17,952 research outputs found
A skewer survey of the Galactic halo from deep CFHT and INT images
We study the density profile and shape of the Galactic halo using deep
multicolour images from the MENeaCS and CCCP projects, over 33 fields selected
to avoid overlap with the Galactic plane. Using multicolour selection and PSF
homogenization techniques we obtain catalogues of F stars (near-main sequence
turnoff stars) out to Galactocentric distances up to 60kpc. Grouping nearby
lines of sight, we construct the stellar density profiles through the halo in
eight different directions by means of photometric parallaxes. Smooth halo
models are then fitted to these profiles. We find clear evidence for a
steepening of the density profile power law index around R=20 kpc, from -2.50
+- 0.04 to -4.85 +- 0.04, and for a flattening of the halo towards the poles
with best-fit axis ratio 0.63 +- 0.02. Furthermore, we cannot rule out a mild
triaxiality (w>=0.8). We recover the signatures of well-known substructure and
streams that intersect our lines of sight. These results are consistent with
those derived from wider but shallower surveys, and augur well for upcoming,
wide-field surveys of comparable depth to our pencil beam surveys.Comment: 14 pages, 8 figures, 6 table
Finding halo streams with a pencil-beam survey: new wraps in the Sagittarius stream
We use data from two CFHT-MegaCam photometric pencil-beam surveys in the g'
and the r' bands to measure distances to the Sagittarius, the Palomar 5 and the
Orphan stream. We show that, using a cross-correlation algorithm to detect the
turnoff point of the main sequence, it is possible to overcome the main
limitation of a two-bands pencil-beam survey, namely the lack of adjacent
control-fields that can be used to subtract the foreground and background stars
to enhance the signal on the colour-magnitude diagrams (CMDs). We describe the
cross-correlation algorithm and its implementation. We combine the resulting
main sequence turnoff points with theoretical isochrones to derive photometric
distances to the streams. Our results (31 detections on the Sagittarius stream
and one each for the Palomar 5 and the Orphan streams) confirm the findings by
previous studies, expand the distance trend for the Sagittarius faint southern
branch and, for the first time, trace the Sagittarius faint branch of the
northern-leading arm out to 56 kpc. In addition, they show evidence for new
substructure: we argue that these detections trace the continuation of the
Sagittarius northern-leading arm into the southern hemisphere, and find a
nearby branch of the Sagittarius trailing wrap in the northern hemisphere.Comment: 16 pages, 15 figures, 2 table
Ambipolar Drift Heating in Turbulent Molecular Clouds
Although thermal pressure is unimportant dynamically in most molecular gas,
the temperature is an important diagnostic of dynamical processes and physical
conditions. This is the first of two papers on thermal equilibrium in molecular
clouds. We present calculations of frictional heating by ion-neutral (or
ambipolar) drift in three-dimensional simulations of turbulent, magnetized
molecular clouds.
We show that ambipolar drift heating is a strong function of position in a
turbulent cloud, and its average value can be significantly larger than the
average cosmic ray heating rate. The volume averaged heating rate per unit
volume due to ambipolar drift, H_AD ~ |JxB|^2 ~ B^4/L_B^2, is found to depend
on the rms Alfvenic Mach number, M_A, and on the average field strength, as
H_AD ~ M_A^2^4. This implies that the typical scale of variation of the
magnetic field, L_B, is inversely proportional to M_A, which we also
demonstrate.Comment: 37 pages, 9 figures include
Evolution in the Dust Lane Fraction of Edge-on L* Spiral Galaxies since z=0.8
The presence of a well-defined and narrow dust lane in an edge-on spiral
galaxy is the observational signature of a thin and dense molecular disk, in
which gravitational collapse has overcome turbulence. Using a sample of
galaxies out to z~1 extracted from the COSMOS survey, we identify the fraction
of massive disks that display a dust lane. Our goal is to explore the evolution
in the stability of the molecular ISM disks in spiral galaxies over a cosmic
timescale. We check the reliability of our morphological classifications
against changes in restframe wavelength, resolution, and cosmic dimming with
(artificially redshifted) images of local galaxies from SDSS. We find that the
fraction of L* disks with dust lanes in COSMOS is consistent with the local
fraction (~80%) out to z~0.7. At z=0.8, the dust lane fraction is only slightly
lower. A somewhat lower dust lane fraction in starbursting galaxies tentatively
supports the notion that a high specific star formation rate can efficiently
destroy or inhibit a dense molecular disk. A small subsample of higher redshift
COSMOS galaxies display low internal reddening (E[B-V]), as well as a low
incidence of dust lanes. These may be disks in which the growth of the dusty
ISM disk lags behind that of the stellar disk. We note that at z=0.8, the most
massive galaxies display a lower dust lane fraction than lower mass galaxies. A
small contribution of recent mergers or starbursts to this most massive
population may be responsible. The fact that the fraction of galaxies with dust
lanes in COSMOS is consistent with little or no evolution implies that models
to explain the Spectral Energy Distribution or the host galaxy dust extinction
of supernovae based on local galaxies are still applicable to higher redshift
spirals. It also suggests that dust lanes are long lived phenomena or can be
reformed over very short time-scales.Comment: 14 pages, 9 figures, 2 tables, accepted for publication by Ap
BAMBOO for RefugeesâA culturally sensitive positive psychology intervention:A protocol for a pilot Randomized Controlled Trial
This protocol investigates the efficacy of a mental health and psychosocial support (MHPSS) program named BAMBOO. The goal of this program is to increase resilience and mental well-being among refugees. In recent years, there has been an increasing number of refugees fleeing to Europe and applying for permanent residence. In the Netherlands, a wide variety of programs that focus on increasing mental well-being among refugees is available. BAMBOO is a strengths-based intervention, which consists of five weekly two-hour group-based sessions. It is based on the theoretical framework of positive psychology and adapted according to the guidelines for the implementation of culturally sensitive cognitive behavioral therapy among refugees. The program does not focus on the treatment of trauma, mental disorders, or underlying problems, but aims to increase factors such as personal strengths, positive emotions, positive relations, and self-esteem. The first version of the program (BAMBOO 1.0) has been conducted at over 50 asylum centers in the Netherlands since January 2020. In the time period from September 2020 to September 2022, qualitative and quantitative data were collected among BAMBOO trainers and participants and the program was adapted, leading to a revised version (BAMBOO 2.0). A single-blinded parallel pilot randomized controlled trial, with an intervention group and awaiting list control group, with 34 participants per group will be used to test the programâs efficacy. Outcomes include resilience, satisfaction with life, positive and negative affect, and self-esteem. The study will be conducted among adult Arabic speaking refugees, residing at an asylum center in the Netherlands and/ or in a Dutch municipality, Baseline, post-intervention, and five-week follow-up assessments will be conducted. This paper describes the protocol for the evaluation of a novel and culturally adapted program. The study will shed light on the efficacy of a culturally sensitive strengths-based intervention as a preventive approach to increase refugee well-bein
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)
We study the K-correction for the case of emission lines formed in the X-ray
illuminated atmosphere of a Roche lobe filling star. We compute the
K-correction as function of the mass ratio 'q' and the disc flaring angle
'alpha' using a compact binary code where the companion's Roche lobe is divided
into 10^5 resolution elements. We also study the effect of the inclination
angle in the results. We apply our model to the case of the neutron star
low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity
K_em=300 +-8 km/s has been measured by Casares et al. (2003). Our numerical
results, combined with previous determination of system parameters, yields
1.61Msun < M_NS < 2.32Msun and 0.44Msun < M_2 < 0.56Msun for the two binary
components(i. e. 0.24 < q < 0.27), which provide a compelling evidence for a
massive neutron star in this system. We also discuss the implications of these
masses into the evolutionary history of the binary.Comment: 6 pages, 5 figures. Accepted for publication in Ap
Exploring the Structure of Distant Galaxies with Adaptive Optics on the Keck-II Telescope
We report on the first observation of cosmologically distant field galaxies
with an high order Adaptive Optics (AO) system on an 8-10 meter class
telescope. Two galaxies were observed at 1.6 microns at an angular resolution
as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial
profiles of both objects are consistent with those of local spiral galaxies and
are decomposed into a classic exponential disk and a central bulge. A
star-forming cluster or companion galaxy as well as a compact core are detected
in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible
explanations for the core including a small bulge, a nuclear starburst, or an
active nucleus. The same galaxy shows a peak disk surface brightness that is
brighter than local disks of comparable size. These observations demonstrate
the power of AO to reveal details of the morphology of distant faint galaxies
and to explore galaxy evolution.Comment: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.
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