7,314 research outputs found
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
Detection of a population gradient in the Sagittarius Stream
We present a quantitative comparison between the Horizontal Branch morphology
in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide
field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc
away from the center of the parent galaxy. We find that the Blue Horizontal
Branch (BHB) stars in that part of the Stream are five times more abundant than
in the Sgr core, relative to Red Clump stars. The difference in the ratio of
BHB to RC stars between the two fields is significant at the 4.8 sigma level.
This indicates that the old and metal-poor population of Sgr was preferentially
stripped from the galaxy in past peri-Galactic passages with respect to the
intermediate-age metal rich population that presently dominates the bound core
of Sgr, probably due to a strong radial gradient that was settled within the
galaxy before its disruption. The technique adopted in the present study allows
to trace population gradients along the whole extension of the Stream.Comment: 4 pages, 3 .ps figures (fig. 1 at low resolution); Accepted for
publication by A&A Letter
Spontaneous Fluxoid Formation in Superconducting Loops
We report on the first experimental verification of the Zurek-Kibble scenario
in an isolated superconducting ring over a wide parameter range. The
probability of creating a single flux quantum spontaneously during the fast
normal-superconducting phase transition of a wide Nb loop clearly follows an
allometric dependence on the quenching time , as one would expect if
the transition took place as fast as causality permits. However, the observed
Zurek-Kibble scaling exponent is two times larger than
anticipated for large loops. Assuming Gaussian winding number densities we show
that this doubling is well-founded for small annuli.Comment: 10 pages, 2 figures, submitted to Phys. Rev. Lett
Zurek-Kibble domain structures: The Dynamics of Spontaneous Vortex formation in Annular Josephson Tunnel Junctions
Phase transitions executed in a finite time show a domain structure with
defects, that has been argued by Zurek and Kibble to depend in a characteristic
way on the quench rate. In this letter we present an experiment to measure the
Zurek-Kibble scaling exponent sigma. Using symmetric and long Josephson Tunnel
Junctions, for which the predicted index is sigma = 0.25, we find sigma = 0.27
+/- 0.05. Further, there is agreement with the ZK prediction for the overall
normalisation.Comment: To be published in Phys. Rev. Lett
Zurek-Kibble Mechanism for the Spontaneous Vortex Formation in Josephson Tunnel Junctions: New Theory and Experiment
New scaling behavior has been both predicted and observed in the spontaneous
production of fluxons in quenched annular Josephson tunnel
junctions as a function of the quench time, . The probability
to trap a single defect during the N-S phase transition clearly follows an
allometric dependence on with a scaling exponent , as
predicted from the Zurek-Kibble mechanism for {\it realistic} JTJs formed by
strongly coupled superconductors. This definitive experiment replaces one
reported by us earlier, in which an idealised model was used that predicted
, commensurate with the then much poorer data. Our experiment
remains the only condensed matter experiment to date to have measured a scaling
exponent with any reliability.Comment: Four pages, one figur
Constraining Cosmological Models by the Cluster Mass Function
We present a comparison between two observational and three theoretical mass
functions for eight cosmological models suggested by the data from the recently
completed BOOMERANG-98 and MAXIMA-1 cosmic microwave background (CMB)
anisotropy experiments as well as peculiar velocities (PVs) and type Ia
supernovae (SN) observations. The cosmological models have been proposed as the
best fit models by several groups. We show that no model is in agreement with
the abundances of X-ray clusters at .On the
other hand, we find that the BOOM+MAX+{\sl COBE}:I, Refined Concordance and
MDM are in a good agreement with the abundances of optical clusters.
The P11 and especially Concordance models predict a slightly lower abundances
than observed at . The BOOM+MAX+{\sl COBE}:II
and PV+CMB+SN models predict a slightly higher abundances than observed at
. The nonflat MAXIMA-1 is in a fatal conflict
with the observational cluster abundances and can be safely ruled out.Comment: 17 pages, 2 figures, reference added, figures changes, substantial
revision mad
Spontaneous Fluxon Production in Annular Josephson Tunnel Junctions in the Presence of a Magnetic Field
We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity of the probability
to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.Comment: Submitted to Phys. Rev. B with 5 figures on Nov. 15, 200
A super lithium-rich red-clump star in the open cluster Trumpler 5
Context. The existence of lithium-rich low-mass red giant stars still
represents a challenge for stellar evolution models. Stellar clusters are
privileged environments for this kind of investigation. Aims. To investigate
the chemical abundance pattern of the old open cluster Trumpler\,5, we observed
a sample of four red-clump stars with high-resolution optical spectrographs.
One of them (#3416) reveals extremely strong lithium lines in its spectrum.
Methods. One-dimensional, local thermodynamic equilibrium analysis was
performed on the spectra of the observed stars. A 3D-NLTE analysis was
performed to derive the lithium abundance of star #3416. Results. Star #3416 is
super Li-rich with A(Li)=3.75\,dex. The lack of Li enrichment
(Li/Li2%), the low carbon isotopic ratio
(C/C=143), and the lack of evidence for radial velocity
variation or enhanced rotational velocity (\kms) all suggest
that lithium production has occurred in this star through the Cameron & Fowler
mechanism. Conclusions. We identified a super Li-rich core helium-burning,
red-clump star in an open cluster. Internal production is the most likely cause
of the observed enrichment. Given the expected short duration of a star's
Li-rich phase, enrichment is likely to have occurred at the red clump or in the
immediately preceding phases, namely during the He-flash at the tip of the red
giant branch (RGB) or while ascending the brightest portion of the RGB.Comment: 7 pages, 2 figures. Accepted for publication in A&A Letter
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