51,433 research outputs found
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
FIP Bias Evolution in a Decaying Active Region
Solar coronal plasma composition is typically characterized by first
ionization potential (FIP) bias. Using spectra obtained by Hinode's EUV Imaging
Spectrometer (EIS) instrument, we present a series of large-scale, spatially
resolved composition maps of active region (AR) 11389. The composition maps
show how FIP bias evolves within the decaying AR from 2012 January 4-6.
Globally, FIP bias decreases throughout the AR. We analyzed areas of
significant plasma composition changes within the decaying AR and found that
small-scale evolution in the photospheric magnetic field is closely linked to
the FIP bias evolution observed in the corona. During the AR's decay phase,
small bipoles emerging within supergranular cells reconnect with the
pre-existing AR field, creating a pathway along which photospheric and coronal
plasmas can mix. The mixing time scales are shorter than those of plasma
enrichment processes. Eruptive activity also results in shifting the FIP bias
closer to photospheric in the affected areas. Finally, the FIP bias still
remains dominantly coronal only in a part of the AR's high-flux density core.
We conclude that in the decay phase of an AR's lifetime, the FIP bias is
becoming increasingly modulated by episodes of small-scale flux emergence, i.e.
decreasing the AR's overall FIP bias. Our results show that magnetic field
evolution plays an important role in compositional changes during AR
development, revealing a more complex relationship than expected from previous
well-known Skylab results showing that FIP bias increases almost linearly with
age in young ARs (Widing Feldman, 2001, ApJ, 555, 426)
Dynamical arrest and replica symmetry breaking in attractive colloids
Within the Replica Symmetry Breaking (RSB) framework developed by M.Mezard
and G.Parisi we investigate the occurrence of structural glass transitions in a
model of fluid characterized by hard sphere repulsion together with short range
attraction. This model is appropriate for the description of a class of
colloidal suspensions. The transition line in the density-temperature plane
displays a reentrant behavior, in agreement with Mode Coupling Theory (MCT), a
dynamical approach based on the Mori-Zwanzig formalism. Quantitative
differences are however found, together with the absence of the predicted
glass-glass transition at high density. We also perform a systematic study of
the pure hard sphere fluid in order to ascertain the accuracy of the adopted
method and the convergence of the numerical procedure.Comment: 7 pages, 6 figure
Cooperative Chiral Order in Copolymers of Chiral and Achiral Units
Polyisocyanates can be synthesized with chiral and achiral pendant groups
distributed randomly along the chains. The overall chiral order, measured by
optical activity, is strongly cooperative and depends sensitively on the
concentration of chiral pendant groups. To explain this cooperative chiral
order theoretically, we map the random copolymer onto the one-dimensional
random-field Ising model. We show that the optical activity as a function of
composition is well-described by the predictions of this theory.Comment: 13 pages, including 3 postscript figures, uses REVTeX 3.0 and
epsf.st
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
TFD Approach to Bosonic Strings and -branes
In this work we explain the construction of the thermal vacuum for the
bosonic string, as well that of the thermal boundary state interpreted as a
-brane at finite temperature. In both case we calculate the respective
entropy using the entropy operator of the Thermo Field Dynamics Theory. We show
that the contribution of the thermal string entropy is explicitly present in
the -brane entropy. Furthermore, we show that the Thermo Field approach
is suitable to introduce temperature in boundary states.Comment: 6 pages, revtex, typos are corrected. Prepared for the Second
Londrina Winter School-Mathematical Methods in Physics, August 25-30, 2002,
Londrina-Pr, Brazil. To appear in a special issue of IJMP
Extremal Black Attractors in 8D Maximal Supergravity
Motivated by the new higher D-supergravity solutions on intersecting
attractors obtained by Ferrara et al. in [Phys.Rev.D79:065031-2009], we focus
in this paper on 8D maximal supergravity with moduli space
[SL(3,R)/SO(3)]x[SL(2,R)/SO(2)] and study explicitly the attractor mechanism
for various configurations of extremal black p- branes (anti-branes) with the
typical near horizon geometries AdS_{p+2}xS^{m}xT^{6-p-m} and p=0,1,2,3,4;
2<=m<=6. Interpretations in terms of wrapped M2 and M5 branes of the 11D
M-theory on 3-torus are also given.
Keywords: 8D supergravity, black p-branes, attractor mechanism, M-theory.Comment: 37 page
Plasma composition in a sigmoidal anemone active region
Using spectra obtained by the EIS instrument onboard Hinode, we present a
detailed spatially resolved abundance map of an active region (AR)-coronal hole
(CH) complex that covers an area of 359 arcsec x 485 arcsec. The abundance map
provides first ionization potential (FIP) bias levels in various coronal
structures within the large EIS field of view. Overall, FIP bias in the small,
relatively young AR is 2-3. This modest FIP bias is a consequence of the AR
age, its weak heating, and its partial reconnection with the surrounding CH.
Plasma with a coronal composition is concentrated at AR loop footpoints, close
to where fractionation is believed to take place in the chromosphere. In the
AR, we found a moderate positive correlation of FIP bias with nonthermal
velocity and magnetic flux density, both of which are also strongest at the AR
loop footpoints. Pathways of slightly enhanced FIP bias are traced along some
of the loops connecting opposite polarities within the AR. We interpret the
traces of enhanced FIP bias along these loops to be the beginning of
fractionated plasma mixing in the loops. Low FIP bias in a sigmoidal channel
above the AR's main polarity inversion line where ongoing flux cancellation is
taking place, provides new evidence of a bald patch magnetic topology of a
sigmoid/flux rope configfiuration.Comment: For on-line animation, see
http://www.mssl.ucl.ac.uk/~db2/fip_intensity.gif. Accepted by Ap
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