445 research outputs found
Measurement of the Total (p,Pi) Cross Sections Through Residual Activity
Supported by the National Science Foundation and Indiana Universit
Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
We examine 20 diffuse and translucent Galactic sight lines and extract the
column densities of the ^12CO and ^13CO isotopologues from their ultraviolet
A--X absorption bands detected in archival Space Telescope Imaging Spectrograph
data with lambda/Deltalambda geq 46,000. Five more targets with Goddard
High-Resolution Spectrograph data are added to the sample that more than
doubles the number of sight lines with published Hubble Space Telescope
observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are
not significantly different from the local value of 70 for ^12C/^13C, which is
based on mm-wave observations of rotational lines in emission from CO and H_2CO
inside dense molecular clouds, as well as on results from optical measurements
of CH^+. Five of the 25 sight lines are found to be fractionated toward lower
12-to-13 values, while three sight lines in the sample are fractionated toward
higher ratios, signaling the predominance of either isotopic charge exchange or
selective photodissociation, respectively. There are no obvious trends of the
^12CO-to-^13CO ratio with physical conditions such as gas temperature or
density, yet ^12CO/^13CO does vary in a complicated manner with the column
density of either CO isotopologue, owing to varying levels of competition
between isotopic charge exchange and selective photodissociation in the
fractionation of CO. Finally, rotational temperatures of H_2 show that all
sight lines with detected amounts of ^13CO pass through gas that is on average
colder by 20 K than the gas without ^13CO. This colder gas is also sampled by
CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28
K, enough to facilitate an efficient charge exchange reaction that lowers the
value of ^12CO/^13CO.Comment: 1-column emulateapj, 23 pages, 9 figure
Magnetically Driven Warping, Precession and Resonances in Accretion Disks
The inner region of the accretion disk onto a rotating magnetized central
star (neutron star, white dwarf or T Tauri star) is subjected to magnetic
torques which induce warping and precession of the disk. The origin of these
torques lies in the interaction between the (induced) surface current on the
disk and the horizontal magnetic field (parallel to the disk) produced by the
inclined magnetic dipole. Under quite general conditions, there exists a
magnetic warping instability in which the magnetic torque drives the disk plane
away from the equatorial plane of the star toward a state where the disk normal
vector is perpendicular to the spin axis. Viscous stress tends to suppress the
warping instability at large radii, but the magnetic torque always dominates as
the disk approaches the magnetosphere boundary. The magnetic torque also drives
the tilted inner disk into retrograde precession around the stellar spin axis.
Moreover, resonant magnetic forcing on the disk can occur which may affect the
dynamics of the disk. The magnetically driven warping instability and
precession may be related to a number observational puzzles, including: (1)
Spin evolution (torque reversal) of accreting X-ray pulsars; (2) Quasi-periodic
oscillations in low-mass X-ray binaries; (3) Super-orbital periods in X-ray
binaries; (4) Photometric period variations of T Tauri stars.Comment: 39 pages including 1 ps figure; Published version; ApJ, 524,
1030-1047 (1999
Measurement of two-halo neutron transfer reaction p(Li,Li)t at 3 MeV
The p(\nuc{11}{Li},\nuc{9}{Li})t reaction has been studied for the first time
at an incident energy of 3 MeV delivered by the new ISAC-2 facility at
TRIUMF. An active target detector MAYA, build at GANIL, was used for the
measurement. The differential cross sectionshave been determined for
transitions to the \nuc{9}{Li} ground andthe first excited states in a wide
range of scattering angles. Multistep transfer calculations using different
\nuc{11}{Li} model wave functions, shows that wave functions with strong
correlations between the halo neutrons are the most successful in reproducing
the observation.Comment: 6 pages, 3 figures, submitted to Physical Review Letter
Recommended from our members
Motivations for innovation in the built environment: new directions for research
Innovation in the built environment involves multiple actors with diverse motivations. Policy-makers find it difficult to promote changes that require cooperation from these numerous and dispersed actors and to align their sometimes divergent interests. Established research traditions on the economics and management of innovation pay only limited attention to stakeholder choices, engagement and motivation. This paper reviews the insights that emerge as research in these traditions comes into contact with work on innovation from sociological and political perspectives. It contributes by highlighting growing areas of research on user involvement in complex innovation, collective action, distributed innovation and transition management. To differing extents, these provide approaches to incorporate the motivations of different actors into theoretical understanding. These indicate new directions for research that promise to enrich understanding of innovation
A search for two body muon decay signals
Lepton family number violation is tested by searching for
decays among the 5.8 positive muon decay events analyzed by the
TWIST collaboration. Limits are set on the production of both massless and
massive bosons. The large angular acceptance of this experiment allows
limits to be placed on anisotropic decays, which can arise
from interactions violating both lepton flavor and parity conservation.
Branching ratio limits of order are obtained for bosons with masses
of 13 - 80 MeV/c and with different decay asymmetries. For bosons with
masses less than 13 MeV/c the asymmetry dependence is much stronger and
the 90% limit on the branching ratio varies up to . This is
the first study that explicitly evaluates the limits for anisotropic two body
muon decays.Comment: 7 pages, 5 figures, 2 tables, accepted by PR
Measurement of the Muon Decay Parameter delta
The muon decay parameter delta has been measured by the TWIST collaboration.
We find delta = 0.74964 +- 0.00066(stat.) +- 0.00112(syst.), consistent with
the Standard Model value of 3/4. This result implies that the product Pmuxi of
the muon polarization in pion decay, Pmu, and the muon decay parameter xi falls
within the 90% confidence interval 0.9960 < Pmuxi < xi < 1.0040. It also has
implications for left-right-symmetric and other extensions of the Standard
Model.Comment: Extended to 5 pages. Referee's comments answere
Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310
We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII
detected in UVES spectra of GRB 080310, with the afterglow radiation exciting
and ionizing the interstellar medium in the host galaxy at a redshift of
z=2.42743. To estimate the rest-frame afterglow brightness as a function of
time, we use a combination of the optical VRI photometry obtained by the
RAPTOR-T telescope array -- which are presented in this paper -- and Swift's
X-Ray Telescope observations. Excitation alone, which has been successfully
applied for a handful of other GRBs, fails to describe the observed
column-density evolution in the case of GRB 080310. Inclusion of ionization is
required to explain the column-density decrease of all observed FeII levels
(including the ground state 6D9/2) and increase of the FeIII 7S3 level. The
large population of ions in this latter level (up to 10% of all FeIII) can only
be explained through ionization of FeII, whereby a large fraction of the
ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan
codes -- initially populate the 7S3 level of FeIII rather than the ground
state. This channel for producing a significant FeIII 7S3 level population may
be relevant for other objects in which absorption lines from this level -- the
UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This
provides conclusive evidence for time-variable ionization in the circumburst
medium, which to date has not been convincingly detected. However, the best-fit
distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar
to GRB-absorber distance estimates for GRBs without any evidence for
ionization. We find that the presence of time-varying ionization in GRB 080310
is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2)
and the low HI column density (log N(HI)=18.7). [abridged]Comment: 14 pages, 6 figures; accepted for publication in A&A (on August 8,
2012
Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas
We discuss the detection of absorption by interstellar hydrogen fluoride (HF)
along the sight line to the submillimeter continuum sources W49N and W51. We
have used Herschel's HIFI instrument in dual beam switch mode to observe the
1232.4762 GHz J = 1 - 0 HF transition in the upper sideband of the band 5a
receiver. We detected foreground absorption by HF toward both sources over a
wide range of velocities. Optically thin absorption components were detected on
both sight lines, allowing us to measure - as opposed to obtain a lower limit
on - the column density of HF for the first time. As in previous observations
of HF toward the source G10.6-0.4, the derived HF column density is typically
comparable to that of water vapor, even though the elemental abundance of
oxygen is greater than that of fluorine by four orders of magnitude. We used
the rather uncertain N(CH)-N(H2) relationship derived previously toward diffuse
molecular clouds to infer the molecular hydrogen column density in the clouds
exhibiting HF absorption. Within the uncertainties, we find that the abundance
of HF with respect to H2 is consistent with the theoretical prediction that HF
is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen
fluoride has the potential to become an excellent tracer of molecular hydrogen,
and provides a sensitive probe of clouds of small H2 column density. Indeed,
the observations of hydrogen fluoride reported here reveal the presence of a
low column density diffuse molecular cloud along the W51 sight line, at an LSR
velocity of ~ 24kms-1, that had not been identified in molecular absorption
line studies prior to the launch of Herschel.Comment: 4 pages, 3 figures, A&A Letter special issue, accepted on 07/13/201
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