764 research outputs found
Hiding in the Shadows: Searching for Planets in Pre--transitional and Transitional Disks
Transitional and pre--transitional disks can be explained by a number of
mechanisms. This work aims to find a single observationally detectable marker
that would imply a planetary origin for the gap and, therefore, indirectly
indicate the presence of a young planet. N-body simulations were conducted to
investigate the effect of an embedded planet of one Jupiter mass on the
production of instantaneous collisional dust derived from a background
planetesimal disk. Our new model allows us to predict the dust distribution and
resulting observable markers with greater accuracy than previous work.
Dynamical influences from a planet on a circular orbit are shown to enhance
dust production in the disk interior and exterior to the planet orbit while
removing planetesimals from the the orbit itself creating a clearly defined
gap. In the case of an eccentric planet the gap opened by the planet is not as
clear as the circular case but there is a detectable asymmetry in the dust
disk.Comment: Accepted to ApJL 25th September 2013. 4 figures, 1 tabl
Isolation by genomic subtraction of subspecies-specific DNA probes from Verticillium dahliae
The vascular wilt pathogen Verticillium dahliae Kleb. has a very broad host range, including more than 200 different plant species in 45 families. Most isolates cannot be discriminated morphologically, and reliable, rapid methods for classifying and differentiating strains are needed. We have used a genomic subtraction method to isolate DNA probes that can be used to differentiate V. dahliae isolates and to investigate the genetic variation within this fungal species
Barra Fan Project : Geophysical operations report - project 92/02
There is significant geochemical, geological and biological interest in the source and fate of methane
in marine sediments. Where production rates are high in the near seabed sediments, or pathways
through the sediment column allow the migration of deep biogenic or thermogenic gas, methane may
escape into the bottom water where gas plumes may be detected by geophysical techniques. Such
plumes were potentially identified on seismic records obtained by BGS at the foot of the Barra Fan,
west of the Hebrides during surveys in 1985.
A multidisciplinary project has been established to examine geological, chemical and biological aspects
. of active gas seeps specifically in this Barra Fan area. The project is funded by the Natural
Environment Research Council and involyes scientists from British Geological Survey, Marine
Biological Association and Dunstaffnage Marine Laboratory.
The geological objectives of the project include: searching for and mapping shallow gas and methane
hydrate deposits and gas seepage areas on the continental slope east of the Rockall Trough and
defining their geological and oceanographic setting; determination of the source of the gas and
comparison of the physical and geotechnical properties of sediments at the seep sites with those from
the surrounding areas.
Survey methodology comprised: an initial survey of the area with seismic and sonar equipment to
locate seeps and gas charged sediments; sediment coring at identified control sites; photography of the
seabed; CTD profiles and subsampling of the cored samples for geological, chemical, physical,
microbiological and infaunal measurements. The wide range of equipment and large numbers of scientists required to undertake the various
components of this multidisciplinary project presented severe logistic and accommodation problems. It was therefore decided that the most cost efficient way of meeting the objectives of the project was
to undertake the geophysical component of the programme as a separate exercise. This was made
possible by extending the charter of the vessel mv Kommandor Michael which was already on contract
to BGS undertaking geophysical survey work in the Rockall Trough and Rockall Continental Margin
areas. The vessel was already fully mobilised and operational and requiring minimal transit time to
the project area.
A total of 17 lines were surveyed on 5 and 6 July 1992, representing a total of 201 line km.
Bathymetry and pinger data were acquired on all lines and deep-tow boomer information was acquired
concurrently on the first 11 lines. The weather throughout the survey period was excellent.
This summary operations report contains brief details of the survey including a daily log of events,
details of the lines surveyed and a description of the equipment used.
The financial contribution from Statoil towards the cost of the geophysical survey is gratefully
acknowledged
Hiding in the Shadows II: Collisional Dust as Exoplanet Markers
Observations of the youngest planets (1-10 Myr for a transitional disk)
will increase the accuracy of our planet formation models. Unfortunately,
observations of such planets are challenging and time-consuming to undertake
even in ideal circumstances. Therefore, we propose the determination of a set
of markers that can pre-select promising exoplanet-hosting candidate disks. To
this end, N-body simulations were conducted to investigate the effect of an
embedded Jupiter mass planet on the dynamics of the surrounding planetesimal
disk and the resulting creation of second generation collisional dust. We use a
new collision model that allows fragmentation and erosion of planetesimals, and
dust-sized fragments are simulated in a post process step including
non-gravitational forces due to stellar radiation and a gaseous protoplanetary
disk. Synthetic images from our numerical simulations show a bright double ring
at 850 m for a low eccentricity planet, whereas a high eccentricity planet
would produce a characteristic inner ring with asymmetries in the disk. In the
presence of first generation primordial dust these markers would be difficult
to detect far from the orbit of the embedded planet, but would be detectable
inside a gap of planetary origin in a transitional disk.Comment: Accepted for publication in Ap
Hazy Blue Worlds:A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots
We present a reanalysis (using the Minnaert limb-darkening approximation) of
visible/near-infrared (0.3 - 2.5 micron) observations of Uranus and Neptune
made by several instruments. We find a common model of the vertical aerosol
distribution that is consistent with the observed reflectivity spectra of both
planets, consisting of: 1) a deep aerosol layer with a base pressure > 5-7 bar,
assumed to be composed of a mixture of H2S ice and photochemical haze; 2) a
layer of photochemical haze/ice, coincident with a layer of high static
stability at the methane condensation level at 1-2 bar; and 3) an extended
layer of photochemical haze, likely mostly of the same composition as the
1-2-bar layer, extending from this level up through to the stratosphere, where
the photochemical haze particles are thought to be produced. For Neptune, we
find that we also need to add a thin layer of micron-sized methane ice
particles at ~0.2 bar to explain the enhanced reflection at longer
methane-absorbing wavelengths. We suggest that methane condensing onto the haze
particles at the base of the 1-2-bar aerosol layer forms ice/haze particles
that grow very quickly to large size and immediately 'snow out' (as predicted
by Carlson et al. 1988), re-evaporating at deeper levels to release their core
haze particles to act as condensation nuclei for H2S ice formation. In
addition, we find that the spectral characteristics of 'dark spots', such as
the Voyager-2/ISS Great Dark Spot and the HST/WFC3 NDS-2018, are well modelled
by a darkening or possibly clearing of the deep aerosol layer only.Comment: 58 pages, 23 figures, 4 table
Hazy Blue Worlds:A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots
We present a reanalysis (using the Minnaert limb-darkening approximation) of
visible/near-infrared (0.3 - 2.5 micron) observations of Uranus and Neptune
made by several instruments. We find a common model of the vertical aerosol
distribution that is consistent with the observed reflectivity spectra of both
planets, consisting of: 1) a deep aerosol layer with a base pressure > 5-7 bar,
assumed to be composed of a mixture of H2S ice and photochemical haze; 2) a
layer of photochemical haze/ice, coincident with a layer of high static
stability at the methane condensation level at 1-2 bar; and 3) an extended
layer of photochemical haze, likely mostly of the same composition as the
1-2-bar layer, extending from this level up through to the stratosphere, where
the photochemical haze particles are thought to be produced. For Neptune, we
find that we also need to add a thin layer of micron-sized methane ice
particles at ~0.2 bar to explain the enhanced reflection at longer
methane-absorbing wavelengths. We suggest that methane condensing onto the haze
particles at the base of the 1-2-bar aerosol layer forms ice/haze particles
that grow very quickly to large size and immediately 'snow out' (as predicted
by Carlson et al. 1988), re-evaporating at deeper levels to release their core
haze particles to act as condensation nuclei for H2S ice formation. In
addition, we find that the spectral characteristics of 'dark spots', such as
the Voyager-2/ISS Great Dark Spot and the HST/WFC3 NDS-2018, are well modelled
by a darkening or possibly clearing of the deep aerosol layer only.Comment: 58 pages, 23 figures, 4 table
Spectral determination of the colour and vertical structure of dark spots in Neptune's atmosphere
Previous observations of dark vortices in Neptune's atmosphere, such as
Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength
channels, which has hampered efforts to pinpoint their pressure level and what
makes them dark. Here, we present Very Large Telescope (Chile) MUSE
spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made
in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to
show that dark spots are caused by a darkening at short wavelengths (< 700 nm)
of a deep ~5-bar aerosol layer, which we suggest is the HS condensation
layer. A deep bright spot, named DBS-2019, is also visible on the edge of
NDS-2018, whose spectral signature is consistent with a brightening of the same
5-bar layer at longer wavelengths (> 700 nm). This bright feature is much
deeper than previously studied dark spot companion clouds and may be connected
with the circulation that generates and sustains such spots.Comment: 1 table. 3 figures. Nature Astronomy (2023
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