2,601 research outputs found
Front-mediated melting of ultrastable glasses
Ultrastable vapor-deposited glasses display uncommon material properties.
Most remarkably, upon heating they are believed to melt via a liquid front that
originates at the free surface and propagates over a mesoscopic crossover
length, before crossing over to bulk melting. We combine swap Monte Carlo with
molecular dynamics simulations to prepare and melt isotropic amorphous films of
unprecedendtly high kinetic stability. We are able to directly observe both
bulk and front melting, and the crossover between them. We measure the front
velocity over a broad range of conditions, and a crossover length scale that
grows to nearly particle diameters in the regime accessible to
simulations. Our results disentangle the relative roles of kinetic stability
and vapor deposition in the physical properties of stable glasses.Comment: 7 pages, 6 figures; accepted for publication in Phys. Rev. Let
[N]pT ensemble and finite-size scaling study of the GEM-4 critical isostructural transition
First-order transitions of system where both lattice site occupancy and
lattice spacing fluctuate, such as cluster crystals, cannot be efficiently
studied by traditional simulation methods. These methods necessarily fix one of
these two degrees of freedom, but this difficulty is surmounted by the
generalized [N]pT ensemble [J. Chem. Phys. 136, 214106 (2012)]. Here it is
shown that histogram reweighting and the [N]pT ensemble can be used to study an
isostructural transition between cluster crystals of different occupancy in the
generalized exponential model of index 4 (GEM-4). Extending this scheme to
finite-size scaling studies also allows to accurately determine the critical
point parameters and to verify that it belongs to the Ising universality class.Comment: 5 pages, 4 figure
Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
By targeting nearby M dwarfs, a transit search using modest equipment is
capable of discovering planets as small as 2 Earth radii in the habitable zones
of their host stars. The MEarth Project, a future transit search, aims to
employ a network of ground-based robotic telescopes to monitor M dwarfs in the
northern hemisphere with sufficient precision and cadence to detect such
planets. Here we investigate the design requirements for the MEarth Project. We
evaluate the optimal bandpass, and the necessary field of view, telescope
aperture, and telescope time allocation on a star-by-star basis, as is possible
for the well-characterized nearby M dwarfs. Through these considerations, 1,976
late M dwarfs (R < 0.33 Rsun) emerge as favorable targets for transit
monitoring. Based on an observational cadence and on total telescope time
allocation tailored to recover 90% of transit signals from planets in habitable
zone orbits, we find that a network of ten 30 cm telescopes could survey these
1,976 M dwarfs in less than 3 years. A null result from this survey would set
an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets
larger than 2 Earth radii in the habitable zones of late M dwarfs, and even
stronger constraints for planets lying closer than the habitable zone. If the
true occurrence rate of habitable planets is 10%, the expected yield would be
2.6 planets.Comment: accepted to PAS
Geometrical Frustration and Static Correlations in Hard-Sphere Glass Formers
We analytically and numerically characterize the structure of hard-sphere
fluids in order to review various geometrical frustration scenarios of the
glass transition. We find generalized polytetrahedral order to be correlated
with increasing fluid packing fraction, but to become increasingly irrelevant
with increasing dimension. We also find the growth in structural correlations
to be modest in the dynamical regime accessible to computer simulations.Comment: 21 pages; part of the "Special Topic Issue on the Glass Transition
Hard sphere crystallization gets rarer with increasing dimension
We recently found that crystallization of monodisperse hard spheres from the
bulk fluid faces a much higher free energy barrier in four than in three
dimensions at equivalent supersaturation, due to the increased geometrical
frustration between the simplex-based fluid order and the crystal [J.A. van
Meel, D. Frenkel, and P. Charbonneau, Phys. Rev. E 79, 030201(R) (2009)]. Here,
we analyze the microscopic contributions to the fluid-crystal interfacial free
energy to understand how the barrier to crystallization changes with dimension.
We find the barrier to grow with dimension and we identify the role of
polydispersity in preventing crystal formation. The increased fluid stability
allows us to study the jamming behavior in four, five, and six dimensions and
compare our observations with two recent theories [C. Song, P. Wang, and H. A.
Makse, Nature 453, 629 (2008); G. Parisi and F. Zamponi, Rev. Mod. Phys, in
press (2009)].Comment: 15 pages, 5 figure
Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the dayside atmosphere of HD 209458b have a thermal
inversion and a stratosphere. This inversion is caused by the capture of
optical stellar flux by an absorber of uncertain origin that resides at
altitude. One consequence of stratospheric heating and temperature inversion is
the flipping of water absorption features into emission features from the near-
to the mid-infrared and we see evidence of such a water emission feature in the
recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na D feature
seen in transit and the fact that the transit radius at 24 m is smaller
than the corresponding radius in the optical. Moreover, it may be a factor in
why HD 209458b's optical transit radius is as large as it is. We speculate on
the nature of this absorber and the planets whose atmospheres may, or may not,
be affected by its presence.Comment: Accepted to the Astrophysical Journal Letters on August 28, 2007, six
pages in emulateapj forma
[N]pT Monte Carlo Simulations of the Cluster-Crystal-Forming Penetrable Sphere Model
Certain models with purely repulsive pair interactions can form cluster
crystals with multiply-occupied lattice sites. Simulating these models'
equilibrium properties is, however, quite challenging. Here, we develop an
expanded isothermal-isobaric ensemble that surmounts this problem by
allowing both particle number and lattice spacing to fluctuate. We apply the
method with a Monte Carlo simulation scheme to solve the phase diagram of a
prototypical cluster-crystal former, the penetrable sphere model (PSM), and
compare the results with earlier theoretical predictions. At high temperatures
and densities, the equilibrium occupancy of
face-centered cubic (FCC) crystal increases linearly. At low temperatures,
although plateaus at integer values, the crystal
behavior changes continuously with density. The previously ambiguous crossover
around is resolved
A Spitzer Spectrum of the Exoplanet HD 189733b
We report on the measurement of the 7.5-14.7 micron spectrum for the
transiting extrasolar giant planet HD 189733b using the Infrared Spectrograph
on the Spitzer Space Telescope. Though the observations comprise only 12 hours
of telescope time, the continuum is well measured and has a flux ranging from
0.6 mJy to 1.8 mJy over the wavelength range, or 0.49 +/- 0.02% of the flux of
the parent star. The variation in the measured fractional flux is very nearly
flat over the entire wavelength range and shows no indication of significant
absorption by water or methane, in contrast with the predictions of most
atmospheric models. Models with strong day/night differences appear to be
disfavored by the data, suggesting that heat redistribution to the night side
of the planet is highly efficient.Comment: 12 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
A cool starspot or a second transiting planet in the TrES-1 system?
We investigate the origin of a flux increase found during a transit of
TrES-1, observed with the HST. This feature in the HST light curve cannot be
attributed to noise and is supposedly a dark area on the stellar surface of the
host star eclipsed by TrES-1 during its transit. We investigate the likeliness
of two possible hypothesis for its origin: A starspot or a second transiting
planet. We made use of several transit observations of TrES-1 from space with
the HST and from ground with the IAC-80 telescope. On the basis of these
observations we did a statistical study of flux variations in each of the
observed events, to investigate if similar flux increases are present in other
parts of the data set. The HST observation presents a single clear flux rise
during a transit whereas the ground observations led to the detection of two
such events but with low significance. In the case of having observed a
starspot in the HST data, assuming a central impact between the spot and
TrES-1, we would obtain a lower limit for the spot radius of 42000 km. For this
radius the spot temperature would be 4690 K, 560 K lower then the stellar
surface of 5250 K. For a putative second transiting planet we can set a lower
limit for its radius at 0.37 R and for periods of less than 10.5 days, we
can set an upper limit at 0.72 R. Assuming a conventional interpretation,
then this HST observation constitutes the detection of a starspot.
Alternatively, this flux rise might also be caused by an additional transiting
planet. The true nature of the origin can be revealed if a wavelength
dependency of the flux rise can be shown or discarded with a higher certainty.
Additionally, the presence of a second planet can also be detected by radial
velocity measurements.Comment: 8 pages, 6 figures, accepted for publication in A&
Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS J13142039+1320011)
We present multi-epoch simultaneous radio, optical, H{\alpha}, UV, and X-ray
observations of the active, young, low-mass binary NLTT 33370 AB (blended
spectral type M7e). This system is remarkable for its extreme levels of
magnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,
and here we show that it is also one of the most X-ray luminous UCDs known. We
detect the system in all bands and find a complex phenomenology of both flaring
and periodic variability. Analysis of the optical light curve reveals the
simultaneous presence of two periodicities, 3.7859 0.0001 and 3.7130
0.0002 hr. While these differ by only ~2%, studies of differential
rotation in the UCD regime suggest that it cannot be responsible for the two
signals. The system's radio emission consists of at least three components:
rapid 100% polarized flares, bright emission modulating periodically in phase
with the optical emission, and an additional periodic component that appears
only in the 2013 observational campaign. We interpret the last of these as a
gyrosynchrotron feature associated with large-scale magnetic fields and a cool,
equatorial plasma torus. However, the persistent rapid flares at all rotational
phases imply that small-scale magnetic loops are also present and reconnect
nearly continuously. We present an SED of the blended system spanning more than
9 orders of magnitude in wavelength. The significant magnetism present in NLTT
33370 AB will affect its fundamental parameters, with the components' radii and
temperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we
suggest spatially resolved observations that could clarify many aspects of this
system's nature.Comment: emulateapj, 22 pages, 15 figures, ApJ in press; v2: fixes low-impact
error in Figure 15; v3: now in-pres
- …