27,928 research outputs found
Comparison of medium frequency pulsed radar interferometer and correlation analysis winds, part 2
In order to test whether the chosen Doppler peaks represent localized scatters in motion, as opposed to some sort of integrated composite, an attempt was made to determine the change in position of single scatterers over a series of sequential records. A four-antenna system was employed which had 1 degree of freedom in phase. Due to limitations N-S linear transmission and E-W linear reception were used. The Doppler frequency peak selection criteria were that at least two of the four power spectra should have a local peak, and that normalized phase discrepancy, should be less than 0.3. The lack of success in tracking individual scatters seems to suggest a short lifetime. If this is the case, then the present experiment is not able to resolve the difference found between the correlation analysis true velocity and the interferometer value. On the other hand, it appears that the interferometer may be of some use in tracking waves
Singular del Pezzo fibrations and birational rigidity
A known conjecture of Grinenko in birational geometry asserts that a Mori
fibre space with the structure of del Pezzo fibration of low degree is
birationally rigid if and only if its anticanonical class is an interior point
in the cone of mobile divisors. The conjecture is proved to be true for smooth
models (with a generality assumption for degree 3). It is speculated that the
conjecture holds for, at least, Gorenstein models in degree 1 and 2. In this
article, I present a (Gorenstein) counterexample in degree 2 to this
conjecture.Comment: This is essentially a more detailed version of the second section of
arXiv:1310.5548. To appear in the proceedings of the conference 'Groups of
Automorphisms in Birational and Affine Geometry', held in Trento, Italy, 201
Towards Proper Motions in the Local Group
Key and still largely missing parameters for measuring the mass content and
distribution of the Local Group are the proper motion vectors of its member
galaxies. The problem when trying to derive the gravitational potential of the
Local Group is that usually only radial velocities are known, and hence
statistical approaches have to be used. The expected proper motions for
galaxies within the Local Group, ranging from 20 to 100 as/yr, are
detectable with VLBI using the phase-referencing technique. We present
phase-referencing observations of bright masers in IC~10 and M33 with respect
to background quasars. We observed the HO masers in IC10 three times over a
period of two months to check the accuracy of the relative positions. The
relative positions were obtained by modeling the interferometer phase data for
the maser sources referenced to the background quasars. The model allowed for a
relative position shift for the source and a single vertical atmospheric delay
error in the correlator model for each antenna. The rms of the relative
positions for the three observations is only 0.01 mas, which is approximately
the expected position error due to thermal noise. Also, we present a method to
measure the geometric distance to M33. This will allow re-calibration of the
extragalactic distance scale based on Cepheids. The method is to measure the
relative proper motions of two HO maser sources on opposite sides of M33.
The measured angular rotation rate, coupled with other measurements of the
inclination and rotation speed of the galaxy, yields a direct distance
measurement.Comment: 4 pages, Proceedings of the 6th European VLBI Network Symposium, Ros,
E., Porcas, R.W., Zensus, J.A. (eds.), MPIfR, Bonn, Germany (2002); Also
availabe http://www.mpifr-bonn.mpg.de/staff/abrunthaler/brunthal01.p
Using bijective maps to improve free energy estimates
We derive a fluctuation theorem for generalized work distributions, related
to bijective mappings of the phase spaces of two physical systems, and use it
to derive a two-sided constraint maximum likelihood estimator of their free
energy difference which uses samples from the equilibrium configurations of
both systems. As an application, we evaluate the chemical potential of a dense
Lennard-Jones fluid and study the construction and performance of suitable
maps.Comment: 17 pages, 11 figure
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
5-micron photometry of late-type dwarfs
We present narrowband-M photometry of nine low-mass dwarfs with spectral
types ranging from M2.5 to L0.5. Combining the (L'-M') colours derived from our
observations with data from the literature, we find colours consistent with a
Rayleigh-Jeans flux distribution for spectral types earlier than M5, but
enhanced F_3.8/F_4.7 flux ratios (negative (L'-M') colours) at later spectral
types. This probably reflects increased absorption at M' due to the CO
fundamental band. We compare our results against recent model predictions and
briefly discuss the implications.Comment: accepted for the Astronomical Journa
On the Computation of Power in Volume Integral Equation Formulations
We present simple and stable formulas for computing power (including
absorbed/radiated, scattered and extinction power) in current-based volume
integral equation formulations. The proposed formulas are given in terms of
vector-matrix-vector products of quantities found solely in the associated
linear system. In addition to their efficiency, the derived expressions can
guarantee the positivity of the computed power. We also discuss the application
of Poynting's theorem for the case of sources immersed in dissipative
materials. The formulas are validated against results obtained both with
analytical and numerical methods for scattering and radiation benchmark cases
Astrometric Galactic maser measurements cross-matched with Gaia
Using the VLBA, the BeSSeL survey has provided distances and proper motions
of young massive stars, allowing an accurate measure of the Galactic spiral
structure. By the same technique, we are planning to map the inner Galaxy using
positions and velocities of evolved stars (provided by the BAaDE survey). These
radio astrometric measurements (BeSSeL and BAaDE) will be complementary to Gaia
results and the overlap will provide important clues on the intrinsic
properties and population distribution of the stars in the bulge.Comment: To appear in the Proceedings of the IAU Symposium No. 334:
"Rediscovering our Galaxy
Maser, infrared and optical emission for late-type stars in the Galactic plane
Radio astrometric campaigns using VLBI have provided distances and proper
motions for masers associated with young massive stars (BeSSeL survey). The
ongoing BAaDE project plans to obtain astrometric information of SiO maser
stars located in the inner Galaxy. These stars are associated with evolved,
mass-losing stars. By overlapping optical (Gaia), infrared (2MASS, MSX and
WISE) and radio (BAaDE) sources, we expect to obtain important clues on the
intrinsic properties and population distribution of late-type stars. Moreover,
a comparison of the Galactic parameters obtained with Gaia and VLBI can be done
using radio observations on different targets: young massive stars (BeSSeL) and
evolved stars (BAaDE).Comment: To appear in the Proceedings of the IAU Symposium No. 336:
Astrophysical Masers: Unlocking the Mysteries of the Univers
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