3,404 research outputs found
Leadership behaviors and personal transitions that occur in the lives of pastors who have led churches through significant growth
https://place.asburyseminary.edu/ecommonsatsdissertations/1418/thumbnail.jp
Solar-insolation-induced changes in the coma morphology of comet 67P/Churyumov-Gerasimenko. Optical monitoring with the Nordic Optical Telescope
Context. 67P/Churyumov-Gerasimenko (67P/C-G) is a short-period Jupiter family comet with an orbital period of 6.55 years. Being
the target comet of ESA’s Rosetta mission, 67P/C-G has become one of the most intensively studied minor bodies of the Solar System.
The Rosetta Orbiter and the Philae Lander have brought us unique information about the structure and activity of the comet nucleus,
as well as its activity along the orbit, composition of gas, and dust particles emitted into the coma. However, as Rosetta stayed in very
close proximity to the cometary nucleus (less than 500 km with a few short excursions reaching up to 1500 km), it could not see the
global picture of a coma at the scales reachable by telescopic observations (103 - 105 km).
Aims. In this work we aim to connect in-situ observations made by Rosetta with the morphological evolution of the coma structures monitored by the ground-based observations. In particular, we concentrate on causal relationships between the coma morphology and evolution observed with the Nordic Optical Telescope (NOT) in the Canary Islands, and the seasonal changes of the insolation and the activity of the comet observed by the Rosetta instruments.
Methods. Comet 67P/C-G was monitored with the NOT in imaging mode in two colors. Imaging optical observations were performed roughly on a weekly basis, which provides good coverage of short- and long-term variability. With the three dimensional modeling of the coma produced by active regions on the Southern Hemisphere, we aim to qualify the observed morphology by connecting it to the activity observed by Rosetta.
Results. During our monitoring program, we detected major changes in the coma morphology of comet 67P/C-G. These were longterm and long-lasting changes. They do not represent any sudden outburst or short transient event, but are connected to seasonal changes of the surface insolation and the emergence of new active regions on the irregular shaped comet nucleus. We have also found significant deviations in morphological changes from the prediction models based on previous apparitions of 67P/C-G, like the time delay of the morphology changes and the reduced activity in the Northern Hemisphere. According to our modeling of coma structures and geometry of observations, the changes are clearly connected with the activity in the Southern Hemisphere observed by the Rosetta spacecraft
The Refractory-to-Ice Mass Ratio in Comets
We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc
Ground-based monitoring of comet 67P/Churyumov-Gerasimenko gas activity throughout the <i>Rosetta</i> mission
Simultaneously to the ESA Rosetta mission, a world-wide ground-based campaign provided measurements of the large scale activity of comet 67P/Churyumov-Gerasimenko through measurement of optically active gas species and imaging of the overall dust coma. We present more than two years of observations performed with the FORS2 low resolution spectrograph at the VLT, TRAPPIST, and ACAM at the WHT. We focus on the evolution of the CN production, as a tracer of the comet activity. We find that it is asymmetric with respect to perihelion and different from that of the dust. The CN emission is detected for the first time at 1.34 au pre-perihelion and production rates then increase steeply to peak about two weeks after perihelion at (1.00±0.10) ×1025 molecules s−1, while the post-perihelion decrease is more shallow. The evolution of the comet activity is strongly influenced by seasonal effects, with enhanced CN production when the Southern hemisphere is illuminated
Using Kepler transit observations to measure stellar spot belt migration rates
Planetary transits provide a unique opportunity to investigate the surface
distributions of star spots. Our aim is to determine if, with continuous
observation (such as the data that will be provided by the Kepler mission), we
can in addition measure the rate of drift of the spot belts. We begin by
simulating magnetic cycles suitable for the Sun and more active stars,
incorporating both flux emergence and surface transport. This provides the
radial magnetic field distribution on the stellar surface as a function of
time. We then model the transit of a planet whose orbital axis is misaligned
with the stellar rotation axis. Such a planet could occult spots at a range of
latitudes. This allows us to complete the forward modelling of the shape of the
transit lightcurve. We then attempt the inverse problem of recovering spot
locations from the transit alone. From this we determine if transit lightcurves
can be used to measure spot belt locations as a function of time. We find that
for low-activity stars such as the Sun, the 3.5 year Kepler window is
insufficient to determine this drift rate. For more active stars, it may be
difficult to distinguish subtle differences in the nature of flux emergence,
such as the degree of overlap of the "butterfly wings". The rate and direction
of drift of the spot belts can however be determined for these stars. This
would provide a critical test of dynamo theory.Comment: 5 pages. Accepted for publication in Monthly Notices of the Royal
Astronomical Society Letter
Elite Scientists and the Global Brain Drain
There are signs – one is world university league tables – that people increasingly think globally when choosing the university in which they wish to work and study. This paper is an exploration of data on the international brain drain. We study highly-cited physicists, highly-cited bio-scientists, and assistant professors of economics. First, we demonstrate that talented researchers are being systematically funnelled into a small number of countries. Among young economists in the top American universities, for example, 75% did their undergraduate degree outside the United States. Second, the extent of the elite brain drain is considerable. Among the world’s top physicists, nearly half no longer work in the country in which they were born. Third, the USA and Switzerland are per capita the largest net-importers of elite scientists. Fourth, we estimate the migration ‘funnelling coefficient’ at approximately 0.2 (meaning that 20% of top researchers tend to leave their country at each professional stage). Fifth, and against our prior expectations, the productivity of top scientists, as measured by the Hirsch h-index, is similar between the elite movers and stayers. Thus it is apparently not true that it is disproportionately the very best people who emigrate. Sixth, there is extreme clustering of ISI Highly Cited Researchers into particular fields in different universities. Seventh, we debate the questions: are the brain drain and this kind of funnelling good or bad for the world, and how should universities and governments respond?
- …