Amos Harpaz's research while affiliated with Technion - Israel Institute of Technology and other places
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Publications (51)
The effects of using different treatments of the surface boundary conditions are investigated in the context of the mass of He White Dwarfs. We find that since the White Dwarf progenitor is a star with a very extended atmosphere, the results are sensitive to the degree of accuracy implemented in the handling of the boundary conditions.
We propose a scenario to account for the surprising orbital properties of the planet orbiting the metal-poor red horizontal branch star HIP 13044. The orbital period of 16.2 days implies that the planet went through a common envelope phase inside the red giant branch (RGB) stellar progenitor of HIP 13044. The present properties of the star imply th...
Calculations are presented of the evolutionary track of an asymptotic giant branch (AGB) star, after most of its envelope is consumed and it starts moving to the right in the Hertzsprung-Russell (HR) diagram. On looking for the conditions that may create a helium-shell flash at this stage, it is found that a necessary condition for the creation of...
We study the runaway mass loss process of major eruptions of luminous blue variables (LBVs) stars, such as the 1837–1856 Great Eruption of η Carinae. We follow the evolution of a massive star with a spherical stellar evolution numerical code. After the star exhausted most of the hydrogen in the core and had developed a large envelope, we remove mas...
We examine the influence of eccentric binary progenitors on the morphologies of their descendant planetary nebulae. In particular, we consider how mass loss via a stellar wind by an asymptotic giant branch (AGB) star in an eccentric binary can lead to the displacement of the central star in the equatorial plane. We postulate that the mass-loss rate...
The conditions for release of the ionization energy in the envelope of an asymptotic giant branch star are studied. It is shown that the recombination that releases the ionization energy also causes a sharp drop of the opacity, thus enabling the released energy to flow outward freely. The possibility that the ionization energy, when released, drive...
The role of the characteristic length that characterizes linear acceleration is studied, in order to find how does this length
determine the characteristic wavelength of the radiation created by the accelerated charge. Unruh equation for the temperature
observed by a detector accelerated relative to the vacuum is used to determine the wavelength di...
We construct a speculative scenario for rotation-induced extra helium mixing
to the envelope of horizontal branch (HB) stars. This scenario differs from
previous ones in that the mixing occurs after the star has left the red giant
branch (RGB). We follow the evolution of a low metallicity star from the RGB to
the HB, and examine the density profile...
Detailed evolutionary calculations show that Coulomb interactions between the charged particles of a stellar plasma reduce the core mass at which a low mass red giant undergoes the helium flash (contrary to a recent claim). This has implications for the determination of the rate of mass loss from red giants.
We analyze the situation of an observer coaccelerated relative to a linearly accelerated charge, in order to find whether
he can observe the radiation emitted from the accelerated charge. It is found that the seemingly special situation of the
coaccelerated observer relative to any other observer, is deduced from a wrong use of the retarded coordin...
The interaction between a charge and its electric field are analyzed in different situatuations. It is found that whether the charge is accelerated and its electric field lags behind, or in the case where the charge is static and its electric field falls in a gravitational field --- in both cases the electric field curves, a stress force is created...
The potential of a static electric charge located in a Schwarzschild gravitational field is given by Linet. The expressions for the field lines derived from this potential are calculated by numerical integration and drawn for different locations of the static charge in the gravitational field. The field lines calculated for a charge located very cl...
We examine whether a charge supported statically in a gravitational field radiates, and find the answer to this question to be positive. Based on our earlier results we find that the important condition for the creation of radiation is the existence of a relative acceleration between the charge and its electric field, where such an acceleration cau...
Late asymptotic giant branch (AGB) and early post-AGB stars which are progenitors of planetary nebulae lose mass at extremely high rate, in what is termed a superwind. We show that the existence of this superwind during the post-AGB phase cannot be explained with models where the stellar effective temperature is the sole main physical parameter whi...
The appearance of the time derivative of the acceleration in the equation of motion (EOM) of an electric charge is studied. It is shown that when an electric charge is accelerated, a stress force exists in the curved electric field of the accelerated charge, and this force is proportional to the acceleration. This stress force acts as a reaction fo...
We re-examine a recent claim by Han et al. (2002) that the ionization energy in the envelope has to be included in the ejection criterion of common envelopes. In particular, we argue that (1) they include a mass loss rate prior to the onset of the common envelope that is too low; (2) They do not include the energy radiated by the accreting white dw...
We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early
post-AGB phase. Because of the high mass-loss rate, the envelope mass decreases by more than an order of magnitude. This makes
the density profile below the photosphere much shallower, and the entropy profile much steeper....
The concept of a `field' is analysed, in connection with different
physical situations, specifically, concerning the creation of
radiation. The role of a field in relativistic physics shows that it
is an independent physical entity that should be considered on the
same grounds as matter particles.
We suggest that the gap observed at 20,000 K in the horizontal branches of several Galactic globular clusters is caused by a small amount of extra mass loss which occurs when stars start to "peel off" the red giant branch (RGB), i.e., when their effective temperature starts to increase, even though they may still be on the RGB. We show that the env...
The conditions in which electromagnetic radiation is formed are discussed. It is found that the main condition for the emission of radiation by an electric charge is the existence of a relative acceleration between the charge and its electric field. Such a situation exists both for a charge accelerated in a free space, and for a charge supported at...
The emission of radiation from an accelerated charge is analyzed. It is foundthat at zero velocity, the radiation emitted from the charge imparts no countermomentum to the emitting charge, and no radiation reaction force is created bythe radiation. A reaction force is created by the stress force that exists in thecurved electric field of the charge...
We analyze the angular momentum evolution from the red giant branch (RGB) to the horizontal branch (HB) and along the HB. Using rotation velocities for stars in the globular cluster M13, we find that the required angular momentum for the fast rotators is up to 1-3 orders of magnitude (depending on some assumptions) larger than that of the sun. Plan...
We present an eccentric binary model for the formation of the proto-planetary nebula CRL 2688 (the Egg Nebula) that exhibits multiple concentric shells. Given the apparent regularity of the structure in the Egg Nebula, we postulate that the shells are caused by the periodic passages of a companion star. Such an orbital period would have to lie in t...
The cyclic external heating of a low-mass 0.4 solar mass star is studied
in detail. The intermittent heating intervals simulate the irradiation
of an asynchronously rotating star by the X-ray flux from an accreting
companion neutron star in a binary system. Each point on the stellar
surface resides for a certain time interval in the heated hemisphe...
The evolution of low-mass X-ray binary (LMXB) systems is investigated
with X-ray heating of the secondary star included in the calculations.
We take into account shielding of the secondary star from X-radiation by
the accretion disk via an ad hoc prescription. The effects of X-ray
heating and the removal of mass from the secondary are calculated in...
We apply a method, which is traditionally used for the solar magnetic field, to estimate the magnetic activity of AGB stars. We find that any magnetic-field model which tries to explain axisymmetrical mass loss from single AGB stars encounters severe difficulties. This order-of-magnitude calculation suggests that magnetic activity in AGB stars coul...
The evolutionary track of low-mass red giant stars (0.7–0.9M
) is computed with the aim to demonstrate the conditions under which low-mass white dwarfs (WDs) can form through the evolution of single stars. Also, the influence of the mixing length to the scale height ratio on the radius of the star is calculated and the coupling between the mixing-l...
Results of calculations of the evolution of low-mass X-ray binary
systems with the inclusion of heating of the secondary star by the
X-radiation from the primary are presented. The secondary is assumed to
be nonsynchronously rotating with the orbit, so that the X-ray heating
may be taken to be approximately uniform over its surface. The evolution
o...
According to the bimetric general relativity theory, after a star has undergone gravitational collapse, it fills its Schwarzschild sphere and reaches hydrostatic equilibrium. The question investigated is whether the equilibrium is stable. This question was considered in an earlier paper, where it was assumed that the star had zero pressure. In the...
According to the bimetric general relativity theory, when a star
undergoes gravitational collapse, it fills its Schwarzschild sphere and
reaches hydrostatic equilibrium. The question investigated here is
whether the equilibrium is stable. For simplicity, the case of zero
pressure is considered, with equilibrium corresponding to uniform
density. It...
Detailed evolutionary calculations show that Coulomb interactions between the charged particles of a stellar plasma reduce the core mass at which a low mass red giant undergoes the helium flash (contrary to a recent claim). This has implications for the determination of the rate of mass loss from red giants.
The evolution of a 0.7-solar mass red giant is followed along the red giant branch, with mass loss included according to Reimers's formula. The red giant star completes its evolution by totally exhausting its envelope, without igniting the helium in the core. At this point, the core mass, which turns into a white dwarf, is 0.43 solar mass. The cons...
The effects of using different treatments of the surface boundary conditions are investigated in the context of the mass of He white dwarfs. The authors find that since the white dwarf progenitor is a star with a very extended atmosphere, the results are sensitive to the degree of accuracy implemented in the handling of the boundary conditions.
The authors followed the evolution of a 0.7 M_sun; red giant, including
mass loss in the computation code. The evolution was computed until the
stellar envelope was totally expelled, and a WD is likely to be formed,
with a mass equal to the core mass of the red giant. The WD mass
obtained in the present work was 0.43 M_sun;.
In the bimetric general relativity theory, one can have a collapsed
star, filling its Schwarzschild sphere in hydrostatic equilibrium.
Particles and light rays falling toward it bounce back from its surface.
If it is surrounded by a cloud of gas, the falling particles may lose
energy by collisions, thus arriving at the stellar surface with zero
vel...
The field equations of the bimetric general relativity theory proposed
by one of the authors (N. Rosen), in the static form, are solved in
order to investigate the structure of a star. It is found that for an
ordinary star the bimetric theory gives the same results as the Einstein
general relativity theory. However, for a collapsed star the two
the...
The structure of a typical star is investigated analytically both in the
framework of general relativity (GR) and in the bimetric version of GR
developed by Rosen (1980) and Falik and Rosen (1980) to eliminate
singularities appearing in the solutions of the Einstein field
equations. The results of the two analyses are shown to be identical for
an o...
The phenomenon of density inversion in the recombination zone is
discussed through detailed analysis of a particular envelope of a red
giant star. It is found that the completion of the recombination of the
hydrogen causes a drop in the efficiency of energy transfer by
convection. This drop raises the need for a steep temperature gradient,
which re...
The spiralling-in of a black dwarf ("planet") inside the envelope of a
red giant is studied. The calculation involves both the interaction of
the black dwarf with the envelope and the evolution of the giant. The
structural changes induced in the giant by an energy deposition and mass
ablation are followed using a fully evolutionary code. The author...
The eigenvalue problem for linear adiabatic radial perturbations of
relativistic gas spheres is presented in a tridiagonal matrix
formulation. Two classes of relativistic polytropes are studied. Curves
of marginal stability are found in the (n, q) parameter plane, where n
is the polytropic index and q is the ratio of central pressure to
density. Je...
Evolutionary calculations of a central star of a planetary nebula are
presented, and the evolutionary path is compared to the Harman-Seaton
sequence. It is found that the crossing of the H-R diagram is along a
horizontal line. The reasons for other possible paths are discussed. A
minute expansion of the core is found at the beginning of this stage,...
The evolution of a 1.2Mʘ star along the asymptotic branch with continuous mass loss is presented, showing that this mass loss leads to the formation of a PN with a typical central star in its center.
A former investigation (Harpaz and Kovetz, 1980) has shown that mechanisms for PN creation based on sudden violent processes are not likely to work in...
The evolutionary calculations of a 1.2 solar mass star along the
asymptotic branch with a continuous mass loss using Reimer's empirical
formula show the final stage loss rate to be 2.7 x 10 to the -6th solar
mass/yr. The final mass is 0.6 solar mass. Analysis of other mechanisms
for the creation of planetary nebulae (P.N.) along the evolutionary pa...
6. Summary 53 Abstract The behaviour of an electric field in a gravitational field is analyzed. The electric field of a charge accelerated in free space is studied using the equations of STR, and it is found to be curved due to the acceleration of the charge. The electric field of a charge located statically in a gravitational field curves too due...
Citations
... Whilst their luminosity will rise they cannot reach the highest phase of the RGB and so helium flash will not occur. Furthermore, (Harpaz et al., 1987) theorised that stars with mass < 0.7 M☉ are unable to become asymptotic giant stars as the majority of their mass has been lost in the red giant phase of evolution. ...
... As is so often pointed out, the causality will be violated if there are tachyons in our world [2] . In fact, nature has exhibited many superluminal phenomena to us, especially in astronomy [3,4] , but they do not imply the genuine existence of superluminal objects. Furthermore, there exist various proposals for observing faster-than-c propagation of light using anomalous dispersion near an absorption line and linear gain lines, or tunnelling barriers [5,6,7,8] . ...
... De Almeida et al [13] obtained a similar result by means of a different analysis. On the other hand, Harpaz and Soker [14][15][16] claimed that a supported charge at rest in a static gravitational field does radiate, thus satisfying the EP. Finally, Licata and Elmo [17] discussed the electric field of a charge falling in a more realistic, non-uniform gravitational field. ...
Reference: Falling charges and Lemaître observers
![[object Object]](https://i1.rgstatic.net/ii/profile.image/278398283862022-1443386731811_Q64/Noam-Soker.jpg)
... The importance of irradiation feedback in BH X-ray binaries (especially those with a large accretion disk, which casts a shadow on the star) has been under debate (Vilhu et al. 1994;Ritter et al. 2000;Ritter 2008), but observations have shown that the process does operate in binary systems containing a white-dwarf accretor (Mróz et al. 2016). Persistent X-ray irradiation could drive cycles of high and low mass transfer rates in low-mass Xray binaries, with peaks reaching 10 −5 M⊙ yr −1 or higher (Harpaz & Rappaport 1994), but the characteristic duty cycle of > 10 4 yr implies that this mechanism is irrelevant for the outbursts seen in HLX-1. ...
![[object Object]](https://i1.rgstatic.net/ii/profile.image/592980612239360-1518389003629_Q64/Saul-Rappaport.jpg)
... In the year 1978, Rosen [30] has initially written the static field equations in the form of Einstein's field equations with an additional bimetric term. In the year 1985, Harpaz and Rosen [18] solved the field equations, which are obtained by using the procedure of Rosen [31] corresponding to de Sitter universe of constant curvature, to obtain a model of compact star. They also proved that for an ordinary star, the results obtained from BGTR have no difference from the result obtained from Einstein's GTR. ...
Reference: Tolman IV fluid sphere in bigravity
... The graphical behavior of adiabatic index is illustrated in Fig. 10 (bottom panels). It is worth mentioning here that the adiabatic index ought be greater than 4∕3 in a stable polytropic stellar body by an amount that depends upon that ratio ∕ at the center of the compact celestial object [58]. Also, the value of the ratio ∕ lies between two to four for the EoS linked to neutron star matter [59]. ...
... This can be caused by the fact that the star does not have time to relax back to its thermal equilibrium (Knigge et al. 2011). In case one component is a NS or BH the effect of X-ray irradiation, which heats up (hence bloats) the star, can play a role (Podsiadlowski 1991, Harpaz & Rappaport 1991. . Radius and mass of the seven short-period black-hole low-mass X-ray binaries. ...
... The established rotation velocity seems to be sufficient to form an equatorially enhanced wind from U Ori through the wind compression effect proposed byBjorkman & Cassinelli (1993)with the equator/pole contrast exceeding 2 or 3. Additional shaping of the U Ori envelope might be produced by the magnetic field that is probably responsible for the bipolar outflow. The origin of the asymmetry seen in the majority of the planetary nebulae has been the subject of much discussion (see, e.g., the series of meetings on Asymmetric Planetary Nebulae;Meixner et al. (2004);Kastner et al. (2000);Harpaz & Soker (1995)). In 1980s, the paradigm emerged that this was the result of a symmetric, fast hot wind plowing into an asymmetric AGB wind remnant. ...
... Many previous studies have explored the effects of an engulfed planet or brown dwarf on the internal structure and observable properties of a giant star, including , Soker et al. (1984), Harpaz & Soker (1994), Soker (1998a), Nelemans & Tauris (1998), Siess & Livio (1999a, 1999b, Retter & Marom (2003), Carlberg et al. (2009Carlberg et al. ( , 2012, and Staff et al. (2016). Others have considered the possible role of substellar companions in the origin of hot subdwarf stars (e.g. , Soker 1998b;Nelemans 2010;Bear & Soker 2011) and single helium-core WDs (e.g., Nelemans & Tauris 1998;Zorotovic & Schreiber 2022), as well as the morphology of planetary nebulae (e.g., Nordhaus & Blackman 2006;Clyne et al. 2014;Boyle 2018). ...
... Owing to numerical difficulties, the timesteps become exceedingly small, preventing further evolution of the star. While less-massive stars are only affected by this process in their late evolutionary phases (Harpaz 1984 ;Lau et al. 2012 ), very massive stars can exceed the Eddington limit already during the corehydrogen-burning phase (Gr äfener, Owocki & Vink 2012 ;Sanyal et al. 2015 ) and inhibit computation of their evolution. Therefore, 1D stellar evolution codes have to employ various solutions to compute further evolution of very massive stars. ...
