235 research outputs found
The History of Astrometry
The history of astrometry, the branch of astronomy dealing with the positions
of celestial objects, is a lengthy and complex chronicle, having its origins in
the earliest records of astronomical observations more than two thousand years
ago, and extending to the high accuracy observations being made from space
today. Improved star positions progressively opened up and advanced fundamental
fields of scientific enquiry, including our understanding of the scale of the
solar system, the details of the Earth's motion through space, and the
comprehension and acceptance of Newtonianism. They also proved crucial to the
practical task of maritime navigation. Over the past 400 years, during which
positional accuracy has improved roughly logarithmically with time, the
distances to the nearest stars were triangulated, making use of the extended
measurement baseline given by the Earth's orbit around the Sun. This led to
quantifying the extravagantly vast scale of the Universe, to a determination of
the physical properties of stars, and to the resulting characterisation of the
structure, dynamics and origin of our Galaxy. After a period in the middle
years of the twentieth century in which accuracy improvements were greatly
hampered by the perturbing effects of the Earth's atmosphere, ultra-high
accuracies of star positions from space platforms have led to a renewed advance
in this fundamental science over the past few years.Comment: 52 pages, 14 figures. To appear in The European Physical Journal:
Historical Perspectives on Contemporary Physic
Hipparcos: a Retrospective
The Hipparcos satellite was launched in 1989. It was the first, and remains
to date the only, attempt at performing large-scale astrometric measurements
from space. Hipparcos marked a fundamentally new approach to the field of
astrometry, revolutionising our knowledge of the positions, distances, and
space motions of the stars in the solar neighbourhood. In this retrospective, I
look back at the processes which led to the mission's acceptance, provide a
short summary of the underlying measurement principles and the experiment's
scientific achievements, and a conclude with a brief summary of its principal
legacy - the Gaia mission.Comment: European Astronomical Society Tycho Brahe Prize Lecture 2011 (18
pages, 4 figures
The Gaia inertial reference frame and the tilting of the Milky Way disk
While the precise relationship between the Milky Way disk and the symmetry
planes of the dark matter halo remains somewhat uncertain, a time-varying disk
orientation with respect to an inertial reference frame seems probable.
Hierarchical structure formation models predict that the dark matter halo is
triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30
muas/yr). These models also predict a time-dependent accretion of gas, such
that the angular momentum vector of the disk should be misaligned with that of
the halo. These effects, as well as tidal effects of the LMC, will result in
the rotation of the angular momentum vector of the disk population with respect
to the quasar reference frame. We assess the accuracy with which the positions
and proper motions from Gaia can be referred to a kinematically non-rotating
system, and show that the spin vector of the transformation from any rigid
self-consistent catalog frame to the quasi-inertial system defined by quasars
should be defined to better than 1 muas/yr. Determination of this inertial
frame by Gaia will reveal any signature of the disk orientation varying with
time, improve models of the potential and dynamics of the Milky Way, test
theories of gravity, and provide new insights into the orbital evolution of the
Sagittarius dwarf galaxy and the Magellanic Clouds.Comment: 16 pages; accepted for publication in Ap
The use of aerial photographs for estimating school sizes of cetaceans
ENGLISH: The accuracy and precision of dolphin school size estimates based on aerial photograph counts were examined using data collected on recent aerial and ship surveys. These estimates were found to be accurate during a 1979research cruise aboard a tuna purse-seiner; dolphin schools were photographed from the ship’s helicopter, encircled with the purse-seine, and then counted as the dolphins were released from the net. A comparison of photographic estimates with these counts indicated that the relationship was fairly close and gave no indication of significantly differing from 1:1. During a 1980 aerial study, photographic estimates from different schools, passes, and camera formats were compared and were found to be quite precise with a standard deviation of approximately 60/0 of school size. Photographic estimates were also compared with estimates made by aerial observers. Most observers tended to underestimate school size, particularly for large schools. The variability among observers was high, indicating that observers should be individually calibrated. SPANISH: Se examinó la exactitud y la precisión de las estimaciones de la magnitud de los cardúmenes de delfines basadas en el cálculo de las fotografías aéreas, usando los datos obtenidos en los últimos reconocimientos aéreos y de los barcos. En 1979, durante un crucero de investigación en un cerquero atunero, se encontró que estas estimaciones eran acertadas; se fotografiaron los cardúmenes de delfines desde un helicóptero del barco, cercados con la red y luego se contaron a medida que se libraban los delfines de la red. Una comparación de las estimaciones fotográficas con estos cálculos indicó que la relación era bastante aproximada y no hubo indicación que se diferenció significativamente de la razón 1:1. Durante un estudio aéreo en 1980, se compararon las estimaciones fotográficas de diferentes del cardúmenes, en los pases y los formatos de las cámaras y se encontró que eran bastante precisos, con una desviación normal de cerca del 60/0 de la magnitud cardumen. Se compararon también las estimaciones fotográficas con las estimaciones realizadas por los observadores aéreos. La mayoría de los observadores tienden a subestimar la magnitud de los cardúmenes, especialmente los cardúmenes grandes. La variabilidad entre los observadores fue elevada, lo que indica que se deben calibrar individualmente los datos de observadores.
(PDF contains 39 pages.
Astrometric exoplanet detection with Gaia
We provide a revised assessment of the number of exoplanets that should be
discovered by Gaia astrometry, extending previous studies to a broader range of
spectral types, distances, and magnitudes. Our assessment is based on a large
representative sample of host stars from the TRILEGAL Galaxy population
synthesis model, recent estimates of the exoplanet frequency distributions as a
function of stellar type, and detailed simulation of the Gaia observations
using the updated instrument performance and scanning law. We use two
approaches to estimate detectable planetary systems: one based on the S/N of
the astrometric signature per field crossing, easily reproducible and allowing
comparisons with previous estimates, and a new and more robust metric based on
orbit fitting to the simulated satellite data.
With some plausible assumptions on planet occurrences, we find that some
21,000 (+/-6000) high-mass (1-15M_J) long-period planets should be discovered
out to distances of ~500pc for the nominal 5-yr mission (including at least
1000-1500 around M dwarfs out to 100pc), rising to some 70,000 (+/-20,000) for
a 10-yr mission. We indicate some of the expected features of this exoplanet
population, amongst them ~25-50 intermediate-period (P~2-3yr) transiting
systems.Comment: 41 pages, 12 figures; accepted for publication in Ap
A Ring of Warm Dust in the HD 32297 Debris Disk
We report the detection of a ring of warm dust in the edge-on disk
surrounding HD 32297 with the Gemini-N/MICHELLE mid-infrared imager. Our
N'-band image shows elongated structure consistent with the orientation of the
scattered-light disk. The Fnu(11.2 um) = 49.9+/-2.1 mJy flux is significantly
above the 28.2+/-0.6 mJy photosphere. Subtraction of the stellar point spread
function reveals a bilobed structure with peaks 0.5"-0.6" from the star. An
analysis of the stellar component of the SED suggests a spectral type later
than A0, in contrast to commonly cited literature values. We fit
three-dimensional, single-size grain models of an optically thin dust ring to
our image and the SED using a Markov chain Monte Carlo algorithm in a Bayesian
framework. The best-fit effective grain sizes are submicron, suggesting the
same dust population is responsible for the bulk of the scattered light. The
inner boundary of the warm dust is located 0.5"-0.7" (~65 AU) from the star,
which is approximately cospatial with the outer boundary of the scattered-light
asymmetry inward of 0.5". The addition of a separate component of larger,
cooler grains that provide a portion of the 60 um flux improves both the
fidelity of the model fit and consistency with the slopes of the
scattered-light brightness profiles. Previous indirect estimates of the stellar
age (~30 Myr) indicate the dust is composed of debris. The peak vertical
optical depths in our models (~0.3-1 x 1e-2) imply that grain-grain collisions
likely play a significant role in dust dynamics and evolution. Submicron grains
can survive radiation pressure blow-out if they are icy and porous. Similarly,
the inferred warm temperatures (130-200 K) suggest that ice sublimation may
play a role in truncating the inner disk.Comment: ApJ accepted, 8 pages, 4 figure
A Search for Exozodiacal Dust and Faint Companions Near Sirius, Procyon, and Altair with the NICMOS Coronagraph
We observed Sirius, Altair, and Procyon with the NICMOS Coronagraph on the
Hubble Space Telescope to look for scattered light from exozodiacal dust and
faint companions within 10 AU from these stars. We did not achieve enough
dynamic range to surpass the upper limits set by IRAS on the amount of
exo-zodiacal dust in these systems, but we did set strong upper limits on the
presence of nearby late-type and sub-stellar companions.Comment: 10 pages, 4 figure
Challenges in the onboard oil condition monitoring
Engine oil is an essential component in modern engines and must fulfil a broad range of
duties. The engine oil condition is closely related to the engine performance and to the engine
reliability. Different effects during the engine operation cause the formation of contaminants reaching
into the oil and degrading the oil condition. Monitoring the oil condition with an onboard
sensor
could provide information about the current oil status. The available sensor data and their correlation
to oil contamination effects is further illustrated and examined. The major challenge to derive a
reliable oil condition statement from these data is also discussed in this paper
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