441 research outputs found
Modeling the physical and excitation conditions of the molecular envelope of NGC 7027
The link between the shaping of bipolar planetary nebulae and the mass
ejection activity of their central stars is still poorly understood.
Appropriately characterizing the evolution of the shells ejected during the
late stages of evolution is vital to gain insight into the mechanism of nebular
shaping.
Herschel/HIFI provides an invaluable tool by opening a new window from which
to probe warm molecular gas (~50-1000 K). We present a radiative-transfer,
spatio-kinematic modeling of the molecular envelope of the young planetary
nebula NGC 7027 in high- and low-J 12CO and 13CO transitions observed by
Herschel/HIFI and IRAM 30-m, and discuss the structure and dynamics of the
molecular envelope.
We have developed a code which, used along with the existing SHAPE software,
implements spatio-kinematic modeling with accurate non-LTE calculations of line
excitation and radiative transfer in molecular species. We have used this code
to build a relatively simple "russian doll" model of the molecular envelope of
NGC 7027.
The model nebula consists of four nested, mildly bipolar shells plus a pair
of high-velocity blobs. The innermost shell is the thinnest and shows a
significant jump in physical conditions (temperature, density, abundance and
velocity) with respect to the adjacent shell. This is a clear indication of a
shock front in the system. Each of the high-velocity blobs is divided into two
sections with different physical conditions. The presence of H2O in NGC 7027, a
C-rich nebula, is likely due to photo-induced chemistry from the hot central
star. The computed molecular mass of the nebula is 1.3 Msun, compatible with
estimates from previous works.Comment: 11 pages, 3 figures. Accepted by Astronomy & Astrophysics on July
3rd, 201
Accurate Wavenumbers for Mid-Infrared Fine-Structure Lines
We present accurate new wavenumbers for a set of 13 mid-infrared
fine-structure lines. The wavenumbers were determined from observations of the
planetary nebula NGC 7027 and of the red supergiant Alpha Scorpii. Most of the
new wavenumbers are good to within 0.0025%, or 8 km/s. We provide details on
the measurements and present an analysis of the errors. In addition, we present
the first observations of hyperfine splitting in the [Na IV] 1106 cm-1 line.Comment: 12 pages text, 2 postscript figures, uses AASTeX macros, figures are
gzipped and uuencode
Continuous-time spike-based reinforcement learning for working memory tasks
As the brain purportedly employs on-policy reinforcement learning compatible with SARSA learning, and most interesting cognitive tasks require some form of memory while taking place in continuous-time, recent work has developed plausible reinforcement learning schemes that are compatible with these requirements. Lacking is a formulation of both computation and learning in terms of spiking neurons. Such a formulation creates both a closer mapping to biology, and also expresses such learning in terms of asynchronous and sparse neural computation. We present a spiking neural network with memory that learns cognitive tasks in continuous time. Learning is biologically plausibly implemented using the AuGMeNT framework, and we show how separate spiking forward and feedback networks suffice for learning the tasks just as fast the analog CT-AuGMeNT counterpart, while computing efficiently using very few spikes: 1–20 Hz on average
Synchronisation in networks of delay-coupled type-I excitable systems
We use a generic model for type-I excitability (known as the SNIPER or SNIC
model) to describe the local dynamics of nodes within a network in the presence
of non-zero coupling delays. Utilising the method of the Master Stability
Function, we investigate the stability of the zero-lag synchronised dynamics of
the network nodes and its dependence on the two coupling parameters, namely the
coupling strength and delay time. Unlike in the FitzHugh-Nagumo model (a model
for type-II excitability), there are parameter ranges where the stability of
synchronisation depends on the coupling strength and delay time. One important
implication of these results is that there exist complex networks for which the
adding of inhibitory links in a small-world fashion may not only lead to a loss
of stable synchronisation, but may also restabilise synchronisation or
introduce multiple transitions between synchronisation and desynchronisation.
To underline the scope of our results, we show using the Stuart-Landau model
that such multiple transitions do not only occur in excitable systems, but also
in oscillatory ones.Comment: 10 pages, 9 figure
The cooling of atomic and molecular gas in DR21
We present an overview of a high-mass star formation region through the major
(sub-)mm, and far-infrared cooling lines to gain insight into the physical
conditions and the energy budget of the molecular cloud. We used the KOSMA 3m
telescope to map the core () of the Galactic star forming region
DR 21/DR 21 (OH) in the Cygnus X region in the two fine structure lines of
atomic carbon CI and four mid- transitions of CO and CO, and CS
J=7\TO6. These observations have been combined with FCRAO J=1\TO0
observations of CO and CO. Five positions, including DR21, DR21
(OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the
\OI 63 and 145 m lines, the \CII 158 m line, and four high- CO
lines. We discuss the intensities and line ratios at these positions and apply
Local Thermal Equilibrium (LTE) and non-LTE analysis methods in order to derive
physical parameters such as masses, densities and temperatures. The CO line
emission has been modeled up to J=20. From non-LTE modeling of the low- to
high- CO lines we identify two gas components, a cold one at temperatures of
T_\RM{kin}\sim 30-40 K, and one with T_\RM{kin}\sim 80-150 K at a local
clump density of about n(H) cm. While the cold
quiescent component is massive containing typically more than 94 % of the mass,
the warm, dense, and turbulent gas is dominated by mid- and high- CO line
emission and its large line widths. The medium must be clumpy with a
volume-filling of a few percent. The CO lines are found to be important for the
cooling of the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the
UV-heated source DR21, the gas cooling is dominated by line emission of atomic
oxygen and of CO
Reef Cover, a coral reef classification for global habitat mapping from remote sensing
Coral reef management and conservation stand to benefit from improved high-resolution global mapping. Yet classifications underpinning large-scale reef mapping to date are typically poorly defined, not shared or region-specific, limiting end-users’ ability to interpret outputs. Here we present Reef Cover, a coral reef geomorphic zone classification, developed to support both producers and end-users of global-scale coral reef habitat maps, in a transparent and version-based framework. Scalable classes were created by focusing on attributes that can be observed remotely, but whose membership rules also reflect deep knowledge of reef form and functioning. Bridging the divide between earth observation data and geo-ecological knowledge of reefs, Reef Cover maximises the trade-off between applicability at global scales, and relevance and accuracy at local scales. Two case studies demonstrate application of the Reef Cover classification scheme and its scientific and conservation benefits: 1) detailed mapping of the Cairns Management Region of the Great Barrier Reef to support management and 2) mapping of the Caroline and Mariana Island chains in the Pacific for conservation purposes
Herschel / HIFI observations of CO, H2O and NH3 in Mon R2
Context. Mon R2 is the only ultracompact HII region (UCHII) where the
associated photon-dominated region (PDR) can be resolved with Herschel. Due to
its brightness and proximity, it is the best source to investigate the
chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to
estimate the abundance of H2O and NH3 in this region and investigate their
origin. Methods. We present new observations obtained with HIFI and the
IRAM-30m telescope. Using a large velocity gradient approach, we model the line
intensities and derive an average abundance of H2O and NH3 across the region.
Finally, we model the line profiles with a non-local radiative transfer model
and compare these results with the abundance predicted by the Meudon PDR code.
Results. The variations of the line profiles and intensities indicate complex
geometrical and kinematical patterns. The H2O lines present a strong absorption
at the ambient velocity and emission in high velocity wings towards the HII
region. The spatial distribution of the o-H2^18O line shows that the its
emission arises in the PDR surrounding the HII region. By modeling the o-H2^18O
emission we derive a mean abundance of o-H2O of ~10^-8 relative to H2. The
ortho-H2O abundance is however larger, ~1x10^-7, in the high velocity wings.
Possible explanations for this larger abundance include an expanding hot PDR
and/or an outflow. Ammonia seems to be present only in the envelope with an
average abundance of ~2x10^-9 relative to H2. Conclusions. The Meudon PDR code
can account for the measured water abundance in the high velocity gas as long
as we assume that it originates from a <1 mag hot expanding layer of the PDR,
i.e. that the outflow has only a minor contribution to this emission. To
explain the abundances in the rest of the cloud the molecular freeze out and
grain surface chemistry would need to be included.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in A&A.
Abstract shortened. Updated references, language editing applied in v
The spine of the swan: A Herschel study of the DR21 ridge and filaments in Cygnus X
In order to characterise the cloud structures responsible for the formation
of high-mass stars, we present Herschel observations of the DR21 environment.
Maps of the column density and dust temperature unveil the structure of the
DR21 ridge and several connected filaments. The ridge has column densities
larger than 1e23/cm^2 over a region of 2.3 pc^2. It shows substructured column
density profiles and branching into two major filaments in the north. The
masses in the studied filaments range between 130 and 1400 Msun whereas the
mass in the ridge is 15000 Msun. The accretion of these filaments onto the DR21
ridge, suggested by a previous molecular line study, could provide a continuous
mass inflow to the ridge. In contrast to the striations seen in e.g., the
Taurus region, these filaments are gravitationally unstable and form cores and
protostars. These cores formed in the filaments potentially fall into the
ridge. Both inflow and collisions of cores could be important to drive the
observed high-mass star formation. The evolutionary gradient of star formation
running from DR21 in the south to the northern branching is traced by
decreasing dust temperature. This evolution and the ridge structure can be
explained by two main filamentary components of the ridge that merged first in
the south.Comment: 8 pages, 5 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
How Much Shallow Coral Habitat Is There on the Great Barrier Reef?
Australia’s Great Barrier Reef (GBR) is a globally unique and precious national resource; however, the geomorphic and benthic composition and the extent of coral habitat per reef are greatly understudied. However, this is critical to understand the spatial extent of disturbance impacts and recovery potential. This study characterizes and quantifies coral habitat based on depth, geomorphic and benthic composition maps of more than 2164 shallow offshore GBR reefs. The mapping approach combined a Sentinel-2 satellite surface reflectance image mosaic and derived depth, wave climate, reef slope and field data in a random-forest machine learning and object-based protocol. Area calculations, for the first time, incorporated the 3D characteristic of the reef surface above 20 m. Geomorphic zonation maps (0–20 m) provided a reef extent estimate of 28,261 km2 (a 31% increase to current estimates), while benthic composition maps (0–10 m) estimated that ~10,600 km2 of reef area (~57% of shallow offshore reef area) was covered by hard substrate suitable for coral growth, the first estimate of potential coral habitat based on substrate availability. Our high-resolution maps provide valuable information for future monitoring and ecological modeling studies and constitute key tools for supporting the management, conservation and restoration efforts of the GBR
Strong CH+ J=1-0 emission and absorption in DR21
We report the first detection of the ground-state rotational transition of
the methylidyne cation CH+ towards the massive star-forming region DR21 with
the HIFI instrument onboard the Herschel satellite. The line profile exhibits a
broad emission line, in addition to two deep and broad absorption features
associated with the DR21 molecular ridge and foreground gas. These observations
allow us to determine a CH+ J=1-0 line frequency of 835137 +/- 3 MHz, in good
agreement with a recent experimental determination. We estimate the CH+ column
density to be a few 1e13 cm^-2 in the gas seen in emission, and > 1e14 cm^-2 in
the components responsible for the absorption, which is indicative of a high
line of sight average abundance [CH+]/[H] > 1.2x10^-8. We show that the CH+
column densities agree well with the predictions of state-of-the-art C-shock
models in dense UV-illuminated gas for the emission line, and with those of
turbulent dissipation models in diffuse gas for the absorption lines.Comment: Accepted for publication in A&
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