Amos Harpaz’s research while affiliated with Technion – Israel Institute of Technology and other places

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Publications (58)


The effects of boundary conditions on stellar evolution
  • Article

August 2017

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2 Reads

Symposium - International Astronomical Union

Amos Harpaz

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Giora Shaviv

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.


A “Fine Structure Constant” for Inertia
  • Article
  • Full-text available

January 2013

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39 Reads

International Journal of Astronomy and Astrophysics

Download

Two Tests for the Equivalence Principle

June 2011

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6 Reads

International Journal of Modern Physics Conference Series

The question: "Is the equivalence principle (EP) a general principle" is examined by analyzing solutions to two cases: 1. The Twin Paradox, and 2. Does a static charge located in a gravitational field radiate? The solutions to these two cases are given first by using EP, and then by physical analysis of the system involved. The fact that the two methods yield the same solutions, may be considered as test cases for the validity of the EP.


Possible Implications of the Planet Orbiting the Red Horizontal Branch Star HIP 13044

April 2011

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15 Reads

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26 Citations

The Astrophysical Journal Letters

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 that it maintained a substantial envelope mass of 0.3 M ☉, raising the question of how the planet survived the common envelope before the envelope itself was lost? If such a planet enters the envelope of an RGB star, it is expected to spiral-in to the very inner region within 100 yr, and be evaporated or destroyed by the core. We speculate that the planet was engulfed by the star as a result of the core helium flash that caused this metal-poor star to swell by a factor of ~3-4. The evolution following the core helium flash is very rapid, and some of the envelope is lost due to the interaction with the planet, and the rest of the envelope shrinks within about a hundred years. This is about equal to the spiraling-in time, and the planet survived.


The role of ballast in the helium-shell flash

January 2010

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8 Reads

Monthly Notices of the Royal Astronomical Society

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 a helium-shell flash at this stage is the presence of mass `ballast' over the helium-burning shell. The effects of different amounts of such ballast are studied, and the detailed evolution of such a flash is presented.


Triggering Eruptive Mass Ejection in Luminous Blue Variables

August 2009

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42 Reads

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18 Citations

New Astronomy

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 mass at a rate of from the outer envelope for 20 years. We find that after removing a small amount of mass at a high rate, the star contracts and releases a huge amount of gravitational energy. We suggest that this energy can sustain the high mass loss rate. The triggering of this runaway mass loss process might be a close stellar companion or internal structural changes. We show that a strong magnetic field region can be built in the radiative zone above the convective core of the evolved massive star. When this magnetic energy is released it might trigger a fast removal of mass, and by that trigger an eruption. Namely, LBV major eruptions might be triggered by magnetic activity cycles. The prediction is that LBV stars that experience major eruptions should be found to have a close companion and/or have signatures of strong magnetic activity during or after the eruption.


Eccentric Binary Model for Off-Center Planetary Nebula Nuclei

January 2009

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14 Reads

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22 Citations

The Astrophysical Journal

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 from the AGB star varies systematically with orbital phase. Such variations may be due to several effects, including a tidal enhancement of the stellar wind near periastron and a cessation of the stellar wind when the Roche lobe of the AGB star encroaches on its extended atmosphere. Our results may pertain to binary systems with semimajor axes in the range of a 7-80 AU, which corresponds to orbital periods in the range P 15-500 yr. We apply the results to planetary nebulae in general, and MyCn 18 (the Hourglass Nebula) in particular, where the central star was recently found by the Hubble Space Telescope to be displaced from the center of the nebula. The results of this paper may be applied to circumstellar matter around more massive stars, such as progenitors of supernovae, by rescaling the physical properties of the binary stars and the wind velocities.


The Role of Ionization Energy during Planetary Nebula Ejection

January 2009

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12 Reads

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18 Citations

The Astrophysical Journal

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, drives the ejection of planetary nebula (PN) is discussed. Tests suggested to validate the hypothesis that the ionization energy drives PN ejection are examined, and it is found that these tests are not sensitive to details of the hypothesis they are supposed to validate.


Triggering Eruptive Mass Ejection in Luminous Blue Variables

January 2009

We study the runaway mass loss process of major eruptions of luminous blue variables (LBVs) stars, such as the 1837-1856 Great Eruption of Eta 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 mass at a rate of 1 Mo/year from the outer envelope for 20 years. We find that after removing a small amount of mass at a high rate, the star contracts and releases a huge amount of gravitational energy. We suggest that this energy can sustain the high mass loss rate. The triggering of this runaway mass loss process might be a close stellar companion or internal structural changes. We show that a strong magnetic field region can be built in the radiative zone above the convective core of the evolved massive star. When this magnetic energy is released it might trigger a fast removal of mass, and by that trigger an eruption. Namely, LBV major eruptions might be triggered by magnetic activity cycles. The prediction is that LBV stars that experience major eruptions should be found to have a close companion and/or have signatures of strong magnetic activity during or after the eruption.


Characteristic Length in a Linear Acceleration

March 2007

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10 Reads

International Journal of Theoretical Physics

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 distribution of the radiation emitted by a linearly accelerated charge, and it is found that this distribution is peaked close to the characteristic length that characterizes the linear acceleration, which is the radius of curvature of the curved electric field created by the acelerated charge.


Citations (26)


... One of the main questions yet to be studied with modern numerical tools is what occurs to the stellar structure of a massive star during a GE, and how such an event affects the future evolution of the star. Harpaz & Soker (2009) simulated GE for a 190 M ⊙ star, using the stellar evolution code Evolve (Harpaz et al. 1987). They removed mass at a rate of 1 M ⊙ yr −1 from the outer envelope for 20 years, and found large fluctuations in luminosity and stellar radius. ...

Reference:

Giant Eruptions in Massive Stars and their Effect on the Stellar Structure
Evolution of a 0.7-solar mass red giant
  • Citing Article
  • December 1987

The Astrophysical Journal

... 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] . ...

Superluminal velocities in astronomy?
  • Citing Article
  • November 1996

The Physics Teacher

... 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. ...

AN ELECTRIC CHARGE IN A GRAVITATIONAL FIELD

... Then Falik extended the theory [43] in 1980. The field equations corresponding to de Sitter spacetime, which were obtained using Rosen's method [44], were resolved by Harpaz and Rosen [45] in 1985 for modeling of a compact star. Also, they demonstrated that, for an ordinary star, the outcomes of BGR and Einstein's GR are identical. ...

Compact objects in bimetric general relativity
  • Citing Article
  • March 1985

The Astrophysical Journal

... According to the Refs. [129,130], a stellar system will be stable when at each point of whole interior region is greater than 4/3 otherwise stellar configurations will be unstable against radial perturbation. Finally, through the Fig. 8, we display the plot of as a function of r in three panels for three different values of α. ...

The stability of relativistic gas spheres
  • Citing Article
  • December 1982

Monthly Notices of the Royal Astronomical Society

... 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. ...

Evolution of Compact Binary Systems with X-Ray Heating
  • Citing Article
  • November 1991

The Astrophysical Journal

... 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. ...

Asymmetrical planetary nebulae : University of Haifa at Oranim conference
  • Citing Article
  • January 1995

... Theory suggests that exoplanets should exist around WDs. Outer planets (those in orbits beyond approximately the asteroid belt) should survive unscathed, while planets inward of ∼1 au should be engulfed or tidally disrupted during the red giant phase (Soker et al. 1984;Mustill & Villaver 2012). Those that survive are expected to migrate outward due to the mass loss of the star. ...

The evolution of a star-'planet' system in the double core phase
  • Citing Article
  • August 1984

Monthly Notices of the Royal Astronomical Society

... 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. ...

Density inversion in a red giant envelope
  • Citing Article
  • September 1984

Monthly Notices of the Royal Astronomical Society