7,724 research outputs found
Mira's wind explored in scattering infrared CO lines
We have observed the intermediate regions of the circumstellar envelope of
Mira (o Ceti) in photospheric light scattered by three vibration-rotation
transitions of the fundamental band of CO, from low-excited rotational levels
of the ground vibrational state, at an angular distance of beta = 2"-7" away
from the star. The data were obtained with the Phoenix spectrometer mounted on
the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately
0.5" and seeing limited. Our observations provide absolute fluxes, leading to
an independent new estimate of the mass-loss rate of approximately 3e-7
Msun/yr, as derived from a simple analytic wind model. We find that the
scattered intensity from the wind of Mira for 2" < beta < 7" decreases as
beta^-3, which suggests a time constant mass-loss rate, when averaged over 100
years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa
Oscillations and instabilities of fast and differentially rotating relativistic stars
We study non-axisymmetric oscillations of rapidly and differentially rotating
relativistic stars in the Cowling approximation. Our equilibrium models are
sequences of relativistic polytropes, where the differential rotation is
described by the relativistic -constant law. We show that a small degree of
differential rotation raises the critical rotation value for which the
quadrupolar f-mode becomes prone to the CFS instability, while the critical
value of at the mass-shedding limit is raised even more. For softer
equations of state these effects are even more pronounced. When increasing
differential rotation further to a high degree, the neutral point of the CFS
instability first reaches a local maximum and is lowered afterwards. For stars
with a rather high compactness we find that for a high degree of differential
rotation the absolute value of the critical is below the corresponding
value for rigid rotation. We conclude that the parameter space where the CFS
instability is able to drive the neutron star unstable is increased for a small
degree of differential rotation and for a large degree at least in stars with a
higher compactness.Comment: 16 pages, 11 figures; paper accepted for publication in Phys. Rev. D
(81.084019
Scaling Limits for Internal Aggregation Models with Multiple Sources
We study the scaling limits of three different aggregation models on Z^d:
internal DLA, in which particles perform random walks until reaching an
unoccupied site; the rotor-router model, in which particles perform
deterministic analogues of random walks; and the divisible sandpile, in which
each site distributes its excess mass equally among its neighbors. As the
lattice spacing tends to zero, all three models are found to have the same
scaling limit, which we describe as the solution to a certain PDE free boundary
problem in R^d. In particular, internal DLA has a deterministic scaling limit.
We find that the scaling limits are quadrature domains, which have arisen
independently in many fields such as potential theory and fluid dynamics. Our
results apply both to the case of multiple point sources and to the
Diaconis-Fulton smash sum of domains.Comment: 74 pages, 4 figures, to appear in J. d'Analyse Math. Main changes in
v2: added "least action principle" (Lemma 3.2); small corrections in section
4, and corrected the proof of Lemma 5.3 (Lemma 5.4 in the new version);
expanded section 6.
The Initial-Boundary Value Problem in General Relativity
In this article we summarize what is known about the initial-boundary value
problem for general relativity and discuss present problems related to it.Comment: 11 pages, 2 figures. Contribution to a special volume for Mario
Castagnino's seventy fifth birthda
On the Use of Blanketed Atmospheres as Boundary Conditions for Stellar Evolutionary Models
Stellar models have been computed for stars having [Fe/H] = 0.0 and -2.0 to
determine the effects of using boundary conditions derived from the latest
MARCS model atmospheres. The latter were fitted to the interior models at both
the photosphere and at tau = 100, and at least for the 0.8-1.0 solar mass stars
considered here, the resultant evolutionary tracks were found to be nearly
independent of the chosen fitting point. Particular care was taken to treat the
entire star as consistently as possible; i.e., both the interior and atmosphere
codes assumed the same abundances and the same treatment of convection. Tracks
were also computed using either the classical gray T(tau,T_eff) relation or
that derived by Krishna Swamy (1966) to derive the boundary pressure. The
latter predict warmer giant branches (by ~150 K) at solar abundances than those
based on gray or MARCS atmospheres, which happens to be in good agreement with
the inferred temperatures of giants in the open cluster M67 from the latest
(V-K)-T_eff relations. Most of the calculations assumed Z=0.0125 (Asplund et
al.), though a few models were computed for Z=0.0165 (Grevesse & Sauval) to
determine the dependence of the tracks on Z_\odot. Grids of "scaled solar,
differentially corrected" (SDC) atmospheres were also computed to try to
improve upon theoretical MARCS models. When they were used as boundary
conditions, the resultant tracks agreed very well with those based on a
standard scaled-solar (e.g., Krishna Swamy) T(tau,T_eff) relation,
independently of the assumed metal abundance. Fits of isochrones to the C-M
diagram of the [Fe/H] = -2 globular cluster M68 were examined, as was the
possibility that the mixing-length parameter varies with stellar parameters.Comment: 54 pages, including 20 figures and 3 tables; accepted (July 2007) for
publication in the Astrophysical Journa
Reply to the "Comment on 'Phase diagram of an impurity in the spin-1/2 chain: two channel Kondo effect versus Curie law'"
In a comment by A.A. Zvyagin the phase diagram in our Letter [Phys. Rev.
Lett. 86, 516 (2001)] was critisized of being incomplete and a new fixed point
was suggested. We show that this point is in fact not a fixed point and that
the phase diagram is correct as presented.Comment: Reply to a comment by A.A. Zvyagin. 1 page, 1 figure. The latest
version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/reply.pd
CN Bimodality at Low Metallicity: The Globular Cluster M53
We present low resolution UV-blue spectroscopic observations of red giant
stars in the globular cluster M53 ([Fe/H]=-1.84), obtained to study primordial
abundance variations and deep mixing via the CN and CH absorption bands. The
metallicity of M53 makes it an attractive target: a bimodal distribution of
3883 angstrom CN bandstrength is common in moderate- and high-metallicity
globular clusters ([Fe/H] > -1.6) but unusual in those of lower metallicity
([Fe/H] < -2.0). We find that M53 is an intermediate case, and has a broad but
not strongly bimodal distribution of CN bandstrength, with CN and CH
bandstrengths anticorrelated in the less-evolved stars. Like many other
globular clusters, M53 also exhibits a general decline in CH bandstrength and
[C/Fe] abundance with rising luminosity on the red giant branch.Comment: 8 pages including 11 figures and 1 table, accepted by PAS
The resultant on compact Riemann surfaces
We introduce a notion of resultant of two meromorphic functions on a compact
Riemann surface and demonstrate its usefulness in several respects. For
example, we exhibit several integral formulas for the resultant, relate it to
potential theory and give explicit formulas for the algebraic dependence
between two meromorphic functions on a compact Riemann surface. As a particular
application, the exponential transform of a quadrature domain in the complex
plane is expressed in terms of the resultant of two meromorphic functions on
the Schottky double of the domain.Comment: 44 page
High accuracy binary black hole simulations with an extended wave zone
We present results from a new code for binary black hole evolutions using the
moving-puncture approach, implementing finite differences in generalised
coordinates, and allowing the spacetime to be covered with multiple
communicating non-singular coordinate patches. Here we consider a regular
Cartesian near zone, with adapted spherical grids covering the wave zone. The
efficiencies resulting from the use of adapted coordinates allow us to maintain
sufficient grid resolution to an artificial outer boundary location which is
causally disconnected from the measurement. For the well-studied test-case of
the inspiral of an equal-mass non-spinning binary (evolved for more than 8
orbits before merger), we determine the phase and amplitude to numerical
accuracies better than 0.010% and 0.090% during inspiral, respectively, and
0.003% and 0.153% during merger. The waveforms, including the resolved higher
harmonics, are convergent and can be consistently extrapolated to
throughout the simulation, including the merger and ringdown. Ringdown
frequencies for these modes (to ) match perturbative
calculations to within 0.01%, providing a strong confirmation that the remnant
settles to a Kerr black hole with irreducible mass and spin $S_f/M_f^2 = 0.686923 \pm 10\times10^{-6}
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