3,297 research outputs found
Domain of validity of some atmospheric mesoscale models
The usual coordinate system in the mesoscale literature is a Cartesian system xyz with its origin at a point on a spherical earth model with the z-axis normal and exterior to the earth. The main form of the momentum equation for
theoretical analysis has been dv/dt = âÏâ1â p + gâ2Ω Ă v + f r where g is approximated by âgËz. Several computational models use a version of this equation where z is replaced by a Ï-type coordinate, and applications of such models have
used a horizontal domain D(L) = 2L Ă 2L with L 650 km but the results of this paper suggest that the equation is valid with L 100 km. However, the necessity of including the effects of synoptic disturbances and reducing the errors from lateral boundaries impose the use of a large(L). This conflict is solved with the use of the correct gravitational acceleration g = âga2Rrâ3 which provides a momentum equation valid on any domain D(L). This is confirmed with an example which shows that the resulting momentum equation can yield the correct pressure field on the whole earth surface. Practical problems limit the use of the coordinate system xyz to L ïżœ 500 km. In this case, it is shown that the approximation g ⌠âg(xËx + yËy + aËz)/a can be applied. Some mesoscale models incorporate map
projections into model equations to consider the earth curvature. This has motivated the use of such models on a domain with L ⌠882, 1665 km. Formally, the governing equations from map projections are written in terms of a curvilinear coordinate system xpypzp but it is shown that if xp, yp, zp are taken as x, y, z the resulting momentum equation is valid on a region with L ïżœ 100 km
Determination of transition frequencies in a single Ba ion
Transition frequencies between low-lying energy levels in a single trapped
Ba ion have been measured with laser spectroscopy referenced to
an optical frequency comb. By extracting the frequencies of one-photon and
two-photon components of the line shape using an eight-level optical Bloch
model, we achieved 0.1 MHz accuracy for the 5d D - 6p
P and 6s S - 5d D transition
frequencies, and 0.2 MHz for the 6s S - 6p P
transition frequency.Comment: 5 pages, 7 figures, submitted to Phys. Rev.
Simulated Galactic methanol maser distribution to constrain Milky Way parameters
Using trigonometric parallaxes and proper motions of masers associated with
massive young stars, the Bar and Spiral Structure Legacy (BeSSeL) survey has
reported the most accurate values of the Galactic parameters so far. The
determination of these parameters with high accuracy has a widespread impact on
Galactic and extragalactic measurements. This research is aimed at establishing
the confidence with which such parameters can be determined. This is relevant
for the data published in the context of the BeSSeL survey collaboration, but
also for future observations, in particular from the Southern Hemisphere. In
addition, some astrophysical properties of the masers can be constrained,
notably the luminosity function. We have simulated the population of
maser-bearing young stars associated with Galactic spiral structure, generating
several samples and comparing them with the observed samples used in the BeSSeL
survey. Consequently, we checked the determination of Galactic parameters for
observational biases introduced by the sample selection. Galactic parameters
obtained by the BeSSeL survey do not seem to be biased by the sample selection
used. In fact, the published error estimates appear to be conservative for most
of the parameters. We show that future BeSSeL data and future observations with
Southern arrays will improve the Galactic parameters estimates and smoothly
reduce their mutual correlation. Moreover, by modeling future parallax data
with larger distance and, thus, greater relative uncertainties for a larger
numbers of sources, we found that parallax-distance biasing is an important
issue. Hence, using fractional parallax uncertainty in the weighting of the
motion data is imperative. Finally, the luminosity function for 6.7 GHz
methanol masers was determined, allowing us to estimate the number of Galactic
methanol masers.Comment: Accepted for publication in A&A. Language edition include
Finding evolved stars in the inner Galactic disk with Gaia
The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey will provide
positions and line-of-sight velocities of ~20,000 evolved, maser bearing stars
in the Galactic plane. Although this Galactic region is affected by optical
extinction, BAaDE targets may have Gaia cross-matches, eventually providing
additional stellar information. In an initial attempt to cross-match BAaDE
targets with Gaia, we have found more than 5,000 candidates. Of these, we may
expect half to show SiO emission, which will allow us to obtain velocity
information. The cross-match is being refined to avoid false positives using
different criteria based on distance analysis, flux variability, and color
assessment in the mid- and near-IR. Once the cross-matches can be confirmed, we
will have a unique sample to characterize the stellar population of evolved
stars in the Galactic bulge, which can be considered fossils of the Milky Way
formation.Comment: To appear in the Proceedings of the IAU Symposium No. 330:
"Astrometry and Astrophysics in the Gaia sky
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