1,143 research outputs found
Frame Combination Techniques for Ultra High-Contrast Imaging
We summarize here an experimental frame combination pipeline we developed for
ultra high-contrast imaging with systems like the upcoming VLT SPHERE
instrument. The pipeline combines strategies from the Drizzle technique, the
Spitzer IRACproc package, and homegrown codes, to combine image sets that may
include a rotating field of view and arbitrary shifts between frames. The
pipeline is meant to be robust at dealing with data that may contain non-ideal
effects like sub-pixel pointing errors, missing data points, non-symmetrical
noise sources, arbitrary geometric distortions, and rapidly changing point
spread functions. We summarize in this document individual steps and
strategies, as well as results from preliminary tests and simulations.Comment: 9 pages, 4 figures, SPIE conference pape
VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre
The dust enshrouded star IRS~3 in the central light year of our galaxy was
partially resolved in a recent VLTI experiment. The presented observation is
the first step in investigating both IRS~3 in particular and the stellar
population of the Galactic Centre in general with the VLTI at highest angular
resolution. We will outline which scientific issues can be addressed by a
complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge
Supernova Remnant in a Stratified Medium: Explicit, Analytical Approximations for Adiabatic Expansion and Radiative Cooling
We propose simple, explicit, analytical approximations for the kinematics of
an adiabatic blast wave propagating in an exponentially stratified ambient
medium, and for the onset of radiative cooling, which ends the adiabatic era.
Our method, based on the Kompaneets implicit solution and the Kahn
approximation for the radiative cooling coefficient, gives straightforward
estimates for the size, expansion velocity, and progression of cooling times
over the surface, when applied to supernova remnants (SNRs). The remnant shape
is remarkably close to spherical for moderate density gradients, but even a
small gradient in ambient density causes the cooling time to vary substantially
over the remnant's surface, so that for a considerable period there will be a
cold dense expanding shell covering only a part of the remnant. Our
approximation provides an effective tool for identifying the approximate
parameters when planning 2-dimensional numerical models of SNRs, the example of
W44 being given in a subsequent paper.Comment: ApJ accepted, 11 pages, 2 figures embedded, aas style with
ecmatex.sty and lscape.sty package
SPOTS: The Search for Planets Orbiting Two Stars. I. Survey description and first observations
Direct imaging surveys for exoplanets commonly exclude binary stars from
their target lists, leaving a large part of the overall planet demography
unexplored. To address this gap in our understanding of planet formation and
evolution, we have launched the first direct imaging survey dedicated to
circumbinary planets: SPOTS, the Search for Planets Orbiting Two Stars. In this
paper, we discuss the theoretical context, scientific merit, and technical
feasibility of such observations, describe the target sample and observational
strategy of our survey, and report on the first results from our pilot survey
of 26 targets with the VLT NaCo facility. While we have not found any confirmed
substellar companions to date, a number of promising candidate companions
remain to be tested for common proper motion in upcoming follow-up
observations. We also report on the astrometry of the three resolved binaries
in our target sample. This pilot survey constitutes a successful proof of
concept for our survey strategy and paves the way for a second stage of
exploratory observations with VLT SPHERE.Comment: 12 pages, 4 figures, 3 tables. Accepted for publication in A&
Subarcsecond Submillimeter Imaging of the Ultracompact HII Region G5.89-0.39
We present the first subarcsecond submillimeter images of the enigmatic
ultracompact HII region (UCHII) G5.89-0.39. Observed with the SMA, the 875
micron continuum emission exhibits a shell-like morphology similar to longer
wavelengths. By using images with comparable angular resolution at five
frequencies obtained from the VLA archive and CARMA, we have removed the
free-free component from the 875 micron image. We find five sources of dust
emission: two compact warm objects (SMA1 and SMA2) along the periphery of the
shell, and three additional regions further out. There is no dust emission
inside the shell, supporting the picture of a dust-free cavity surrounded by
high density gas. At subarcsecond resolution, most of the molecular gas tracers
encircle the UCHII region and appear to constrain its expansion. We also find
G5.89-0.39 to be almost completely lacking in organic molecular line emission.
The dust cores SMA1 and SMA2 exhibit compact spatial peaks in optically-thin
gas tracers (e.g. 34SO2), while SMA1 also coincides with 11.9 micron emission.
In CO(3-2), we find a high-velocity north/south bipolar outflow centered on
SMA1, aligned with infrared H2 knots, and responsible for much of the maser
activity. We conclude that SMA1 is an embedded intermediate mass protostar with
an estimated luminosity of 3000 Lsun and a circumstellar mass of ~1 Msun.
Finally, we have discovered an NH3 (3,3) maser 12 arcsec northwest of the UCHII
region, coincident with a 44 GHz CH3OH maser, and possibly associated with the
Br gamma outflow source identified by Puga et al. (2006).Comment: 41 pages, 11 figures, published in The Astrophysical Journal (2008)
Volume 680, Issue 2, pp. 1271-1288. An error in the registration of the
marker positions in Figure 11 has been corrected in this versio
Early phase of massive star formation: A case study of Infrared dark cloud G084.81-01.09
We mapped the MSX dark cloud G084.81-01.09 in the NH3 (1,1) - (4,4) lines and
in the J = 1-0 transitions of 12CO, 13CO, C18O and HCO+ in order to study the
physical properties of infrared dark clouds, and to better understand the
initial conditions for massive star formation. Six ammonia cores are identified
with masses ranging from 60 to 250 M_sun, a kinetic temperature of 12 K, and a
molecular hydrogen number density n(H2) ~ 10^5 cm^-3. In our high mass cores,
the ammonia line width of 1 km/s is larger than those found in lower mass cores
but narrower than the more evolved massive ones. We detected self-reversed
profiles in HCO+ across the northern part of our cloud and velocity gradients
in different molecules. These indicate an expanding motion in the outer layer
and more complex motions of the clumps more inside our cloud. We also discuss
the millimeter wave continuum from the dust. These properties indicate that our
cloud is a potential site of massive star formation but is still in a very
early evolutionary stage
The VLT/NaCo Large program to probe the occurrence of exoplanets and brown dwarfs in wide orbits: I- Sample definition and characterization
Young, nearby stars are ideal targets to search for planets using the direct
imaging technique. The determination of stellar parameters is crucial for the
interpretation of imaging survey results particularly since the luminosity of
substellar objects has a strong dependence on system age. We have conducted a
large program with NaCo at the VLT in order to search for planets and brown
dwarfs in wide orbits around 86 stars. A large fraction of the targets observed
with NaCo were poorly investigated in the literature. We performed a study to
characterize the fundamental properties (age, distance, mass) of the stars in
our sample. To improve target age determinations, we compiled and analyzed a
complete set of age diagnostics. We measured spectroscopic parameters and age
diagnostics using dedicated observations acquired with FEROS and CORALIE
spectrographs at La Silla Observatory. We also made extensive use of archival
spectroscopic data and results available in the literature. Additionally, we
exploited photometric time-series, available in ASAS and Super-WASP archives,
to derive rotation period for a large fraction of our program stars. We
provided updated characterization of all the targets observed in the VLT NaCo
Large program, a survey designed to probe the occurrence of exoplanets and
brown dwarfs in wide orbits. The median distance and age of our program stars
are 64 pc and 100 Myr, respectively. Nearly all the stars have masses between
0.70 and 1.50sun, with a median value of 1.01 Msun. The typical metallicity is
close to solar, with a dispersion that is smaller than that of samples usually
observed in radial velocity surveys. Several stars are confirmed or proposed
here to be members of nearby young moving groups. Eight spectroscopic binaries
are identified.Comment: 64 pages with Appendix, 15 figures, accepted to A&
Shadows cast on the transition disk of HD 135344B. Multiwavelength VLT/SPHERE polarimetric differential imaging
The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in
a transition stage and shows many intriguing structures both in scattered light
and thermal (sub-)millimeter emission which are possibly related to planet
formation processes. We study the morphology and surface brightness of the disk
in scattered light to gain insight into the innermost disk regions, the
formation of protoplanets, planet-disk interactions traced in the surface and
midplane layers, and the dust grain properties of the disk surface. We have
carried out high-contrast polarimetric differential imaging (PDI) observations
with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in R-
and I-band and with IRDIS in Y- and J-band. The scattered light images reveal
with unprecedented angular resolution and sensitivity the spiral arms as well
as the 25 au cavity of the disk. Multiple shadow features are discovered on the
outer disk with one shadow only being present during the second observation
epoch. A positive surface brightness gradient is observed in the stellar
irradiation corrected images in southwest direction possibly due to an
azimuthally asymmetric perturbation of the temperature and/or surface density
by the passing spiral arms. The disk integrated polarized flux, normalized to
the stellar flux, shows a positive trend towards longer wavelengths which we
attribute to large aggregate dust grains in the disk surface. Part of the the
non-azimuthal polarization signal in the Uphi image of the J-band observation
could be the result of multiple scattering in the disk. The detected shadow
features and their possible variability have the potential to provide insight
into the structure of and processes occurring in the innermost disk regions.Comment: Accepted for publication in A&A, 20 pages, 15 figure
Prehospital critical care for out-of-hospital cardiac arrest: An observational study examining survival and a stakeholder-focused cost analysis
© 2016 The Author(s). Background: Survival rates from out-of-hospital cardiac arrest (OHCA) remain low, despite remarkable efforts to improve care. A number of ambulance services in the United Kingdom (UK) have developed prehospital critical care teams (CCTs) which attend critically ill patients, including OHCA. However, current scientific evidence describing CCTs attending OHCA is sparse and research to date has not demonstrated clear benefits from this model of care. Methods: This prospective, observational study will describe the effect of CCTs on survival from OHCA, when compared to advanced-life-support (ALS), the current standard of prehospital care in the UK. In addition, we will describe the association between individual critical care interventions and survival, and also the costs of CCTs for OHCA. To examine the effect of CCTs on survival from OHCA, we will use routine Utstein variables data already collected in a number of UK ambulance trusts. We will use propensity score matching to adjust for imbalances between the CCT and ALS groups. The primary outcome will be survival to hospital discharge, with the secondary outcome of survival to hospital admission. We will record the critical care interventions delivered during CCT attendance at OHCA. We will describe frequencies and aim to use multiple logistic regression to examine possible associations with survival. Finally, we will undertake a stakeholder-focused cost analysis of CCTs for OHCA. This will utilise a previously published Emergency Medical Services (EMS) cost analysis toolkit and will take into account the costs incurred from use of a helicopter and the proportion of these costs currently covered by charities in the UK. Discussion: Prehospital critical care for OHCA is not universally available in many EMS. In the UK, it is variable and largely funded through public donations to charities. If this study demonstrates benefit from CCTs at an acceptable cost to the public or EMS commissioners, it will provide a rationale to increase funding and service provision. If no clinical benefit is found, the public and charities providing these services can consider concentrating their efforts on other areas of prehospital care. Trial registration: ISRCTN registry ID ISRCTN18375201
An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys
We describe algorithms that detect 21cm line HI self-absorption (HISA) in
large data sets and extract it for analysis. Our search method identifies HISA
as spatially and spectrally confined dark HI features that appear as negative
residuals after removing larger-scale emission components with a modified CLEAN
algorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in
(l,b,v) space, and the HI brightness of voxels outside the 3-D feature
boundaries is smoothly interpolated to estimate the absorption amplitude and
the unabsorbed HI emission brightness. The reliability and completeness of our
HISA detection scheme have been tested extensively with model data. We detect
most features over a wide range of sizes, linewidths, amplitudes, and
background levels, with poor detection only where the absorption brightness
temperature amplitude is weak, the absorption scale approaches that of the
correlated noise, or the background level is too faint for HISA to be
distinguished reliably from emission gaps. False detection rates are very low
in all parts of the parameter space except at sizes and amplitudes approaching
those of noise fluctuations. Absorption measurement biases introduced by the
method are generally small and appear to arise from cases of incomplete HISA
detection. This paper is the third in a series examining HISA at high angular
resolution. A companion paper (Paper II) uses our HISA search and extraction
method to investigate the cold atomic gas distribution in the Canadian Galactic
Plane Survey.Comment: 39 pages, including 14 figure pages; to appear in June 10 ApJ, volume
626; figure quality significantly reduced for astro-ph; for full resolution,
please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey
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