517 research outputs found
The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
We present VLA HI spectral line imaging of 5 sources discovered by ALFALFA.
These targets are drawn from a larger sample of systems that were not uniquely
identified with optical counterparts during ALFALFA processing, and as such
have unusually high HI mass to light ratios. These candidate "Almost Dark"
objects fall into 4 categories: 1) objects with nearby HI neighbors that are
likely of tidal origin; 2) objects that appear to be part of a system of
multiple HI sources, but which may not be tidal in origin; 3) objects isolated
from nearby ALFALFA HI detections, but located near a gas-poor early-type
galaxy; 4) apparently isolated sources, with no object of coincident redshift
within ~400 kpc. Roughly 75% of the 200 objects without identified counterparts
in the .40 database (Haynes et al. 2011) fall into category 1. This
pilot sample contains the first five sources observed as part of a larger
effort to characterize HI sources with no readily identifiable optical
counterpart at single dish resolution. These objects span a range of HI mass
[7.41 < log(M) < 9.51] and HI mass to B-band luminosity ratios (3 <
M/L < 9). We compare the HI total intensity and velocity
fields to SDSS optical imaging and to archival GALEX UV imaging. Four of the
sources with uncertain or no optical counterpart in the ALFALFA data are
identified with low surface brightness optical counterparts in SDSS imaging
when compared with VLA HI intensity maps, and appear to be galaxies with clear
signs of ordered rotation. One source (AGC 208602) is likely tidal in nature.
We find no "dark galaxies" in this limited sample. The present observations
reveal complex sources with suppressed star formation, highlighting both the
observational difficulties and the necessity of synthesis follow-up
observations to understand these extreme objects. (abridged)Comment: Astronomical Journal, in pres
Mid-IR spectroscopy of T Tauri stars in Chamealeon I: evidence for processed dust at the earliest stages
We present mid-IR spectroscopy of three T Tauri stars in the young Chamealeon
I dark cloud obtained with TIMMI2 on the ESO 3.6m telescope. In these three
stars, the silicate emission band at 9.7 micron is prominent. We model it with
a mixture of amorphous olivine grains of different size, crystalline silicates
and silica. The fractional mass of these various components change widely from
star to star. While the spectrum of CR Cha is dominated by small amorphous
silicates, in VW Cha (and in a lesser degree in Glass I), there is clear
evidence of a large amount of processed dust in the form of crystalline
silicates and large amorphous grains. This is the first time that processed
dust has been detetected in very young T Tauri stars (~ 1 Myr).Comment: 5 pages, 4 Postscript figures. accepted for A&A Letter
C2D Spitzer-IRS spectra of disks around T Tauri stars V. Spectral decomposition
(Abridged) Dust particles evolve in size and lattice structure in
protoplanetary disks, due to coagulation, fragmentation and crystallization,
and are radially and vertically mixed in disks. This paper aims at determining
the mineralogical composition and size distribution of the dust grains in disks
around 58 T Tauri stars observed with Spitzer/IRS. We present a spectral
decomposition model that reproduces the IRS spectra over the full spectral
range. The model assumes two dust populations: a warm component responsible for
the 10\mu m emission arising from the disk inner regions and a colder component
responsible for the 20-30\mu m emission, arising from more distant regions. We
show evidence for a significant size distribution flattening compared to the
typical MRN distribution, providing an explanation for the usual boxy 10\mu m
feature profile generally observed. We reexamine the crystallinity paradox,
observationally identified by Olofsson et al. (2009), and we find a
simultaneous enrichment of the crystallinity in both the warm and cold regions,
while grain sizes in both components are uncorrelated. Our modeling results do
not show evidence for any correlations between the crystallinity and either the
star spectral type, or the X-ray luminosity (for a subset of the sample). The
size distribution flattening may suggests that grain coagulation is a slightly
more effective process than fragmentation in disk atmospheres, and that this
imbalance may last over most of the T Tauri phase. This result may also point
toward small grain depletion via strong stellar winds or radiation pressure in
the upper layers of disk. The non negligible cold crystallinity fractions
suggests efficient radial mixing processes in order to distribute crystalline
grains at large distances from the central object, along with possible nebular
shocks in outer regions of disks that can thermally anneal amorphous grains
Left-right symmetry at LHC and precise 1-loop low energy data
Despite many tests, even the Minimal Manifest Left-Right Symmetric Model
(MLRSM) has never been ultimately confirmed or falsified. LHC gives a new
possibility to test directly the most conservative version of left-right
symmetric models at so far not reachable energy scales. If we take into account
precise limits on the model which come from low energy processes, like the muon
decay, possible LHC signals are strongly limited through the correlations of
parameters among heavy neutrinos, heavy gauge bosons and heavy Higgs particles.
To illustrate the situation in the context of LHC, we consider the "golden"
process . For instance, in a case of degenerate heavy neutrinos
and heavy Higgs masses at 15 TeV (in agreement with FCNC bounds) we get
fb at TeV which is consistent with muon
decay data for a very limited masses in the range (3008 GeV, 3040 GeV).
Without restrictions coming from the muon data, masses would be in the
range (1.0 TeV, 3.5 TeV). Influence of heavy Higgs particles themselves on the
considered LHC process is negligible (the same is true for the light, SM
neutral Higgs scalar analog). In the paper decay modes of the right-handed
heavy gauge bosons and heavy neutrinos are also discussed. Both scenarios with
typical see-saw light-heavy neutrino mixings and the mixings which are
independent of heavy neutrino masses are considered. In the second case heavy
neutrino decays to the heavy charged gauge bosons not necessarily dominate over
decay modes which include only light, SM-like particles.Comment: 16 pages, 10 figs, KL-KS and new ATLAS limits taken into accoun
Signatures of arithmetic simplicity in metabolic network architecture
Metabolic networks perform some of the most fundamental functions in living
cells, including energy transduction and building block biosynthesis. While
these are the best characterized networks in living systems, understanding
their evolutionary history and complex wiring constitutes one of the most
fascinating open questions in biology, intimately related to the enigma of
life's origin itself. Is the evolution of metabolism subject to general
principles, beyond the unpredictable accumulation of multiple historical
accidents? Here we search for such principles by applying to an artificial
chemical universe some of the methodologies developed for the study of genome
scale models of cellular metabolism. In particular, we use metabolic flux
constraint-based models to exhaustively search for artificial chemistry
pathways that can optimally perform an array of elementary metabolic functions.
Despite the simplicity of the model employed, we find that the ensuing pathways
display a surprisingly rich set of properties, including the existence of
autocatalytic cycles and hierarchical modules, the appearance of universally
preferable metabolites and reactions, and a logarithmic trend of pathway length
as a function of input/output molecule size. Some of these properties can be
derived analytically, borrowing methods previously used in cryptography. In
addition, by mapping biochemical networks onto a simplified carbon atom
reaction backbone, we find that several of the properties predicted by the
artificial chemistry model hold for real metabolic networks. These findings
suggest that optimality principles and arithmetic simplicity might lie beneath
some aspects of biochemical complexity
Discrimination of low missing energy look-alikes at the LHC
The problem of discriminating possible scenarios of TeV scale new physics
with large missing energy signature at the Large Hadron Collider (LHC) has
received some attention in the recent past. We consider the complementary, and
yet unexplored, case of theories predicting much softer missing energy spectra.
As there is enough scope for such models to fake each other by having similar
final states at the LHC, we have outlined a systematic method based on a
combination of different kinematic features which can be used to distinguish
among different possibilities. These features often trace back to the
underlying mass spectrum and the spins of the new particles present in these
models. As examples of "low missing energy look-alikes", we consider
Supersymmetry with R-parity violation, Universal Extra Dimensions with both
KK-parity conserved and KK-parity violated and the Littlest Higgs model with
T-parity violated by the Wess-Zumino-Witten anomaly term. Through detailed
Monte Carlo analysis of the four and higher lepton final states predicted by
these models, we show that the models in their minimal forms may be
distinguished at the LHC, while non-minimal variations can always leave scope
for further confusion. We find that, for strongly interacting new particle
mass-scale ~600 GeV (1 TeV), the simplest versions of the different theories
can be discriminated at the LHC running at sqrt{s}=14 TeV within an integrated
luminosity of 5 (30) fb^{-1}.Comment: 40 pages, 10 figures; v2: Further discussions, analysis and one
figure added, ordering of certain sections changed, minor modifications in
the abstract, version as published in JHE
Crystalline silicate dust around evolved stars III. A correlations study of crystalline silicate features
We have carried out a quantitative trend analysis of the crystalline
silicates observed in the ISO spectra of a sample of 14 stars with different
evolutionary backgrounds. We have modeled the spectra using a simple dust
radiative transfer model and have correlated the results with other known
parameters. We confirm the abundance difference of the crystalline silicates in
disk and in outflow sources, as found by Molster et al. (1999, Nature 401,
563). We found some indication that the enstatite over forsterite abundance
ratio differs, it is slightly higher in the outflow sources with respect to the
disk sources. It is clear that more data is required to fully test this
hypothesis. We show that the 69.0 micron feature, attributed to forsterite, may
be a very suitable temperature indicator. We found that the enstatite is more
abundant than forsterite in almost all sources. The temperature of the
enstatite grains is about equal to that of the forsterite grains in the disk
sources but slightly lower in the outflow sources. Crystalline silicates are on
average colder than amorphous silicates. This may be due to the difference in
Fe content of both materials. Finally we find an indication that the ratio of
ortho to clino enstatite, which is about 1:1 in disk sources, shifts towards
ortho enstatite in the high luminosity (outflow) sources.Comment: 16 pages, 20 figures, accepted by A&A, this paper and others (in this
series) can also be found at http://zon.wins.uva.nl/~frankm/papers.htm
Mid-IR observations of circumstellar disks -- Part III: A mixed sample of PMS stars and Vega-type objects
We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis
object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained
with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members
of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands
are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144
N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the
Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm
wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no
indication for circumstellar matter at mid-infrared wavelengths is found
towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the
T Tauri system HD 155555.
The silicate emission features of the remaining eight sources are modelled
with a mixture of silicates of different grain sizes and composition.
Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous
grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS
144 S. Both small grains and crystalline dust is found for the Vega-type HD
123356, with a dominance of small amorphous grains. We show that the infrared
emission of the binary HD 123356 is dominated by its late-type secondary, but
optical spectroscopy is still required to confirm the age of the system and the
spectral class of the companion. For most targets this is their first
mid-infrared spectroscopic observation. We investigate trends between stellar,
disk and silicate properties and confirm correlations of previous studies.
Several objects present an exciting potential for follow-up high-resolution
disk studies.Comment: Accepted for publication in A&
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