418 research outputs found
The RR Lyrae Star Period - K-band Luminosity Relation of the Globular Cluster M3
That the RR Lyrae star period -- K-band luminosity relation is a promising
tool as a distance indicator in the Milky Way and Local Group of Galaxies is
apparent on observational and theoretical grounds in the literature. Less clear
is the sensitivity of the relation, and consequently the physics of horizontal
branch stars, to differences in stellar environment. In this paper, the first
measurement of the (fundamental) period -- K-band luminosity relation for the
central region of a globular cluster is presented. It is based on a sample of
seven RR Lyrae stars imaged with adaptive optics. In addition, the relation for
the outer region has been reanalyzed, and is found to be in good agreement with
both the previous estimate by Longmore et al. (1990), and with the inner region
relation, especially when irregular and double-mode RR Lyrae stars are
excluded. Importantly, there is no difference between the slope of the inner
and outer region relation within measurement uncertainties, suggesting no
difference in evolutionary state (luminosity). Taking the M3 distance modulus
as 15.0 +/- 0.07mag, the period -- absolute K-band magnitude relation derived
by linear least squares fitting is: M_K = -0.96 (+/- 0.10) - 2.42 (+/- 0.16)Log
Po for the inner region. Excluding irregular variable stars, the outer region
relation is: M_K = -1.07 (+/- 0.10) - 2.38 (+/- 0.15)Log Po. This good
agreement provides further strong support for the near-IR period-luminosity
relation as a distance indicator.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 5
figure
The Color Variability of Quasars
We quantify quasar color-variability using an unprecedented variability
database - ugriz photometry of 9093 quasars from SDSS Stripe 82, observed over
8 years at ~60 epochs each. We confirm previous reports that quasars become
bluer when brightening. We find a redshift dependence of this blueing in a
given set of bands (e.g. g and r), but show that it is the result of the flux
contribution from less-variable or delayed emission lines in the different SDSS
bands at different redshifts. After correcting for this effect, quasar
color-variability is remarkably uniform, and independent not only of redshift,
but also of quasar luminosity and black hole mass. The color variations of
individual quasars, as they vary in brightness on year timescales, are much
more pronounced than the ranges in color seen in samples of quasars across many
orders of magnitude in luminosity. This indicates distinct physical mechanisms
behind quasar variability and the observed range of quasar luminosities at a
given black hole mass - quasar variations cannot be explained by changes in the
mean accretion rate. We do find some dependence of the color variability on the
characteristics of the flux variations themselves, with fast, low-amplitude,
brightness variations producing more color variability. The observed behavior
could arise if quasar variability results from flares or ephemeral hot spots in
an accretion disc.Comment: Accepted for publication in ApJ - in press, 17 pages, 14 figures -
v2: abstract typo corrected & reference clean-u
Think Outside the Color Box: Probabilistic Target Selection and the SDSS-XDQSO Quasar Targeting Catalog
We present the SDSS-XDQSO quasar targeting catalog for efficient flux-based
quasar target selection down to the faint limit of the Sloan Digital Sky Survey
(SDSS) catalog, even at medium redshifts (2.5 <~ z <~ 3) where the stellar
contamination is significant. We build models of the distributions of stars and
quasars in flux space down to the flux limit by applying the
extreme-deconvolution method to estimate the underlying density. We convolve
this density with the flux uncertainties when evaluating the probability that
an object is a quasar. This approach results in a targeting algorithm that is
more principled, more efficient, and faster than other similar methods. We
apply the algorithm to derive low-redshift (z < 2.2), medium-redshift (2.2 <= z
3.5) quasar probabilities for all 160,904,060
point sources with dereddened i-band magnitude between 17.75 and 22.45 mag in
the 14,555 deg^2 of imaging from SDSS Data Release 8. The catalog can be used
to define a uniformly selected and efficient low- or medium-redshift quasar
survey, such as that needed for the SDSS-III's Baryon Oscillation Spectroscopic
Survey project. We show that the XDQSO technique performs as well as the
current best photometric quasar-selection technique at low redshift, and
outperforms all other flux-based methods for selecting the medium-redshift
quasars of our primary interest. We make code to reproduce the XDQSO quasar
target selection publicly available
Photometric redshifts and quasar probabilities from a single, data-driven generative model
We describe a technique for simultaneously classifying and estimating the
redshift of quasars. It can separate quasars from stars in arbitrary redshift
ranges, estimate full posterior distribution functions for the redshift, and
naturally incorporate flux uncertainties, missing data, and multi-wavelength
photometry. We build models of quasars in flux-redshift space by applying the
extreme deconvolution technique to estimate the underlying density. By
integrating this density over redshift one can obtain quasar flux-densities in
different redshift ranges. This approach allows for efficient, consistent, and
fast classification and photometric redshift estimation. This is achieved by
combining the speed obtained by choosing simple analytical forms as the basis
of our density model with the flexibility of non-parametric models through the
use of many simple components with many parameters. We show that this technique
is competitive with the best photometric quasar classification
techniques---which are limited to fixed, broad redshift ranges and high
signal-to-noise ratio data---and with the best photometric redshift techniques
when applied to broadband optical data. We demonstrate that the inclusion of UV
and NIR data significantly improves photometric quasar--star separation and
essentially resolves all of the redshift degeneracies for quasars inherent to
the ugriz filter system, even when included data have a low signal-to-noise
ratio. For quasars spectroscopically confirmed by the SDSS 84 and 97 percent of
the objects with GALEX UV and UKIDSS NIR data have photometric redshifts within
0.1 and 0.3, respectively, of the spectroscopic redshift; this amounts to about
a factor of three improvement over ugriz-only photometric redshifts. Our code
to calculate quasar probabilities and redshift probability distributions is
publicly available
Dose optimisation in paediatric radiography - using regression models to investigate the relative impact of acquisition factors on image quality and radiation dose
Objective: To investigate the optimum pelvis X-ray acquisition factors for a 10-year-old child. Secondly, to evaluate the impact of each acquisition factor on image quality (IQ) and radiation dose.
Method: Images were acquired using a pelvis phantom and a range of acquisition parameters; e.g. tube potential, additional filtration and source-to-image distance (SID). Automatic exposure control (AEC) was used with two orientations (head towards/away from two outer chambers) and three different chamber selections. Visual IQ was evaluated using relative and absolute-VGA methods. Radiation doses were measured by placing a dosimeter on the anterior surface of the phantom. Regression analysis was used to determine optimum parameters.
Results: The optimised technique (178.8 µGy), with diagnostic IQ, was with 89kVp, 130 cm SID and with 1 mm Al + 0.1 mm Cu filtration. This technique was with the head towards the two outer AEC chambers. Regression analysis showed that SID had the lowest impact on IQ (β = 0.002 95% CI −0.001 to 0.005) and dose (β = −0.96 95% CI −0.40 to −1.53). The impact of filtration on dose (β = −76.24 95% CI −86.76 to −85.72) was higher than tube potential (β = −13.44 95% CI −14.34 to −12.53). The following impact ratios were higher on IQ than radiation dose: filtration/kVp; 11.28 times, filtration/SID; 7.01 times and kVp/SID; 0.62 times.
Conclusion: Optimised parameters were identified as 89 kVp, 130 cm SID and with 1 mm Al + 0.1 mm Cu additional filtration. Regression analysis demonstrated that filtration and tube potential had the greatest effect on radiation dose and IQ, respectively
A spectral atlas of post-main-sequence stars in omega Centauri: kinematics, evolution, enrichment and interstellar medium
We present a spectral atlas of the post-main-sequence population of the most
massive Galactic globular cluster, omega Centauri. Spectra were obtained of
more than 1500 stars selected as uniformly as possible from across the (B, B-V)
colour-magnitude diagram of the proper motion cluster member candidates of van
Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre
spectrograph at the Anglo Australian Telescope, and cover the approximate range
lambda~3840-4940 Angstroem. We measure the radial velocities, effective
temperatures, metallicities and surface gravities by fitting ATLAS9 stellar
atmosphere models. We analyse the cluster membership and stellar kinematics,
interstellar absorption in the Ca II K line at 3933 Angstroem, the RR Lyrae
instability strip and the extreme horizontal branch, the metallicity spread and
bimodal CN abundance distribution of red giants, nitrogen and s-process
enrichment, carbon stars, pulsation-induced Balmer line emission on the
asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright
stars. Membership is confirmed for the vast majority of stars, and the radial
velocities clearly show the rotation of the cluster core. We identify
long-period RR Lyrae-type variables with low gravity, and low-amplitude
variables coinciding with warm RR Lyrae stars. A barium enhancement in the
coolest red giants indicates that 3rd dredge-up operates in AGB stars in omega
Cen. This is distinguished from the pre-enrichment by more massive AGB stars,
which is also seen in our data. The properties of the AGB, post-AGB and
UV-bright stars suggest that RGB mass loss may be less efficient at very low
metallicity, [Fe/H]<<-1, increasing the importance of mass loss on the AGB. The
catalogue and spectra are made available via CDS.Comment: accepted for publication in MNRA
The SDSS-III Baryon Oscillation Spectroscopic Survey: Quasar Target Selection for Data Release Nine
The SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), a five-year
spectroscopic survey of 10,000 deg^2, achieved first light in late 2009. One of
the key goals of BOSS is to measure the signature of baryon acoustic
oscillations in the distribution of Ly-alpha absorption from the spectra of a
sample of ~150,000 z>2.2 quasars. Along with measuring the angular diameter
distance at z\approx2.5, BOSS will provide the first direct measurement of the
expansion rate of the Universe at z > 2. One of the biggest challenges in
achieving this goal is an efficient target selection algorithm for quasars over
2.2 < z < 3.5, where their colors overlap those of stars. During the first year
of the BOSS survey, quasar target selection methods were developed and tested
to meet the requirement of delivering at least 15 quasars deg^-2 in this
redshift range, out of 40 targets deg^-2. To achieve these surface densities,
the magnitude limit of the quasar targets was set at g <= 22.0 or r<=21.85.
While detection of the BAO signature in the Ly-alpha absorption in quasar
spectra does not require a uniform target selection, many other astrophysical
studies do. We therefore defined a uniformly-selected subsample of 20 targets
deg^-2, for which the selection efficiency is just over 50%. This "CORE"
subsample will be fixed for Years Two through Five of the survey. In this paper
we describe the evolution and implementation of the BOSS quasar target
selection algorithms during the first two years of BOSS operations. We analyze
the spectra obtained during the first year. 11,263 new z>2.2 quasars were
spectroscopically confirmed by BOSS. Our current algorithms select an average
of 15 z > 2.2 quasars deg^-2 from 40 targets deg^-2 using single-epoch SDSS
imaging. Multi-epoch optical data and data at other wavelengths can further
improve the efficiency and completeness of BOSS quasar target selection.
[Abridged]Comment: 33 pages, 26 figures, 12 tables and a whole bunch of quasars.
Submitted to Ap
The Host Galaxies of Short-Duration Gamma-Ray Bursts: Luminosities, Metallicities, and Star Formation Rates
The association of some short-duration gamma-ray bursts (GRBs) with
elliptical galaxies established that their progenitors, unlike those of long
GRBs, belong to an old stellar population. However, the majority of short GRBs
appear to occur in star forming galaxies, raising the possibility that some
progenitors are related to recent star formation activity. Here we present
optical spectroscopy of these hosts and measure their luminosities, star
formation rates, and metallicities. We find luminosities of L_B~0.1-1.5 L*,
star formation rates of SFR~0.2-6 M_sun/yr, and metallicities of
12+log(O/H)~8.5-8.9 (Z~0.6-1.6 Z_sun). A detailed comparison to the hosts of
long GRBs reveals systematically higher luminosities, lower specific star
formation rates (SFR/L_B) by about an order of magnitude, and higher
metallicities by about 0.6 dex. The K-S probability that the short and long GRB
hosts are drawn from the same underlying galaxy distribution is only 10^-3. On
the other hand, short GRB hosts exhibit excellent agreement with the specific
star formation rates and the luminosity-metallicity relation of field galaxies
at z~0.1-1. We thus conclude that short GRB hosts are not dominated by young
stellar populations like long GRBs hosts. Instead, short GRB hosts appear to be
drawn uniformly from the underlying galaxy distribution, indicating that the
progenitors have a wide age distribution of several Gyr.Comment: Submitted to Ap
The worldwide marine radiocarbon reservoir effect: definitions, mechanisms, and prospects
When a carbon reservoir has a lower radiocarbon content than the atmosphere, this is referred to as a reservoir effect. This is expressed as an offset between the radiocarbon ages of samples from the two reservoirs at a single point in time. The marine reservoir effect (MRE) has been a major concern in the radiocarbon community, as it introduces an additional source of error that is often difficult to accurately quantify. For this reason, researchers are often reluctant to date marine material where they have another option. The influence of this phenomenon makes the study of the MRE important for a broad range of applications. The advent of Accelerator Mass Spectrometry (AMS) has reduced sample size requirements and increased measurement precision, in turn increasing the number of studies seeking to measure marine samples. These studies rely on overcoming the influence of the MRE on marine radiocarbon dates through the worldwide quantification of the local parameter ΔR, that is, the local variation from the global average MRE. Furthermore, the strong dependence on ocean dynamics makes the MRE a useful indicator for changes in oceanic circulation, carbon exchange between reservoirs, and the fate of atmospheric CO2, all of which impact Earth's climate. This article explores data from the Marine Reservoir Database and reviews the place of natural radiocarbon in oceanic records, focusing on key questions (e.g., changes in ocean dynamics) that have been answered by MRE studies and on their application to different subjects
- …