11,495 research outputs found
A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
Recent optical and IR studies have revealed that the heavily-reddened
starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (WR)
stars, comprising the richest WR population of any galactic cluster. We present
results of a senstive Chandra X-ray observation of Wd 1 which detected 12 of
the 22 known WR stars and the mysterious emission line star W9. The fraction of
detected WN stars is nearly identical to that of WC stars. The WN stars WR-A
and WR-B as well as W9 are exceptionally luminous in X-rays and have similar
hard heavily-absorbed spectra with strong Si XIII and S XV emission lines. The
luminous high-temperature X-ray emission of these three stars is characteristic
of colliding wind binary systems but their binary status remains to be
determined. Spectral fits of the X-ray bright sources WR-A and W9 with
isothermal plane-parallel shock models require high absorption column densities
log N = 22.56 (cm) and yield characteristic shock temperatures
kT_shock ~ 3 keV (T ~ 35 MK).Comment: ApJL, 2006, in press (3 figures, 1 table
Observations of Binary and Single Wolf-Rayet Stars with XMM-Newton and Chandra
We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars
with XMM-Newton and Chandra. A new XMM spectrum of the nearby WN8 + OB binary
WR 147 shows hard absorbed X-ray emission, including the Fe K-alpha line
complex, characteristic of colliding wind shock sources. In contrast, sensitive
observations of four of the closest known single WC (carbon-rich) WR stars have
yielded only non-detections. These results tentatively suggest that single WC
stars are X-ray quiet. The presence of a companion may thus be an essential
factor in elevating the X-ray emission of WC + OB stars to detectable levels.Comment: To appear in conf. proceedings: Close Binaries in the 21st Century -
New Opportunities and Challenges, eds. A. Gimenez, E. Guinan, P. Niarchos, S.
Rucinski; Astrophys. and Space Sci. (special issue), 2006. 4 pages, 2 figure
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
Populations of Pear Thrips, \u3ci\u3eTaeniothrips Inconsequens\u3c/i\u3e (Thysanoptera: Thripidae) in Sugar Maple Stands in Vermont: 1989-2005
Development of an effective IPM strategy for pear thrips, Taeniothrips inconsequens (Uzel) (Thysanoptera: Thripidae), a pest of sugar maple, Acer saccharum Marshall, demands an understanding of their population fluctuations over time. Pear thrips populations were monitored using a standardized soil sampling method every fall from 1989 â 2005 in 14 counties of Vermont (U.S.). Data from individual sites were combined into north, central and south regions. High numbers of thrips emerged from soil sampled in 1989, 1990, 1993 and 2001, particularly in the north region (Washington, Lamoille, and Franklin counties). The central and south regions had lower pear thrips populations over all years. These results provide, for the first time, fundamental knowledge of pear thrips populations across a wide geographical area of Vermont and will assist in the design of suitable control strategies for pear thrips in the future
The Chandra X-ray Spectrum of the 10.6 s Pulsar in Westerlund 1: Testing the Magnetar Hypothesis
Two sensitive Chandra X-ray observations of the heavily-reddened galactic
starburst cluster Westerlund 1 in May and June 2005 detected a previously
unknown X-ray pulsar (CXO J164710.20-455217). Its slow 10.6 s pulsations,
moderate X-ray temperature kT 0.5 keV, and apparent lack of a massive
companion tentatively suggest that it is an Anomalous X-ray Pulsar (AXP). An
isothermal blackbody model yields an acceptable spectral fit but the inferred
source radius is much less than that of a neutron star, a result that has also
been found for other AXPs. We analyze the X-ray spectra with more complex
models including a model that assumes the pulsar is a strongly magnetized
neutron star (``magnetar'') with a light element atmosphere. We conclude that
the observed X-ray emission cannot be explained as global surface emission
arising from the surface of a cooling neutron star or magnetar. The emission
likely arises in one or more localized regions (``hot spots'') covering a small
fraction of the surface. We discuss these new results in the context of both
accretion and magnetar interpretations for the X-ray emission.Comment: 14 pages, 5 figures; to appear in Ap
Anomalously large capacitance of a plane capacitor with a two-dimensional electron gas
In electronic devices where a two-dimensional electron gas (2DEG) comprises
one or both sides of a plane capacitor, the resulting capacitance can be
larger than the "geometric capacitance" determined by the physical
separation between electrodes. This larger capacitance is known to result
from the Coulomb correlations between individual electrons within the low
density 2DEG, which lead to a negative thermodynamic density of states
(negative compressibility). Experiments on such systems generally operate in
the regime where the average spacing between electrons in the 2DEG
is smaller than , and these experiments observe by only a few
percent. A recent experiment [1], however, has observed larger than
by almost 40% while operating in the regime . In this paper we argue
that at correlations between the electronic charge of opposite
electrodes become important. We develop a theory of the capacitance for the
full range of . We show that, in the absence of disorder, the capacitance
can be times larger than the geometric value, where is the
electron Bohr radius. Our results compare favorably with the experiment of Ref.
[1] without the use of adjustable parameters.Comment: 8 pages, 6 figures; revised discussion of the zero density limit;
some typos fixe
Probing Residue-Specific Interactions in the Stabilization of Proteins Using High-Resolution NMR: A Study of Disulfide Bond Compensation
It is well established that the oxidation state of cysteine residues in proteins are critical to overall physical stability. The presence of disulfide bonds most often imparts thermodynamic stability, and as such, engineered disulfide bonds have become a means for improving the viability of protein therapeutics. In some cases, however, disulfide bonds can diminish stability. Because proteins are held together by numerous weak interactions, understanding the mechanisms by which stabilization is achieved is important to the design of new biotechnology products that better resist unfolding and aggregation. Mechanistic information describing how specific interactions influence stability is lacking, in part because the techniques typically used to study inherent stability do not provide sufficient detail. In the present study, a model protein system, phosphatase of regenerating liver (PRL-1), was used to investigate the role of cysteine residues on physical stability. A combination of chemical modulation and mutagenesis was employed to alter the redox state of the protein, and the effects were observed using a combination of low- and high-resolution methods. Specifically, solution NMR data revealed the stability of PRL-1 depends on cooperation between local interactions with the Cys side chains. This approach provides a means to better understand how protein stabilization is achieved
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