304 research outputs found
Multivariate characterization of hydrogen Balmer emission in cataclysmic variables
The ratios of hydrogen Balmer emission line intensities in cataclysmic
variables are signatures of the physical processes that produce them. To
quantify those signatures relative to classifications of cataclysmic variable
types, we applied the multivariate statistical analysis methods of principal
components analysis and discriminant function analysis to the spectroscopic
emission data set of Williams (1983). The two analysis methods reveal two
different sources of variation in the ratios of the emission lines. The source
of variation seen in the principal components analysis was shown to be
correlated with the binary orbital period. The source of variation seen in the
discriminant function analysis was shown to be correlated with the equivalent
width of the H line. Comparison of the data scatterplot with
scatterplots of theoretical models shows that Balmer line emission from T CrB
systems is consistent with the photoionization of a surrounding nebula.
Otherwise, models that we considered do not reproduce the wide range of Balmer
decrements, including "inverted" decrements, seen in the data.Comment: Accepted by PAS
Recommended from our members
Sun-shades as a passive cooling element
A discussion of the importance of proper sun-shade design is presented by showing results of thermal performance evaluation of a building with and without sun-shades. The analysis was carried out by using a simulation model developed by Shaviv and Shaviv. The criteria for evaluating sunshades is based on the total energy consumption of the building for cooling, heating, and artificial lighting. We find that the best solution for shading is external sun-shades and we present a method developed by Shaviv for their design. Several designs of external sun-shades are presented
Variability in GRBs - A Clue
We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.Comment: 15 page latex file with 5 PS figure. Complete uuencoded compressed PS
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/SaP_aclue.u
The role of electron-screening deformations in solar nuclear fusion reactions and the solar neutrino puzzle
Thermonuclear fusion reaction rates in the solar plasma are enhanced by the
presence of the electron cloud that screens fusing nuclei. The present work
studies the influence of electron screening deformations on solar reaction
rates in the framework of the Debye-Huckel model. These electron-ion cloud
deformations, assumed here to be static and axially symmetric, are shown to be
able to considerably influence the solar neutrino fluxes of the pp and the CNO
chains, with reasonable changes in the macroscopic parameters of the standard
solar model (SSM) . Various known deformation sources are discussed but none of
them is found strong enough to have a significant impact on the SSM neutrino
fluxes.Comment: Revised version (14 RevTeX pages, 3 ps figures). Accepted for
publication in Nuclear Physics
Observations and simulations of recurrent novae: U Sco and V394 CrA
Observations and analysis of the Aug. 1987 outburst of the recurrent nova V394 CrA are presented. This nova is extremely fast and its outburst characteristics closely resemble those of the recurrent nova U Sco. Hydrodynamic simulations of the outbursts of recurrent novae were performed. Results as applied to the outbursts of V394 CrA and U Sco are summarized
Astrophysical factors:Zero energy vs. Most effective energy
Effective astrophysical factors for non-resonant astrophysical nuclear
reaction are invariably calculated with respect to a zero energy limit. In the
present work that limit is shown to be very disadvantageous compared to the
more natural effective energy limit. The latter is used in order to modify the
thermonuclear reaction rate formula so that it takes into account both plasma
and laboratory screening effects.Comment: 7 RevTex pages. Accepted for publication in Phys.Rev.
Thermophysical properties of the lanthanide sesquisulfides. III. Determination of Schottky and lattice heatâcapacity contributions of Îłâphase Sm2S3 and evaluation of the thermophysical properties of the Îłâphase Ln2S3 subset
We report the experimental heat capacity of Îłâphase Sm2S3 and derived thermophysical properties at selected temperatures. The entropy, enthalpy increments, and Gibbs energy function are 21.50R, 3063Râ
K, and 11.23R at 298.15 K. The experimental heat capacity is made up of lattice and electronic (Schottky) contributions. The lattice contribution is determined for all Îłâphase lanthanide sesquisulfides (Ln2S3 ) using the Komada/Westrum model. The difference between the experimental heat capacity and the deduced lattice heat capacity is analyzed as the Schottky contribution. Comparisons are made between the calorimetric Schottky contributions and those determined based on crystalâfield electronic energy levels of Ln3+ ions in the lattice and between the Schottky contributions obtained from the empirical volumetric priority approach and from the Komada/Westrum theoretical approach. Predictions for the thermophysical properties of Îłâphase Eu2S3 and Îłâphase Pm2S3 (unavailable for experimental determination) are also presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71137/2/JCPSA6-96-8-6149-1.pd
Flow instabilities of magnetic flux tubes IV. Flux storage in the solar overshoot region
We consider the effects of material flows on the dynamics of toroidal
magnetic flux tubes located close to the base of the solar convection zone,
initially within the overshoot region. The problem is to find the physical
conditions in which magnetic flux can be stored for periods comparable to the
dynamo amplification time, which is of the order of a few years. We carry out
nonlinear numerical simulations to investigate the stability and dynamics of
thin flux tubes subject to perpendicular and longitudinal flows. We compare the
simulations with the results of simplified analytical approximations. We
determine ranges of the flow parameters for which a linearly Parker-stable
magnetic flux tube is stored in the middle of the overshoot region for a period
comparable to the dynamo amplification time. The residence time for magnetic
flux tubes with fluxes of 2x10^{21} Mx in the convective overshoot layer is
comparable to the dynamo amplification time, provided that the average speed
and the duration of the downflow do not exceed about 50 m/s and 100 days,
respectively, and that the lateral extension of the flow is smaller than about
10 degrees.Comment: Accepted to be published in Astronomy and Astrophysics. 16 pages, 16
figures. To access GIF animations, use
http://www.mps.mpg.de/homes/ishik/flute/frict_inst.gif,
http://www.mps.mpg.de/homes/ishik/flute/TF60.gif and
http://www.mps.mpg.de/homes/ishik/flute/TF180.gi
Non-linear screening corrections of stellar nuclear reaction rates and their effects on solar neutrino fluxes
Non-linear electron screening corrections of stellar nuclear fusion rates are
calculated analytically in the framework of the Debye-Huckel model and compared
with the respective ones of Salpeter's weak screening approximation. In typical
solar conditions, the deviation from Salpeter's screening factor is less than
one percent, while for hotter stars such corrections turn out to be of the
order of one percent only over the limits of the Debye-Huckel model. Moreover,
an investigation of the impact of the derived non-linear screening effects on
the solar neutrino fluxes yields insignificant corrections for both the pp and
CNO chain reactions.Comment: To appear in Phys.Rev.
Physical parameters and emission mechanism in Gamma-Ray Bursts
Detailed information on the physical parameters in the sources of
cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions
consistent with observations. Model-independent requirements posed by these
assumptions on the emission mechanism in GRBs are formulated. It is found that
the observed radiation in sub-MeV energy range is generated by the synchrotron
emission mechanism, though about ten per cent of the total GRB energy should be
converted via the inverse Compton process into ultra-hard spectral domain
(above 100 GeV). We estimate the magnetic field strength in the emitting
region, the Lorentz factor of accelerated electrons, and the typical energy of
IC photons.
We show that there is a "line-of-death" relation for GRBs and derive from
this relation the lower limits on both GRB duration and GRB variability
timescale. The upper limit on the Lorentz factor of GRB fireballs is also
found. We demonstrate that steady-state electron distribution consistent with
the Compton losses may produce different spectral indices, e.g., 3/4 as opposed
to the figure 1/2 widely discussed in the literature. It is suggested that the
changes in the decline rate observed in the lightcurves of several GRB
afterglows may be due to the time evolution of spectral break, which appears in
the synchrotron emission generated by steady-state self-consistent electron
distribution.Comment: Journal reference added, introduction extended, minor changes in
notation
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