148 research outputs found
Structure, bonding and morphology of hydrothermally synthesised xonotlite
The authors have systematically investigated the role of synthesis conditions upon the structure and morphology of xonotlite. Starting with a mechanochemically prepared, semicrystalline phase with Ca/Si=1, the authors have prepared a series of xonotlite samples hydrothermally, at temperatures between 200 and 250 degrees C. Analysis in each case was by X-ray photoelectron spectroscopy, environmental scanning electron microscopy and X-ray diffraction. The authors’ use of a much lower water/solid ratio has indirectly confirmed the ‘through solution’ mechanism of xonotlite formation, where silicate dissolution is a key precursor of xonotlite formation. Concerning the role of temperature, too low a temperature (~200 degrees C) fails to yield xonotlite or leads to increased number of structural defects in the silicate chains of xonotlite and too high a temperature (>250 degrees C) leads to degradation of the xonotlite structure, through leaching of interchain calcium. Synthesis duration meanwhile leads to increased silicate polymerisation due to diminishing of the defects in the silicate chains and more perfect crystal morphologies
Participation of women scientists in ESA solar system missions: A historical trend
We analyzed the participation of women scientists in 10 ESA (European Space Agency) Solar System missions over a period of 38 years. Being part of a spacecraft mission science team can be considered a proxy to measure the "success"in the field. Participation of women in PI (Principal Investigators) teams varied between 4% and 25 %, with several missions with no women as PI. The percentage of female scientists as Co-I (Co-Investigators) is always less than 16 %. This number is lower than the percentage of women in the International Astronomical Union from all ESA's Member State (24 %), which can give us an indication of the percentage of women in the field. We encountered many difficulties to gather the data for this study. The list of team members were not always easily accessible. An additional difficulty was to determine the percentage of female scientists in planetary science in Europe. We would like to encourage the planetary community as a whole, as well as international organizations, universities and societies to continuously gather statistics over many years. Detailed statistics are only the first step to closely monitor the development of achievement gaps and initiate measures to tackle potential causes of inequity, leading to gender inequalities in STEM careers
UBVRI Night sky brightness during sunspot maximum at ESO-Paranal
In this paper we present and discuss for the first time a large data set of
UBVRI night sky brightness measurements collected at ESO-Paranal from April
2000 to September 2001. A total of about 3900 images obtained on 174 different
nights with FORS1 were analysed using an automatic algorithm specifically
designed for this purpose. This led to the construction of an unprecedented
database that allowed us to study in detail a number of effects such as
differential zodiacal light contamination, airmass dependency, daily solar
activity and moonlight contribution. Particular care was devoted to the
investigation of short time scale variations and micro-auroral events. The
typical dark time night sky brightness values found for Paranal are similar to
those reported for other astronomical dark sites at a similar solar cycle
phase. The zenith-corrected values averaged over the whole period are 22.3,
22.6, 21.6 20.9 and 19.7 mag arcsec-2 in U, B, V, R and I respectively. In
particular, there is no evidence of light pollution either in the broadband
photometry or in the high-airmass spectra we have analysed. Finally, possible
applications for the exposure time calculators are discussed.Comment: Accepted for publication in A&A; 22 pages, 22 figures. Paper with
full resolution figures available at
http://www.eso.org/~fpatat/science/skybright/paperII.ps.g
Theoretical Limits on Extrasolar Terrestrial Planet Detection with Coronagraphs
Many high contrast coronagraph designs have recently been proposed. In this
paper, their suitability for direct imaging of extrasolar terrestrial planets
is reviewed. We also develop a linear-algebra based model of coronagraphy that
can both explain the behavior of existing coronagraphs and quantify the
coronagraphic performance limit imposed by fundamental physics. We find that
the maximum theoretical throughput of a coronagraph is equal to one minus the
non-aberrated non-coronagraphic PSF of the telescope. We describe how a
coronagraph reaching this fundamental limit may be designed, and how much
improvement over the best existing coronagraph design is still possible. Both
the analytical model and numerical simulations of existing designs also show
that this theoretical limit rapidly degrades as the source size is increased:
the ``highest performance'' coronagraphs, those with the highest throughput and
smallest Inner Working Angle (IWA), are the most sensitive to stellar angular
diameter. This unfortunately rules out the possibility of using a small IWA
(lambda/d) coronagraph for a terrestrial planet imaging mission.
Finally, a detailed numerical simulation which accurately accounts for
stellar angular size, zodiacal and exozodiacal light is used to quantify the
efficiency of coronagraph designs for direct imaging of extrasolar terrestrial
planets in a possible real observing program. We find that in the photon noise
limited regime, a 4m telescope with a theoretically optimal coronagraph is able
to detect Earth-like planets around 50 stars with 1hr exposure time per target
(assuming 25% throughput and exozodi levels similar to our solar system). We
also show that at least 2 existing coronagraph design can approach this level
of performance in the ideal monochromatic case considered in this study.Comment: Accepted for publication to ApJ Sup
The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (II): Measurement of Foreground Zodiacal Light
We present a measurement of the absolute surface brightness of the zodiacal
light (3900-5100A) toward a fixed extragalactic target at high ecliptic
latitude based on moderate resolution (~1.3A per pixel) spectrophotometry
obtained with the du Pont 2.5m telescope at Las Campanas Observatory in Chile.
This measurement and contemporaneous Hubble Space Telescope data from WFPC2 and
FOS comprise a coordinated program to measure the mean flux of the diffuse
extragalactic background light (EBL). The zodiacal light at optical wavelengths
results from scattering by interplanetary dust, so that the zodiacal light flux
toward any extragalactic target varies seasonally with the position of the
Earth. This measurement of zodiacal light is therefore relevant to the specific
observations (date and target field) under discussion. To obtain this result,
we have developed a technique that uses the strength of the zodiacal Fraunhofer
lines to identify the absolute flux of the zodiacal light in the
multiple-component night sky spectrum. Statistical uncertainties in the result
are 0.6% (1 sigma). However, the dominant source of uncertainty is systematic
errors, which we estimate to be 1.1% (1 sigma). We discuss the contributions
included in this estimate explicitly. The systematic errors in this result
contribute 25% in quadrature to the final error in our coordinated EBL
measurement, which is presented in the first paper of this series.Comment: Accepted for publication in ApJ, 22 pages using emulateapj.sty,
version with higher resolution figures available at
http://www.astro.lsa.umich.edu/~rab/publications.html or at
http://nedwww.ipac.caltech.edu/level5/Sep01/Bernstein2/frames.htm
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SMART-1 Impact Ground-based campaign
Based on predictions of impact magnitude and cloud ejecta dynamics, we organized a SMART-1 ground-based observation campaign to perform coordinated measurements of the impact. Results from the coordinated multi-site campaign will be discussed
Cometary Origin of the Zodiacal Cloud and Carbonaceous Micrometeorites
The zodiacal cloud is a thick circumsolar disk of small debris particles
produced by asteroid collisions and comets. Here, we present a zodiacal cloud
model based on the orbital properties and lifetimes of comets and asteroids,
and on the dynamical evolution of dust after ejection. The model is
quantitatively constrained by IRAS observations of thermal emission, but also
qualitatively consistent with other zodiacal cloud observations. We find that
85-95% of the observed mid-infrared emission is produced by particles from the
Jupiter-family comets (JFCs) and 10% by dust from long period comets.
Asteroidal dust is found to be present at 10%. We suggest that spontaneous
disruptions of JFCs, rather than the usual cometary activity driven by
sublimating volatiles, is the main mechanism that librates cometary particles
into the zodiacal cloud. Our results imply that JFC particles represent
85% of the total mass influx at Earth. Since their atmospheric entry
speeds are typically low (14.5 km s mean for D=100-200 m
with 12 km s being the most common case), many JFC grains
should survive frictional heating and land on the Earth's surface. This
explains why most micrometeorites collected in antarctic ice have primitive
carbonaceous composition. The present mass of the inner zodiacal cloud at 5
AU is estimated to be 1- g, mainly in D=100-200 m
particles. The inner zodiacal cloud should have been times brighter
during the Late Heavy Bombardment (LHB) epoch 3.8 Gyr ago, when the
outer planets scattered numerous comets into the inner solar system. The bright
debris disks with a large 24-m excess observed around mature stars may be
an indication of massive cometary populations existing in those systems
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