138 research outputs found
The stellar coronal X-ray Explorer: STCOEX
The status of the newly born field of stellar X-ray astronomy is considered. The need for further observational capability, the study of stellar surface activity and the development of design criteria for a follow-up mission, with primary emphasis on stellar observations and with general capabilities within the scope of the Explorer program are examined. The criteria which lead to the desired instrumental capabilities are reviewed; and an overview of the proposed instrument and some of its capabilities, as indicated by simulations is presented. Two versions of a trial payload the stellar coronal. Explorer (STCOEX) which meet the observational desirable are developed
Continuing data analysis of the AS/E grazing incidence X-ray telescope experiment on the OSO-4 satellite
The work to correct and extend the calculation of the theoretical solar X-ray spectrum produced during earlier OSO-4 data analysis is reported along with the work to formulate models of active regions, and compare these models with the experimental values. An atlas of solar X-ray photographs is included, and solar X-ray observations are correlated with the solar wind
Folding of VemP into translation-arresting secondary structure is driven by the ribosome exit tunnel
The ribosome is a fundamental biomolecular complex that synthesizes proteins in cells. Nascent proteins emerge from the ribosome through a tunnel, where they may interact with the tunnel walls or small molecules such as antibiotics. These interactions can cause translational arrest with notable physiological consequences. Here, we studied the arrest caused by the regulatory peptide VemP, which is known to form alpha-helices inside the ribosome tunnel near the peptidyl transferase center under specific conditions. We used all-atom molecular dynamics simulations of the entire ribosome and circular dichroism spectroscopy to study the driving forces of helix formation and how VemP causes the translational arrest. To that aim, we compared VemP dynamics in the ribosome tunnel with its dynamics in solution. We show that the VemP peptide has a low helical propensity in water and that the propensity is higher in mixtures of water and trifluorethanol. We propose that helix formation within the ribosome is driven by the interactions of VemP with the tunnel and that a part of VemP acts as an anchor. This anchor might slow down VemP progression through the tunnel enabling alpha-helix formation, which causes the elongation arrest
High resolution studies of the solar X-ray corona from Aerobee rockets
The research in high resolution solar X-ray astronomy is reported. The payload for the Aerobee 150 launch vehicle, which included a 23 cm diameter mirror whose polished surface was a nickel-phosphorus alloy is discussed along with the high resolution measurements, by Flight 13.028 CS, of the temperature and density structure of the lower corona. Flight 13.029 CS is also discussed
A Contemporary View of Coronal Heating
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures orders of
magnitude higher than their surface temperatures has long been a key problem.
For decades the problem has been known as the coronal heating problem, but it
is now clear that `coronal heating' cannot be treated or explained in isolation
and that the heating of the whole solar atmosphere must be studied as a highly
coupled system. The magnetic field of the star is known to play a key role,
but, despite significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question of which
mechanism or mechanisms are the dominant supplier of energy to the chromosphere
and corona is still open. Following substantial recent progress, we consider
the most likely contenders and discuss the key factors that have made, and
still make, determining the actual (coronal) heating mechanism (or mechanisms)
so difficult
The flaring and quiescent components of the solar corona
The solar corona is a template to understand stellar activity. The Sun is a
moderately active star, and its corona differs from active stars: active
stellar coronae have a double-peaked EM(T) with the hot peak at 8-20 MK, while
the non flaring solar corona has one peak at 1-2 MK. We study the average
contribution of flares to the solar EM(T) to investigate indirectly the
hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a
large number of unresolved solar-like flares, and to infer properties on the
flare distribution from nano- to macro-flares. We measure the disk-integrated
time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares
analyzing uninterrupted GOES/XRS light curves over time intervals of one month.
We obtain the EM_Q(T) of quiescent corona for the same time intervals from the
Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary with the solar
cycle, we evaluate them at different phases of the cycle (from Dec. 1991 to
Apr. 1998). Irrespective of the solar cycle phase, EM_F(T) appears like a peak
of the distribution significantly larger than the values of EM_Q(T) for T~5-10
MK. As a result the time-averaged EM(T) of the whole solar corona is
double-peaked, with the hot peak, due to time-averaged flares, located at
temperature similar of that of active stars, but less enhanced. The EM_F(T)
shape supports the hypothesis that the hot EM(T) peak of active coronae is due
to unresolved solar-like flares. If this is the case, quiescent and flare
components should follow different scaling laws for increasing stellar
activity. In the assumption that the heating of the corona is entirely due to
flares, from nano- to macro-flares, then either the flare distribution or the
confined plasma response to flares, or both, are bimodal.Comment: 8 pages, 7 postscript figures, accepted for publication in Astronomy
and Astrophysic
The structure and dynamics of a bright point as seen with Hinode, SoHO and TRACE
Our aim is to determine the plasma properties of a coronal bright point and
compare its magnetic topology extrapolated from magnetogram data with its
appearance in X-ray images. We analyse spectroscopic data obtained with
EIS/Hinode, Ca II H and G-band images from SOT/Hinode, UV images from TRACE,
X-ray images from XRT/Hinode and high-resolution/high-cadence magnetogram data
from MDI/SoHO. The BP comprises several coronal loops as seen in the X-ray
images, while the chromospheric structure consists of tens of small bright
points as seen in Ca II H. An excellent correlation exists between the Ca II
BPs and increases in the magnetic field, implying that the Ca II H passband is
a good indicator for the concentration of magnetic flux. Doppler velocities
between 6 and 15 km/s are derived from the Fe XII and Fe XIII lines for the BP
region, while for Fe XIV and Si VII they are in the range from -15 to +15 km/s.
The coronal electron density is 3.7x10^9 cm^-3. An excellent correlation is
found between the positive magnetic flux and the X-ray light-curves. The
remarkable agreement between the extrapolated magnetic field configuration and
some of the loops composing the BP as seen in the X-ray images suggests that a
large fraction of the magnetic field in the bright point is close to potential.
The close correlation between the positive magnetic flux and the X-ray emission
suggests that energy released by magnetic reconnection is stimulated by flux
emergence or cancellation.Comment: 10 pages with 11 figures. Accepted in Astronomy and Astrophysic
Investigating ChaMPlane X-ray sources in the Galactic Bulge with Magellan LDSS2 spectra
We have carried out optical and X-ray spectral analyses on a sample of 136
candidate optical counterparts of X-ray sources found in five Galactic-bulge
fields included in our Chandra Multi-wavelength Plane Survey. We use a
combination of optical spectral fitting and quantile X-ray analysis to obtain
the hydrogen column density towards each object, and a three-dimensional dust
model of the Galaxy to estimate the most probable distance in each case. We
present the discovery of a population of stellar coronal emission sources,
likely consisting of pre-main sequence, young main sequence and main sequence
stars, as well as a component of active binaries of RS CVn or BY Dra type. We
identify one candidate quiescent low-mass X-ray binary with a sub-giant
companion; we note that this object may also be an RS CVn system. We report the
discovery of three new X-ray detected cataclysmic variables (CVs) in the
direction of the Galactic Center (at distances ~2kpc). This number is in excess
of predictions made with a simple CV model based on a local CV space density of
<~ 10^-5 pc^-3, and a scale height ~200pc. We discuss several possible reasons
for this observed excess.Comment: 41 pages, 11 figures. Accepted for publication in Astrophysical
Journal, September 10 editio
High antiplasmodial activity of novel plasmepsins I and II inhibitors
The aim of this study was to develop new antiplasmodial compounds acting through distinct mechanisms during both the liver and the blood stages of the parasite life cycle. Compounds were designed on the basis of the "double-drug" approach: primaquine, which has been linked to statine-based inhibitors of plasmepsins (PLMs), the plasmodial aspartic proteases involved in degradation of hemeoglobin. The compounds were tested in vitro for anti-PLM I/PLM II activities and against chloroquine-sensitive (D10) and chloroquine-resistant (W2) strains of P. falciparum. An antiplasmodial activity (IC50) as low as 0.1 M was obtained, an excellent improvement in comparison with inhibitors previously reported (IC50 = 2-20 M). The killing activity was equally directed against both P. falciparum strains and was correlated to lipophilicity (calculated as ALogP), for all compounds but one (9). All compounds inhibited PLM I and PLM II in the nanomolar range (Ki = 1-700 nM). The most promising compounds (2, 6, 10) were not cytotoxic against human fibroblasts at 100 M and were highly selective for PLMs vs human cathepsin
Dynamics of Coronal Bright Points as seen by Sun Watcher using Active Pixel System detector and Image Processing (SWAP), Atmospheric Imaging Assembly AIA), and Helioseismic and Magnetic Imager (HMI)
The \textit{Sun Watcher using Active Pixel system detector and Image
Processing}(SWAP) on board the \textit{PRoject for OnBoard Autonomy\todash 2}
(PROBA\todash 2) spacecraft provides images of the solar corona in EUV channel
centered at 174 \AA. These data, together with \textit{Atmospheric Imaging
Assembly} (AIA) and the \textit{Helioseismic and Magnetic Imager} (HMI) on
board \textit{Solar Dynamics Observatory} (SDO), are used to study the dynamics
of coronal bright points. The evolution of the magnetic polarities and
associated changes in morphology are studied using magnetograms and
multi-wavelength imaging. The morphology of the bright points seen in
low-resolution SWAP images and high-resolution AIA images show different
structures, whereas the intensity variations with time show similar trends in
both SWAP 174 and AIA 171 channels. We observe that bright points are seen in
EUV channels corresponding to a magnetic-flux of the order of Mx. We
find that there exists a good correlation between total emission from the
bright point in several UV\todash EUV channels and total unsigned photospheric
magnetic flux above certain thresholds. The bright points also show periodic
brightenings and we have attempted to find the oscillation periods in bright
points and their connection to magnetic flux changes. The observed periods are
generally long (10\todash 25 minutes) and there is an indication that the
intensity oscillations may be generated by repeated magnetic reconnection
- …