6,931 research outputs found
Evidence for tidal interaction and merger as the origin of galaxy morphology evolution in compact groups
We present the results of a morphological study based on NIR images of 25
galaxies, with different levels of nuclear activity, in 8 Compact Groups of
Galaxies (CGs). We perform independently two different analysis: a isophotal
study and a study of morphological asymmetries. The results yielded by the two
analysis are highly consistent. For the first time, it is possible to show that
deviations from pure ellipses are produced by inhomogeneous stellar mass
distributions related to galaxy interactions and mergers. We find evidence of
mass asymmetries in 74% of the galaxies in our sample. In 59% of these cases,
the asymmetries come in pairs, and are consistent with tidal effects produced
by the proximity of companion galaxies. The symmetric galaxies are generally
small in size or mass, inactive, and have an early-type morphology. In 20% of
the galaxies we find evidence for cannibalism. In 36% of the early-type
galaxies the color gradient is positive (blue nucleus) or flat. Summing up
these results, as much as 52% of the galaxies in our sample could show evidence
of an on going or past mergers. Our observations suggest that galaxies in CGs
merge more frequently under ``dry'' conditions. The high frequency of
interacting and merging galaxies observed in our study is consistent with the
bias of our sample towards CGs of type B, which represents the most active
phase in the evolution of the groups. In these groups we also find a strong
correlation between asymmetries and nuclear activity in early-type galaxies.
This correlation allows us to identify tidal interactions and mergers as the
cause of galaxy morphology transformation in CGs.[abridge]Comment: 64 pages, 35 figures. Accepted for publication in Ap
A multiwavelength study of Galactic HII region Sh2-294
We present the observational results of Galactic HII region S294, using
optical photometry, narrow-band imaging and radio continuum mapping at 1280
MHz, together with archival data from 2MASS, MSX and IRAS surveys. The stellar
surface density profile indicates that the radius of the cluster associated
with the S294 region is ~ 2.3 arcmin. We found an anomalous reddening law for
the dust inside the cluster region and the ratio of total-to-selective
extinction is found to be 3.8+-0.1. We estimate the minimum reddening E (B-V) =
1.35 mag and distance of 4.8+-0.2 kpc to the region from optical CC and CM
diagrams. We identified the ionizing source of the HII region, and spectral
type estimates are consistent with a star of spectral type ~ B0 V. The 2MASS
JHKs images reveal a partially embedded cluster associated with the ionizing
source along with a small cluster towards the eastern border of S294. The
ionization front seen along the direction of small cluster in radio continuum
and Halpha images, might be due to the interaction of ionizing sources with the
nearby molecular cloud. We found an arc shaped diffuse molecular hydrogen
emission at 2.12 micron and a half ring of MSX dust emission which surrounds
the ionized gas in the direction of the ionization front. Self consistent
radiative transfer model of mid- to far-infrared continuum emission detected
near small cluster is in good agreement with the observed spectral energy
distribution of a B1.5 ZAMS star. The morphological correlation between the
ionised and molecular gas, along with probable time scale involved between the
ionising star, evolution of HII region and small cluster, indicates that the
star-formation activity observed at the border is probably triggered by the
expansion of HII region.Comment: 50 pages, 21 figures: Accepted by The Astrophysical Journal; Also
available at http://www.tifr.res.in/~ojha/S294.pd
Monetary Policy Nominalization in Chile: an Evaluation
In August of 2001, the Central Bank of Chile modified its main monetary policy instrument, the inflation-indexed monetary policy interest rate MPR it had used so far, to a nominal or peso-denominated rate. A number of consequences, both in the conduct of monetary policy and in the countryâs financial markets, can be associated to this new way of taking monetary policy actions. This article evaluates such effects, providing theoretical support and evidence to several hypotheses, some of them found in the existing literature. The main contribution of this so-called ânominalizationââthat has not been discussed earlierâis that it extends the scope of operation of monetary policy, something that becomes important when both the interest rate and the inflation rate are low, as is the case today.
Spitzer Space Telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars
We present Spitzer Space Telescope photometry of six short-period polars, EF
Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the
Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s
photometry to construct the spectral energy distributions of these systems from
the near- to mid-IR (1.235 - 8 microns). We find that five out of the six
polars have flux densities in the mid-IR that are substantially in excess of
the values expected from the stellar components alone. We have modeled the
observed SEDs with a combination of contributions from the white dwarf,
secondary star, and either cyclotron emission or a cool, circumbinary dust disk
to fill in the long-wavelength excess. We find that a circumbinary dust disk is
the most likely cause of the 8 micron excess in all cases, but we have been
unable to rule out the specific (but unlikely) case of completely optically
thin cyclotron emission as the source of the observed 8 micron flux density.
While both model components can generate enough flux at 8 microns, neither dust
nor cyclotron emission alone can match the excess above the stellar components
at all wavelengths. A model combining both cyclotron and dust contributions,
possibly with some accretion-generated flux in the near-IR, is probably
required, but our observed SEDs are not sufficiently well-sampled to constrain
such a complicated model. If the 8 micron flux density is caused by the
presence of a circumbinary dust disk, then our estimates of the masses of these
disks are many orders of magnitude below the mass required to affect CV
evolution.Comment: 58 pages, 14 figures, ApJ accepte
Performance Evaluation of HL-LHC Crab Cavity Prototypes in a CERN Vertical Test Cryostat
Three proof-of-principle compact crab cavity designs have been fabricated in bulk niobium and cold tested at their home labs, as a first validation step towards the High Luminosity LHC project. As a cross check, all three bare cavities have been retested at CERN, in order to cross check their performance, and cross-calibrate the CERN SRF cold test facilities. While achievable transverse deflecting voltage is the key performance indicator, secondary performance aspects derived from multiple cavity monitoring systems are also discussed. Temperature mapping profiles, quench detection, material properties, and trapped magnetic flux effects have been assessed, and the influence on performance discussed. The significant effort invested in developing expertise in preparation and testing of these crab cavities has already been fruitful for all partners, and more is to come within this ongoing program
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
Three Bosons in One Dimension with Short Range Interactions I: Zero Range Potentials
We consider the three-boson problem with -function interactions in
one spatial dimension. Three different approaches are used to calculate the
phase shifts, which we interpret in the context of the effective range
expansion, for the scattering of one free particle a off of a bound pair. We
first follow a procedure outlined by McGuire in order to obtain an analytic
expression for the desired S-matrix element. This result is then compared to a
variational calculation in the adiabatic hyperspherical representation, and to
a numerical solution to the momentum space Faddeev equations. We find excellent
agreement with the exact phase shifts, and comment on some of the important
features in the scattering and bound-state sectors. In particular, we find that
the 1+2 scattering length is divergent, marking the presence of a zero-energy
resonance which appears as a feature when the pair-wise interactions are
short-range. Finally, we consider the introduction of a three-body interaction,
and comment on the cutoff dependence of the coupling.Comment: 9 figures, 2 table
High-spatial-resolution observations of NH3 and CH3OH towards the massive twin cores NGC6334 I & I(N)
Molecular line observations of NH3 (J,K)=(1,1), (2,2) and CH3OH at 24.93GHz
taken with the Australian Telescope Compact Array (ATCA) toward the massive
twin cores NGC6334 I & I(N) reveal significant variations in the line emission
between the two massive cores. The UCHII region/hot core NGC6334 I exhibits
strong thermal NH3 and CH3OH emission adjacent to the UCHII region and
coincident with two mm continuum peaks observed by Hunter et al. (in prep.). In
contrast, we find neither compact NH3 nor thermal CH3OH line emission toward
NGC6334 I(N). There, the NH3 emission is distributed over a broad region (>1')
without a clear peak, and we find Class I CH3OH maser emission with peak
brightness temperatures up to 7000K. The maser emission peaks appear to be
spatially associated with the interfaces between the molecular outflows and the
ambient dense gas. Peak NH3(1,1) line brightness temperatures >= 70K in both
regions indicate gas temperatures of the same order. NH3 emission is also
detected toward the outflow in NGC6334 I resulting in an estimated rotational
temperature of Trot~19K. Furthermore, we observe CH3OH and NH3 absorption
toward the UCHII region, the velocity structure is consistent with expanding
molecular gas around the UCHII region. Thermal and kinematic effects possibly
imposed from the UCHII region on the molecular core are also discussed.Comment: Accepted for the Astrophysical Journa
Quantum States of Topologically Massive Electrodynamics and Gravity
The free quantum states of topologically massive electrodynamics and gravity
in 2+1 dimensions, are explicitly found. It is shown that in both theories the
states are described by infrared-regular polarization tensors containing a
regularization phase which depends on the spin. This is done by explicitly
realizing the quantum algebra on a functional Hilbert space and by finding the
Wightman function to define the scalar product on such a Hilbert space. The
physical properties of the states are analyzed defining creation and
annihilation operators.
For both theories, a canonical and covariant quantization procedure is
developed. The higher order derivatives in the gravitational lagrangian are
treated by means of a preliminary Dirac procedure.
The closure of the Poincar\'e algebra is guaranteed by the
infrared-finiteness of the states which is related to the spin of the
excitations through the regularization phase. Such a phase may have interesting
physical consequences.Comment: 21 page, latex, no figure
Collective vs local measurements in qubit mixed state estimation
We discuss the problem of estimating a general (mixed) qubit state. We give
the optimal guess that can be inferred from any given set of measurements. For
collective measurements and for a large number of copies, we show that the
error in the estimation goes as 1/N. For local measurements we focus on the
simpler case of states lying on the equatorial plane of the Bloch sphere. We
show that standard tomographic techniques lead to an error proportional to
, while with our optimal data processing it is proportional to
.Comment: 4 pages, 1 figure, minor style changes, refs. adde
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