2,265 research outputs found
Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes
We present a novel analysis of time-resolved extended x-ray absorption fine structure (EXAFS) spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments. We apply it to the analysis of picosecond EXAFS data on aqueous [FeII (bpy)3] 2+, which undergoes a light induced conversion from its low-spin (LS) ground state to the short-lived (τ≈650 ps) excited high-spin (HS) state. A series of EXAFS spectra were simulated for a collection of possible HS structures from which the ground state fit spectrum was subtracted to generate transient difference absorption (TA) spectra. These are then compared with the experimental TA spectrum using a least-squares statistical analysis to derive the structural change. This approach reduces the number of required parameters by cancellation in the differences. It also delivers a unique solution for both the fractional population and the extracted excited state structure. We thus obtain a value of the Fe-N bond elongation in the HS state with subpicometer precision (0.203±0.008 Å). © 2009 American Institute of Physics.This work was funded by the Swiss National Science Foundation via Contract Nos. 620–066145, 200021–107956, PP002–110464, 200020–116023, 200021–105239, and 200020-116533.Peer Reviewe
Distribution and kinematics of the HCN(3-2) emission down to the innermost region in the envelope of the O-rich star W Hya
We report high angular resolution observations of the HCN (3-2) line emission
in the circumstellar envelope of the O-rich star W Hya with the Submillimeter
Array. The proximity of this star allows us to image its molecular envelope
with a spatial resolution of just ~40 AU, corresponding to about 10 times the
stellar diameter. We resolve the HCN (3-2) emission and find that it is
centrally peaked and has a roughly spherically symmetrical distribution. This
shows that HCN is formed in the innermost region of the envelope (within ~10
stellar radii), which is consistent with predictions from pulsation-driven
shock chemistry models, and rules out the scenario in which HCN forms through
photochemical reactions in the outer envelope. Our model suggests that the
envelope decreases steeply in temperature and increases smoothly in velocity
with radius, inconsistent with the standard model for mass-loss driven by
radiative pressure on dust grains. We detect a velocity gradient of ~5 km/s in
the NW--SE direction over the central 40 AU. This velocity gradient is
reminescent of that seen in OH maser lines, and could be caused by the rotation
of the envelope or by a weak bipolar outflow.Comment: accepted for publication in ApJ
Inactivation of the type I interferon pathway reveals long double‐stranded RNA ‐mediated RNA interference in mammalian cells
RNA interference (RNAi) elicited by long double-stranded (ds) or
base-paired viral RNA constitutes the major mechanism of antiviral
defence in plants and invertebrates. In contrast, it is controversial
whether it acts in chordates. Rather, in vertebrates, viral RNAs
induce a distinct defence system known as the interferon (IFN)
response. Here, we tested the possibility that the IFN response
masks or inhibits antiviral RNAi in mammalian cells. Consistent
with that notion, we find that sequence-specific gene silencing can
be triggered by long dsRNAs in differentiated mouse cells rendered
deficient in components of the IFN pathway. This unveiled response
is dependent on the canonical RNAi machinery and is lost upon
treatment of IFN-responsive cells with type I IFN. Notably, transfection with long dsRNA specifically vaccinates IFN-deficient cells
against infection with viruses bearing a homologous sequence.
Thus, our data reveal that RNAi constitutes an ancient antiviral
strategy conserved from plants to mammals that precedes but has
not been superseded by vertebrate evolution of the IFN system
The Anomalous Infrared Emission of Abell 58
We present a new model to explain the excess in mid and near infrared
emission of the central, hydrogen poor dust knot in the planetary nebula (PN)
Abell 58. Current models disagree with ISO measurement because they apply an
average grain size and equilibrium conditions only. We investigate grain size
distributions and temperature fluctuations affecting infrared emission using a
new radiative transfer code and discuss in detail the conditions requiring an
extension of the classical description. The peculiar infrared emission of V605
Aql, the central dust knot in Abell 58, has been modeled with our code. V605
Aql is of special interest as it is one of only three stars ever observed to
move from the evolutionary track of a central PN star back to the post-AGB
state.Comment: 17 pages, 4 figures; accepted and to be published in Ap
Magnetic Fields in Evolved Stars: Imaging the Polarized Emission of High-Frequency SiO Masers
We present Submillimeter Array observations of high frequency SiO masers
around the supergiant VX Sgr and the semi-regular variable star W Hya. The
J=5-4, v=1 28SiO and v=0 29SiO masers of VX Sgr are shown to be highly linearly
polarized with a polarization from ~5-60%. Assuming the continuum emission
peaks at the stellar position, the masers are found within ~60 mas of the star,
corresponding to ~100 AU at a distance of 1.57 kpc. The linear polarization
vectors are consistent with a large scale magnetic field, with position and
inclination angles similar to that of the dipole magnetic field inferred in the
H2O and OH maser regions at much larger distances from the star. We thus show
for the first time that the magnetic field structure in a circumstellar
envelope can remain stable from a few stellar radii out to ~1400 AU. This
provides further evidence supporting the existence of large scale and
dynamically important magnetic fields around evolved stars. Due to a lack of
parallactic angle coverage, the linear polarization of masers around W Hya
could not be determined. For both stars we observed the 28SiO and 29SiO
isotopologues and find that they have a markedly different distribution and
that they appear to avoid each other. Additionally, emission from the SO
5_5-4_4 line was imaged for both sources. Around W Hya we find a clear offset
between the red- and blue-shifted SO emission. This indicates that W Hya is
likely host to a slow bipolar outflow or a rotating disk-like structure.Comment: 8 pages, 7 figures, accepted for publication in ApJ. Online table
will be available with published versio
Correlation between Infrared Colors and Intensity Ratios of SiO Maser Lines
We present the results of SiO millimeter-line observations of a sample of
known SiO maser sources covering a wide dust-temperature range. A cold part of
the sample was selected from the SiO maser sources found in our recent SiO
maser survey of cold dusty objects. The aim of the present research is to
investigate the causes of the correlation between infrared colors and SiO maser
intensity ratios among different transition lines. In particular, the
correlation between infrared colors and SiO maser intensity ratio among the
J=1-0 v=1, 2, and 3 lines are mainly concerned in this paper. We observed in
total 75 SiO maser sources with the Nobeyama 45m telescope quasi-simultaneously
in the SiO J=1-0 v=0, 1, 2, 3, 4 and J=2-1 v=1, 2 lines. We also observed the
sample in the 29SiO J=1-0 v=0 and J=2-1 v=0, and 30SiO J=1-0 v=0 lines, and the
H2O 6(1,6)-5(2,3) line. As reported in previous papers, we confirmed that the
intensity ratios of the SiO J=1-0 v=2 to v=1 lines clearly correlate with
infrared colors. In addition, we found possible correlation between infrared
colors and the intensity ratios of the SiO J=1-0 v=3 to v=1&2 lines. Two
overlap lines of H2O (i.e., 11(6,6) nu_2=1 -> 12(7,5) nu_2=0 and 5(0,5) nu_2=2
-> 6(3,4) nu_2=1) might explain these correlation if these overlap lines become
stronger with increase of infrared colors, although the phenomena also might be
explained by more fundamental ways if we take into account the variation of
opacity from object to object.Comment: 49 pages, 7 figures, 3 tables, accepted for publication in ApJ. Full
resolution version available at
http://www.asiaa.sinica.edu.tw/~junichi/paper
Adaptive Optics Imaging of IRAS 18276-1431: a bipolar pre-planetary nebula with circumstellar "searchlight beams" and "arcs"
We present high-angular resolution images of the post-AGB nebula
IRAS18276-1431 (also known as OH17.7-2.0) obtained with the Keck II Adaptive
Optics (AO) system in its Natural Guide Star (NGS) mode in the Kp, Lp, and Ms
near-infrared bands. We also present supporting optical F606W and F814W HST
images as well as interferometric observations of the 12CO(J=1-0), 13CO(J=1-0),
and 2.6mm continuum emission with OVRO. The envelope of IRAS18276-1431 displays
a clear bipolar morphology in our optical and NIR images with two lobes
separated by a dark waist and surrounded by a faint 4.5"x3.4" halo. Our Kp-band
image reveals two pairs of radial ``searchlight beams'' emerging from the
nebula center and several intersecting, arc-like features. From our CO data we
derive a mass of M>0.38[D/3kpc]^2 Msun and an expansion velocity v_exp=17km/s
for the molecular envelope. The density in the halo follows a radial power-law
proportional to r^-3, which is consistent with a mass-loss rate increasing with
time. Analysis of the NIR colors indicates the presence of a compact central
source of ~300-500K dust illuminating the nebula in addition to the central
star. Modeling of the thermal IR suggests a two-shell structure in the dust
envelope: 1) an outer shell with inner and outer radius R_in~1.6E16cm and
R_out>~1.25E17cm, dust temperature T_d~105-50K, and a mean mass-loss rate of
Mdot~1E-3Msun/yr; and 2) an inner shell with R_in~6.3E14cm, T_dust~500-105K,
and Mdot~3E-5Msun/yr. An additional population of big dust grains (radius
a>~0.4mm) with T_dust=150-20K and mass M_dust=(0.16-1.6)E-3 [D/3kpc]^2 Msun can
account for the observed sub-mm and mm flux excess. The mass of the envelope
enclosed within R_out=1.25E17cm derived from SED modeling is ~1[D/3kpc]^2 Msun.Comment: 46 pages, 14 figures, 3 tables, accepted for publication in ApJ.
Figures 12 & 13 in low resolution. Full resolution versions are available
upon request to the first autho
ICESat Measurements Reveal Complex Pattern of Elevation Changes on Siple Coast Ice Streams, Antarctica
We compare ICESat data (2003-2004) to airborne laser altimetry data (1997-98 and 1999-2000) to monitor surface changes over portions of Van der Veen (VdVIS), Whillans (WIS) and Kamb ice streams (KIS) in the Ross Embayment of the West Antarctic Ice Sheet. The spatial pattern of detected surface changes is generally consistent with earlier observations. However, important changes have occurred during the past decade. For example, areas on the VdVIS and WIS, where large thinning was detected by the airborne surveys, are now closer to being in balance. The upper trunk of KIS continues to build up with thickening rates reaching 0.4 m/year. Our results provide new evidence that the overall mass balance of the region is becoming more positive, but a significant spatial variability exists. They also demonstrate the potential of ICESat data for detecting spatial patterns of surface elevation change in Antarctica
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