381 research outputs found
Present Status of the Theoretical Predictions for the ^(37)Cl Solar-Neutrino Experiment
The theoretical predictions for the ^(37)Cl solar-neutrino experiment are summarized and compared with the experimental results of Davis, Harmer, and Hoffman. Three important conclusions about the sun are shown to follow
Continuum driven winds from super-Eddington stars. A tale of two limits
Continuum driving is an effective method to drive a strong stellar wind. It
is governed by two limits: the Eddington limit and the photon-tiring limit. A
star must exceed the effective Eddington limit for continuum driving to
overcome the stellar gravity. The photon-tiring limit places an upper limit on
the mass loss rate that can be driven to infinity, given the energy available
in the radiation field of the star. Since continuum driving does not require
the presence of metals in the stellar atmosphere it is particularly suited to
removing mass from low- and zero-metallicity stars and can play a crucial part
in their evolution. Using a porosity length formalism we compute numerical
simulations of super-Eddington, continuum driven winds to explore their
behaviour for stars both below and above the photon-tiring limit. We find that
below the photon tiring limit, continuum driving can produce a large, steady
mass loss rate at velocities on the order of the escape velocity. If the star
exceeds the photon-tiring limit, a steady solution is no longer possible. While
the effective mass loss rate is still very large, the wind velocity is much
smallerComment: to be published in the conference proceedings of: First Stars III,
Santa Fe, 200
Solar models and electron screening
We investigate the sensitivity of the solar model to changes in the nuclear
reaction screening factors. We show that the sound speed profile as determined
by helioseismology certainly rules out changes in the screening factors
exceeding more than 10%. A slightly improved solar model could be obtained by
enhancing screening by about 5% over the Salpeter value. We also discuss how
envelope properties of the Sun depend on screening, too. We conclude that the
solar model can be used to help settling the on-going dispute about the
``correct'' screening factors.Comment: accepted for publication by Astron. Astrophy
Numerical simulations of continuum-driven winds of super-Eddington stars
We present the results of numerical simulations of continuum-driven winds of
stars that exceed the Eddington limit and compare these against predictions
from earlier analytical solutions. Our models are based on the assumption that
the stellar atmosphere consists of clumped matter, where the individual clumps
have a much larger optical thickness than the matter between the clumps. This
`porosity' of the stellar atmosphere reduces the coupling between radiation and
matter, since photons tend to escape through the more tenuous gas between the
clumps. This allows a star that formally exceeds the Eddington limit to remain
stable, yet produce a steady outflow from the region where the clumps become
optically thin. We have made a parameter study of wind models for a variety of
input conditions in order to explore the properties of continuum-driven winds.
The results show that the numerical simulations reproduce quite closely the
analytical scalings. The mass loss rates produced in our models are much larger
than can be achieved by line driving. This makes continuum driving a good
mechanism to explain the large mass loss and flow speeds of giant outbursts, as
observed in eta Carinae and other luminous blue variable (LBV) stars. Continuum
driving may also be important in population III stars, since line driving
becomes ineffective at low metalicities. We also explore the effect of photon
tiring and the limits it places on the wind parameters.Comment: Accepted for publication by MNRA
Inhomogeneity in the Supernova Remnant Distribution as the Origin of the PAMELA Anomaly
Recent measurements of the positron/electron ratio in the cosmic ray (CR)
flux exhibits an apparent anomaly, whereby this ratio increases between 10 and
100 GeV. We show that inhomogeneity of CR sources on a scale of order a kpc,
can naturally explain this anomaly. If the nearest major CR source is about a
kpc away, then low energy electrons ( GeV) can easily reach us. At
higher energies ( GeV), the source electrons cool via synchrotron
and inverse-Compton before reaching Earth. Pairs formed in the local vicinity
through the proton/ISM interactions can reach Earth also at high energies, thus
increasing the positron/electron ratio. A natural origin of source
inhomogeneity is the strong concentration of supernovae in the galactic spiral
arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and
taking into account their concentration near the galactic spiral arms, we
consistently recover the observed positron fraction between 1 and 100 GeV.
ATIC's electron excess at GeV is explained, in this picture, as the
contribution of a few known nearby SNRs. The apparent coincident similarity
between the cooling time of electrons at 10 GeV (where the positron/electron
ratio upturn), Myr, and the CRs protons cosmogenic age at the same
energy is predicted by this model
Early evolution of the extraordinary Nova Del 2013 (V339 Del)
We determine the temporal evolution of the luminosity L(WD), radius R(WD) and
effective temperature Teff of the white dwarf (WD) pseudophotosphere of V339
Del from its discovery to around day 40. Another main objective was studying
the ionization structure of the ejecta. These aims were achieved by modelling
the optical/near-IR spectral energy distribution (SED) using low-resolution
spectroscopy (3500 - 9200 A), UBVRcIc and JHKLM photometry. During the fireball
stage (Aug. 14.8 - 19.9, 2013), Teff was in the range of 6000 - 12000 K, R(WD)
was expanding non-uniformly in time from around 66 to around 300 (d/3 kpc)
R(Sun), and L(WD) was super-Eddington, but not constant. After the fireball
stage, a large emission measure of 1.0-2.0E+62 (d/3 kpc)**2 cm**(-3)
constrained the lower limit of L(WD) to be well above the super-Eddington
value. The evolution of the H-alpha line and mainly the transient emergence of
the Raman-scattered O VI 1032 A line suggested a biconical ionization structure
of the ejecta with a disk-like H I region persisting around the WD until its
total ionization, around day 40. It is evident that the nova was not evolving
according to the current theoretical prediction. The unusual non-spherically
symmetric ejecta of nova V339 Del and its extreme physical conditions and
evolution during and after the fireball stage represent interesting new
challenges for the theoretical modelling of the nova phenomenon.Comment: 14 pages, 9 figures, 3 tables, accepted for Astronomy and
Astrophysic
The Salpeter plasma correction for solar fusion reactions
We review five different derivations that demonstrate that the Salpeter
formula for the plasma corrections to fusion rates is valid at the center of
the sun with insignificant errors (~ percent). We point out errors in several
recent papers that have obtained a variety of answers, some even with the wrong
sign or the wrong functional dependence.Comment: Related information at http://www.sns.ias.edu/~jn
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 Brightest Black Holes
I suggest that there are two classes of ultraluminous X-ray sources (ULXs),
corresponding to super-Eddington mass inflow in two situations: (a)
thermal-timescale mass transfer in high-mass X-ray binaries, and (b)
long-lasting transient outbursts in low-mass X-ray binaries. These two classes
are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The
observed ULX population is a varying mixture of the two, depending on the star
formation history of the host galaxy. ULXs in galaxies with vigorous star
formation (such as the Antennae) are generally SS433--like, while ULXs in
elliptical galaxies must be of the microquasar type. The latter probably have
significantly anisotropic radiation patterns. They should also be variable, but
demonstrating this may require observations over decades. The close analogy
between models of X-ray binaries and active galactic nuclei (AGN) suggests that
there should exist an apparently super-Eddington class of the latter, which may
be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray
emission. These are presumably a subset of the ULXs, but remain as yet
unidentified.Comment: 4 pages, no figures; accepted for MNRAS Letter
- âŠ