5,521 research outputs found
Stationary components of HeI in strong magnetic fields - a tool to identify magnetic DB white dwarfs
In only three of the 61 known magnetic white dwarfs helium has been
identified unambiguously while about 20% of all non-magnetic stars of this
class are known to contain HeI or HeII. Until recently, data for HeI data were
available only for magnetic fields below 20MG. This changed with the
publication of extensive data by the group in Heidelberg. The corresponding
calculations have now been completed for the energetically lowest five states
of singlet and triplet symmetry for the subspaces with |m| <= 3; selected
calculations have been performed for even higher excitations. In strongly
magnetized white dwarfs only line components are visible whose wavelengths vary
slowly with respect to the magnetic field, particularly stationary components
which have a wavelength minimum or maximum in the range of the magnetic fields
strengths on the stellar surface. In view of the many ongoing surveys finding
white dwarfs we want to provide the astronomical community with a tool to
identify helium in white dwarfs for fields up to 5.3GG. To this end we present
all calculated helium line components whose wavelengths in the UV, optical, and
near IR vary slowly enough with respect to the field strength to produce
visible absorption features. We also list all stationary line components in
this spectral range. Finally, we find series of minima and maxima which occur
as a result of series of extremal transitions to increasingly higher
excitations. We estimated the limits for 8 series which can possibly give rise
to additional absorption in white dwarf spectra; one strong absorption feature
in GD229 which is yet unexplained by stationary components is very close to two
estimated series limits.Comment: 8 pages, 2 figures, accepted for publication by Astronomy and
Astrophysic
Patterns of variability in voice onset time: a developmental study of motor speech skills in humans
This study investigated the developmental patterns of variability in the speech parameter voice onset time (VOT) in forty six children. Five groups of children participated in the study as follows: i) Group 1 - aged 5 years 8 months (n=6); ii) Group 2 - 7 years 10 months (n=10); iii) Group 3 - 9 years 10 months (n=10); iv) Group 4 - 11 years 10 months (n=10), and v) Group 5 - 13 years 2 months (n=10). Coefficient of variation (COV) values were examined for the VOT values of both "voiceless" (/p t k/) and "voiced" (/b d g/) plosives to determine patterns of variability. Significant effects of age were revealed for both the voiceless and voiced plosives, and levels of variability leveled off for Group 4. The data suggest that although variability in VOT decreases with age, the presence of residual variability may be a prerequisite for the further refinement of motor speech skills
Cosmological Expansion in the Randall-Sundrum Brane World Scenario
The cosmology of the Randall-Sundrum scenario for a positive tension brane in
a 5-D Universe with localized gravity has been studied previously. In the
radiation-dominated Universe, it was suggested that there are two solutions for
the cosmic scale factor a(t) : the standard solution , and a
solution , which is incompatible with standard big bang
nucleosynthesis. In this note, we reconsider expansion of the Universe in this
scenario. We derive and solve a first order, linear differential equation for
H^2, the square of the expansion rate of the Universe, as a function of a. The
differences between our equation for H^2 and the relationship found in standard
cosmology are (i) there is a term proportional to density squared (a fact
already known), which is small when the density is small compared to the brane
tension, and (ii) there is a contribution which acts like a relativistic fluid.
We show that this second contribution is due to gravitational degrees of
freedom in the bulk. Thus, we find that there need not be any conflict between
cosmology of the Randall-Sundrum scenario and the standard model of cosmology.
We discuss how reheating at the end of inflation leads to the correct
relationship between matter density and expansion rate, , and the conditions that must be met for the expansion rate of the
Universe to be close to its standard model value around the epoch of
cosmological nucleosynthesis.Comment: 14 pages, 1 figure. Error in interpretation corrected, references
adde
Chandra study of an overdensity of X-ray sources around two distant (z~0.5) clusters
We present results from a Chandra X-ray Observatory study of the field X-ray
source populations in 4 different observations: two high-redshift (z~0.5)
clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields
with similar exposure time. Surprisingly, the 0.5-2 keV source surface
densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg
cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a
factor of ~2 the value expected on the basis of the ROSAT and Chandra
logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2
fields are combined (i.e. a probability to be a statistical fluctuation of <1%
and <0.04%, respectively). The same analysis performed on the non-cluster
fields and on the outer chips of the cluster fields does NOT show evidence of
such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the
detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs
and shows no sign of intrinsic absorption. The few (~10 out of 35) optical
identifications available to date confirm that most of them are, as expected,
AGNs but the number of redshifts available is too small to allow conclusions on
their nature. We discuss possible interpretations of the overdensity in terms
of: a statistical variation of Cosmic Background sources; a concentration of
AGNs and/or powerful starburst galaxies associated with the clusters; and g
ravitational lensing of background QSO's by the galaxy clusters. All
explanations are however difficult to reconcile with the large number of excess
sources detected. Deeper X-ray observations and more redshifts measurements are
clearly required to settle the issue.Comment: 22 LateX pages (including Tables and Figures), uses psfig.sty and
emulateapj.sty. Accepted for publication in Astrophysical Journa
Constraining Lyman-alpha spatial offsets at from VANDELS slit spectroscopy
We constrain the distribution of spatially offset Lyman-alpha emission
(Ly) relative to rest-frame ultraviolet emission in high
redshift () Lyman-break galaxies (LBGs) exhibiting Ly emission
from VANDELS, a VLT/VIMOS slit-spectroscopic survey of the CANDELS Ultra Deep
Survey and Chandra Deep Field South fields (
total). Because slit spectroscopy compresses two-dimensional spatial
information into one spatial dimension, we use Bayesian inference to recover
the underlying Ly spatial offset distribution. We model the
distribution using a 2D circular Gaussian, defined by a single parameter
, the standard deviation expressed in polar
coordinates. Over the entire redshift range of our sample (), we find
kpc ( conf.),
corresponding to arcsec at . We also find that
decreases significantly with redshift. Because
Ly spatial offsets can cause slit-losses, the decrease in
with redshift can partially explain the increase
in the fraction of Ly emitters observed in the literature over this
same interval, although uncertainties are still too large to reach a strong
conclusion. If continues to decrease into the
reionization epoch, then the decrease in Ly transmission from galaxies
observed during this epoch might require an even higher neutral hydrogen
fraction than what is currently inferred. Conversely, if spatial offsets
increase with the increasing opacity of the IGM, slit losses may explain some
of the drop in Ly transmission observed at . Spatially resolved
observations of Ly and UV continuum at are needed to settle the
issue.Comment: Submitted to MNRA
Through the looking GLASS: HST spectroscopy of faint galaxies lensed by the Frontier Fields cluster MACS0717.5+3745
The Grism Lens-Amplified Survey from Space (GLASS) is a Hubble Space
Telescope (HST) Large Program, which will obtain 140 orbits of grism
spectroscopy of the core and infall regions of 10 galaxy clusters, selected to
be among the very best cosmic telescopes. Extensive HST imaging is available
from many sources including the CLASH and Frontier Fields programs. We
introduce the survey by analyzing spectra of faint multiply-imaged galaxies and
galaxy candidates obtained from the first seven orbits out of
fourteen targeting the core of the Frontier Fields cluster MACS0717.5+3745.
Using the G102 and G141 grisms to cover the wavelength range 0.8-1.7m, we
confirm 4 strongly lensed systems by detecting emission lines in each of the
images. For the 9 galaxy candidates clear from contamination, we do
not detect any emission lines down to a seven-orbit 1 noise level of
510erg scm. Taking lensing magnification
into account, our flux sensitivity reaches 0.2-510erg
scm. These limits over an uninterrupted wavelength range rule out
the possibility that the high- galaxy candidates are instead strong line
emitters at lower redshift. These results show that by means of careful
modeling of the background - and with the assistance of lensing magnification -
interesting flux limits can be reached for large numbers of objects, avoiding
pre-selection and the wavelength restrictions inherent to ground-based
multi-slit spectroscopy. These observations confirm the power of slitless HST
spectroscopy even in fields as crowded as a cluster core.Comment: Accepted by ApJ letters, 8 pages, 4 figures, GLASS website at
http://glass.physics.ucsb.ed
The 160 Square Degree ROSAT Survey: the Revised Catalog of 201 Clusters with Spectroscopic Redshifts
We present the revised catalog of galaxy clusters detected as extended X-ray
sources in the 160 Square Degree ROSAT Survey, including spectroscopic
redshifts and X-ray luminosities for 200 of the 201 members. The median
redshift is z~0.25 and the median X-ray luminosity is 4.2e+43 erg/s/h50^2
(0.5-2.0 keV). This is the largest high-redshift sample of X-ray selected
clusters published to date. There are 73 objects at z>0.3 and 22 objects at
z>0.5 drawn from a statistically complete flux-limited survey with a median
object flux of 1.4d-13 erg/cm^2/s. We describe the optical follow-up of these
clusters with an emphasis on our spectroscopy which has yielded 155 cluster
redshifts, 110 of which are presented here for the first time. These
measurements combined with 45 from the literature and other sources provide
near-complete spectroscopic coverage for our survey. We discuss the final
optical identifications for the extended X-ray sources in the survey region and
compare our results to similar X-ray cluster searches.Comment: 17 pages, 7 figs, accepted for publication in ApJ, a version with
full resolution images is available at
http://www.eso.org/~cmullis/papers/160sd-catalog.ps.gz, machine-readable
versions of the catalog are available at
http://www.eso.org/~cmullis/research/160sd-catalog.htm
Measuring Cation Dependent DNA Polymerase Fidelity Landscapes by Deep Sequencing
High-throughput recording of signals embedded within inaccessible micro-environments is a technological challenge. The ideal recording device would be a nanoscale machine capable of quantitatively transducing a wide range of variables into a molecular recording medium suitable for long-term storage and facile readout in the form of digital data. We have recently proposed such a device, in which cation concentrations modulate the misincorporation rate of a DNA polymerase (DNAP) on a known template, allowing DNA sequences to encode information about the local cation concentration. In this work we quantify the cation sensitivity of DNAP misincorporation rates, making possible the indirect readout of cation concentration by DNA sequencing. Using multiplexed deep sequencing, we quantify the misincorporation properties of two DNA polymerases – Dpo4 and Klenow exo[subscript −] – obtaining the probability and base selectivity of misincorporation at all positions within the template. We find that Dpo4 acts as a DNA recording device for Mn[superscript 2+] with a misincorporation rate gain of ~2%/mM. This modulation of misincorporation rate is selective to the template base: the probability of misincorporation on template T by Dpo4 increases >50-fold over the range tested, while the other template bases are affected less strongly. Furthermore, cation concentrations act as scaling factors for misincorporation: on a given template base, Mn[superscript 2+] and Mg[superscript 2+] change the overall misincorporation rate but do not alter the relative frequencies of incoming misincorporated nucleotides. Characterization of the ion dependence of DNAP misincorporation serves as the first step towards repurposing it as a molecular recording device.Damon Runyon Cancer Research FoundationNational Institutes of Health (U.S.)National Science Foundation (U.S.)McGovern Institute for Brain Research at MITMassachusetts Institute of Technology. Media LaboratoryNew York Stem Cell Foundation (Robertson Neuroscience Investigator Award)Paul G. Allen Family Foundation (Distinguished Investigator in Neuroscience Award
The Grism Lens-Amplified Survey from Space (GLASS). I. Survey overview and first data release
We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a
large Hubble Space Telescope program aimed at obtaining grism spectroscopy of
the fields of ten massive clusters of galaxies at redshift z=0.308-0.686,
including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near
infrared spectra of the cluster cores, covering the wavelength range
0.81-1.69mum through grisms G102 and G141, while the Advanced Camera for
Surveys in parallel mode provides G800L spectra of the infall regions of the
clusters. The WFC3 spectra are taken at two almost orthogonal position angles
in order to minimize the effects of confusion. After summarizing the scientific
drivers of GLASS, we describe the sample selection as well as the observing
strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF
cluster and the first one observed by GLASS, to illustrate the data quality and
the high-level data products. Each spectrum brighter than H_AB=23 is visually
inspected by at least two co-authors and a redshift is measured when sufficient
information is present in the spectra. Furthermore, we conducted a thorough
search for emission lines through all the GLASS WFC3 spectra with the aim of
measuring redshifts for sources with continuum fainter than H_AB=23. We provide
a catalog of 139 emission-line based spectroscopic redshifts for extragalactic
sources, including three new redshifts of multiple image systems (one probable,
two tentative). In addition to the data itself we also release software tools
that are helpful to navigate the data.Comment: ApJ in press. GLASS data available at
https://archive.stsci.edu/prepds/glass/ . More info on GLASS available at
http://glass.physics.ucsb.edu
Propagation of squeezed radiation through amplifying or absorbing random media
We analyse how nonclassical features of squeezed radiation (in particular the
sub-Poissonian noise) are degraded when it is transmitted through an amplifying
or absorbing medium with randomly located scattering centra. Both the cases of
direct photodetection and of homodyne detection are considered. Explicit
results are obtained for the dependence of the Fano factor (the ratio of the
noise power and the mean current) on the degree of squeezing of the incident
state, on the length and the mean free path of the medium, the temperature, and
on the absorption or amplification rate.Comment: 8 pages, 4 figure
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