3,106 research outputs found
Bound states of 3He at the edge of a 4He drop on a cesium surface
We show that small amounts of 3He atoms, added to a 4He drop deposited on a
flat cesium surface at zero temperature, populate bound states localized at the
contact line. These edge states show up for drops large enough to develop well
defined surface and bulk regions together with a contact line, and they are
structurally different from the well-known Andreev states that appear at the
free surface and at the liquid-solid interface of films. We illustrate the
one-body density of 3He in a drop with 1000 4He atoms, and show that for
sufficiently large number of impurities, the density profiles spread beyond the
edge, coating both the curved drop surface and its flat base and eventually
isolating it from the substrate.Comment: 10 pages and 7 figures. Submitted to PR
Photometric Variability in the Ultracool Dwarf BRI 0021-0214: Possible Evidence for Dust Clouds
We report CCD photometric monitoring of the nonemission ultracool dwarf BRI
0021-0214 (M9.5) obtained during 10 nights in 1995 November and 4 nights in
1996 August, with CCD cameras at 1 m class telescopes on the observatories of
the Canary Islands. We present differential photometry of BRI 0021-0214, and we
report significant variability in the I-band light curve obtained in 1995. A
periodogram analysis finds a strong peak at a period of 0.84 day. This
modulation appears to be transient because it is present in the 1995 data but
not in the 1996 data. We also find a possible period of 0.20 day, which appears
to be present in both the 1995 and 1996 datasets. However, we do not find any
periodicity close to the rotation period expected from the spectroscopic
rotational broadening (< 0.14 day). BRI 0021-0214 is a very inactive object,
with extremely low levels of Halpha and X-ray emission. Thus, it is unlikely
that magnetically induced cool spots can account for the photometric
variability. The photometric variability of BRI 0021-0214 could be explained by
the presence of an active meteorology that leads to inhomogeneous clouds on the
surface. The lack of photometric modulation at the expected rotational period
suggests that the pattern of surface features may be more complicated than
previously anticipated.Comment: Accepted for publication in ApJ. 26 pages, 13 figures include
Proton tracking in a high-granularity Digital Tracking Calorimeter for proton CT purposes
Radiation therapy with protons as of today utilizes information from x-ray CT
in order to estimate the proton stopping power of the traversed tissue in a
patient. The conversion from x-ray attenuation to proton stopping power in
tissue introduces range uncertainties of the order of 2-3% of the range,
uncertainties that are contributing to an increase of the necessary planning
margins added to the target volume in a patient. Imaging methods and
modalities, such as Dual Energy CT and proton CT, have come into consideration
in the pursuit of obtaining an as good as possible estimate of the proton
stopping power. In this study, a Digital Tracking Calorimeter is benchmarked
for proof-of-concept for proton CT purposes. The Digital Tracking Calorimeteris
applied for reconstruction of the tracks and energies of individual high energy
protons. The presented prototype forms the basis for a proton CT system using a
single technology for tracking and calorimetry. This advantage simplifies the
setup and reduces the cost of a proton CT system assembly, and it is a unique
feature of the Digital Tracking Calorimeter. Data from the AGORFIRM beamline at
KVI-CART in Groningen in the Netherlands and Monte Carlo simulation results are
used to in order to develop a tracking algorithm for the estimation of the
residual ranges of a high number of concurrent proton tracks. The range of the
individual protons can at present be estimated with a resolution of 4%. The
readout system for this prototype is able to handle an effective proton
frequency of 1 MHz by using 500 concurrent proton tracks in each readout frame,
which is at the high end range of present similar prototypes. A future further
optimized prototype will enable a high-speed and more accurate determination of
the ranges of individual protons in a therapeutic beam.Comment: 21 pages, 8 figure
Path integral Monte Carlo simulation of helium at negative pressures
Path integral Monte Carlo (PIMC) simulations of liquid helium at negative
pressure have been carried out for a temperature range from the critical
temperature to below the superfluid transition. We have calculated the
temperature dependence of the spinodal line as well as the pressure dependence
of the isothermal sound velocity in the region of the spinodal. We discuss the
slope of the superfluid transition line and the shape of the dispersion curve
at negative pressures.Comment: 6 pages, 7 figures, submitted to Physical Review B Revised: new
reference, replaced figure
The spectroscopic binary system Gl 375. I. Orbital parameters and chromospheric activity
We study the spectroscopic binary system Gl 375. We employ medium resolution
echelle spectra obtained at the 2.15 m telescope at the Argentinian observatory
CASLEO and photometric observations obtained from the ASAS database. We
separate the composite spectra into those corresponding to both components. The
separated spectra allow us to confirm that the spectral types of both
components are similar (dMe3.5) and to obtain precise measurements of the
orbital period (P = 1.87844 days), minimum masses (M_1 sin^3 i = 0.35 M_sun and
M_2 sin^3 i =0.33 M_sun) and other orbital parameters. The photometric
observations exhibit a sinusoidal variation with the same period as the orbital
period. We interpret this as signs of active regions carried along with
rotation in a tidally synchronized system, and study the evolution of the
amplitude of the modulation in longer timescales. Together with the mean
magnitude, the modulation exhibits a roughly cyclic variation with a period of
around 800 days. This periodicity is also found in the flux of the Ca II K
lines of both components, which seem to be in phase. The periodic changes in
the three observables are interpreted as a sign of a stellar activity cycle.
Both components appear to be in phase, which implies that they are magnetically
connected. The measured cycle of approximately 2.2 years (800 days) is
consistent with previous determinations of activity cycles in similar stars.Comment: 10 pages, including 11 figures and 3 tables. Accepted for publication
in Astronomy & Astrophysic
Recommended from our members
Microwave signatures of ice hydrometeors from ground-based observations above Summit, Greenland
Multi-instrument, ground-based measurements provide unique and comprehensive
data sets of the atmosphere for a specific location over long periods of time
and resulting data compliment past and existing global satellite
observations. This paper explores the effect of ice hydrometeors on
ground-based, high-frequency passive microwave measurements and attempts to
isolate an ice signature for summer seasons at Summit, Greenland, from 2010
to 2013. Data from a combination of passive microwave, cloud radar,
radiosonde, and ceilometer were examined to isolate the ice signature at
microwave wavelengths. By limiting the study to a cloud liquid water path of
40 g m−2 or less, the cloud radar can identify cases where the
precipitation was dominated by ice. These cases were examined using liquid
water and gas microwave absorption models, and brightness temperatures were
calculated for the high-frequency microwave channels: 90, 150, and 225 GHz.
By comparing the measured brightness temperatures from the microwave
radiometers and the calculated brightness temperature using only gas and
liquid contributions, any residual brightness temperature difference is due
to emission and scattering of microwave radiation from the ice hydrometeors
in the column. The ice signature in the 90, 150, and 225 GHz channels for
the Summit Station summer months was isolated. This measured ice signature
was then compared to an equivalent brightness temperature difference
calculated with a radiative transfer model including microwave
single-scattering properties for several ice habits. Initial model results
compare well against the 4 years of summer season isolated ice signature in
the high-frequency microwave channels
Anomaly in the stability limit of liquid helium 3
We propose that the liquid-gas spinodal line of helium 3 reaches a minimum at
0.4 K. This feature is supported by our cavitation measurements. We also show
that it is consistent with extrapolations of sound velocity measurements.
Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar
behavior for the spinodal of water and related it to a change in sign of the
expansion coefficient alpha, i. e. a line of density maxima. Helium 3 exhibits
such a line at positive pressure. We consider its extrapolation to negative
pressure. Our discussion raises fundamental questions about the sign of alpha
in a Fermi liquid along its spinodal.Comment: 5 pages, 3 figure
- …