6,341 research outputs found
Young Binary Stars and Associated Disks
The typical product of the star formation process is a binary star. Binaries
have provided the first dynamical measures of the masses of pre-main-sequence
(PMS) stars, providing support for the calibrations of PMS evolutionary tracks.
Surprisingly, in some star-forming regions PMS binary frequencies are higher
than among main-sequence solar-type stars. The difference in PMS and
main-sequence binary frequencies is apparently not an evolutionary effect;
recent attention has focussed on correlations between binary frequency and
stellar density or cloud temperatures. Accretion disks are common among young
binary stars. Binaries with separations between 1 AU and 100 AU have
substantially less submillimeter emission than closer or wider binaries,
suggesting that they have truncated their disks. Evidence of dynamical clearing
has been seen in several binaries. Remarkably, PMS binaries of all separations
show evidence of circumstellar disks and continued accretion. This suggests
that the circumstellar disks are replenished from circumbinary disks or
envelopes. The frequent presence of disks suggests that planet formation can
occur in binary environments, and formation of planets in wide binaries is
already established by their discovery. Circumbinary disk masses around very
short period binaries are ample to form planetary systems such as our own. The
nature of planetary systems among the most common binaries, with separations
between 10 AU and 100 AU, is less clear given the observed reduction in disk
mass, though they may have disk masses adequate for the formation of
terrestrial-like planets.Comment: 32 pages, including 6 Postscript figures (TeX, uses psfig.sty); to
appear in "Protostars & Planets IV". Gif figures with captions and high-res
Postscript color figure available at
http://hven.swarthmore.edu/~jensen/preprints/ppiv.htm
Weakly bound states of polar molecules in bilayers
We investigate a system of two polarized molecules in a layered trap. The
molecules reside in adjacent layers and interact purely via the dipole-dipole
interaction. We determine the properties of the ground state of the system as a
function of the dipole moment and polarization angle. A bound state is always
present in the system and in the weak binding limit the bound state extends to
a very large distance and shows universal behavior.Comment: Presented at the 21st European Conference on Few-Body Problems in
Physics, Salamanca, Spain, 30 August - 3 September 201
The pre-main sequence spectroscopic binary UZ Tau East: improved orbital parameters and accretion phase dependence
We present radial-velocity measurements obtained using high- and
intermediate-resolution spectroscopic observations of the classical T Tauri
star UZ Tau East obtained from 1994 to 1996. We also provide measurements of
H equivalent widths and optical veiling. Combining our radial-velocity
data with those recently reported by Prato et al. (2002), we improve the
orbital elements for this spectroscopic binary. The orbital period is
18.9790.007 days and the eccentricity is e=0.14. We find variability in
the H emission and veiling, signposts of accretion, but at periastron
passage the accretion is not as clearly enhanced as in the case of the binary
DQ Tau. The difference in the behaviour of these two binaries is consistent
with the hydrodynamical models of accretion from circumbinary disks because UZ
Tau East has lower eccentricity than DQ Tau. It seems that enhanced periastron
accretion may occur only in systems with very high eccentricity (e0.5).Comment: accepted for publication in A&
Cluster-mining: An approach for determining core structures of metallic nanoparticles from atomic pair distribution function data
We present a novel approach for finding and evaluating structural models of
small metallic nanoparticles. Rather than fitting a single model with many
degrees of freedom, the approach algorithmically builds libraries of
nanoparticle clusters from multiple structural motifs, and individually fits
them to experimental PDFs. Each cluster-fit is highly constrained. The
approach, called cluster-mining, returns all candidate structure models that
are consistent with the data as measured by a goodness of fit. It is highly
automated, easy to use, and yields models that are more physically realistic
and result in better agreement to the data than models based on cubic
close-packed crystallographic cores, often reported in the literature for
metallic nanoparticles
Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe
Local symmetry breaking in complex materials is emerging as an important
contributor to materials properties but is inherently difficult to study. Here
we follow up an earlier structural observation of such a local symmetry broken
phase in the technologically important compound PbTe with a study of the
lattice dynamics using inelastic neutron scattering (INS). We show that the
lattice dynamics are responsive to the local symmetry broken phase, giving key
insights in the behavior of PbTe, but also revealing INS as a powerful tool for
studying local structure. The new result is the observation of the unexpected
appearance on warming of a new zone center phonon branch in PbTe. In a harmonic
solid the number of phonon branches is strictly determined by the contents and
symmetry of the unit cell. The appearance of the new mode indicates a crossover
to a dynamic lower symmetry structure with increasing temperature. No
structural transition is seen crystallographically but the appearance of the
new mode in inelastic neutron scattering coincides with the observation of
local Pb off-centering dipoles observed in the local structure. The observation
resembles relaxor ferroelectricity but since there are no inhomogeneous dopants
in pure PbTe this anomalous behavior is an intrinsic response of the system. We
call such an appearance of dipoles out of a non-dipolar ground-state
"emphanisis" meaning the appearance out of nothing. It cannot be explained
within the framework of conventional phase transition theories such as
soft-mode theory and challenges our basic understanding of the physics of
materials
Uncovering Extreme Nonlinear Dynamics in Solids Through Time-Domain Field Analysis
Time-domain analysis of harmonic fields with sub-cycle resolution is now
experimentally viable due to the emergence of sensitive, on-chip techniques for
petahertz-scale optical-field sampling. We demonstrate how such a time-domain,
field-resolved analysis uncovers the extreme nonlinear electron dynamics
responsible for high-harmonic generation within solids. Time-dependent density
functional theory was used to simulate harmonic generation from a solid-state
band-gap system driven by near- to mid-infrared waveforms. Particular attention
was paid to regimes where both intraband and interband emission mechanisms play
a critical role in shaping the nonlinear response. We show that a time-domain
analysis of the harmonic radiation fields identifies the interplay between
intra- and interband dynamical processes underlying the nonlinear light
generation. With further analysis, we show that changes to the dominant
emission regime can occur after only slight changes to the peak driving
intensity and central driving wavelength. Time-domain analysis of harmonic
fields also reveals, for the first time, the possibility of rapid changes in
the dominant emission mechanism within the temporal window of the driving pulse
envelope. Finally, we examine the experimental viability of performing
time-domain analysis of harmonic fields with sub-cycle resolution using
realistic parameters
The build-up of the colour-magnitude relation in galaxy clusters since z~0.8
Using galaxy clusters from the ESO Distant Cluster Survey, we study how the
distribution of galaxies along the colour-magnitude relation has evolved since
z~0.8. While red-sequence galaxies in all these clusters are well described by
an old, passively evolving population, we confirm our previous finding of a
significant evolution in their luminosity distribution as a function of
redshift. When compared to galaxy clusters in the local Universe, the high
redshift EDisCS clusters exhibit a significant "deficit" of faint red galaxies.
Combining clusters in three different redshift bins, and defining as `faint'
all galaxies in the range 0.4 > L/L* > 0.1, we find a clear decrease in the
luminous-to-faint ratio of red galaxies from z~0.8 to z~0.4. The amount of such
a decrease appears to be in qualitative agreement with predictions of a model
where the blue bright galaxies that populate the colour-magnitude diagram of
high redshift clusters, have their star formation suppressed by the hostile
cluster environment. Although model results need to be interpreted with
caution, our findings clearly indicate that the red-sequence population of
high-redshift clusters does not contain all progenitors of nearby red-sequence
cluster galaxies. A significant fraction of these must have moved onto the
red-sequence below z~0.8.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Inverse Scattering at a Fixed Quasi-Energy for Potentials Periodic in Time
We prove that the scattering matrix at a fixed quasi--energy determines
uniquely a time--periodic potential that decays exponentially at infinity. We
consider potentials that for each fixed time belong to in space. The
exponent 3/2 is critical for the singularities of the potential in space. For
this singular class of potentials the result is new even in the
time--independent case, where it was only known for bounded exponentially
decreasing potentials.Comment: In this revised version I give a more detailed motivation of the
class of potentials that I consider and I have corrected some typo
Hunting for millimeter flares from magnetic reconnection in pre-main sequence spectroscopic binaries
Recent observations of the low-mass pre-main sequence, eccentric
spectroscopic binaries DQ Tau and V773 Tau A reveal that their millimeter
spectrum is occasionally dominated by flares from non-thermal emission
processes. The transient activity is believed to be synchrotron in nature,
resulting from powerful magnetic reconnection events when the separate magnetic
structures of the binary components are capable of interacting and forced to
reorganize, typically near periastron. We conducted the first systematic study
of the millimeter variability toward a sample of 12 PMS spectroscopic binaries
with the aim to characterize the proliferation of flares amongst sources likely
to experience similar interbinary reconnection events. The source sample
consists of short-period, close-separation binaries that possess either a high
orbital eccentricity or a circular orbit. Using the MAMBO2 array on the IRAM
30m telescope, we carried out continuous monitoring at 1.25 mm over a 4-night
period during which all of the high-eccentricity binaries approached
periastron. We also obtained simultaneous optical VRI measurements, since a
strong link is often observed between stellar reconnection events and optical
brightenings. UZ Tau E is the only source to be detected at millimeter
wavelengths: it exhibited significant variation; it is also the only source to
undergo strong simultaneous optical variability. The binary possesses the
largest orbital eccentricity in the current sample, a predicted factor in
star-star magnetic interaction events. With orbital parameters and variable
accretion activity similar to DQ Tau, the millimeter behavior of UZ Tau E draws
many parallels to the DQ Tau model for colliding magnetospheres. However, on
the basis of our observations alone, we cannot determine whether the
variability is repetitive, or if it could also be due to variable free-free
emission in an ionized wind.Comment: 19 pages in referee format, 3 figures, 1 table, 3 on-line tables,
accepted for publication in Astronomy and Astrophysic
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