5,388 research outputs found
Thermal Expansion and Magnetostriction Studies of a Kondo Lattice Compound: Ceagsb2
We have investigated a single crystal of CeAgSb2 using low field
ac-susceptibility, thermal expansion and magnetostriction measurements in the
temperature range 1.5K to 90K. The ac-susceptibility exhibits a sharp peak at
9.7K for both B//c and B perp c due to the magnetic ordering of the Ce moment.
The thermal expansion coefficient alpha, exhibits highly anisotropic behaviour
between 3K and 80K : alpha is positive for dL/L perp c, but negative for dL/L
// c. Furthermore, alpha (for dL/L) perp c (i.e. in ab-plane) exhibits a sharp
peak at TN followed by a broad maximum at 20K, while a sharp negative peak at
TN followed by a minimum at 20K has been observed for (dL/L //) the c
direction. The observed maximum and minimum in alpha(T) at 20K have been
attributed to the crystalline field effect on the J=5/2 state of the Ce3+ ion.
The magnetostriction also exhibits anisotropic behaviour with a large
magnetostriction along the c-axis. The ab-plane magnetostriction exhibits a
peak at B=3.3T at 3K, which is consistent with the observed peak in the
magnetoresistance measurements.Comment: 4 Pages (B5), 3 figures, submitted to SCES200
Quantifying orbital migration from exoplanet statistics and host metallicities
We investigate how the statistical distribution of extrasolar planets may be
combined with knowledge of the host stars' metallicity to yield constraints on
the migration histories of gas giant planets. At any radius, planets that
barely manage to form around the lowest metallicity stars accrete their
envelopes just as the gas disk is being dissipated, so the lower envelope of
planets in a plot of metallicity vs semi-major axis defines a sample of
non-migratory planets that will have suffered less than average migration
subsequent to gap opening. Under the assumption that metallicity largely
controls the initial surface density of planetesimals, we use simplified core
accretion models to calculate how the minimum metallicity needed for planet
formation varies as a function of semi-major axis. Models that do not include
core migration prior to gap opening (Type I migration) predict that the
critical metallicity is largely flat between the snow line and a semimajor axis
of about 6 AU, with a weak dependence on the initial surface density profile of
planetesimals. When slow Type I migration is included, the critical metallicity
is found to increase steadily from 1-10 AU. Large planet samples, that include
planets at modestly greater orbital radii than present surveys, therefore have
the potential to quantify the extent of migration in both Type I and Type II
regimes.Comment: ApJ, in pres
Investigating fragmentation conditions in self-gravitating accretion discs
The issue of fragmentation in self-gravitating gaseous accretion discs has
implications both for the formation of stars in discs in the nuclei of active
galaxies, and for the formation of gaseous planets or brown dwarfs in
circumstellar discs. It is now well established that fragmentation occurs if
the disc is cooled on a timescale smaller than the local dynamical timescale,
while for longer cooling times the disc reaches a quasi-steady state in thermal
equilibrium, with the cooling rate balanced by the heating due to gravitational
stresses. We investigate here how the fragmentation boundary depends on the
assumed equation of state. We find that the cooling time required for
fragmentation increases as the specific heat ratio, gamma, decreases, exceeding
the local dynamical timescale for gamma = 7/5. This result can be easily
interpreted as a consequence of there being a maximum stress (in units of the
local disc pressure) that can be sustained by a self-gravitating disc in
quasi-equilibrium. Fragmentation occurs if the cooling time is such that the
stress required to reach thermal equilibrium exceeds this value, independent of
gamma. This result suggest that a quasi-steady, self-gravitating disc can never
produce a stress that results in the viscous alpha parameter exceeding ~ 0.06.Comment: 5 pages, MNRAS, accepte
Absolute differential positronium-formation cross sections
The first absolute experimental determinations of the differential cross-sections for the formation of ground-state positronium are presented for He, Ar, H2 and CO2 near 0â—‹. Results are compared with available theories. The ratio of the differential and integrated cross-sections for the targets exposes the higher propensity for forward-emission of positronium formed from He and H2
Evidence for a merger of binary white dwarfs: the case of GD 362
GD 362 is a massive white dwarf with a spectrum suggesting a H-rich
atmosphere which also shows very high abundances of Ca, Mg, Fe and other
metals. However, for pure H-atmospheres the diffusion timescales are so short
that very extreme assumptions have to be made to account for the observed
abundances of metals. The most favored hypothesis is that the metals are
accreted from either a dusty disk or from an asteroid belt. Here we propose
that the envelope of GD 362 is dominated by He, which at these effective
temperatures is almost completely invisible in the spectrum. This assumption
strongly alleviates the problem, since the diffusion timescales are much larger
for He-dominated atmospheres. We also propose that the He-dominated atmosphere
of GD 362 is likely to be the result of the merger of a binary white dwarf.Comment: 4 pages, 3 figures. Accepted for publication in Astrophysical Journal
Letter
Gauss Sums and Quantum Mechanics
By adapting Feynman's sum over paths method to a quantum mechanical system
whose phase space is a torus, a new proof of the Landsberg-Schaar identity for
quadratic Gauss sums is given. In contrast to existing non-elementary proofs,
which use infinite sums and a limiting process or contour integration, only
finite sums are involved. The toroidal nature of the classical phase space
leads to discrete position and momentum, and hence discrete time. The
corresponding `path integrals' are finite sums whose normalisations are derived
and which are shown to intertwine cyclicity and discreteness to give a finite
version of Kelvin's method of images.Comment: 14 pages, LaTe
Accretion disc-stellar magnetosphere interaction: field line inflation and the effect on the spin-down torque
We calculate the structure of a force-free magnetosphere which is assumed to
corotate with a central star and which interacts with an embedded
differentially rotating accretion disc. The magnetic and rotation axes are
aligned and the stellar field is assumed to be a dipole. We concentrate on the
case when the amount of field line twisting through the disc-magnetosphere
interaction is large and consider different outer boundary conditions. In
general the field line twisting produces field line inflation (eg. Bardou &
Heyvaerts 1996) and in some cases with large twisting many field lines can
become open. We calculate the spin-down torque acting between the star and the
disc and we find that it decreases significantly for cases with large field
line twisting. This suggests that the oscillating torques observed for some
accreting neutron stars could be due to the magnetosphere varying between
states with low and high field line inflation. Calculations of the spin
evolution of T Tauri stars may also have to be revised in light of the
significant effect that field line twisting has on the magnetic torque
resulting from star-disc interactions.Comment: Accepted by MNRAS. 21 pages, 15 figures. LaTeX2e in the MN style.
PostScript files are also available from http://www-star.qmw.ac.uk/~va/ or by
e-mail: [email protected]
A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods
We use mid-IR (primarily 10 m) photometry as a diagnostic for the
presence of disks with inner cavities among 32 pre-main sequence stars in Orion
and Taurus-Auriga for which rotation periods are known and which do not show
evidence for inner disks at near-IR wavelengths. Disks with inner cavities are
predicted by magnetic disk-locking models that seek to explain the regulation
of angular momentum in T Tauri stars. Only three stars in our sample show
evidence for excess mid-IR emission. While these three stars may possess
truncated disks consistent with magnetic disk-locking models, the remaining 29
stars in our sample do not. Apparently, stars lacking near-IR excesses in
general do not possess truncated disks to which they are magnetically coupled.
We discuss the implications of this result for the hypothesis of
disk-regulated angular momentum. Evidently, young stars can exist as slow
rotators without the aid of present disk-locking, and there exist very young
stars already rotating near breakup velocity whose subsequent angular momentum
evolution will not be regulated by disks. Moreover, we question whether disks,
when present, truncate in the manner required by disk-locking scenarios.
Finally, we discuss the need for rotational evolution models to take full
account of the large dispersion of rotation rates present at 1 Myr, which may
allow the models to explain the rotational evolution of low-mass pre-main
sequence stars in a way that does not depend upon braking by disks.Comment: 20 pages, 4 figure
Planetesimal formation via fragmentation in self-gravitating protoplanetary discs
An unsolved issue in the standard core accretion model for gaseous planet formation is how kilometre-sized planetesimals form from, initially, micron-sized dust grains. Solid growth beyond metre sizes can be difficult both because the sticking efficiency becomes very small, and because these particles should rapidly migrate into the central star. We consider here how metre-sized particles evolve in self-gravitating accretion discs using simulations in which the gravitational influence of the solid particles is also included. Metre-sized particles become strongly concentrated in the spiral structures present in the disc and, if the solid to gas density ratio is sufficiently high, can fragment due to their own self-gravity to form planetesimals directly. This result suggests that planetesimal formation may occur very early in the star formation process while discs are still massive enough to be self-gravitating. The dependence of this process on the surface density of the solids is also consistent with the observation that extrasolar planets are preferentially found around high metallicity stars
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