7,317 research outputs found
The nearby Galaxy structure toward the Vela Gum nebula
We report on photometry and spectroscopy for MK classification
purposes carried out in the fields of five open clusters projected against the
Vela Gum in the Third Galactic Quadrant of the Galaxy. They are Ruprecht 20,
Ruprecht 47, Ruprecht 60, NGC 2660 and NGC 2910. We could improve/confirm the
parameters of these objects derived before. The spectroscopic parallax method
has been applied to several stars located in the fields of four out of the five
clusters to get their distances and reddenings. With this method we found two
blue stars in the field of NGC 2910 at distances that make them likely members
of Vela OB1 too. Also, projected against the fields of Ruprecht 20 and Ruprecht
47 we have detected other young stars favoring not only the existence of Puppis
OB1 and OB2 but conforming a young stellar group at kpc from the Sun
and extending for more than 6 kpc outward the Galaxy. If this is the case,
there is a thickening of the thin Galactic disk of more than 300 pc at just 2-3
kpc from the Sun. Ruprecht 60 and NGC 2660 are too old objects that have no
physical relation with the associations under discussion. An astonishing result
has been the detection in the background of Ruprecht 47 of a young star at the
impressive distance of 9.5 kpc from the Sun that could be a member of the
innermost part of the Outer Arm. Another far young star in the field of NGC
2660, at near 6.0 kpc, may become a probable member of the Perseus Arm or of
the inner part of the Local Arm. The distribution of young clusters and stars
onto the Third Galactic Quadrant agrees with recent findings concerning the
extension of the Local Arm as revealed by parallaxes of regions of star
formation. We show evidences too that added to previous ones found by our group
explain the thickening of the thin disk as a combination of flare and warp.Comment: Accepted for publication in New Astronom
Numerical analysis of the stability of the Electrohydrodynamic (EHD) electroconvection between two plates
The time evolution of the problem of Electrohydrodynamic (EHD)
convection in a liquid between two plates is analysed numerically. The equations are
nondimensionalized using the ion drift velocity and the viscous time scales. Following
the non-dimensionalisation of the respective model, two different techniques have been
used to describe the charge evolution, namely the Finite-Element Flux-Corrected
Transport Method and the Particle-In-Cell technique. The results obtained with the
two schemes, apart from showing good agreement, have revealed the appearance of a
two-roll structure not described in previous works. This is investigated in detail for
both strong and weak injection.Ministerio de ciencia y tecnología FQM-42
Pressure-induced structural, electronic, and magnetic effects in BiFeO3
We present a first-principles study of multiferroic BiFeO3 at high pressures.
Our work reveals the main structural (change in Bi's coordination and loss of
ferroelectricity), electronic (spin crossover and metallization), and magnetic
(loss of order) effects favored by compression and how they are connected. Our
results are consistent with the striking manifold transition observed
experimentally by Gavriliuk et al. [Phys. Rev. B 77, 155112 (2008)] and provide
an explanation for it.Comment: 4 pages with 4 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Examples of signature (2,2) manifolds with commuting curvature operators
We exhibit Walker manifolds of signature (2,2) with various commutativity
properties for the Ricci operator, the skew-symmetric curvature operator, and
the Jacobi operator. If the Walker metric is a Riemannian extension of an
underlying affine structure A, these properties are related to the Ricci tensor
of A
Variable Stars in Local Group Galaxies. I: Tracing the Early Chemical Enrichment and Radial Gradients in the Sculptor dSph with RR Lyrae Stars
We identified and characterized the largest (536) RR Lyrae (RRL) sample in a
Milky Way dSph satellite (Sculptor) based on optical photometry data collected
over 24 years.
The RRLs display a spread in V-magnitude (0.35 mag) which appears
larger than photometric errors and the horizontal branch (HB) luminosity
evolution of a mono-metallic population. Using several calibrations of two
different reddening free and metal independent Period-Wesenheit relations we
provide a new distance estimate =19.62 mag (=0.04 mag) that
agrees well with literature estimates. We constrained the metallicity
distribution of the old population, using the Period-Luminosity relation,
and we found that it ranges from -2.3 to -1.5 dex. The current estimate is
narrower than suggested by low and intermediate spectroscopy of RGBs
([Fe/H] 1.5).
We also investigated the HB morphology as a function of the galactocentric
distance. The HB in the innermost regions is dominated by red HB stars and by
RRLs, consistent with a more metal-rich population, while in the outermost
regions it is dominated by blue HB stars and RRLs typical of a metal-poor
population. Our results suggest that fast chemical evolution occurred in
Sculptor, and that the radial gradients were in place at an early epoch.Comment: 10 pages, 5 figures, MNRAS accepte
Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a
The detailed spiral structure in the outer Galactic disk is still poorly
known, and for several Galactic directions we rely on model extrapolations. One
of these regions is the fourth Galactic quadrant, in the sector comprised
between Vela and Carina (270 <l< 300) where no spiral arms have been detected
so far in the optical beyond 270. By means of deep UBVI photometry, we search
for spiral features in known low absorption windows.U photometry, although
demanding, constitutes a powerful tool to detect and characterize distant
aggregates, and allows to derive firmer distance estimates. We studied a
direction close to the tangent (l=290) to the Carina arm, in an attempt to
detect optical spiral tracers beyond the Carina branch, where radio
observations and models predictions indicate the presence of the extension of
the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line
of sight, we detect three distinct groups of young stars. Two of them, at 2.5
and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this
particular direction).The latter is here detected for the first time. The third
group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond
the Carina arm, and constitutes the first optical detection of this arm in the
fourth Galactic quadrant. The position of this feature is compatible with HI
observations and model predictions. We furthermore present evidence that this
extremely distant group, formerly thought to be a star cluster (Shorlin 1), is
in fact a diffuse young population. In addition, our data-set does not support
the possible presence of the Monoceros Ring toward this direction. This study
highlights how multicolor optical studies can be effective to probe the spiral
structure in the outer Galactic disk.Comment: 9 pages, 13 eps figure, in press in A&A, abstract rephrased and a few
figures degraded in resolution to fit i
Open Wilson Lines and Chiral Condensates in Thermal Holographic QCD
We investigate various aspects of a proposal by Aharony and Kutasov
arXiv:0803.3547 [hep-th] for the gravity dual of an open Wilson line in the
Sakai-Sugimoto model or its non-compact version. In particular, we use their
proposal to determine the effect of finite temperature, as well as background
electric and magnetic fields, on the chiral symmetry breaking order parameter.
We also generalize their prescription to more complicated worldsheets and
identify the operators dual to such worldsheets.Comment: 45 pages, 18 figures; added reference
Probing the early chemical evolution of the Sculptor dSph with purely old stellar tracers
We present the metallicity distribution of a sample of 471 RR Lyrae (RRL)
stars in the Sculptor dSph, obtained from the -band Period-Luminosity
relation. It is the first time that the early chemical evolution of a dwarf
galaxy is characterized in such a detailed and quantitative way, using
photometric data alone. We find a broad metallicity distribution (FWHM=0.8 dex)
that is peaked at [Fe/H]-1.90 dex, in excellent agreement with
literature values obtained from spectroscopic data. Moreover, we are able to
directly trace the metallicity gradient out to a radius of 55 arcmin. We
find that in the outer regions (r32 arcmin) the slope of the metallicity
gradient from the RRLs (-0.025 dex arcmin) is comparable to the
literature values based on red giant (RG) stars. However, in the central part
of Sculptor we do not observe the latter gradients. This suggests that there is
a more metal-rich and/or younger population in Sculptor that does not produce
RRLs. This scenario is strengthened by the observation of a metal-rich peak in
the metallicity distribution of RG stars by other authors, which is not present
in the metallicity distribution of the RRLs within the same central area.Comment: 5 pages, 4 figures. Accepted for publication on MNRAS Letter
Strongly-Coupled Quarks and Colorful Black Holes
We use the AdS/CFT correspondence to study the behavior of strongly-coupled
quarks in a black hole background. The supergravity background consists of a
six-dimensional Schwarzschild-black string AdS soliton, for which the bulk
horizon extends from the AdS boundary down to an infra-red floor. By going to
higher energy scales, the regime of validity of the classical supergravity
background can be extended closer to the singularity than might be expected
from the four-dimensional perspective. Small black holes potentially created by
the Large Hadron Collider could typically carry color charges inherited from
their parton progenitors. The dynamics of quarks near such a black hole depends
on the curved spacetime geometry as well as the strong interaction with the
color-charged black hole. We study the resulting behavior of quarks and compute
the rate at which a quark rotating around the black hole loses energy. We also
investigate how the interaction between a quark and an antiquark is altered by
the presence of the black hole, which results in a screening length.Comment: Proceedings of the DPF-2011 Conference, 8 pages, 5 figures, added
reference
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