaningfull and homogeneous sample: Of 11 stars observed
sufficiently deep in beta Pic (12 Myrs), not more than one has a massive planet
outside of ~ 100 AU, i.e. massive planets at large separations are rare (< 9
%).Comment: Astronomische Nachrichten, in pres
Infrared Spectra of Meteoritic SiC Grains
We present here the first infrared spectra of meteoritic SiC grains. The
mid-infrared transmission spectra of meteoritic SiC grains isolated from the
Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in
order to make available the optical properties of presolar SiC grains. These
grains are most likely stellar condensates with an origin predominately in
carbon stars. Measurements were performed on two different extractions of
presolar SiC from the Murchison meteorite. The two samples show very different
spectral appearance due to different grain size distributions. The spectral
feature of the smaller meteoritic SiC grains is a relatively broad absorption
band found between the longitudinal and transverse lattice vibration modes
around 11.3 micron, supporting the current interpretation about the presence of
SiC grains in carbon stars. In contrast to this, the spectral feature of the
large (> 5 micron) grains has an extinction minimum around 10 micron. The
obtained spectra are compared with commercially available SiC grains and the
differences are discussed. This comparison shows that the crystal structure
(e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the
optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&
Features of the Acoustic Mechanism of Core-Collapse Supernova Explosions
In the context of 2D, axisymmetric, multi-group, radiation/hydrodynamic
simulations of core-collapse supernovae over the full 180 domain, we
present an exploration of the progenitor dependence of the acoustic mechanism
of explosion. All progenitor models we have tested with our Newtonian code
explode. We investigate the roles of the Standing-Accretion-Shock-Instability
(SASI), the excitation of core g-modes, the generation of core acoustic power,
the ejection of matter with r-process potential, the wind-like character of the
explosion, and the fundamental anisotropy of the blasts. We find that the
breaking of spherical symmetry is central to the supernova phenomenon and the
blasts, when top-bottom asymmetric, are self-collimating. We see indications
that the initial explosion energies are larger for the more massive
progenitors, and smaller for the less massive progenitors, and that the
neutrino contribution to the explosion energy may be an increasing function of
progenitor mass. The degree of explosion asymmetry we obtain is completely
consistent with that inferred from the polarization measurements of Type Ic
supernovae. Furthermore, we calculate for the first time the magnitude and sign
of the net impulse on the core due to anisotropic neutrino emission and suggest
that hydrodynamic and neutrino recoils in the context of our asymmetric
explosions afford a natural mechanism for observed pulsar proper motions.
[abridged]Comment: Accepted to the Astrophysical Journal, 23 pages in emulateapj format,
including 12 figure
Continuous control of ionization wave chaos by spatially derived feedback signals
In the positive column of a neon glow discharge, two different types of
ionization waves occur simultaneously. The low-dimensional chaos arising from
the nonlinear interaction between the two waves is controlled by a continuous
feedback technique. The control strategy is derived from the time-delayed
autosynchronization method. Two spatially displaced points of observation are
used to obtain the control information, using the propagation characteristics
of the chaotic wave.Comment: Elsevier-Tex-File, 8 pages, 6 figures, submitted to PL
Phase Diagram of the Two-Channel Kondo Lattice
The phase diagram of the two-channel Kondo lattice model is examined with a
Quantum Monte Carlo simulation in the limit of infinite dimensions.
Commensurate (and incommensurate) antiferromagnetic and superconducting states
are found. The antiferromagnetic transition is very weak and continuous;
whereas the superconducting transition is discontinuous to an odd-frequency
channel-singlet and spin-singlet pairing state.Comment: 5 pages, LaTeX and 4 PS figures (see also cond-mat/9609146 and
cond-mat/9605109
Semiquantum Chaos in the Double-Well
The new phenomenon of semiquantum chaos is analyzed in a classically regular
double-well oscillator model. Here it arises from a doubling of the number of
effectively classical degrees of freedom, which are nonlinearly coupled in a
Gaussian variational approximation (TDHF) to full quantum mechanics. The
resulting first-order nondissipative autonomous flow system shows energy
dependent transitions between regular behavior and semiquantum chaos, which we
monitor by Poincar\'e sections and a suitable frequency correlation function
related to the density matrix. We discuss the general importance of this new
form of deterministic chaos and point out the necessity to study open
(dissipative) quantum systems, in order to observe it experimentally.Comment: LaTeX, 25 pages plus 7 postscript figures. Replaced figure 3 with a
non-bitmapped versio
Two-Channel Kondo Lattice: An Incoherent Metal
The two-channel Kondo lattice model is examined with a Quantum Monte Carlo
simulation in the limit of infinite dimensions. We find non-fermi-liquid
behavior at low temperatures including a finite low-temperature single-particle
scattering rate, the lack of a fermi edge and Drude weight. However, the
low-energy density of electronic states is finite. Thus, we identify this
system as an incoherent metal. We discuss the relevance of our results for
concentrated heavy fermion metals with non-Fermi-Liquid behavior.Comment: LaTex, 5 pages, 3 Postscript files. Revision - in reference 5 and
6(a
Pulsar spins from an instability in the accretion shock of supernovae
Rotation-powered radio pulsars are born with inferred initial rotation
periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae.
In the traditional picture, this fast rotation is the result of conservation of
angular momentum during the collapse of a rotating stellar core. This leads to
the inevitable conclusion that pulsar spin is directly correlated with the
rotation of the progenitor star. So far, however, stellar theory has not been
able to explain the distribution of pulsar spins, suggesting that the birth
rotation is either too slow or too fast. Here we report a robust instability of
the stalled accretion shock in core-collapse supernovae that is able to
generate a strong rotational flow in the vicinity of the accreting
proto-neutron star. Sufficient angular momentum is deposited on the
proto-neutron star to generate a final spin period consistent with
observations, even beginning with spherically symmetrical initial conditions.
This provides a new mechanism for the generation of neutron star spin and
weakens, if not breaks, the assumed correlation between the rotational periods
of supernova progenitor cores and pulsar spin.Comment: To be published in Natur
Thermal conductivity and specific heat of the linear chain cuprate SrCuO: Evidence for thermal transport via spinons
We report measurements of the specific heat and the thermal conductivity of
the model Heisenberg spin-1/2 chain cuprate SrCuO at low
temperatures. In addition to a nearly isotropic phonon heat transport, we find
a quasi one-dimensional excess thermal conductivity along the chain direction,
most likely associated with spin excitations (spinons). The spinon energy
current is limited mainly by scattering on defects and phonons. Analyzing the
specific heat data, the intrachain magnetic exchange is estimated to
be 2650 K.Comment: 4 RevTeX pages, 3 figures, to appear in Phys. Rev.
Parametrized 3D models of neutrino-driven supernova explosions: Neutrino emission asymmetries and gravitational-wave signals
Time-dependent and direction-dependent neutrino and gravitational-wave (GW)
signatures are presented for a set of 3D hydrodynamic models of parametrized,
neutrino-driven supernova explosions of non-rotating 15 and 20 solar mass
stars. We employ an approximate treatment of neutrino transport. Due to the
excision of the high-density core of the proto-neutron star and the use of an
axis-free overset grid, the models can be followed from the post-bounce
accretion phase for more than one second without imposing any symmetry
restrictions. GW and neutrino emission exhibit the generic time-dependent
features known from 2D models. Non-radial hydrodynamic mass motions in the
accretion layer and their interaction with the outer layers of the
proto-neutron star together with anisotropic neutrino emission give rise to a
GW signal with an amplitude of ~5-20 cm and frequencies 100--500 Hz. The GW
emission from mass motions reaches a maximum before the explosion sets in.
Afterwards the GW signal exhibits a low-frequency modulation, in some cases
describing a quasi-monotonic growth, associated with the non-spherical
expansion of the explosion shock wave and the large-scale anisotropy of the
escaping neutrino flow. Variations of the mass-quadrupole moment due to
convective activity inside the nascent neutron star contribute a high-frequency
component to the GW signal during the post-explosion phase. The GW signals
exhibit strong variability between the two polarizations, different explosion
simulations and different observer directions, and does not possess any
template character. The neutrino emission properties show fluctuations over the
neutron star surface on spatial and temporal scales that reflect the different
types of non-spherical mass motions. The modulation amplitudes of the
measurable neutrino luminosities and mean energies are significantly smaller
than predicted by 2D simulations.Comment: revised version: 20 pages, 17 figures, Astronomy & Astrophysics in
pres
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