18,943 research outputs found
A systematic observational study on Galactic interstellar ratio 18O/17O: I. C18O and C17O J=1-0 data analysis
The interstellar oxygen isotopic ratio of 18O/17O can reflect the relative
amount of the secular enrichment by ejecta from high-mass versus
intermediate-mass stars. Previous observations found a Galactic gradient of
18O/17O, i.e., low ratios in the Galactic center and large values in the
Galactic disk, which supports the insideout formation scenario of our Galaxy.
However, the observed objects are not many and, in particular, not so many at
large galactocentric distances. Thus we started a systematic study on Galactic
interstellar 18O/17O, through observations of C18O and C17O multi-transition
lines toward a large sample of 286 sources (at least one order of magnitude
larger than previous ones), from the Galactic center region to the far outer
Galaxy (~22 kpc). Here we present our observations of J=1-0 lines of C18O and
C17O, with the ARO12m and the IRAM 30m telescope. We detected successfully both
C18O and C17O 1-0 lines for 34 sources among our IRAM30m sample of 50 targets
and for 166 sources among our ARO12m sample of 260 targets. The C18O optical
depth effect on our ratio results, evaluated by fitting results of C17O spectra
with hyperfine components and our RADEX non-LTE model calculation for the
strongest source, was found to be insignificant. Beam dilution does not seem to
be a problem either, which was supported by the fact of no systematic variation
between the isotopic ratio and the heliocentric distance, and consistent
measured ratios from two telescopes for most of those detected sources. Our
results, though there are still very few detections made for sources in the
outer Galaxy, confirm the apparent 18O/17O gradient of 18O/17O =
(0.10+-0.03)R_GC+(2.95+-0.30), with a Pearson's rank correlation coefficient R
= 0.69. This is supported by the newest Galactic chemical evolution model
including the impact of massive stellar rotators and novae.Comment: 35 pages, 7 figures, published in ApJ
Precise determination of ground-state hyperfine splitting and calculation of Zeeman coefficients for 171Yb+ microwave frequency standard
We report precise measurement of the hyperfine splitting and calculation of
the Zeeman coefficients of the Yb ground state. The absolute
hyperfine splitting frequency is measured using high-resolution laser-microwave
double-resonance spectroscopy at 0.1 mHz level, and evaluated using more
accurate Zeeman coefficients. These Zeeman coefficients are derived using
Land\'{e} factors calculated by two atomic-structure methods,
multiconfiguration Dirac-Hartree-Fock, and multireference configuration
interaction. The cross-check of the two calculations ensures an accuracy of the
Zeeman coefficients at Hz/G level. The results provided in this
paper improve the accuracy and reliability of the second-order Zeeman shift
correction, thus further improving the accuracy of the microwave frequency
standards based on Yb. The high-precision hyperfine splitting and
Zeeman coefficients could also support could also support further experiments
to improve the constraints of fundamental constants through clock frequency
comparison of the Yb system
Measurement of the branching fractions of psi(2S) -> 3(pi+pi-) and J/psi -> 2(pi+pi-)
Using data samples collected at sqrt(s) = 3.686GeV and 3.650GeV by the BESII
detector at the BEPC, the branching fraction of psi(2S) -> 3(pi+pi-) is
measured to be [4.83 +- 0.38(stat) +- 0.69(syst)] x 10^-4, and the relative
branching fraction of J/psi -> 2(pi+pi-) to that of J/psi -> mu+mu- is measured
to be [5.86 +- 0.19(stat) +- 0.39(syst)]% via psi(2S) -> (pi+pi-)J/psi, J/psi
-> 2(pi+pi-). The electromagnetic form factor of 3(pi+pi-) is determined to be
0.21 +- 0.02 and 0.20 +- 0.01 at sqrt(s) = 3.686GeV and 3.650GeV, respectively.Comment: 17pages, 7 figures, submitted to Phys. Rev.
Bursts of terahertz radiation from large-scale plasmas irradiated by relativistic picosecond laser pulses
Powerful terahertz (THz) radiation is observed from large-scale underdense preplasmas in front of a solid target irradiated obliquely with picosecond relativistic intense laser pulses. The radiation covers an extremely broad spectrum with about 70% of its energy located in the high frequency regime over 10 THz. The pulse energy of the radiation is found to be above 100  μJ per steradian in the laser specular direction at an optimal preplasma scale length around 40–50  μm. Particle-in-cell simulations indicate that the radiation is mainly produced by linear mode conversion from electron plasma waves, which are excited successively via stimulated Raman scattering instability and self-modulated laser wakefields during the laser propagation in the preplasma. This radiation can be used not only as a powerful source for applications, but also as a unique diagnostic of parametric instabilities of laser propagation in plasmas
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