580 research outputs found
Mystery of the Lyα Blobs
We present Spitzer Space Telescope observations of the extended Lyman α blobs associated with the z=2.38 over-density J2143-4423, the largest known structure (110 Mpc) above z=2. We detect all 4 of the Lyα blobs in all four IRAC channels and we also detect 3 out of 4 of the blobs with MIPS 24μm. Conversion from rest-wavelength 7μm to total far-infrared luminosity using locally derived correlations suggests all the detected sources are in the class of ULIRGs or even Hyper-LIRGs. We find a weak correlation between Lyα and mid-infrared emission for the Lyα blobs (L_(Lyα)/L_(bol) = 0.05-0.2%). Nearly all Lyα blobs show some evidence for interaction, either in HST imaging, or the proximity of multiple MIPS sources within the Lyα cloud. This suggests that interaction or even mergers may be related to the production of Lyα blobs. Optical through infrared SEDs of the Lyα blobs do not show a clear 1.6μm bump, but rather are indicative of a composite of star formation and AGN energy sources
Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38
We present high contrast images of the hydrogen white dwarf G 29-38 taken in
the near infrared with the Hubble Space Telescope and the Gemini North
Telescope as part of a high contrast imaging search for substellar objects in
orbit around nearby white dwarfs.
We review the current limits on planetary companions for G29-38, the only
nearby white dwarf with an infrared excess due to a dust disk. We add our
recent observations to these limits to produce extremely tight constraints on
the types of possible companions that could be present. No objects 6
M are detected in our data at projected separations 12 AU, and no
objects 16 M are detected for separations from 3 to 12 AU, assuming
a total system age of 1 Gyr. Limits for companions at separations 3 AU come
from a combination of 2MASS photometry and previous studies of G29-38's
pulsations. Our imaging with Gemini cannot confirm a tentative claim for the
presence of a low mass brown dwarf. These observations demonstrate that a
careful combination of several techniques can probe nearby white dwarfs for
large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap
Interplay between magnetism and short-range order in medium- and high-entropy alloys: CrCoNi, CrFeCoNi, and CrMnFeCoNi
The impact of magnetism on predicted atomic short-range order in three
medium- and high-entropy alloys is studied using a first-principles,
all-electron, Landau-type linear response theory, coupled with lattice-based
atomistic modelling. We perform two sets of linear-response calculations: one
in which the paramagnetic state is modelled within the disordered local moment
picture, and one in which systems are modelled in a magnetically ordered state,
which is ferrimagnetic for the alloys considered in this work. We show that the
treatment of magnetism can have significant impact both on the predicted
temperature of atomic ordering and also the nature of atomic order itself. In
CrCoNi, we find that the nature of atomic order changes from being
-like when modelled in the paramagnetic state to MoPt-like
when modelled assuming the system has magnetically ordered. In CrFeCoNi, atomic
correlations between Fe and the other elements present are dramatically
strengthened when we switch from treating the system as magnetically disordered
to magnetically ordered. Our results show it is necessary to consider the
magnetic state when modelling multicomponent alloys containing mid- to
late- elements. Further, we suggest that there may be high-entropy alloy
compositions containing transition metals that will exhibit specific
atomic short-range order when thermally treated in an applied magnetic field.
This has the potential to provide a route for tuning physical and mechanical
properties in this class of materials.Comment: 26 pages, 4 figures, 2 table
Revisiting N\'eel 60 years on: the magnetic anisotropy of FeNi (tetrataenite)
The magnetocrystalline anisotropy energy of atomically ordered
FeNi (the meteoritic mineral tetrataenite) is studied within a
first-principles electronic structure framework. Two compositions are examined:
equiatomic FeNi and an Fe-rich composition,
FeNi. It is confirmed that, for the single crystals modelled
in this work, the leading-order anisotropy coefficient dominates the
higher-order coefficients and . To enable comparison with
experiment, the effects of both imperfect atomic long-range order and finite
temperature are included. While our computational results initially appear to
undershoot the measured experimental values for this system, careful scrutiny
of the original analysis due to N\'{e}el et al. [J. Appl. Phys. 35, 873 (1964)]
suggests that our computed value of is, in fact, consistent with
experimental values, and that the noted discrepancy has its origins in the
nanoscale polycrystalline, multivariant nature of experimental samples, that
yields much larger values of and than expected a priori. These
results provide fresh insight into the existing discrepancies in the literature
regarding the value of tetrataenite's uniaxial magnetocrystalline anisotropy in
both natural and synthetic samples.Comment: 9 pages, 3 figures, 2 table
Translation selectively destroys non-functional transcription complexes
Transcription elongation stalls at lesions in the DNA template(1). For the DNA lesion to be repaired, the stalled transcription elongation complex (EC) has to be removed from the damaged site(2). Here we show that translation, which is coupled to transcription in bacteria, actively dislodges stalled ECs from the damaged DNA template. By contrast, paused, but otherwise elongation-competent, ECs are not dislodged by the ribosome. Instead, they are helped back into processive elongation. We also show that the ribosome slows down when approaching paused, but not stalled, ECs. Our results indicate that coupled ribosomes functionally and kinetically discriminate between paused ECs and stalled ECs, ensuring the selective destruction of only the latter. This functional discrimination is controlled by the RNA polymerase\u27s catalytic domain, the Trigger Loop. We show that the transcription-coupled DNA repair helicase UvrD, proposed to cause backtracking of stalled ECs(3), does not interfere with ribosome-mediated dislodging. By contrast, the transcription-coupled DNA repair translocase Mfd(4) acts synergistically with translation, and dislodges stalled ECs that were not destroyed by the ribosome. We also show that a coupled ribosome efficiently destroys misincorporated ECs that can cause conflicts with replication(5). We propose that coupling to translation is an ancient and one of the main mechanisms of clearing non-functional ECs from the genome
Cool Customers in the Stellar Graveyard II: Limits to Substellar Objects Around Nearby DAZ White Dwarfs
Results from a concerted Hubble Space Telescope (HST) survey of nearby white
dwarfs for substellar objects is presented. A total of 7 DAZ white dwarfs with
distances of 50 pc had high contrast and high spatial resolution NICMOS
coronagraphic images taken to search for candidate substellar objects at
separations \ltorder10\arcsec away. Limits to unresolved companions are
derived through analysis of 2MASS photometry of the white dwarfs compared to
expected fluxes based on the WDs effective temperature, distance, and gravity.
From our survey with HST we find several candidate companions, which have
been or will be followed up with second epoch observations. We find that for
four of the white dwarfs we are sensitive to planetary companions \gtorder10
M, and sensitive to companions 18 M for all of the targets.
The lack of significant near infrared excesses for our targets limits any kind
of unresolved companions present to be substellar. In light of these results we
make several comments on the possibility of determining the origin of white
dwarfs with metals in their atmospheres.Comment: 11 pages, 7 figures, emulateapj, revisions to figures and text, to
appear in September A
Magnetic field diagnostics and spatio-temporal variability of the solar transition region
Magnetic field diagnostics of the transition region from the chromosphere to
the corona faces us with the problem that one has to apply extreme UV
spectro-polarimetry. While for coronal diagnostic techniques already exist
through infrared coronagraphy above the limb and radio observations on the
disk, for the transition region one has to investigate extreme UV observations.
However, so far the success of such observations has been limited, but there
are various projects to get spectro-polarimetric data in the extreme UV in the
near future. Therefore it is timely to study the polarimetric signals we can
expect for such observations through realistic forward modeling.
We employ a 3D MHD forward model of the solar corona and synthesize the
Stokes I and Stokes V profiles of C IV 1548 A. A signal well above 0.001 in
Stokes V can be expected, even when integrating for several minutes in order to
reach the required signal-to-noise ratio, despite the fact that the intensity
in the model is rapidly changing (just as in observations). Often this
variability of the intensity is used as an argument against transition region
magnetic diagnostics which requires exposure times of minutes. However, the
magnetic field is evolving much slower than the intensity, and thus when
integrating in time the degree of (circular) polarization remains rather
constant. Our study shows the feasibility to measure the transition region
magnetic field, if a polarimetric accuracy on the order of 0.001 can be
reached, which we can expect from planned instrumentation.Comment: Accepted for publication in Solar Physics (4.Mar.2013), 19 pages, 9
figure
The Diagnostic Potential of Transition Region Lines under-going Transient Ionization in Dynamic Events
We discuss the diagnostic potential of high cadence ultraviolet spectral data
when transient ionization is considered. For this we use high cadence UV
spectra taken during the impulsive phase of a solar flares (observed with
instruments on-board the Solar Maximum Mission) which showed excellent
correspondence with hard X-ray pulses. The ionization fraction of the
transition region ion O V and in particular the contribution function for the O
V 1371A line are computed within the Atomic Data and Analysis Structure, which
is a collection of fundamental and derived atomic data and codes which
manipulate them. Due to transient ionization, the O V 1371A line is enhanced in
the first fraction of a second with the peak in the line contribution function
occurring initially at a higher electron temperature than in ionization
equilibrium. The rise time and enhancement factor depend mostly on the electron
density. The fractional increase in the O V 1371A emissivity due to transient
ionization can reach a factor of 2--4 and can explain the fast response in the
line flux of transition regions ions during the impulsive phase of flares
solely as a result of transient ionization. This technique can be used to
diagnostic the electron temperature and density of solar flares observed with
the forth-coming Interface Region Imaging Spectrograph.Comment: 18 pages, 6 figure
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