1,438 research outputs found
Blunting the Spike: the CV Minimum Period
The standard picture of CV secular evolution predicts a spike in the CV
distribution near the observed short-period cutoff P_0 ~ 78 min, which is not
observed. We show that an intrinsic spread in minimum (`bounce') periods P_b
resulting from a genuine difference in some parameter controlling the evolution
can remove the spike without smearing the sharpness of the cutoff. The most
probable second parameter is different admixtures of magnetic stellar wind
braking (at up to 5 times the GR rate) in a small tail of systems, perhaps
implying that the donor magnetic field strength at formation is a second
parameter specifying CV evolution. We suggest that magnetic braking resumes
below the gap with a wide range, being well below the GR rate in most CVs, but
significantly above it in a small tail.Comment: 5 pages, 4 figures; accepted for publication in MNRA
The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings
The macroscopic mechanical properties of colloidal particle gels strongly
depend on the local arrangement of the powder particles. Experiments have shown
that more heterogeneous microstructures exhibit up to one order of magnitude
higher elastic properties than their more homogeneous counterparts at equal
volume fraction. In this paper, packings of spherical particles are used as
model structures to computationally investigate the elastic properties of
coagulated particle gels as a function of their degree of heterogeneity. The
discrete element model comprises a linear elastic contact law, particle bonding
and damping. The simulation parameters were calibrated using a homogeneous and
a heterogeneous microstructure originating from earlier Brownian dynamics
simulations. A systematic study of the elastic properties as a function of the
degree of heterogeneity was performed using two sets of microstructures
obtained from Brownian dynamics simulation and from the void expansion method.
Both sets cover a broad and to a large extent overlapping range of degrees of
heterogeneity. The simulations have shown that the elastic properties as a
function of the degree of heterogeneity are independent of the structure
generation algorithm and that the relation between the shear modulus and the
degree of heterogeneity can be well described by a power law. This suggests the
presence of a critical degree of heterogeneity and, therefore, a phase
transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February
2012
H\"older equicontinuity of the integrated density of states at weak disorder
H\"older continuity, , with
a constant independent of the disorder strength is proved for the
integrated density of states associated to a discrete random
operator consisting of a translation invariant hopping
matrix and i.i.d. single site potentials with an absolutely
continuous distribution, under a regularity assumption for the hopping term.Comment: 15 Pages, typos corrected, comments and ref. [1] added, theorems 3,4
combine
A room temperature 19-channel magnetic field mapping device for cardiac signals
We present a multichannel cardiac magnetic field imaging system built in
Fribourg from optical double-resonance Cs vapor magnetometers. It consists of
25 individual sensors designed to record magnetic field maps of the beating
human heart by simultaneous measurements on a grid of 19 points over the chest.
The system is operated as an array of second order gradiometers using
sophisticated digitally controlled feedback loops.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter
The minimum orbital period in thermal-timescale mass transfer
We show that the usual picture of supersoft X-ray binary evolution as driven
by conservative thermal-timescale mass transfer cannot explain the short
orbital periods of RX J0537.7-7034 (3.5 hr) and 1E 0035.4-7230 (4.1 hr).
Non-conservative evolution may produce such periods, but requires very
significant mass loss, and is highly constrained.Comment: 6 pages, 5 figures; to appear in MNRA
The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks
Observations of HO masers from circumnuclear disks in active galaxies for
the Megamaser Cosmology Project allow accurate measurement of the mass of
supermassive black holes (BH) in these galaxies. We present the Very Long
Baseline Interferometry (VLBI) images and kinematics of water maser emission in
six active galaxies: NGC~1194, NGC~2273, NGC~2960 (Mrk~1419), NGC~4388,
NGC~6264 and NGC~6323. We use the Keplerian rotation curves of these six
megamaser galaxies, plus a seventh previously published, to determine accurate
enclosed masses within the central pc of these galaxies, smaller than
the radius of the sphere of influence of the central mass in all cases. We also
set lower limits to the central mass densities of between 0.12 and 60 ~pc. For six of the seven disks, the high central
densities rule out clusters of stars or stellar remnants as the central
objects, and this result further supports our assumption that the enclosed mass
can be attributed predominantly to a supermassive black hole. The seven BHs
have masses ranging between 0.76 and 6.510. The BH mass
errors are \%, dominated by the uncertainty of the Hubble constant.
We compare the megamaser BH mass determination with other BH mass measurement
techniques. The BH mass based on virial estimation in four galaxies is
consistent with the megamaser BH mass given the latest empirical value of
, but the virial mass uncertainty is much greater. MCP
observations continue and we expect to obtain more maser BH masses in the
future.Comment: 18 pages, 4 figures. This paper has been submitted to ApJ. An updated
version of this paper will be posted when it gets accepte
Keck Spectroscopy of Faint 3<z<8 Lyman Break Galaxies:- Evidence for a Declining Fraction of Emission Line Sources In the Redshift Range 6<z<8
Using deep Keck spectroscopy of Lyman break galaxies selected from infrared
imaging data taken with WFC3/IR onboard the Hubble Space Telescope, we present
new evidence for a reversal in the redshift-dependent fraction of star forming
galaxies with detectable Lyman alpha emission in the redshift range 6.3 < z <
8.8. Our earlier surveys with the DEIMOS spectrograph demonstrated a
significant increase with redshift in the fraction of line emitting galaxies
over the interval 4 < z < 6, particularly for intrinsically faint systems which
dominate the luminosity density. Using the longer wavelength sensitivities of
LRIS and NIRSPEC, we have targeted 19 Lyman break galaxies selected using
recent WFC3/IR data whose photometric redshifts are in the range 6.3 < z < 8.8
and which span a wide range of intrinsic luminosities. Our spectroscopic
exposures typically reach a 5-sigma sensitivity of < 50 A for the rest-frame
equivalent width (EW) of Lyman alpha emission. Despite the high fraction of
emitters seen only a few hundred million years later, we find only 2 convincing
and 1 possible line emitter in our more distant sample. Combining with
published data on a further 7 sources obtained using FORS2 on the ESO VLT, and
assuming continuity in the trends found at lower redshift, we discuss the
significance of this apparent reversal in the redshift-dependent Lyman alpha
fraction in the context of our range in continuum luminosity. Assuming all the
targeted sources are at their photometric redshift and our assumptions about
the Lyman alpha EW distribution are correct, we would expect to find so few
emitters in less than 1% of the realizations drawn from our lower redshift
samples. Our new results provide further support for the suggestion that, at
the redshifts now being probed spectroscopically, we are entering the era where
the intergalactic medium is partially neutral.Comment: 8 pages, 5 figures, Accepted to ApJ 10/1/1
Generation of folk song melodies using Bayes transforms
The paper introduces the `Bayes transform', a mathematical procedure for putting data into a hierarchical representation. Applicable to any type of data, the procedure yields interesting results when applied to sequences. In this case, the representation obtained implicitly models the repetition hierarchy of the source. There are then natural applications to music. Derivation of Bayes transforms can be the means of determining the repetition hierarchy of note sequences (melodies) in an empirical and domain-general way. The paper investigates application of this approach to Folk Song, examining the results that can be obtained by treating such transforms as generative models
Hamiltonian structure for dispersive and dissipative dynamical systems
We develop a Hamiltonian theory of a time dispersive and dissipative
inhomogeneous medium, as described by a linear response equation respecting
causality and power dissipation. The proposed Hamiltonian couples the given
system to auxiliary fields, in the universal form of a so-called canonical heat
bath. After integrating out the heat bath the original dissipative evolution is
exactly reproduced. Furthermore, we show that the dynamics associated to a
minimal Hamiltonian are essentially unique, up to a natural class of
isomorphisms. Using this formalism, we obtain closed form expressions for the
energy density, energy flux, momentum density, and stress tensor involving the
auxiliary fields, from which we derive an approximate, ``Brillouin-type,''
formula for the time averaged energy density and stress tensor associated to an
almost mono-chromatic wave.Comment: 68 pages, 1 figure; introduction revised, typos correcte
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