4,284 research outputs found
First Time-dependent Study of H2 and H3+ Ortho-Para Chemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions
The chemistry in the diffuse interstellar medium initiates the gradual
increase of molecular complexity during the life cycle of matter. A key
molecule that enables build-up of new molecular bonds and new molecules via
proton-donation is H3+. Its evolution is tightly related to molecular hydrogen
and thought to be well understood. However, recent observations of ortho and
para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in
nuclear spin excitation temperatures and populations between these two key
species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to
the predictions of modern astrochemical models. We conduct the first
time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM
and compare our results to a set of line-of-sight observations, including new
measurements presented in this study. We isolate a set of key reactions for H3+
and find that the destruction of the lowest rotational states of H3+ by
dissociative recombination largely control its ortho/para ratio. A plausible
agreement with observations cannot be achieved unless a ratio larger than 1:5
for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally,
an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas
variations of other individual physical parameters, such as density and
chemical age, have only a minor effect on the predicted ortho/para ratios. Thus
our study calls for new laboratory measurements of the dissociative
recombination rate and branching ratio of the key ion H3+ under interstellar
conditions.Comment: 27 pages, 6 figures, 3 table
MgB2 tunnel junctions and 19 K low-noise dc superconducting quantum interference devices
Point contact junctions made from two pieces of MgB2 can be adjusted to
exhibit either superconductor-insulator-superconductor (SIS) or
superconductor-normal metal-superconductor (SNS) current-voltage
characteristics. The SIS characteristics are in good agreement with the
standard tunneling model for s-wave superconductors, and yield an energy gap of
(2.02 +/- 0.08) meV. The SNS characteristics are in good agreement with the
predictions of the resistively-shunted junction model. DC Superconducting
QUantum Interference Devices made from two SNS junctions yield magnetic field
noise as low as 35 fT/Hz^{1/2} at 19 K.Comment: 4 pages, 4 figure
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