555 research outputs found
Absolute and differential measurement of water vapor supersaturation using a commercial thin-film sensor
We describe a technique for measuring the water vapor supersaturation of normal air over a temperature range of –40<~T<~0 °C. The measurements use an inexpensive commercial hygrometer which is based on a thin-film capacitive sensor. The time required for the sensor to reach equilibrium was found to increase exponentially with decreasing sensor temperature, exceeding 2 min for T = –30 °C; however, the water vapor sensitivity of the device remained high down to this temperature. After calibrating our measurement procedure, we found residual scatter in the data corresponding to an uncertainty in the absolute water vapor pressure of about ±15%. This scatter was due mainly to long-term drift, which appeared to be intrinsic to the capacitive thin-film sensor. The origin of this drift is not clear, but it effectively limits the applicability of this instrument for absolute measurements. We also found, however, that the high sensitivity of the thin-film sensor makes it rather well suited for differential measurements. By comparing supersaturated and saturated air at the same temperature we obtained a relative measurement uncertainty of about ±1.5%, an order of magnitude better than the absolute measurements
A basic lock-in amplifier experiment for the undergraduate laboratory
We describe a basic experiment for the undergraduate laboratory that demonstrates aspects of both, the science and the art of precision electronic measurements. The essence of the experiment is to measure the resistance of a small length of brass-wire to high accuracy using a simple voltage divider and a lock-in amplifier. By performing the measurement at different frequencies and different drive currents, one observes various random noise sources and systematic measurement effects
High-frequency modes in solar-like stars
p-mode oscillations in solar-like stars are excited by the outer convection
zone in these stars and reflected close to the surface. The p-modes are trapped
inside an acoustic cavity, but the modes only stay trapped up to a given
frequency (known as the acoustic cut-off frequency) as modes with larger
frequencies are generally not reflected at the surface. This means that modes
with frequency larger than the acoustic cut-off frequency must be traveling
waves. The high-frequency modes may provide information about the physics in
the outer layers of the stars and the excitation source and are therefore
highly interesting as it is the estimation of these two phenomena that causes
some of the largest uncertainties when calculating stellar oscillations.
High-frequency modes have been detected in the Sun, beta Hydri and in alpha Cen
A & B by smoothing the so-called echelle diagram and the large frequency
separation as a function of frequency have been estimated. The large frequency
separation has been compared with a simple model of the acoustic cavity which
suggests that the reflectivity of the photosphere is larger at high frequency
than predicted by standard models of the solar atmosphere and that the depth of
the excitation source is larger than what has been estimated by other models
and might depend on the order n and degree l of the modes.Comment: 11 pages, 5 figures. Accepted for publication in MNRA
Numerical computations of facetted pattern formation in snow crystal growth
Facetted growth of snow crystals leads to a rich diversity of forms, and
exhibits a remarkable sixfold symmetry. Snow crystal structures result from
diffusion limited crystal growth in the presence of anisotropic surface energy
and anisotropic attachment kinetics. It is by now well understood that the
morphological stability of ice crystals strongly depends on supersaturation,
crystal size and temperature. Until very recently it was very difficult to
perform numerical simulations of this highly anisotropic crystal growth. In
particular, obtaining facet growth in combination with dendritic branching is a
challenging task. We present numerical simulations of snow crystal growth in
two and three space dimensions using a new computational method recently
introduced by the authors. We present both qualitative and quantitative
computations. In particular, a linear relationship between tip velocity and
supersaturation is observed. The computations also suggest that surface energy
effects, although small, have a larger effect on crystal growth than previously
expected. We compute solid plates, solid prisms, hollow columns, needles,
dendrites, capped columns and scrolls on plates. Although all these forms
appear in nature, most of these forms are computed here for the first time in
numerical simulations for a continuum model.Comment: 12 pages, 28 figure
Amplitude modulation of solar p-modes by surface magnetic fields
Context.It is known from Doppler velocity measurements that the amplitudes of solar p-modes are modulated by strong photospheric magnetic field.
Aims.The aim of this paper is to investigate amplitude modulation by model surface magnetic fields.
Methods.Linearised magnetohydrodynamics equations, in the absence of gravity, are used to derive the inhomogeneous wave equation which is then solved using the Born Approximation.
Results.The amount of modulation depends on the plasma beta, the distance from the magnetic region and the wavenumber. It is also found that the direction of observation could also have an effect on the amount of modulation. Finally, the applicability of the findings to the observational data suggests that the modulation depends on the properties of the magnetic field region and measuring it is an un-contaminating probe for the magnetic field
Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing
We present the results of two-band CCD photometric monitoring of the
gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La
Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 -
September 1999) and in the Johnson V-band (October 1998 to December 2001), were
analyzed using three different photometric methods. The light-curves obtained
with all methods show variations, with a peak-to-peak amplitude of 0.14
magnitude in . Although it was not possible to measure the time delay
between the two lensed QSO images, the brighter component displays possible
evidence for microlensing: it becomes bluer as it gets brighter, as expected
under the assumption of differential magnification of a quasar accretion diskComment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 7
figure
QSO 2237+0305 VR light curves from Gravitational Lenses International Time Project optical monitoring
We present VR observations of QSO 2237+0305 conducted by the GLITP
collaboration from 1999 October 1 to 2000 February 3. The observations were
made with the 2.56 m Nordic Optical Telescope at Roque de los Muchachos
Observatory, La Palma (Spain). The PSF fitting method and an adapted version of
the ISIS subtraction method have been used to derive the VR light curves of the
four components (A-D) of the quasar. The mean errors range in the intervals
0.01-0.04 mag (PSF fitting) and 0.01-0.02 mag (ISIS subtraction), with the
faintest component (D) having the largest uncertainties. We address the
relatively good agreement between the A-D light curves derived using different
filters, photometric techniques, and telescopes. The new VR light curves of
component A extend the time coverage of a high magnification microlensing peak,
which was discovered by the OGLE team.Comment: 15 pages, 3 figures, ApJ accepted (Feb 19
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