980 research outputs found
Six supersoft X-ray binaries: system parameters and twin-jet outflows
A comparison is made between the properties of CAL 83, CAL 87, RX
J0513.9-6951, 1E 0035.4-7230 (SMC 13), RX J0019.8+2156, and RX J0925.7-4758,
all supersoft X-ray binaries. Spectra with the same resolution and wavelength
coverage of these systems are compared and contrasted. Some new photometry is
also presented. The equivalent widths of the principal emission lines of H and
He II differ by more than an order of magnitude among these sources, although
those of the highest ionization lines (e.g. O VI) are very similar. In
individual systems, the velocity curves derived from various ions often differ
in phasing and amplitude, but those whose phasing is consistent with the light
curves (implying the lines are formed near the compact star) give masses of
and for the degenerate and mass-losing
stars, respectively. This finding is in conflict with currently prevailing
theoretical models for supersoft binaries. The three highest luminosity sources
show evidence of "jet" outflows, with velocities of .
In CAL 83 the shape of the He II 4686\AA profile continues to show evidence
that these jets may precess with a period of days.Comment: 27 pages including 5 tables, plus 6 figures. To appear in Ap
The role of initial density profiles in simulations of coronal wave - coronal hole interaction
Interactions between global coronal waves (CWs) and coronal holes (CHs)
reveal many interesting features of reflected waves and coronal hole boundaries
(CHB) but have fairly been studied so far. Magnetohydrodynamic (MHD)
simulations can help us to better understand what is happening during these
interaction events, and therefore, to achieve a broader understanding of the
parameters involved. In this study, we perform for the first time 2D MHD
simulations of a CW-CH interaction including a realistic initial wave density
profile that consists of an enhanced as well as a depleted wave part. We vary
several initial parameters, such as the initial density amplitudes of the
incoming wave, the CH density, and the CHB width, which are all based on actual
measurements. We analyse the effects of different incident angles on the
interaction features and we use the corresponding time-distance plots to detect
specific features of the incoming and the reflected wave. We found that a
particular combination of a small CH density, a realistic initial density
profile and a sufficiently small incident angle leads to remarkable interaction
features, such as a large density amplitude of the reflected wave with respect
to the incoming one. The parameter studies in this paper provide a tool to
compare time-distance plots based on observational measurements to those
created from simulations and therefore enable us to derive interaction
parameters from observed CW-CH interaction events that usually cannot be
obtained directly. The simulation results in this study are augmented by
analytical expressions for the reflection coefficient of the CW-CH interaction
which allows us to verify the simulations results in an additional way. This
work is the first of a series of studies aiming to finally reconstruct actual
observed CW-CH interaction events by means of MHD-simulations
Classical Cepheid Pulsation Models. X. The Period-Age Relation
We present new Period-Age (PA) and Period-Age-Color (PAC) relations for
fundamental and first overtone classical Cepheids. Current predictions rely on
homogeneous sets of evolutionary and pulsation models covering a broad range of
stellar masses and chemical compositions. We found that PA and PAC relations
present a mild dependence upon metal content. Moreover, the use of different PA
and PAC relation for fundamental and first overtone Cepheids improves the
accuracy of age estimates in the short-period (log P < 1) range (old Cepheids),
because they present smaller intrinsic dispersions. At the same time, the use
of the PAC relations improves the accuracy in the long-period (log P >= 1)
range (young Cepheids), since they account for the position of individual
objects inside the instability strip. We performed a detailed comparison
between evolutionary and pulsation ages for a sizable sample of LMC (15) and
SMC (12) clusters which host at least two Cepheids. In order to avoid deceptive
uncertainties in the photometric absolute zero-point, we adopted the
homogeneous set of B,V,I data for clusters and Cepheids collected by OGLE. We
also adopted the same reddening scale. The different age estimates agree at the
level of 20 % for LMC clusters and of 10 % for SMC clusters. We also performed
the same comparison for two Galactic clusters (NGC6067, NGC7790) and the
difference in age is smaller than 20 %. These findings support the use of PA
and PAC relations to supply accurate estimates of individual stellar ages in
the Galaxy and in external Galaxies. The main advantage of this approach is its
independence from the distance.Comment: 16 pages, 10 postscript figures, accepted for publication on Ap
A New View of the Circumstellar Environment of SN 1987A
We summarize the analysis of a uniform set of both previously-known and
newly-discovered scattered-light echoes, detected within 30" of SN 1987A in ten
years of optical imaging, and with which we have constructed the most complete
three-dimensional model of the progenitor's circumstellar environment.
Surrounding the SN is a richly-structured bipolar nebula. An outer,
double-lobed ``peanut,'' which we believe is the contact discontinuity between
the red supergiant and main sequence winds, is a prolate shell extending 28 ly
along the poles and 11 ly near the equator. Napoleon's Hat, previously believed
to be an independent structure, is the waist of this peanut, which is pinched
to a radius of 6 ly. Interior, the innermost circumstellar material lies along
a cylindrical hourglass, 1 ly in radius and 4 ly long, which connects to the
peanut by a thick equatorial disk. The nebulae are inclined 41o south and 8o
east of the line of sight, slightly elliptical in cross section, and marginally
offset west of the SN. The 3-D geometry of the three circumstellar rings is
studied, suggesting the equatorial ring is elliptical (b/a<0.98), and spatially
offset in the same direction as the hourglass. Dust-scattering models suggest
that between the hourglass and bipolar lobes: the gas density drops from 1--3
cm^{-3} to >0.03 cm^{-3}; the maximum dust-grain size increases from ~0.2
micron to 2 micron; and the Si:C dust ratio decreases. The nebulae have a total
mass of ~1.7 Msun, yielding a red-supergiant mass loss around 5*10^{-6} Msun
yr^{-1}.Comment: Accepted for publication in ApJ 2/14/05. 16 pages in emualteapj
forma
The source ambiguity problem: Distinguishing the effects of grammar and processing on acceptability judgments
Judgments of linguistic unacceptability may theoretically arise from either grammatical deviance or significant processing difficulty. Acceptability data are thus naturally ambiguous in theories that explicitly distinguish formal and functional constraints. Here, we consider this source ambiguity problem in the context of Superiority effects: the dispreference for ordering a wh-phrase in front of a syntactically “superior” wh-phrase in multiple wh-questions, e.g., What did who buy? More specifically, we consider the acceptability contrast between such examples and so-called D-linked examples, e.g., Which toys did which parents buy? Evidence from acceptability and self-paced reading experiments demonstrates that (i) judgments and processing times for Superiority violations vary in parallel, as determined by the kind of wh-phrases they contain, (ii) judgments increase with exposure, while processing times decrease, (iii) reading times are highly predictive of acceptability judgments for the same items, and (iv) the effects of the complexity of the wh-phrases combine in both acceptability judgments and reading times. This evidence supports the conclusion that D-linking effects are likely reducible to independently motivated cognitive mechanisms whose effects emerge in a wide range of sentence contexts. This in turn suggests that Superiority effects, in general, may owe their character to differential processing difficulty
On Orbital Period Changes in Nova Outbursts
We propose a new mechanism that produces an orbital period change during a
nova outburst. When the ejected material carries away the specific angular
momentum of the white dwarf, the orbital period increases. A magnetic field on
the surface of the secondary star forces a fraction of the ejected material to
corotate with the star, and hence the binary system. The ejected material thus
takes angular momentum from the binary orbit and the orbital period decreases.
We show that for sufficiently strong magnetic fields on the surface of the
secondary star, the total change to the orbital period could even be negative
during a nova outburst, contrary to previous expectations. Accurate
determinations of pre- and post-outburst orbital periods of recurrent nova
systems could test the new mechanism, in addition to providing meaningful
constraints on otherwise difficult to measure physical quantities. We apply our
mechanism to outbursts of the recurrent nova U Sco.Comment: Accepted for publication in MNRA
Can management intensity be more important than environmental factors? A case study along an extreme elevation gradient from central Italian cereal fields
This paper aims to assess the importance of environmental and management factors determining the weed species
composition along a strong elevation gradient. A total of 76 cereal fields (39 low input and 37 intensively managed) were
sampled along an elevation gradient in central Italy. Explanatory variables were recorded for each field to elucidate the role of
large-scale spatial trends, of site-specific abiotic environmental conditions and of field management characters. Redundancy
analysis was used to assess the relative importance of each environmental variable in explaining the variation in species
composition. Our results indicate that variation in weed species composition is strongly determined by altitude, mean annual
precipitation, mean annual temperature and also by soil characteristics. However, the level of intensification proved to be the
most influential variable. There was a significant difference in species richness and composition between low-input and
intensively managed fields. Intensification leads to considerable species loss at both lower and higher elevations. Low-input
fields had 296 species in total, while intensively managed fields had only 196
Heat flow of the Earth and resonant capture of solar 57-Fe axions
In a very conservative approach, supposing that total heat flow of the Earth
is exclusively due to resonant capture inside the Earth of axions, emitted by
57-Fe nuclei on Sun, we obtain limit on mass of hadronic axion: m_a<1.8 keV.
Taking into account release of heat from decays of 40-K, 232-Th, 238-U inside
the Earth, this estimation could be improved to the value: m_a<1.6 keV. Both
the values are less restrictive than limits set in devoted experiments to
search for 57-Fe axions (m_a<216-745 eV), but are much better than limits
obtained in experiments with 83-Kr (m_a<5.5 keV) and 7-Li (m_a<13.9-32 keV).Comment: 8 page
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