3,379 research outputs found
Focal plane transport assembly for the HEAO-B X-ray telescope
The High Energy Astronomy Observatory - Mission B (HEAO-B), an earth orbiting X-ray telescope facility capable of locating and imaging celestial X-ray sources within one second of arc in the celestial sphere, is considered. The Focal Plane Transport Assembly (FPTA) is one of the basic structural elements of the three thousand pound HEAO-B experiment payload. The FPTA is a multifunctional assembly which supports seven imaging X-ray detectors circumferentially about a central shaft and accurately positions any particular one into the focus of a high resolution mirror assembly. A drive system, position sensor, rotary coupler, and detent alignment system, all an integral part of the rotatable portion which in turn is supported by main bearings to the stationary focal plane housing are described
A high resolution spectral atlas of brown dwarfs
We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf
system, spectral classes at which most of the objects are brown dwarfs. Our
atlas covers the optical region from H up to the near infrared at 1
m. We present spectral details of ultra-cool atmospheres at very high
resolution () and compare the spectra to model calculations. Our
comparison shows that molecular features from VO and CaH, and atomic features
from Cs and Rb are reasonably well fit by current models. On the other hand,
features due to TiO, CrH, and water, and atomic Na and K reveal large
discrepancies between model calculations and our observations.Comment: 17 pages, 11 figures, accepted by A&A, reduced figure quality for
arXi
Non-linear diffusive acceleration of heavy nuclei in supernova remnant shocks
We describe a semi-analytical approach to non-linear diffusive shock
acceleration in the case in which nuclei other than protons are also
accelerated. The structure of the shock is determined by the complex interplay
of all nuclei, and in turn this shock structure determines the spectra of all
components. The magnetic field amplification upstream is described as due to
streaming instability of all nuclear species. The amplified magnetic field is
then taken into account for its dynamical feedback on the shock structure as
well as in terms of the induced modification of the velocity of the scattering
centers that enters the particle transport equation. The spectra of accelerated
particles are steep enough to be compared with observed cosmic ray spectra only
if the magnetic field is sufficiently amplified and the scattering centers have
high speed in the frame of the background plasma. We discuss the implications
of this generalized approach on the structure of the knee in the all-particle
cosmic ray spectrum, which we interpret as due to an increasingly heavier
chemical composition above eV. The effects of a non trivial chemical
composition at the sources on the gamma ray emission from a supernova remnant
when gamma rays are of hadronic origin are also discussed.Comment: 23 pages, 5 figures, minor changes to reflect the published versio
A library of near-infrared integral field spectra of young M-L dwarfs
We present a library of near-infrared (1.1-2.45 microns) medium-resolution
(R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of
companions with known ages and of isolated objects. We use it to (re)derive the
NIR spectral types, luminosities and physical parameters of the targets, and to
test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral
types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and
M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B,
DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX
models yield close Teff and log g estimates for each sources. The models seem
to evidence a 600-300+600 K drop of the effective temperature at the M-L
transition. Assuming the former temperatures are correct, we derive new mass
estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4,
OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range.
We combine the empirical luminosities of the M9.5-L0 sources to the Teff to
derive semi-empirical radii estimates that do not match "hot-start"
evolutionary models predictions at 1-3 Myr. We use complementary data to
demonstrate that atmospheric models are able to reproduce the combined optical
and infrared spectral energy distribution, together with the near-infrared
spectra of these sources simultaneously. But the models still fail to represent
the dominant features in the optical. This issue casts doubts on the ability of
these models to predict correct effective temperatures from near-infrared
spectra alone. We advocate the use of photometric and spectroscopic data
covering a broad range of wavelengths to study the properties of very low mass
young companions to be detected with the planet imagers (Subaru/SCExAO,
LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).Comment: 27 pages, 14 tables, 19 figures, accepted for publication in
Astronomy & Astrophysic
Quasi-molecular Satellites of Lyman Beta in the Spectrum of the DA White Dwarf WOLF 1346
We present new FUV/UV observations of the DA white dwarf Wolf 1346 obtained
with the Hopkins Ultraviolet Telescope. The atmospheric parameters of this
object are estimated from a fit of model atmospheres to several optical spectra
to be Teff = 20000 K, log g = 7.90. From the optical spectrum this star is a
normal DA without any indications for chemical elements other than hydrogen.
The hydrogen line L beta, however, shows a very unusual shape, with a steep red
wing and two absorption features on this wing. The shape is reminiscent of the
effects of quasi-molecular line broadening, as observed in L alpha in cooler DA
white dwarfs. We show that this is indeed the correct explanation, by
identifying 4 quasi-molecular satellites caused through perturbations by the H+
ion (H2+ quasi-molecule). The steep red wing is caused by the exponential
decline of the line profile beyond the satellite most distant from the line
center at 1078 A.Comment: 11 pages Latex with aaspp4 style, 4 postscript figures, as compressed
tar file, ApJ Letters, in pres
FUSE observations of G226-29: First detection of the H_2 quasi-molecular satellite at 1150A
We present new FUV observations of the pulsating DA white dwarf G226-29
obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This ZZ Ceti
star is the brightest one of its class and the coolest white dwarf observed by
FUSE. We report the first detection of the broad quasi-molecular
collision-induced satellite of Ly-beta at 1150 A, an absorption feature that is
due to transitions which take place during close collisions of hydrogen atoms.
The physical interpretation of this feature is based on recent progress of the
line broadening theory of the far wing of Ly-beta. This predicted feature had
never been observed before, even in laboratory spectra.Comment: Accepted for publication in ApJ Letters; 6 pages, 3 figure
Found: The missing blue opacity in atmosphere models of cool hydrogen white dwarfs
We investigate the importance of the far red wing of the Lyman line
of hydrogen in the atmospheres of cool white dwarfs of pure hydrogen
composition. We find that this absorption process dominates all important
sources of opacity in the blue part of the optical spectrum of these stars. Our
successful fits to the spectra of cool DA/DC white dwarfs indicate that the far
red wing of the line is the source of opacity that had been
missing in the models. The observed sequence of cool white dwarfs in
color-color diagrams is very well reproduced by our new pure hydrogen
atmosphere models, suggesting that the atmospheric composition of the coolest
DC white dwarfs must be revisited.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
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