405 research outputs found
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
Low Hanging Fruit: Use of Virtual Reality Driving Simulation in Department of Motor Vehicles to Assess Minimal Competence of Novice Drivers
Nationally, Departments of Motor Vehicles (DMV) license novice drivers based in part on on-road assessments. Intuitively it is assumed that such assessments are fair, reliable and valid measures of minimal driving competency. Upon further reflection, this would be difficult, given the subjectivity of a huge range of driving examiners that approach this assessment with different training backgrounds, life distractions and biases from examination to examination, the different road, traffic, lighting and weather conditions from one examination and DMV center to the next, and the minimal driving challenges in such assessments. For example, a typical on-road test involves only a 4 mile road segment with 2 left turns, 4 right turns, 1 lane change, pulling into a turn lane, and 1 speed limit change. It does not include highway driving nor defensive driving maneuvers. Additionally, such on-road assessments are both potentially dangerous and time demanding/expensive. A less expensive, safer, more challenging, objective, reliable, and valid procedure may be the use of Virtual Reality Driving Simulation (VRDS) that administers consistent and more extensive driving challenges to all examinees, which is evaluated in an objective manner based on normative data from current safe drivers. This presentation describes the experience and presents the data from a project where VRDSs were set up in two DMV facilities and a Research facility. The goals of this project were to determine whether VRDS assessments are just as reliable, discriminating and acceptable to the public as on-road assessments, and whether performance on the simulator predicts future on-road driving mishaps
Spectral line survey of the ultracompact HII region Mon R2
Ultracompact (UC) HII regions constitute one of the earliest phases in the
formation of a massive star and are characterized by extreme physical
conditions (Go>10^5 Habing field and n>10^6 cm^-3). The UC HII Mon R2 is the
closest one and therefore an excellent target to study the chemistry in these
complex regions.
We carried out a 3mm and 1mm spectral survey using the IRAM 30-m telescope
towards three positions that represent different physical environments in Mon
R2: (i) the ionization front (IF) at (0",0"); two peaks in the molecular cloud
(ii) MP1 at the offset (+15",-15") and (iii) MP2 at the farther offset
(0",40"). In addition, we carried out extensive modeling to explain the
chemical differences between the three observed regions.
We detected more than thirty different species. We detected SO+ and C4H
suggesting that UV radiation plays an important role in the molecular chemistry
of this region. We detected the typical PDR molecules CN, HCN, HCO, C2H, and
c-C3H2. While the IF and the MP1 have a chemistry similar to that found in high
UV field and dense PDRs like the Orion Bar, the MP2 is more similar to lower
UV/density PDRs like the Horsehead nebula.
We also detected complex molecules that are not usually found in PDRs (CH3CN,
H2CO, HC3N, CH3OH and CH3C2H). Sulfur compounds CS, HCS+, C2S, H2CS, SO and SO2
and the deuterated species DCN and C2D were also identified. [DCN]/[HCN]=0.03
and [C2D]/[C2H]=0.05, are among the highest in warm regions.
Our results show that the high UV/dense PDRs present a different chemistry
from that of the low UV case. Abundance ratios like [CO+]/[HCO+] or
[HCO]/[HCO+] are good diagnostics to differentiate between them. In Mon R2 we
have the two classes of PDRs, a high UV PDR towards the IF and the adjacent
molecular bar and a low-UV PDR which extends towards the north-west following
the border of the cloud.Comment: 31 page
Inducing Metallicity in Graphene Nanoribbons via Zero-Mode Superlattices
The design and fabrication of robust metallic states in graphene nanoribbons
(GNRs) is a significant challenge since lateral quantum confinement and
many-electron interactions tend to induce electronic band gaps when graphene is
patterned at nanometer length scales. Recent developments in bottom-up
synthesis have enabled the design and characterization of atomically-precise
GNRs, but strategies for realizing GNR metallicity have been elusive. Here we
demonstrate a general technique for inducing metallicity in GNRs by inserting a
symmetric superlattice of zero-energy modes into otherwise semiconducting GNRs.
We verify the resulting metallicity using scanning tunneling spectroscopy as
well as first-principles density-functional theory and tight binding
calculations. Our results reveal that the metallic bandwidth in GNRs can be
tuned over a wide range by controlling the overlap of zero-mode wavefunctions
through intentional sublattice symmetry-breaking.Comment: The first three authors listed contributed equall
Neural distinctiveness of fatigue and low sleep quality in multiple sclerosis
Background and purpose
Fatigue and low sleep quality in multiple sclerosis (MS) are closely related symptoms. Here, the associations between the brain's functional connectivity (FC) and fatigue and low sleep quality were investigated to determine the degree of neural distinctiveness of these symptoms.
Method
A hundred and four patients with relapsing–remitting MS (age 38.9 ± 10.2 years, 66 females) completed the Modified Fatigue Impact Scale and the Pittsburgh Sleep Quality Index and underwent resting-state functional magnetic resonance imaging. FC was analyzed using independent-component analysis in sensorimotor, default-mode, fronto-parietal and basal-ganglia networks. Multiple linear regression models allowed us to test the association between FC and fatigue and sleep quality whilst controlling for one another as well as for demographic, disease-related and imaging variables.
Results
Higher fatigue correlated with lower sleep quality (r = 0.54, p < 0.0001). Higher fatigue was associated with lower FC of the precentral gyrus in the sensorimotor network, the precuneus in the posterior default-mode network and the superior frontal gyrus in the left fronto-parietal network, independently of sleep quality. Lower sleep quality was associated with lower FC of the left intraparietal sulcus in the left fronto-parietal network, independently of fatigue. Specific associations were found between fatigue and the sensorimotor network's global FC and between low sleep quality and the left fronto-parietal network's global FC.
Conclusion
Despite the high correlation between fatigue and low sleep quality in the clinical picture, our findings clearly indicate that, on the neural level, fatigue and low sleep quality in MS are associated with decreased FC in distinct functional brain networks
Topological Band Engineering of Graphene Nanoribbons
Topological insulators (TIs) are an emerging class of materials that host
highly robust in-gap surface/interface states while maintaining an insulating
bulk. While most notable scientific advancements in this field have been
focused on TIs and related topological crystalline insulators in 2D and 3D,
more recent theoretical work has predicted the existence of 1D
symmetry-protected topological phases in graphene nanoribbons (GNRs). The
topological phase of these laterally-confined, semiconducting strips of
graphene is determined by their width, edge shape, and the terminating unit
cell, and is characterized by a Z2 invariant (similar to 1D solitonic systems).
Interfaces between topologically distinct GNRs characterized by different Z2
are predicted to support half-filled in-gap localized electronic states which
can, in principle, be utilized as a tool for material engineering. Here we
present the rational design and experimental realization of a
topologically-engineered GNR superlattice that hosts a 1D array of such states,
thus generating otherwise inaccessible electronic structure. This strategy also
enables new end states to be engineered directly into the termini of the 1D GNR
superlattice. Atomically-precise topological GNR superlattices were synthesized
from molecular precursors on a Au(111) surface under ultra-high vacuum (UHV)
conditions and characterized by low temperature scanning tunneling microscopy
(STM) and spectroscopy (STS). Our experimental results and first-principles
calculations reveal that the frontier band structure of these GNR superlattices
is defined purely by the coupling between adjacent topological interface
states. This novel manifestation of 1D topological phases presents an entirely
new route to band engineering in 1D materials based on precise control of their
electronic topology, and is a promising platform for future studies of 1D
quantum spin physics.Comment: Contains main manuscript and supplemental informatio
Gauge coupling unification with large extra dimensions
We make a detailed study of the unification of gauge couplings in the MSSM
with large extra dimensions. We find some scenarios where unification can be
achieved (with the strong coupling constant at the Z mass within one standard
deviation of the experimental value) with both the compactification scale and
the SUSY breaking scale in the few TeV range. No enlargement of the gauge group
or particle content is needed. One particularly interesting scenario is when
the SUSY breaking scale is larger than the compactification scale, but both are
small enough to be probed at the CERN LHC. Unification in two scales scenarios
is also investigated and found to give results within the LHC.Comment: 17 pages, 3 figures, some discussions added, few additional
references included. Version to appear in Phys. Rev.
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