501 research outputs found
Flexible temperature probe for biological systems
Probe is sufficiently flexible so that it can be worn comfortably for long periods of time, but relatively rigid to permit easy insertion. Body and electrical leads of small thermistor are imbedded in flexible fluorosilicone matrix contained in vinyl plastic tubing
Preliminary Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology
For many years, astronomers have promised that the study of pulsating white
dwarfs would ultimately lead to useful information about the physics of matter
under extreme conditions of temperature and pressure. In this paper we finally
make good on that promise. Using observational data from the Whole Earth
Telescope and a new analysis method employing a genetic algorithm, we
empirically determine that the central oxygen abundance in the
helium-atmosphere variable white dwarf GD 358 is 84+/-3 percent. We use this
value to place preliminary constraints on the 12C(alpha,gamma)16O nuclear
reaction cross-section. More precise constraints will be possible with
additional detailed simulations. We also show that the pulsation modes of our
best-fit model probe down to the inner few percent of the stellar mass. We
demonstrate the feasibility of reconstructing the internal chemical profiles of
white dwarfs from asteroseismological data, and find an oxygen profile for GD
358 that is qualitatively similar to recent theoretical calculations.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 6
figures, 2 tables, uses emulateapj5.st
White Dwarfs In Ngc6397 And M4: Constraints On The Physics Of Crystallization
We explore the physics of crystallization in the dense Coulomb plasma of the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from Hubble Space Telescope photometry of the globular cluster M 4 and compare it with our results for proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with a binary mixture of carbon and oxygen crystallizing at a value of Gamma higher than the theoretical value for a One Component Plasma (OCP). We show that this result is in line with the latest Molecular Dynamics simulations for binary mixtures of C/O. We discuss implications for future work.Astronom
A Dark Spot on a Massive White Dwarf
We present the serendipitous discovery of eclipse-like events around the
massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We
selected J1529+2928 for time-series photometry based on its spectroscopic
temperature and surface gravity, which place it near the ZZ Ceti instability
strip. Instead of pulsations, we detect photometric dips from this white dwarf
every 38 minutes. Follow-up optical spectroscopy observations with Gemini
reveal no significant radial velocity variations, ruling out stellar and brown
dwarf companions. A disintegrating planet around this white dwarf cannot
explain the observed light curves in different filters. Given the short period,
the source of the photometric dips must be a dark spot that comes into view
every 38 min due to the rotation of the white dwarf. Our optical spectroscopy
does not show any evidence of Zeeman splitting of the Balmer lines, limiting
the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display
kG magnetic fields, such eclipse-like events should be common around white
dwarfs. We discuss the potential implications of this discovery on transient
surveys targeting white dwarfs, like the K2 mission and the Large Synoptic
Survey Telescope.Comment: ApJ Letters, in pres
Circadian, endocrine, and metabolic effects of prolonged bedrest: Two 56-day bedrest studies
Two bedrest studies of 56 days each have been conducted to evaluate the effects of prolonged bedrest on circadian synchrony and endocrine and metabolic function. Measurements included the pituitary-adrenal, thyroid, parathyroid, insulin-glucose-growth hormones, catecholamine excretion, body temperature, and heart rate. The results indicated that a rigorous regimen of exercise did not prevent the endocrine and metabolic effects of prolonged bedrest. Changes in circadian, endocrine, and metabolic functions in bedrest appear to be due to changes in hydrostatic pressure and lack of postural cues rather than to inactivity, confinement, or the bleeding schedule. Prolonged bedrest, particularly beyond 24 days, resulted in rhythm desynchronization in spite of well regulated light/dark cycles, temperature, humidity, activity, and meal times and meal composition and in increased lability of all endocrine parameter measured. It also resulted in an apparent insensitivity of the glucose response to insulin, of cortisol secretion to ACTH, and of growth hormone secretion to hypoglycemia
The search for ZZ Ceti stars in the original Kepler mission
We report the discovery of 42 white dwarfs in the original Kepler mission
field, including nine new confirmed pulsating hydrogen-atmosphere white dwarfs
(ZZ Ceti stars). Guided by the Kepler-INT Survey (KIS), we selected white dwarf
candidates on the basis of their U-g, g-r, and r-H_alpha photometric colours.
We followed up these candidates with high-signal-to-noise optical spectroscopy
from the 4.2-m William Herschel Telescope. Using ground-based, time-series
photometry, we put our sample of new spectroscopically characterized white
dwarfs in the context of the empirical ZZ Ceti instability strip. Prior to our
search, only two pulsating white dwarfs had been observed by Kepler.
Ultimately, four of our new ZZ Cetis were observed from space. These rich
datasets are helping initiate a rapid advancement in the asteroseismic
investigation of pulsating white dwarfs, which continues with the extended
Kepler mission, K2.Comment: 9 pages, 6 figures, accepted for publication in MNRA
The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry
The pulsating DA white dwarfs are the coolest degenerate stars that undergo
self-driven oscillations. Understanding their interior structure will help to
understand the previous evolution of the star. To this end, we report the
analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA
white dwarf star EC 14012-1446 acquired during four observing epochs in three
different years, including a coordinated three-site campaign. A total of 19
independent frequencies in the star's light variations together with 148
combination signals up to fifth order could be detected. We are unable to
obtain the period spacing of the normal modes and therefore a mass estimate of
the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d,
assuming the rotationally split modes are l=1. The pulsation modes of the star
undergo amplitude and frequency variations, in the sense that modes with higher
radial overtone show more pronounced variability and that amplitude changes are
always accompanied by frequency variations. Most of the second-order
combination frequencies detected have amplitudes that are a function of their
parent mode amplitudes, but we found a few cases of possible resonantly excited
modes. We point out the complications in the analysis and interpretation of
data sets of pulsating white dwarfs that are affected by combination
frequencies of the form f_A+f_B-f_C intruding into the frequency range of the
independent modes.Comment: 14 pages, 6 figures, 6 tables. MNRAS, in pres
Shear viscosity of degenerate electron matter
We calculate the partial electron shear viscosity limited by
electron-electron collisions in a strongly degenerate electron gas taking into
account the Landau damping of transverse plasmons. The Landau damping strongly
suppresses in the domain of ultrarelativistic degenerate electrons
and modifies its %asymptotic temperature behavior. The efficiency of the
electron shear viscosity in the cores of white dwarfs and envelopes of neutron
stars is analyzed.Comment: 16 pages, 4 figures, accepted to Journal of Physics
Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data
With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti
variables (pulsating hydrogen-atmosphere white dwarf stars) are severely
undersampled by long-cadence (29.42 minutes per exposure) K2 observations.
Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling
precision asteroseismology. We report the discovery of two new ZZ Cetis in
long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights
of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we
recover accurate pulsation frequencies for K2 signals that reflected 4-5 times
off the Nyquist with the full precision of over 70 days of monitoring (~0.01
muHz). In turn, the K2 observations enable us to select the correct peaks from
the alias structure of the ground-based signals caused by gaps in the
observations. We identify at least seven independent pulsation modes in the
light curves of each of these stars. For EPIC 220274129, we detect three
complete sets of rotationally split ell=1 (dipole mode) triplets, which we use
to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We
also detect two sub-Nyquist K2 signals that are likely combination (difference)
frequencies. We attribute our inability to match some of the K2 signals to the
ground-based data to changes in pulsation amplitudes between epochs of
observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC
220274129 near the middle of the ZZ Ceti instability strip, with Teff =
11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03
Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap
Intelligent chilled mirror humidity sensor
A new, intelligent, chilled mirror humidity instrument has been designed
for use on buoys and ships. The design goal is to make high quality dew point
temperature measurements for a period of up to one year from an unattended
platform, while consuming as little power as possible. Nominal system accuracy
is 0.3°C, and a measure of data quality is provided to indicate possible drift in
calibration. Energy consumption is typically 800 Joules per measurement; standby
power consumption is 0.05 watts. Control of the instrument is managed by an onboard
central processing unit which is programmable in BASIC, and communication
to an external data logger is provided through an RS232 compatible interface. This
report describes the preliminary sensor tests that led to this new design and provides
the complete technical description required for fabrication.Funding was provided by the Office of Naval Research
under contract Number N00014-84-C-0134,
and the National Science Foundation
through grant Number OCE87- 09614
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