14,753 research outputs found
Near-Infrared Kinetic Spectroscopy of the HO_2 and C_2H_5O_2 Self-Reactions and Cross Reactions
The self-reactions and cross reactions of the peroxy radicals HO_2 and C_2H_5O_2 and HO_2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO_2, and UV absorption monitored HO_2 and C_2H_5O_2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221−296 K. The Arrhenius expression determined for the cross reaction, k_2(T) = (6.01^(+1.95)_(−1.47)) × 10^(−13) exp((638 ± 73)/T) cm^3 molecules^(−1) s^(−1) is in agreement with other work from the literature. The measurements of the HO_2 self-reaction agreed with previous work from this lab and were not further refined.(1) The C_2H_5O_2 self-reaction is complicated by secondary production of HO_2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO_2. The Arrhenius expression for the self-reaction rate constant is k_3(T) = (1.29^(+0.34)_(−0.27)) × 10^(−13)exp((−23 ± 61)/T) cm^3 molecules^(−1) s^(−1), and the branching fraction value is α = 0.28 ± 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO_2 self-reactions are required
Computation of nucleation of a non-equilibrium first-order phase transition using a rare-event algorithm
We introduce a new Forward-Flux Sampling in Time (FFST) algorithm to
efficiently measure transition times in rare-event processes in non-equilibrium
systems, and apply it to study the first-order (discontinuous) kinetic
transition in the Ziff-Gulari-Barshad model of catalytic surface reaction. The
average time for the transition to take place, as well as both the spinodal and
transition points, are clearly found by this method.Comment: 12 pages, 10 figure
Diamond thin Film Detectors for Beam Monitoring Devices
Diamonds offer radiation hard sensors, which can be used directly in primary
beams. Here we report on the use of a polycrystalline CVD diamond strip sensor
as beam monitor of heavy ion beams with up to 2.10^9 lead ions per bunch. The
strips allow for a determination of the transverse beam profile to a fraction
of the pitch of the strips, while the timing information yields the
longitudinal bunch length with a resolution of the order of a few mm.Comment: 6 pages, 7 figures, to appear in the Proceedings of the Hasselt
Diamond Workshop (Hasselt, Belgium, Feb. 2006), v4: accidentally submitted
figure, appearing at end, remove
Dynamics and pattern formation in invasive tumor growth
In this work, we study the in-vitro dynamics of the most malignant form of
the primary brain tumor: Glioblastoma Multiforme. Typically, the growing tumor
consists of the inner dense proliferating zone and the outer less dense
invasive region. Experiments with different types of cells show qualitatively
different behavior. Wild-type cells invade a spherically symmetric manner, but
mutant cells are organized in tenuous branches. We formulate a model for this
sort of growth using two coupled reaction-diffusion equations for the cell and
nutrient concentrations. When the ratio of the nutrient and cell diffusion
coefficients exceeds some critical value, the plane propagating front becomes
unstable with respect to transversal perturbations. The instability threshold
and the full phase-plane diagram in the parameter space are determined. The
results are in a good agreement with experimental findings for the two types of
cells.Comment: 4 pages, 4 figure
A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized
atmospheric magnetic field has been detected. To gain insights into the
interplay between atmospheres, winds, and magnetic fields of massive stars, we
acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the
XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to
study the variability of HD 54879. A multiwavelength (X-ray to optical)
spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model
atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The
microturbulent, macroturbulent, and projected rotational velocities are lower
than previously suggested (<4 km/s). An initial mass of 16 and an
age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray
emitting temperature of [K] and an X-ray luminosity of
[erg/s]. Short- and long-scale variability is seen in
the H-alpha line, but only a very long period of yr could be
estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we
can roughly estimate the mass-loss rate HD 54879 would have in the absence of a
magnetic field as . The
magnetic field traps the stellar wind up to the Alfv\'en radius >
, implying that its true mass-loss rate is . Hence, density enhancements around magnetic stars
can be exploited to estimate mass-loss rates of non-magnetic stars of similar
spectral types, essential for resolving the weak wind problem.
Conclusions: Our study confirms that strongly magnetized stars lose little or
no mass, and supplies important constraints on the weak-wind problem of massive
main sequence stars.Comment: Accepted for publication in A&A on Aug. 9, 2017, 12 + 1 pages, 15
figures. Paper replaced due to typos and missing acknowledgment
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