9,416 research outputs found
Understanding the central kinematics of globular clusters with simulated integrated-light IFU observations
The detection of intermediate mass black holes in the centres of globular
clusters is highly controversial, as complementary observational methods often
deliver significantly different results. In order to understand these
discrepancies, we develop a procedure to simulate integral field unit (IFU)
observations of globular clusters: Simulating IFU Star Cluster Observations
(SISCO). The input of our software are realistic dynamical models of globular
clusters that are then converted in a spectral data cube. We apply SISCO to
Monte Carlo cluster simulations from Downing et al. (2010), with a realistic
number of stars and concentrations. Using independent realisations of a given
simulation we are able to quantify the stochasticity intrinsic to the problem
of observing a partially resolved stellar population with integrated-light
spectroscopy. We show that the luminosity-weighted IFU observations can be
strongly biased by the presence of a few bright stars that introduce a scatter
in the velocity dispersion measurements up to 40% around the expected
value, preventing any sound assessment of the central kinematic and a sensible
interpretation of the presence/absence of an intermediate mass black hole.
Moreover, we illustrate that, in our mock IFU observations, the average
kinematic tracer has a mass of 0.75 solar masses, only slightly lower
than the mass of the typical stars examined in studies of resolved
line-of-sight velocities of giant stars. Finally, in order to recover unbiased
kinematic measurements we test different masking techniques that allow us to
remove the spaxels dominated by bright stars, bringing the scatter down to a
level of only a few percent. The application of SISCO will allow to investigate
state-of-the-art simulations as realistic observations.Comment: 13 pages, 9 figures, 1 table. Accepted for publication in MNRA
Being WISE I: Validating Stellar Population Models and M/L ratios at 3.4 and 4.6 microns
Using data from the WISE mission, we have measured near infra-red (NIR)
photometry of a diverse sample of dust-free stellar systems (globular clusters,
dwarf and giant early-type galaxies) which have metallicities that span the
range -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample size
and broadens the metallicity regime over which the 3.4 (W1) and 4.6 micron (W2)
photometry of stellar populations have been examined.
We find that the W1 - W2 colors of intermediate and old (> 2 Gyr) stellar
populations are insensitive to the age of the stellar population, but that the
W1 - W2 colors become bluer with increasing metallicity, a trend not well
reproduced by most stellar population synthesis (SPS) models. In common with
previous studies, we attribute this behavior to the increasing strength of the
CO absorption feature located in the 4.6 micron bandpass with metallicity.
Having used our sample to validate the efficacy of some of the SPS models, we
use these models to derive stellar mass-to-light ratios in the W1 and W2 bands.
Utilizing observational data from the SAURON and ATLAS3D surveys, we
demonstrate that these bands provide extremely simple, yet robust stellar mass
tracers for dust free older stellar populations that are freed from many of the
uncertainties common among optical estimators.Comment: 11 pages, 6 figures, submitted to Ap
The star catalogues of Ptolemaios and Ulugh Beg: Machine-readable versions and comparison with the modern Hipparcos Catalogue
In late antiquity and throughout the middle ages, the positions of stars on
the celestial sphere were obtained from the star catalogue of Ptolemaios. A
catalogue based on new measurements appeared in 1437, with positions by Ulugh
Beg, and magnitudes from the 10th-century astronomer al-Sufi. We provide
machine-readable versions of these two star catalogues, based on the editions
by Toomer (1998) and Knobel (1917), and determine their accuracies by
comparison with the modern Hipparcos Catalogue. The magnitudes in the
catalogues correlate well with modern visual magnitudes; the indication `faint'
by Ptolemaios is found to correspond to his magnitudes 5 and 6. Gaussian fits
to the error distributions in longitude / latitude give widths sigma ~ 27
arcmin / 23 arcmin in the range |Delta lambda, Delta beta|<50 arcmin for
Ptolemaios and sigma ~ 22 arcmin /18 arcmin in Ulugh Beg. Fits to the range
|Delta lambda, Delta beta|<100 arcmin gives 10-15 per cent larger widths,
showing that the error distributions are broader than gaussians. The fraction
of stars with positions wrong by more than 150 arcmin is about 2 per cent for
Ptolemaios and 0.1 per cent in Ulugh Beg; the numbers of unidentified stars are
1 in Ptolemaios and 3 in Ulugh Beg. These numbers testify to the excellent
quality of both star catalogues (as edited by Toomer and Knobel).Comment: to be published in Astronomy and Astrophysics; 34 pages with 57
Figures. Note changed address and email address of first autho
Configuring the caller in ambiguous encounters: volunteer handling of calls to Samaritans emotional support services
This paper discusses volunteer strategies for handling
and assessing calls to Samaritans emotional support services for the suicidal and despairing. It presents findings from the qualitative components of a two year mixed methods study based on an online caller survey, branch observations and interviews with volunteers and callers throughout the UK. A thematic analysis of the qualitative data analysis was undertaken using the principle of constant comparison. Many calls fell beyond the primary remit of a crisis service, and called for rapid attribution and assessment. Uncertainty about identifying âgoodâ calls and recognizing those which were not caused difficulty, frustration and negative attribution towards some callers. This paper presents our analysis of volunteersâ accounts of how they configure the caller in intrinsically uncertain and ambiguous encounters, and how such strategies relate to the formal principles of unconditional support and non-judgemental active listening espoused by the organization
Radial distribution of the multiple stellar populations in omega Centauri
We present a detailed study of the radial distribution of the multiple
populations identified in the Galactic globular cluster omega Cen. We used both
space-based images (ACS/WFC and WFPC2) and ground-based images (FORS1@VLT and
[email protected] ESO telescopes) to map the cluster from the inner core to the
outskirts (~20 arcmin). These data sets have been used to extract high-accuracy
photometry for the construction of color-magnitude diagrams and astrometric
positions of ~900 000 stars. We find that in the inner ~2 core radii the blue
main sequence (bMS) stars slightly dominate the red main sequence (rMS) in
number. At greater distances from the cluster center, the relative numbers of
bMS stars with respect to rMS drop steeply, out to ~8 arcmin, and then remain
constant out to the limit of our observations. We also find that the dispersion
of the Gaussian that best fits the color distribution within the bMS is
significantly greater than the dispersion of the Gaussian that best fits the
color distribution within the rMS. In addition, the relative number of
intermediate-metallicity red-giant-branch stars which includes the progeny of
the bMS) with respect to the metal-poor component (the progeny of the rMS)
follows a trend similar to that of the main-sequence star-count ratio
N_bMS/N_rMS. The most metal-rich component of the red-giant branch follows the
same distribution as the intermediate-metallicity component. We briefly discuss
the possible implications of the observed radial distribution of the different
stellar components in omega Cen.Comment: 16 pages, 14 figures (6 in low resolution), 3 tables. Accepted for
publication in Astronomy and Astrophysics on 23 September 200
Fully broadband vAPP coronagraphs enabling polarimetric high contrast imaging
We present designs for fully achromatic vector Apodizing Phase Plate (vAPP)
coronagraphs, that implement low polarization leakage solutions and achromatic
beam-splitting, enabling observations in broadband filters. The vAPP is a pupil
plane optic, inducing the phase through the inherently achromatic geometric
phase. We discuss various implementations of the broadband vAPP and set
requirements on all the components of the broadband vAPP coronagraph to ensure
that the leakage terms do not limit a raw contrast of 1E-5. Furthermore, we
discuss superachromatic QWPs based of liquid crystals or quartz/MgF2
combinations, and several polarizer choices. As the implementation of the
(broadband) vAPP coronagraph is fully based on polarization techniques, it can
easily be extended to furnish polarimetry by adding another QWP before the
coronagraph optic, which further enhances the contrast between the star and a
polarized companion in reflected light. We outline several polarimetric vAPP
system designs that could be easily implemented in existing instruments, e.g.
SPHERE and SCExAO.Comment: 11 pages, 5 figures, presented at SPIE Astronomical Telescopes and
Instrumentation 201
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