2,116 research outputs found
The Origin of the Arches Stellar Cluster Mass Function
We investigate the time evolution of the mass distribution of pre-stellar
cores (PSCs) and their transition to the initial stellar mass function (IMF) in
the central parts of a molecular cloud (MC) under the assumption that the
coalescence of cores is important. Our aim is to explain the observed shallow
IMF in dense stellar clusters such as the Arches cluster. The initial
distributions of PSCs at various distances from the MC center are those of
gravitationally unstable cores resulting from the gravo-turbulent fragmentation
of the MC. As time evolves, there is a competition between the PSCs rates of
coalescence and collapse. Whenever the local rate of collapse is larger than
the rate of coalescence in a given mass bin, cores are collapsed into stars.
With appropriate parameters, we find that the coalescence-collapse model
reproduces very well all the observed characteristics of the Arches stellar
cluster IMF; Namely, the slopes at high and low mass ends and the peculiar bump
observed at ~5-6 M_sol. Our results suggest that today's IMF of the Arches
cluster is very similar to the primordial one and is prior to the dynamical
effects of mass segregation becoming importantComment: 5 pages, 2 figures, accepted to MNRAS Letter
The measured compositions of Uranus and Neptune from their formation on the CO iceline
The formation mechanisms of the ice giants Uranus and Neptune, and the origin
of their elemental and isotopic compositions, have long been debated. The
density of solids in the outer protosolar nebula is too low to explain their
formation, and spectroscopic observations show that both planets are highly
enriched in carbon, very poor in nitrogen, and the ices from which they
originally formed might had deuterium-to-hydrogen ratios lower than the
predicted cometary value, unexplained properties observed in no other planets.
Here we show that all these properties can be explained naturally if Uranus and
Neptune both formed at the carbon monoxide iceline. Due to the diffusive
redistribution of vapors, this outer region of the protosolar nebula
intrinsically has enough surface density to form both planets from carbon-rich
solids but nitrogen-depleted gas, in abundances consistent with their observed
values. Water rich interiors originating mostly from transformed CO ices
reconcile the D/H value of Uranus and Neptune's building blocks with the
cometary value. Finally, Our scenario generalizes a well known hypothesis that
Jupiter formed on an iceline (water snowline) for the two ice giants, and might
be a first step towards generalizing this mechanism for other giant planets.Comment: The Astrophysical Journal (in press), 8 pages, 5 figure
An annotated water-filled, and dry potholes dataset for deep learning applications.
Potholes have long posed a challenging risk to automated systems due to their random and stochastic shapes and the reflectiveness of their surface when filled with water, whether it is "muddy" water or clear water. This has formed a significant limitation to autonomous assistive technologies such as Electric-Powered Wheelchairs (EPWs), mobility scooters, etc. due to the risk potholes pose on the user's well-being as it could cause severe falls and injuries as well as neck and back problems. Current research proved that Deep Leaning technologies are one of the most relevant solutions used to detect potholes due to the high accuracy of the detection. One of the main limitations to the datasets currently made available is the lack of photos describing water-filled, rabble-filled, and random coloured potholes. The purpose of our dataset is to provide the answer to this problem as it contains 713 high-quality photos representing 1152 manuall-annotated potholes in different shapes, locations, colours, and conditions, all of which were manually-collected via a mobile phone and within different areas in the United Kingdom along with two additional benchmarking videos recorded via a dashcam
Constraining Radio Emission from Magnetars
We report on radio observations of five magnetars and two magnetar candidates
carried out at 1950 MHz with the Green Bank Telescope in 2006-2007. The data
from these observations were searched for periodic emission and bright single
pulses. Also, monitoring observations of magnetar 4U0142+61 following its 2006
X-ray bursts were obtained. No radio emission was detected was detected for any
of our targets. The non-detections allow us to place luminosity upper limits
(at 1950 MHz) of approximately L < 1.60 mJy kpc^2 for periodic emission and L <
7.6 Jy kpc^2 for single pulse emission. These are the most stringent limits yet
for the magnetars observed. The resulting luminosity upper limits together with
previous results are discussed, as is the importance of further radio
observations of radio-loud and radio-quiet magnetars.Comment: 11 pages, 4 figure
Influence of the C/O ratio on titanium and vanadium oxides in protoplanetary disks
Context. The observation of carbon-rich disks have motivated several studies
questioning the influence of the C/O ratio on their gas phase composition in
order to establish the connection between the metallicity of hot-Jupiters and
that of their parent stars.
Aims. We to propose a method that allows the characterization of the adopted
C/O ratio in protoplanetary disks independently from the determination of the
host star composition. Titanium and vanadium chemistries are investigated
because they are strong optical absorbers and also because their oxides are
known to be sensitive to the C/O ratio in some exoplanet atmospheres.
Methods. We use a commercial package based on the Gibbs energy minimization
technique to compute the titanium and vanadium equilibrium chemistries in
protoplanetary disks for C/O ratios ranging from 0.05 to 10. Our calculations
are performed for pressures ranging from 1e-6 to 1e-2 bar, and for temperatures
ranging from 50 to 2000 K.
Results. We find that the vanadium nitride/vanadium oxide and titanium
hydride/titanium oxide gas phase ratios strongly depend on the C/O ratio in the
hot parts of disks (T > 1000 K). Our calculations suggest that, in these
regions, these ratios can be used as tracers of the C/O value in protoplanetary
disks.Comment: Accepted for publication in A&
Probing Majorana neutrinos in rare K and D, D_s, B, B_c meson decays
We study lepton number violating decays of charged K, D, D_s, B and B_c
mesons of the form M^+\to {M'}^-\ell^+\ell^+, induced by the existence of
Majorana neutrinos. These processes provide information complementary to
neutrinoless double nuclear beta decays, and are sensitive to neutrino masses
and lepton mixing. We explore neutrino mass ranges m_N from below 1 eV to
several hundred GeV. We find that in many cases the branching ratios are
prohibitively small, however in the intermediate range m_\pi < m_N < m_{B_c},
in specific channels and for specific neutrino masses, the branching ratios can
be at the reach of high luminosity experiments like those at the LHC-b and
future Super flavor-factories, and can provide bounds on the lepton mixing
parameters.Comment: 25 page
Reactivity of OH and CH3OH between 22 and 64 K: Modelling the gas phase production of CH3O in Barnard 1b
In the last years, ultra-low temperature chemical kinetic experiments have
demonstrated that some gas-phase reactions are much faster than previously
thought. One example is the reaction between OH and CH3OH, which has been
recently found to be accelerated at low temperatures yielding CH3O as main
product. This finding opened the question of whether the CH3O observed in the
dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and
OH. Several chemical models including this reaction and grain-surface processes
have been developed to explain the observed abundance of CHO with little
success. Here we report for the first time rate coefficients for the gas-phase
reaction of OH and CH3OH down to a temperature of 22 K, very close to those in
cold interstellar clouds. Two independent experimental set-ups based on the
supersonic gas expansion technique coupled to the pulsed laser photolysis-laser
induced fluorescence technique were used to determine rate coefficients in the
temperature range 22-64 K. The temperature dependence obtained in this work can
be expressed as k(22-64 K) = (3.6+/-0.1)e-12 (T/ 300)^(-1.0+/-0.2) cm3
molecule-1 s-1. Implementing this expression in a chemical model of a cold
dense cloud results in CH3O/CH3OH abundance ratios similar or slightly lower
than the value of 3e-3 observed in Barnard 1b. This finding confirms that the
gas-phase reaction between OH and CH3OH is an important contributor to the
formation of interstellar CH3O. The role of grain-surface processes in the
formation of CH3O, although it cannot be fully neglected, remains
controversial.Comment: Accepted for publication in The Astrophysical Journa
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