1,650 research outputs found
Separation and fractionation of order and disorder in highly polydisperse systems
Microcanonical Monte Carlo simulations of a polydisperse soft-spheres model
for liquids and colloids have been performed for very large polydispersity, in
the region where a phase-separation is known to occur when the system (or part
of it) solidifies. By studying samples of different sizes, from N=256 to N=864,
we focus on the nature of the two distinct coexisting phases. Measurements of
crystalline order in particles of different size reveal that the solid phase
segregates between a crystalline solid with cubic symmetry and a disordered
phase. This phenomenon is termed fractionation.Comment: 8 pages, 5 figure
The CORALIE survey for southern extra-solar planets IX. A 1.3-day period brown dwarf disguised as a planet
In this article we present the case of HD 41004 AB, a system composed of a
K0V star and a 3.7-magnitude fainter M-dwarf companion separated by only 0.5
arcsec. An analysis of CORALIE radial-velocity measurements has revealed a
variation with an amplitude of about 50m/s and a periodicity of 1.3days. This
radial-velocity signal is consistent with the expected variation induced by the
presence a very low mass giant planetary companion to HD 41004 A, whose light
dominates the spectra. The radial-velocity measurements were then complemented
with a photometric campaign and with the analysis of the bisector of the
CORALIE Cross-Correlation Function (CCF). While the former revealed no
significant variations within the observational precision of 0.003-0.004 mag
(except for an observed flare event), the bisector analysis showed that the
line profiles are varying in phase with the radial-velocity. This latter
result, complemented with a series of simulations, has shown that we can
explain the observations by considering that HD 41004 B has a brown-dwarf
companion orbiting with the observed 1.3-day period. If confirmed, this
detection represents the first discovery of a brown dwarf in a very short
period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as
a serious warning about the importance of analyzing the bisector when looking
for planets using radial-velocity techniques.Comment: 16 pages, 17 eps figures, A&A in press (Figure 11 not as in original
version due to size
Finite-size scaling analysis of the distributions of pseudo-critical temperatures in spin glasses
Using the results of large scale numerical simulations we study the
probability distribution of the pseudo critical temperature for the
three-dimensional Edwards-Anderson Ising spin glass and for the fully connected
Sherrington-Kirkpatrick model. We find that the behavior of our data is nicely
described by straightforward finite-size scaling relations.Comment: 23 pages, 9 figures. Version accepted for publication in J. Stat.
Mec
Mid-infrared laser light nulling experiment using single-mode conductive waveguides
Aims: In the context of space interferometry missions devoted to the search
of exo-Earths, this paper investigates the capabilities of new single mode
conductive waveguides at providing modal filtering in an infrared and
monochromatic nulling experiment; Methods: A Michelson laser interferometer
with a co-axial beam combination scheme at 10.6 microns is used. After
introducing a Pi phase shift using a translating mirror, dynamic and static
measurements of the nulling ratio are performed in the two cases where modal
filtering is implemented and suppressed. No additional active control of the
wavefront errors is involved. Results: We achieve on average a statistical
nulling ratio of 2.5e-4 with a 1-sigma upper limit of 6e-4, while a best null
of 5.6e-5 is obtained in static mode. At the moment, the impact of external
vibrations limits our ability to maintain the null to 10 to 20 seconds.;
Conclusions: A positive effect of SM conductive waveguide on modal filtering
has been observed in this study. Further improvement of the null should be
possible with proper mechanical isolation of the setup.Comment: Accepted in A&A, 7 pages, 5 figure
Phase diagram of the (bosonic) Double-Exchange Model
The phase diagram of the simplest approximation to Double-Exchange systems,
the bosonic Double-Exchange model with antiferromagnetic super-exchange
coupling, is fully worked out by means of Monte Carlo simulations, large-N
expansions and Variational Mean-Field calculations. We find a rich phase
diagram, with no first-order phase transitions. The most surprising finding is
the existence of a segment like ordered phase at low temperature for
intermediate AFM coupling which cannot be detected in neutron-scattering
experiments. This is signaled by a maximum (a cusp) in the specific heat. Below
the phase-transition, only short-range ordering would be found in
neutron-scattering. Researchers looking for a Quantum Critical Point in
manganites should be wary of this possibility. Finite-Size Scaling estimates of
critical exponents are presented, although large scaling corrections are
present in the reachable lattice sizes.Comment: 17 pages, 14 figure
Metal-macrofauna interactions determine microbial community structure and function in copper contaminated sediments
Peer reviewedPublisher PD
Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling
We analyze HST/STIS spectra (see Paper I) of the central region of the dense
globular cluster M15. We infer the velocities of 64 individual stars,
two-thirds of which have their velocity measured for the first time. This
triples the number of stars with measured velocities in the central 1 arcsec of
M15 and doubles the number in the central 2 arcsec. Combined with existing
ground-based data we obtain the radial profiles of the projected kinematical
quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few
arcsec, somewhat higher than the values of 10-12 km/s inferred previously from
ground-based data. To interpret the results we construct dynamical models based
on the Jeans equation, which imply that M15 must have a central concentration
of non-luminous material. If this is due to a single black hole, then its mass
is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the
relation between M_BH and sigma_RMS that has been established for galaxies.
Also, the existence of intermediate-mass black holes in globular clusters is
consistent with several scenarios for globular cluster evolution proposed in
the literature. Therefore, these results may have important implications for
our understanding of the evolution of globular clusters, the growth of black
holes, the connection between globular cluster and galaxy formation, and the
nature of the recently discovered `ultra-luminous' X-ray sources in nearby
galaxies. Instead of a single black hole, M15 could have a central
concentration of dark remnants (e.g., neutron stars) due to mass segregation.
However, the best-fitting Fokker-Planck models that have previously been
constructed for M15 do not predict a central mass concentration that is
sufficient to explain the observed kinematics.[ABRIDGED]Comment: 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in
press (Dec 2002). Please note that the results reported here are modified by
the Addendum available at astro-ph/0210158 (Astronomical Journal, in press,
Jan 2003). This second version submitted to astro-ph is identical to first,
with the exception of the preceeding remar
Space Velocities of L- and T-type Dwarfs
(Abridged) We have obtained radial velocities of a sample of 18 ultracool
dwarfs (M6.5-T8) using high-resolution, near-infrared spectra obtained with
NIRSPEC and the Keck II telescope. We have confirmed that the radial velocity
of Gl 570 D is coincident with that of the K-type primary star Gl 570 A, thus
providing additional support for their true companionship. The presence of
planetary-mass companions around 2MASS J05591914-1404488 (T4.5V) has been
analyzed using five NIRSPEC radial velocity measurements obtained over a period
of 4.37 yr. We have computed UVW space motions for a total of 21 L and T dwarfs
within 20 pc of the Sun. This population shows UVW velocities that nicely
overlap the typical kinematics of solar to M-type stars within the same spatial
volume. However, the mean Galactic (44.2 km/s) and tangential (36.5 km/s)
velocities of the L and T dwarfs appear to be smaller than those of G to M
stars. A significant fraction (~40%) of the L and T dwarfs lies near the Hyades
moving group (0.4-2 Gyr), which contrasts with the 10-12% found for
earlier-type stellar neighbors. Additionally, the distributions of all three
UVW components (sigma_{UVW} = 30.2, 16.5, 15.8 km/s) and the distributions of
the total Galactic (sigma_{v_tot} = 19.1 km/s) and tangential (sigma_{v_t} =
17.6 km/s) velocities derived for the L and T dwarf sample are narrower than
those measured for nearby G, K, and M-type stars, but similar to the
dispersions obtained for F stars. This suggests that, in the solar
neighborhood, the L- and T-type ultracool dwarfs in our sample (including brown
dwarfs) is kinematically younger than solar-type to early M stars with likely
ages in the interval 0.5-4 Gyr.Comment: Accepted for publication in Ap
Matching microscopic and macroscopic responses in glasses
We first reproduce on the Janus and Janus II computers a milestone experiment
that measures the spin-glass coherence length through the lowering of
free-energy barriers induced by the Zeeman effect. Secondly we determine the
scaling behavior that allows a quantitative analysis of a new experiment
reported in the companion Letter [S. Guchhait and R. Orbach, Phys. Rev. Lett.
118, 157203 (2017)]. The value of the coherence length estimated through the
analysis of microscopic correlation functions turns out to be quantitatively
consistent with its measurement through macroscopic response functions.
Further, non-linear susceptibilities, recently measured in glass-forming
liquids, scale as powers of the same microscopic length.Comment: 6 pages, 4 figure
Calcium II Triplet Spectroscopy of LMC Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Abridged Abstract -
Utilizing the FORS2 instrument on the VLT, we have obtained near infrared
spectra for more than 200 stars in 28 populous LMC clusters. This cluster
sample spans a large range of ages (~ 1-13 Gyr) and metallicities (-0.3 >
[Fe/H] > -2.0) and has good areal coverage of the LMC disk. The strong
absorption lines of the Calcium II triplet are used to derive cluster radial
velocities and abundances. We determine mean cluster velocities to typically
1.6 km/s and mean metallicities to 0.04 dex (random error). For eight of these
clusters, we report the first spectroscopically determined metallicities based
on individual cluster stars, and six of these eight have no published radial
velocity measurements.
(continued in paper)Comment: 26 pages of text plus 14 figures and 6 tables. Accepted for
publication in AJ. Scheduled for Vol. 132, No. 4 (October 2006
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