Equation of Motion of an Electric Charge

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 force which is responsible for the creation of the radiation (instead of the "radiation reaction force" that actually does not exist at low velocities). Thus the initial acceleration should be supplied as an initial condition for the solution of the EOM of an electric charge. It is also shown that in certain cases, like periodic motions of an electric charge, the term that includes the time derivative of the acceleration, represents the stress reaction force.Comment: 12 pages, 2 figure

Radiation from a Uniformly Accelerated Charge

The emission of radiation by a uniformly accelerated charge is analyzed. According to the standard approach, a radiation is observed whenever there is a relative acceleraion between the charge and the observer. Analyzing difficulties that arose in the standard approach, we propose that a radaition is created whenever a relative acceleration between the charge and its own electric field exists. The electric field induced by a charge accelerated by an external (nongravitational) force, is not accelerated with the charge. Hence the electric field is curved in the instantanous rest frame of the accelerated charge. This curvature gives rise to a stress force, and the work done to overcome the stress force is the source of the energy carried by the radiation. In this way, the "energy balance paradox" finds its solution.Comment: Latex, uses aasms4.sty, 14 pages, Accepted for publication in General Relativity and Gravitation. For a postscript file please contact Noam Soker: [email protected]

Triggering Eruptive Mass Ejection in Luminous Blue Variables

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.Comment: Accepted by New Astronom

A Superwind from Early Post-Red Giant Stars?

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 envelope structure of RGB stars which start to peel off is similar to that of late asymptotic giant branch stars known to have a super-wind phase. An analogous super-wind in the RGB peel-off stars could easily lead to the observed gap in the distribution of the hottest HB stars.Comment: 9 pages; Accepted by ApJ Letters; Available also at http://www.astro.puc.cl/~mcatelan

Defining the Termination of the Asymptotic Giant Branch

I suggest a theoretical quantitative definition for the termination of the asymptotic giant branch (AGB) phase and the beginning of the post-AGB phase. I suggest that the transition will be taken to occur when the ratio of the dynamical time scale to the the envelope thermal time scale, Q, reaches its maximum value. Time average values are used for the different quantities, as the criterion does not refer to the short time-scale variations occurring on the AGB and post-AGB, e.g., thermal pulses (helium shell flashes) and magnetic activity. Along the entire AGB the value of Q increases, even when the star starts to contract. Only when a rapid contraction starts does the value of Q start to decrease. This criterion captures the essence of the transition from the AGB to the post AGB phase, because Q is connected to the stellar effective temperature, reaching its maximum value at T~4000-6000 K, it is related to the mass loss properties, and it reaches its maximum value when rapid contraction starts and envelope mass is very low.Comment: Submitted to ApJ Letter

Why Magnetic Fields Cannot be the Main Agent Shaping Planetary Nebulae

An increasing amount of literature reports the detection of magnetic fields in asymptotic giant branch (AGB) stars and in central stars of planetary nebulae (PNs). These detections lead to claims that the magnetic fields are the main agent shaping the PNs. In this paper, I examine the energy and angular momentum carried by magnetic fields expelled from AGB stars, as well as other physical phenomena that accompany the presence of large scale fields, such as those claimed in the literature. I show that a single star cannot supply the energy and angular momentum if the magnetic fields have the large coherent structure required to shape the circumstellar wind. Therefore, the structure of non-spherical planetary nebulae cannot be attributed to dynamically important large scale magnetic fields. I conclude that the observed magnetic fields around evolved stars can be understood by locally enhanced magnetic loops which can have a secondary role in the shaping of the PN. The primary role, I argue, rests with the presence of a companion.Comment: PASP, 2006, in press. (This paper was rejected by MNRAS and ApJ; my criticism of the referee reports are in: Soker, N. astro-ph/0508525.