298 research outputs found
Resistive anode image converter
The invention of an apparatus for imaging soft X-ray and ultraviolet electromagnetic radiation and charged particles was described. The apparatus includes a pair of microchannel electron multiplier plates connected in a cascaded chevron configuration which intercepts an incident beam of radiation or charged particles. Incident photons or charged particles strike the front surface of the chevron configuration causing emission of electrons. The electrons are accelerated by a voltage gradient and strike the inner side walls of the individual channels, causing emission of secondary electrons. Accelerated and multiplied secondary electrons impinge upon a resistive anode after they transverse the chevron configuration. A pulse position circuit converts the magnitude or transit time of the currents flowing from the point of impact of the electrons on the resistive anode to four contact electrodes mounted on their periphery of the resistive anode into the spatial coordinates of electron impact
On the accuracy of the S/N estimates obtained with the exposure time calculator of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope
We have studied the accuracy and reliability of the exposure time calculator
(ETC) of the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space
Telescope (HST) with the objective of determining how well it represents actual
observations and, therefore, how much confidence can be invested in it and in
similar software tools. We have found, for example, that the ETC gives, in
certain circumstances, very optimistic values for the signal-to-noise ratio
(SNR) of point sources. These values overestimate by up to a factor of 2 the
HST performance when simulations are needed to plan deep imaging observations,
thus bearing serious implications on observing time allocation. For this
particular case, we calculate the corrective factors to compute the appropriate
SNR and detection limits and we show how these corrections vary with field
crowding and sky background. We also compare the ETC of the WFPC2 with a more
general ETC tool, which takes into account the real effects of pixel size and
charge diffusion. Our analysis indicates that similar problems may afflict
other ETCs in general showing the limits to which they are bound and the
caution with which their results must be taken.Comment: 14 pages, 13 figures, to be published in PASP on July 200
Arthroscopic synovectomy in chronic inflammatory rheumatism: clinical and functional aspects
By now many authors regard arthroscopic synovectomy an integral part of therapeutic treatment of many rheumatic diseases with favourable results on post operating course and clinical picture in the long term. The pathologic synovial tissue during articular inflammatory rheumatism is well known to have a damaging effect responsible of early cartilage injury, as well as symptomatic action (e.g. articular stiffness, effusion, pain, functional limitation). Therefore to value the removal of such a tissue you should think of the secondary prevention of cartilage injury, besides the symptomatic point of view. Since 1996 we performed 190 arthroscopic synovectomy, the adopted criteria of judgement were: pain (spontaneous, during active and passive movements), effusion or swelling presence, articular range and cartilage state (evaluated during arthroscopy according to Outerbridg's classification). 70% of the cases showed good results and six years later the beginning of this activity we retain arthroscopic synovectomy as a valid help in articular inflammatory rheumatism treatment
Spectra and Diagnostics for the Direct Detection of Wide-Separation Extrasolar Giant Planets
We calculate as a function of orbital distance, mass, and age the theoretical
spectra and orbit-averaged planet/star flux ratios for representative
wide-separation extrasolar giant planets (EGPs) in the optical, near-infrared,
and mid-infrared. Stellar irradiation of the planet's atmosphere and the
effects of water and ammonia clouds are incorporated and handled in a
consistent fashion. We include predictions for 12 specific known EGPs. In the
process, we derive physical diagnostics that can inform the direct EGP
detection and remote sensing programs now being planned or proposed.
Furthermore, we calculate the effects of irradiation on the spectra of a
representative companion brown dwarf as a function of orbital distance.Comment: submitted to the Astrophysical Journal, 19 pages, 11 color figure
HST observations of the metal rich globular clusters NGC6496 and NGC6352
Deep exposures of the metal-rich globular clusters NGC6496 and NGC6352 were
obtained with the WFPC2 camera on board the HST through the F606W and F814W
filters. The resulting colour-magnitude diagrams reach down to absolute
magnitude M_814~10-10.5, approximately 5 magnitudes below the main sequence
(MS) turn-off. The MS of the two clusters are sharp and well defined and their
fiducial lines overlap almost exactly throughout this range. Their colour is,
however, more than 0.1 mag redder than the MS fiducial line of the prototype
metal-rich globular cluster 47 Tuc, after proper correction for the relative
distances and reddening. This provides solid empirical evidence of a higher
metal content, which is not surprising if these objects belong indeed to the
bulge as their present location suggests. A good fit to the upper part of the
MS of both clusters is obtained with a 10 Gyr-old theoretical isochrone from
Baraffe et al. (1998) for a metallicity of [M/H]=-0.5, but at lower luminosity
all models depart considerably from the observations, probably because of a
deficiency in the treatment of the TiO opacity. The luminosity functions
obtained from the observed CMD are rather similar to one another and show a
peak at M_814~9. The present day mass functions (PDMF) are derived down to
m~0.2 Msolar and are consistent with power-law indices alpha=0.7 for NGC6496
and alpha=0.6 for NGC6352. The PDMF of 47 Tuc is twice as steep in the same
mass range (alpha=1.4). We investigate the origin of this discrepancy and show
that it can be understood if the two clusters contain a considerably higher
fraction of primordial binaries amongst their MS population, similar to that
expected in the bulge. We briefly discuss the implications of this finding on
the process of star and binary formation and on the universality of the IMF.Comment: Accepted for publication in Astronomy and Astrophysics. Prepared with
aatex, 12 pages, 7 figure
Why is the mass function of NGC 6218 flat?
We have used the FORS-1 camera on the VLT to study the main sequence (MS) of
the globular cluster NGC 6218 in the V and R bands. The observations cover an
area of 3.4 x 3.4 around the cluster centre and probe the stellar population
out to the cluster's half-mass radius (r_h ~ 2.2). The colour-magnitude diagram
(CMD) that we derive in this way reveals a narrow and well defined MS extending
down to the 5 sigma detection limit at V~25, or about 6 magnitudes below the
turn-off, corresponding to stars of ~ 0.25 Msolar. The luminosity function (LF)
obtained with these data shows a marked radial gradient, in that the ratio of
lower- and higher-mass stars increases monotonically with radius. The mass
function (MF) measured at the half-mass radius, and as such representative of
the clusters global properties, is surprisingly flat. Over the range 0.4 - 0.8
Msolar, the number of stars per unit mass follows a power-law distribution of
the type dN/dm \propto m^{0}, where, for comparison, Salpeter's IMF would be
dN/dm \propto m^{-2.35}. We expect that such a flat MF does not represent the
cluster's IMF but is the result of severe tidal stripping of the stars from the
cluster due to its interaction with the Galaxy's gravitational field. Our
results cannot be reconciled with the predictions of recent theoretical models
that imply a relatively insignificant loss of stars from NGC 6218 as measured
by its expected very long time to disruption. They are more consistent with the
orbital parameters based on the Hipparcos reference system that imply a much
higher degree of interaction of this cluster with the Galaxy than assumed by
those models. Our results indicate that, if the orbit of a cluster is known,
the slope of its MF could be useful in discriminating between the various
models of the Galactic potential.Comment: 11 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Toward a unified light curve model for multi-wavelength observations of V1974 Cygni (Nova Cygni 1992)
We present a unified model for optical, ultraviolet (UV), and X-ray light
curves of V1974 Cygni (Nova Cygni 1992). Based on an optically thick wind model
of nova outbursts, we have calculated light curves and searched for the best
fit model that is consistent with optical, UV, and X-ray observations. Our best
fit model is a white dwarf (WD) of mass 1.05 M_\sun with a chemical composition
of X=0.46, C+N+O=0.15, and Ne = 0.05 by mass weight. Both supersoft X-ray and
continuum UV 1455 \AA light curves are well reproduced. Supersoft X-rays
emerged on day ~ 250 after outburst, which is naturally explained by our model:
our optically thick winds cease on day 245 and supersoft X-rays emerge from
self-absorption by the winds. The X-ray flux keeps a constant peak value for ~
300 days followed by a quick decay on day ~ 600. The duration of X-ray flat
peak is well reproduced by a steady hydrogen shell burning on the WD. Optical
light curve is also explained by the same model if we introduce free-free
emission from optically thin ejecta. A t^{-1.5} slope of the observed optical
and infrared fluxes is very close to the slope of our modeled free-free light
curve during the optically thick wind phase. Once the wind stops, optical and
infrared fluxes should follow a t^{-3} slope, derived from a constant mass of
expanding ejecta. An abrupt transition from a t^{-1.5} slope to a t^{-3} slope
at day ~ 200 is naturally explained by the change from the wind phase to the
post-wind phase on day ~ 200. The development of hard X-ray flux is also
reasonably understood as shock-origin between the wind and the companion star.
The distance to V1974 Cyg is estimated to be ~ 1.7 kpc with E(B-V)= 0.32 from
the light curve fitting for the continuum UV 1455 \AA.Comment: 8 pages, 4 figures, to appear in the Astrophysical Journa
Hydrodynamical simulations of the jet in the symbiotic star MWC 560 III. Application to X-ray jets in symbiotic stars
In papers I and II in this series, we presented hydrodynamical simulations of
jet models with parameters representative of the symbiotic system MWC 560.
These were simulations of a pulsed, initially underdense jet in a high density
ambient medium. Since the pulsed emission of the jet creates internal shocks
and since the jet velocity is very high, the jet bow shock and the internal
shocks are heated to high temperatures and should therefore emit X-ray
radiation. In this paper, we investigate in detail the X-ray properties of the
jets in our models. We have focused our study on the total X-ray luminosity and
its temporal variability, the resulting spectra and the spatial distribution of
the emission. Temperature and density maps from our hydrodynamical simulations
with radiative cooling presented in the second paper are used together with
emissivities calculated with the atomic database ATOMDB. The jets in our models
show extended and variable X-ray emission which can be characterized as a sum
of hot and warm components with temperatures that are consistent with
observations of CH Cyg and R Aqr. The X-ray spectra of our model jets show
emission line features which correspond to observed features in the spectra of
CH Cyg. The innermost parts of our pulsed jets show iron line emission in the
6.4 - 6.7 keV range which may explain such emission from the central source in
R Aqr. We conclude that MWC 560 should be detectable with Chandra or
XMM-Newton, and such X-ray observations will provide crucial for understanding
jets in symbiotic stars.Comment: 10 pages, 12 figures, accepted for publication in ApJ, uses
emulateap
Arthroscopic synovectomy in chronic inflammatory rheumatism: clinical and functional aspects
By now many authors regard arthroscopic synovectomy an integral part of therapeutic treatment of many rheumatic diseases with favourable results on post operating course and clinical picture in the long term. The pathologic synovial tissue during articular inflammatory rheumatism is well known to have a damaging effect responsible of early cartilage injury, as well as symptomatic action (e.g. articular stiffness, effusion, pain, functional limitation). Therefore to value the removal of such a tissue you should think of the secondary prevention of cartilage injury, besides the symptomatic point of view. Since 1996 we performed 190 arthroscopic synovectomy, the adopted criteria of judgement were: pain (spontaneous, during active and passive movements), effusion or swelling presence, articular range and cartilage state (evaluated during arthroscopy according to Outerbridg’s classification). 70% of the cases showed good results and six years later the beginning of this activity we retain arthroscopic synovectomy as a valid help in articular inflammatory rheumatism treatment
- …