4,048 research outputs found
Does the Milky Way have a Maximal Disk?
The Milky Way is often considered to be the best example of a spiral for
which the dark matter not only dominates the outer kinematics, but also plays a
major dynamical role in the inner galaxy: the Galactic disk is therefore said
to be ``sub-maximal.'' This conclusion is important to the understanding of the
evolution of galaxies and the viability of particular dark matter models. The
Galactic evidence rests on a number of structural and kinematic measurements,
many of which have recently been revised. The new constraints indicate not only
that the Galaxy is a more typical member of its class (Sb-Sc spirals) than
previously thought, but also require a re-examination of the question of
whether or not the Milky Way disk is maximal. By applying to the Milky Way the
same definition of ``maximal disk'' that is applied to external galaxies, it is
shown that the new observational constraints are consistent with a Galactic
maximal disk of reasonable . In particular, the local disk column can be
substantially less than the oft-quoted required \Sigma_{\odot} \approx 100
\msolar pc^{-2} - as low as 40 \msolar pc^{-2} in the extreme case - and
still be maximal, in the sense that the dark halo provides negligible rotation
support in the inner Galaxy. This result has possible implications for any
conclusion that rests on assumptions about the potentials of the Galactic disk
or dark halo, and in particular for the interpretation of microlensing results
along both LMC and bulge lines of sight.Comment: Accepted for publication in The Astrophysical Journal. 23
Latex-generated pages, one (new) table, three figures (two new). A few
additions to the bibliography, an expanded discussion, and slight
quantitative changes, none of which affect the conclusion
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Super-KMS functionals for graded-local conformal nets
Motivated by a few preceding papers and a question of R. Longo, we introduce
super-KMS functionals for graded translation-covariant nets over R with
superderivations, roughly speaking as a certain supersymmetric modification of
classical KMS states on translation-covariant nets over R, fundamental objects
in chiral algebraic quantum field theory. Although we are able to make a few
statements concerning their general structure, most properties will be studied
in the setting of specific graded-local (super-) conformal models. In
particular, we provide a constructive existence and partial uniqueness proof of
super-KMS functionals for the supersymmetric free field, for certain subnets,
and for the super-Virasoro net with central charge c>= 3/2. Moreover, as a
separate result, we classify bounded super-KMS functionals for graded-local
conformal nets over S^1 with respect to rotations.Comment: 30 pages, revised version (to appear in Ann. H. Poincare
Single- and few-layer graphene growth on stainless steel substrates by direct thermal chemical vapor deposition
Steeping interest on graphene research in basic sciences and applications
emphasizes the need for an economical means of synthesizing it. We report a
method for the synthesis of graphene on commercially available stainless steel
foils using direct thermal chemical vapor deposition. Our method of synthesis
and the use of relatively cheap precursors such as ethanol (CH3CH2OH) as a
source of carbon and SS 304 as the substrate, proved to be economically viable.
Presence of single- and few-layer graphene was confirmed using confocal Raman
microscopy/spectroscopy. X-ray photoelectron spectroscopic measurements were
further used to establish the influence of various elemental species present in
stainless steel on graphene growth. Role of cooling rate on surface migration
of certain chemical species (oxides of Fe, Cr and Mn) that promote or hinder
the growth of graphene is probed. Such analysis of the chemical species present
on the surface can be promising for graphene based catalytic research
A Study of the Populations of X-ray Sources in the Small Magellanic Cloud with ASCA
The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made
multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images
in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately
constructed, and the latter provides the first hard X-ray view of the SMC. We
extract 39 sources from the two-band images with a criterion of S/N>5, and
conduct timing and spectral analyses for all of these sources. Coherent
pulsations are detected from 12 X-ray sources; five of which are new
discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux
variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the
other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because
their spectra exhibit emission lines from highly ionized atoms. We find that
XBPs and thermal SNRs in the SMC can be clearly separated by their hardness
ratio (the ratio of the count rate between the hard and soft bands). Using this
empirical grouping, we find many XBP candidates in the SMC, although no
pulsations have yet been detected from these sources. Possible implications on
the star-formation history and evolution of the SMC are presented by a
comparison of the source populations in the SMC and our Galaxy.Comment: 11 pages, 39 Figures, to be published in ApJ Supplement. Tables (body
and figures also) are available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job
Probing the Canis Major stellar over-density as due to the Galactic warp
Proper-motion, star counts and photometric catalog simulations are used to
explain the detected stellar over-density in the region of Canis Major (CMa),
claimed to be the core of a disrupted dwarf galaxy (Martin et al. 2004,
Bellazzini et al. 2003), as due to the Galactic warp and flare in the external
disk. We compare the kinematics of CMa M-giant selected sample with surrounding
Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion
signature: CMa stars mimic thick disk kinematics. Moreover, when taking into
account the Galactic warp and flare of the disk, 2MASS star count profiles
reproduce the CMa stellar over-density. This star count analysis is confirmed
by direct comparison with synthetic color-magnitude diagrams simulated with the
Besancon models (Robin et al. 2003) that include the warp and flare of the
disk. The presented evidence casts doubt on the identification of the CMa
over-density as the core of a disrupted Milky Way satellite. This however does
not make clear the origin of over-densities responsible for the ring structure
in the anticenter direction of the Galactic halo (Newberg et al. 2002; Yanny et
al. 2003; Zaggia et al. 2004, in preparation).Comment: Accepted for publication in A&A Letters, 4 page
Starcounts Redivivus. IV. Density Laws Through Photometric Parallaxes
In an effort to more precisely define the spatial distribution of Galactic
field stars, we present an analysis of the photometric parallaxes of 70,000
stars covering nearly 15 square degrees in seven Kapteyn Selected Areas. We
address the affects of Malmquist Bias, subgiant/giant contamination,
metallicity and binary stars upon the derived density laws. The affect of
binary stars is the most significant. We find that while the disk-like
populations of the Milky Way are easily constrained in a simultaneous analysis
of all seven fields, no good simultaneous solution for the halo is found. We
have applied halo density laws taken from other studies and find that the
Besancon flattened power law halo model (c/a=0.6, r^-2.75) produces the best
fit to our data. With this halo, the thick disk has a scale height of 750 pc
with an 8.5% normalization to the old disk. The old disk scale height is
280-300 pc. Corrected for a binary fraction of 50%, these scale heights are 940
pc and 350-375 pc, respectively. Even with this model, there are systematic
discrepancies between the observed and predicted density distributions. Our
model produces density overpredictions in the inner Galaxy and density
underpredictions in the outer Galaxy. A possible solution is modeling the
stellar halo as a two-component system in which the halo has a flattened inner
distribution and a roughly spherical, but substructured outer distribution.
Further reconciliation could be provided by a flared thick disk, a structure
consistent with a merger origin for that population. (Abridged)Comment: 66 pages, accepted to Astrophysical journal, some figures compresse
String Matching and 1d Lattice Gases
We calculate the probability distributions for the number of occurrences
of a given letter word in a random string of letters. Analytical
expressions for the distribution are known for the asymptotic regimes (i) (Gaussian) and such that is finite
(Compound Poisson). However, it is known that these distributions do now work
well in the intermediate regime . We show that the
problem of calculating the string matching probability can be cast into a
determining the configurational partition function of a 1d lattice gas with
interacting particles so that the matching probability becomes the
grand-partition sum of the lattice gas, with the number of particles
corresponding to the number of matches. We perform a virial expansion of the
effective equation of state and obtain the probability distribution. Our result
reproduces the behavior of the distribution in all regimes. We are also able to
show analytically how the limiting distributions arise. Our analysis builds on
the fact that the effective interactions between the particles consist of a
relatively strong core of size , the word length, followed by a weak,
exponentially decaying tail. We find that the asymptotic regimes correspond to
the case where the tail of the interactions can be neglected, while in the
intermediate regime they need to be kept in the analysis. Our results are
readily generalized to the case where the random strings are generated by more
complicated stochastic processes such as a non-uniform letter probability
distribution or Markov chains. We show that in these cases the tails of the
effective interactions can be made even more dominant rendering thus the
asymptotic approximations less accurate in such a regime.Comment: 44 pages and 8 figures. Major revision of previous version. The
lattice gas analogy has been worked out in full, including virial expansion
and equation of state. This constitutes the main part of the paper now.
Connections with existing work is made and references should be up to date
now. To be submitted for publicatio
Non-Riemannian Gravity and the Einstein-Proca System
We argue that all Einstein-Maxwell or Einstein-Proca solutions to general
relativity may be used to construct a large class of solutions (involving
torsion and non-metricity) to theories of non-Riemannian gravitation that have
been recently discussed in the literature.Comment: 9 pages Plain Tex (No Figures), Letter to Editor Classical and
Quantum Gravit
ASTEP South: An Antarctic Search for Transiting ExoPlanets around the celestial South pole
ASTEP South is the first phase of the ASTEP project (Antarctic Search for
Transiting ExoPlanets). The instrument is a fixed 10 cm refractor with a 4kx4k
CCD camera in a thermalized box, pointing continuously a 3.88 degree x 3.88
degree field of view centered on the celestial South pole. ASTEP South became
fully functional in June 2008 and obtained 1592 hours of data during the 2008
Antarctic winter. The data are of good quality but the analysis has to account
for changes in the point spread function due to rapid ground seeing variations
and instrumental effects. The pointing direction is stable within 10 arcseconds
on a daily timescale and drifts by only 34 arcseconds in 50 days. A truly
continuous photometry of bright stars is possible in June (the noon sky
background peaks at a magnitude R=15 arcsec-2 on June 22), but becomes
challenging in July (the noon sky background magnitude is R=12.5 arcsec?2 on
July 20). The weather conditions are estimated from the number of stars
detected in the field. For the 2008 winter, the statistics are between 56.3 %
and 68.4 % of excellent weather, 17.9 % to 30 % of veiled weather and 13.7 % of
bad weather. Using these results in a probabilistic analysis of transit
detection, we show that the detection efficiency of transiting exoplanets in
one given field is improved at Dome C compared to a temperate site such as La
Silla. For example we estimate that a year-long campaign of 10 cm refractor
could reach an efficiency of 69 % at Dome C versus 45 % at La Silla for
detecting 2-day period giant planets around target stars from magnitude 10 to
15. This shows the high potential of Dome C for photometry and future planet
discoveries. [Short abstract
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