212 research outputs found
Why a New Code for Novae Evolution and Mass Transfer in Binaries?
One of the most interesting problems in Cataclysmic Variables is the long time scale evolution. This problem appears in long time evolution, which is also very important in the search for the progenitor of SN Ia. The classical approach to overcome this problem in the simulation of novae evolution is to assume: (1) A constant in time, rate of mass transfer. (2) The mass transfer rate that does not vary throughout the life time of the nova, even when many eruptions are considered. Here we show that these assumptions are valid only for a single thermonuclear flash and such a calculation cannot be the basis for extrapolation of the behavior over many flashes. In particular, such calculation cannot be used to predict under what conditions an accreting WD may reach the Chandrasekhar mass and collapse. We report on a new code to attack this problem. The basic idea is to create two parallel processes, one calculating the mass losing star and the other the accreting white dwarf. The two processes communicate continuously with each other and follow the time depended mass loss
A Shotgun Model for Gamma Ray Bursts
We propose that gamma ray bursts (GRBs) are produced by a shower of heavy
blobs running into circumstellar material at highly relativistic speeds. The
gamma ray emission is produced in the shocks these bullets drive into the
surrounding medium. The short term variability seen in GRBs is set by the
slowing-down time of the bullets while the overall duration of the burst is set
by the lifetime of the central engine. A requirement of this model is that the
ambient medium be dense, consistent with a strong stellar wind. The efficiency
of the burst can be relatively high.Comment: 4 pages, 2 figures, revised version accepted by ApJ Letter
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
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
Increased ionization supports growth of aerosols into cloud condensation nuclei
Ions produced by cosmic rays have been thought to influence aerosol and cloud processes by an unknown mechanism. Here the authors show that the mass flux of ions to aerosols enhances their growth significantly, with implications for the formation of cloud condensation nuclei
The response of clouds and aerosols to cosmic ray decreases
A method is developed to rank Forbush decreases (FDs) in the galactic cosmic ray radiation according to their expected impact on the ionization of the lower atmosphere. Then a Monte Carlo bootstrap-based statistical test is formulated to estimate the significance of the apparent response in physical and microphysical cloud parameters to FDs. The test is subsequently applied to one ground-based and three satellite-based data sets. Responses (>95%) to FDs are found in the following parameters of the analyzed data sets. AERONET: Ångström exponent (cloud condensation nuclei changes), SSM/I: liquid water content, International Satellite Cloud Climate Project (ISCCP): total, high, and middle, IR-detected clouds over the oceans, Moderate Resolution Imaging Spectroradiometer (MODIS): cloud effective emissivity, cloud optical thickness, liquid water, cloud fraction, liquid water path, and liquid cloud effective radius. Moreover, the responses in MODIS are found to correlate positively with the strength of the FDs, and the signs and magnitudes of the responses agree with model-based expectations. The effect is mainly seen in liquid clouds. An impact through changes in UV-driven photo chemistry is shown to be negligible and an impact via UV absorption in the stratosphere is found to have no effect on clouds. The total solar irradiance has a relative decrease in connection with FDs of the order of 10−3, which is too small to have a thermodynamic impact on timescales of a few days. The results demonstrate that there is a real influence of FDs on clouds probably through ions.</p
Super-Eddington Atmospheres that Don't Blow Away
We show that magnetized, radiation dominated atmospheres can support steady
state patterns of density inhomogeneity that enable them to radiate at far
above the Eddington limit, without suffering mass loss. The inhomogeneities
consist of periodic shock fronts bounding narrow, high-density regions,
interspersed with much broader regions of low density. The radiation flux
avoids the regions of high density, which are therefore weighed down by
gravity, while gas in the low-density regions is slammed upward into the shock
fronts by radiation force. As the wave pattern moves through the atmosphere,
each parcel of matter alternately experiences upward and downward forces, which
balance on average. Magnetic tension shares the competing forces between
regions of different densities, preventing the atmosphere from blowing apart.
We calculate the density structure and phase speed of the wave pattern, and
relate these to the wavelength, the density contrast, and the factor by which
the net radiation flux exceeds the Eddington limit. In principle, this factor
can be as large as the ratio of magnetic pressure to mean gas pressure, or the
ratio of radiation pressure to gas pressure, whichever is smaller. Although the
magnetic pressure must be large compared to the mean gas pressure in order to
support a large density contrast, it need not be large compared to the
radiation pressure. These highly inhomogeneous flows could represent the
nonlinear development of the "photon bubble" instability discovered by Gammie.
We briefly discuss the applicability of these solutions to astrophysical
systems.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journa
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
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