899 research outputs found
High-order time-splitting Hermite and Fourier spectral methods
In this paper, we are concerned with the numerical solution of the time-dependent Gross-Pitaevskii Equation (GPE) involving a quasi-harmonic potential. Primarily, we consider discretisations that are based on spectral methods in space and higher-order exponential operator splitting methods in time. The resulting methods are favourable in view of accuracy and efficiency; moreover, geometric properties of the equation such as particle number and energy conservation are well captured. Regarding the spatial discretisation of the GPE, we consider two approaches. In the unbounded domain, we employ a spectral decomposition of the solution into Hermite basis functions: on the other hand. restricting the equation to a sufficiently large bounded domain, Fourier techniques are applicable. For the time integration of the GPE, we study various exponential operator splitting methods of convergence orders two, four, and six. Our main objective is to provide accuracy and efficiency comparisons of exponential operator splitting Fourier and Hermite pseudospectral methods for the time evolution of the GPE. Furthermore, we illustrate the effectiveness of higher-order time-splitting methods compared to standard integrators in a long-term integration
Self-affine surface morphology of plastically deformed metals
We analyze the surface morphology of metals after plastic deformation over a
range of scales from 10 nm to 2 mm, using a combination of atomic force
microscopy and scanning white-light interferometry. We demonstrate that an
initially smooth surface during deformation develops self-affine roughness over
almost four orders of magnitude in scale. The Hurst exponent of
one-dimensional surface profiles is initially found to decrease with increasing
strain and then stabilizes at . By analyzing their statistical
properties we show that the one-dimensional surface profiles can be
mathematically modelled as graphs of a fractional Brownian motion. Our findings
can be understood in terms of a fractal distribution of plastic strain within
the deformed samples
Turbulent Flow-Driven Molecular Cloud Formation: A Solution to the Post-T Tauri Problem?
We suggest that molecular clouds can be formed on short time scales by
compressions from large scale streams in the interstellar medium (ISM). In
particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc
2-5 pc, most have been formed by H I flows in Myr,
explaining the absence of post-T Tauri stars in the region with ages Myr. Observations in the 21 cm line of the H I `halos' around the Taurus
molecular gas show many features (broad asymmetric profiles, velocity shifts of
H I relative to CO) predicted by our MHD numerical simulations, in which
large-scale H I streams collide to produce dense filamentary structures. This
rapid evolution is possible because the H I flows producing and disrupting the
cloud have much higher velocities (5-10 kms) than present in the molecular gas
resulting from the colliding flows. The simulations suggest that such flows can
occur from the global ISM turbulence without requiring a single triggering
event such as a SN explosion.Comment: 26 pages, 12 ps figures. Apj accepte
The Origin of Jovian Planets in Protostellar Disks: The Role of Dead Zones
The final masses of Jovian planets are attained when the tidal torques that
they exert on their surrounding protostellar disks are sufficient to open gaps
in the face of disk viscosity, thereby shutting off any further accretion. In
sufficiently well-ionized disks, the predominant form of disk viscosity
originates from the Magneto-Rotational Instability (MRI) that drives
hydromagnetic disk turbulence. In the region of sufficiently low ionization
rate -- the so-called dead zone -- turbulence is damped and we show that lower
mass planets will be formed. We considered three ionization sources (X-rays,
cosmic rays, and radioactive elements) and determined the size of a dead zone
for the total ionization rate by using a radiative, hydrostatic equilibrium
disk model developed by Chiang et al. (2001). We studied a range of surface
mass density (Sigma_{0}=10^3 - 10^5 g cm^{-2}) and X-ray energy (kT_{x}=1 - 10
keV). We also compared the ionization rate of such a disk by X-rays with cosmic
rays and find that the latter dominate X-rays in ionizing protostellar disks
unless the X-ray energy is very high (5 - 10 keV). Among our major conclusions
are that for typical conditions, dead zones encompass a region extending out to
several AU -- the region in which terrestrial planets are found in our solar
system. Our results suggest that the division between low and high mass planets
in exosolar planetary systems is a consequence of the presence of a dead zone
in their natal protoplanetary disks. We also find that the extent of a dead
zone is mainly dependent on the disk's surface mass density. Our results
provide further support for the idea that Jovian planets in exosolar systems
must have migrated substantially inwards from their points of origin.Comment: 28 pages, 10 figures, accepted by Ap
Raman cooling and heating of two trapped Ba+ ions
We study cooling of the collective vibrational motion of two 138Ba+ ions
confined in an electrodynamic trap and irradiated with laser light close to the
resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of
the ions is monitored by a spatially resolving photo multiplier. Depending on
detuning and intensity of the cooling lasers, macroscopically different
motional states corresponding to different ion temperatures are observed. We
also derive the ions' temperature from detailed analytical calculations of
laser cooling taking into account the Zeeman structure of the energy levels
involved. The observed motional states perfectly match the calculated
temperatures. Significant heating is observed in the vicinity of the dark
resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon
processes dominate the interaction between lasers and ions. Parameter regimes
of laser light are identified that imply most efficient laser cooling.Comment: 8 pages, 5 figure
The Blandford-Znajek mechanism and emission from isolated accreting black holes
In the presence of a magnetic field, rotational energy can be extracted from
black holes via the Blandford-Znajek mechanism. We use self-similar advection
dominated accretion (ADAF) models to estimate the efficiency of this mechanism
for black holes accreting from geometrically thick disks, in the light of
recent magnetohydrodynamic disk simulations, and show that the power from
electromagnetic energy extraction exceeds the accretion luminosity for ADAFs at
sufficiently low accretion rates. We consider the detectability of isolated
stellar mass black holes accreting from the ISM, and show that for any rapidly
rotating holes the efficiency of energy extraction could reach 0.01. The
estimated total luminosity would be consistent with the tentative
identification of some EGRET sources as accreting isolated black holes, if that
energy is radiated primarily as gamma rays. We discuss the importance of
emission from the Blandford-Znajek mechanism for the spectra of other advection
dominated accretion flows, especially those in low luminosity galactic nuclei.Comment: ApJL, in pres
51 Eri and GJ 3305: A 10-15 Myr old binary star system at 30 parsecs
Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we
consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0),
historically classified as unrelated main sequence stars in the solar
neighborhood, are instead a wide physical binary system and members of the
young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of
several groups of young stars with ages around 10 Myr that are kinematically
convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star
association. Combining SAAO optical photometry, Hobby-Eberly Telescope
high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics,
we confirm with high confidence that the system is indeed extremely young. GJ
3305 itself exhibits very strong magnetic activity but has rapidly depleted
most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member
of the OSCA, lying 110 pc from the main subgroups. The system is similar to the
BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting
that dynamically fragile multiple systems can survive the turbulent
environments of their natal giant molecular cloud complexes, while still being
imparted high dispersion velocities. Nearby young systems such as these are
excellent targets for evolved circumstellar disk and planetary studies, having
stellar ages comparable to that of the late phases of planet formation.Comment: 27 pages, 7 figures. Accepted for publication in the Astronomical
Journal. For a version with high resolution figures, see
http://www.astro.psu.edu/users/edf/51Eri.pd
Modeling the power flow in normal conductor-insulator-superconductor junctions
Normal conductor-insulator-superconductor (NIS) junctions promise to be interesting for x-ray and phonon sensing applications, in particular due to the expected self-cooling of the N electrode by the tunneling current. Such cooling would enable the operation of the active element of the sensor below the cryostat temperature and at a correspondingly higher sensitivity. It would also allow the use of MS junctions as microcoolers. At present, this cooling has not been realized in large area junctions (suitable for a number of detector applications). In this article, we discuss a detailed modeling of the heat flow in such junctions; we show how the heat flow into the normal electrode by quasiparticle back-tunneling and phonon absorption from quasiparticle pair recombination can overcompensate the cooling power. This provides a microscopic explanation of the self-heating effects we observe in our large area NIS junctions. The model suggests a number of possible solutions
A Search for Photometric Rotation Periods in Low-Mass Stars and Brown Dwarfs in the Pleiades
We have photometrically monitored (Cousins Ic) eight low mass stars and brown
dwarfs which are probable members of the Pleiades. We derived rotation periods
for two of the stars - HHJ409 and CFHT-PL8 - to be 0.258 d and 0.401 d,
respectively. The masses of these stars are near 0.4 and 0.08 Msun,
respectively; the latter is the second such object near the hydrogen-burning
boundary for which a rotation period has been measured. We also observed HHJ409
in V; the relative amplitude in the two bands shows that the spots in that star
are about 200 K cooler than the stellar effective temperature of 3560 K and
have a filling factor on the order of 13%. With one possible exception, the
remaining stars in the sample do not show photometric variations larger than
the mean error of measurement. We also examined the M9.5V disk star 2MASSJ0149,
which had previously exhibited a strong flare event, but did not detect any
photometric variation.Comment: 13 pages, four figures. Accepted for publication in A
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