James H. Applegate

A mechanism for orbital period modulation in close binaries

Some eclipsing variables are observed to undergo orbital period modulations of amplitude ΔP/P∼10 −5 over time scales of decades or longer. These modulations can be explained by the gravitational coupling of the orbit to variations in the shape of a magnetically active star in the system. The variable deformation of the active star is produced by variations in the distribution of angular momentum as the star goes through its activity cycle. This mechanism typically requires that the active star be variable at the ΔL/L⇒0.1 level, and be differentially rotating at the ΔΩ/Ω⇒0.01 level

The cooling of neutron stars by the direct Urca process

The first results are reported from a program to reanalyze the cooling of neutron stars by including the direct Urca process in calculations. It is found that the surface temperature of a young neutron star drops dramatically after about 100 yr if the direct Urca process is allowed and nucleons do not become superfluid. If nucleon superfluidity occurs throughout the direct Urca region, the surface temperature drops to a value determined by the superfluid transition temperature after about 100 yr and decreases slowly for the next 100,000 yr, at which time surface photon cooling takes over. By comparison with observational data, it is found that superfluid transition temperatures of the order of 10 exp 9 K are required in the whole direct Urca inner core.

Cosmological quantum chromodynamics, neutron diffusion, and the production of primordial heavy elements

A simple one-dimensional model is used to describe the evolution of neutron density before and during nucleosynthesis in a high-entropy bubble left over from the cosmic quark-hadron phase transition. It is shown why cosmic nucleosynthesis in such a neutron-rich environment produces a surfeit of elements heavier than lithium. Analytical and numerical techniques are used to estimate the abundances of carbon, nitrogen, and heavier elements up to Ne-22. A high-density neutron-rich region produces enough primordial N-14 to be observed in stellar atmospheres. It shown that very heavy elements may be created in a cosmological r-process; the neutron exposure in the neutron-rich regions is large enough for the Ne-22 to trigger a catastrophic r-process runaway in which the quantity of heavy elements doubles in much less than an expansion time due to fission cycling. A primordial abundance of r-process elements is predicted to appear as an excess of rare earth elements in extremely metal-poor stars. 42 references.

Orbital period variability in the eclipsing pulsar binary PSR B1957+20: Evidence for a tidally powered star

Recent observations indicate that the eclipsing pulsar binary PSR B1957+20 undergoes alternating epochs of orbital period increase and decrease. We apply a model developed to explain orbital period changes of alternating sign in other binaries to the PSR B1957+20 system and find that it fits the pulsars observations well. The novel feature of the PSR B1957+20 system is that the energy flow in the companion needed to power the orbital period change mechanism can be supplied by tidal dissipation, making the companion the first identified tidally powered star. The flow of energy in the companion drives magnetic activity, which underlies the observed orbital period variations. The magnetic activity and the wind driven by the pulsar irradiation results in a torque on the spin of the companion. This torque holds the companion out of synchronous rotation, causing tidal dissipation of energy. We propose that the progenitor had a approximately 2 hr orbital period and a companion mass of 0.1-0.2 solar mass, and the system is evolving to longer orbital periods by mass and angular momentum loss on a timescale of 10(exp 8) yr.

LONG-TERM PERIODS IN CATACLYSMIC VARIABLES

Recent interest in long periods which may be present in cataclysmic variables has been motivated principally by the idea that solar-like magnetic cycles might be important. For the brighter members of the class, light curves compiled from amateur visual observations are an idea resource for testing the presnece of such periods. Because of the sheer immensity of the data, and the relative constancy of the average human eye over decades, the detection limits are more sensitve than could be achieved by any feasible photographic or photoelectric means. Here we present results for four well-studied stars, and evaluate the case for periods in other CVs. The conclusion from all of this is that none of the stars studied to date exhibits a good case for strictly periodic variability, and it is not even clear whether there is a preferred timescale._ Nevertheless, the observed amplitudes (0.2 mag) and apparent_ timescales (5-40 yrs) of variability are plausible consequences from solar-like magnetic cycles. We propose that the observed light variations on decade timescales are produced by the same mechanism which underlies the decade-timescale orbital period changes seen in eclipsing binaries. In particular, we extend a theory of Porb changes to the case of accretion-powered binaries, and show that it predicts accretion rate variations of amplitude delta-M/M ~= 0.1, consistent with observation. The proximate cause of the Porb and M variations is the cyclic transfer of angular momentum to and from the outer layers of the contact star. The underlying cause of these phenomena is solar-type cycles of the contact star.

Formation of Planets around Pulsars

Pulse arrival-time delays PSR 1257+12 suggest the existence of at least two planets in nearly circular orbits around it. In this paper we discuss different scenarios for the formation of planets in circular orbits around pulsars. Among other topics, we look in some detail at wind emission mechanisms that are particularly relevant to the process of evaporation of planets around pulsars and discuss their possible role in orbit circularization. We conclude that the formation of such planets may occur in a very late phase of low-mass X-ray binary (LMHB) or binary millisecond pulsar (BMP) evolution

Why stars become red giants

It is shown that a radiative envelope in which the Kramers opacity law holds cannot transport a luminosity larger than a critical value, and it is argued that the transition to red giant structure is triggered by the stars luminosity exceeding the critical value. If the Kramers law is used for all temperatures and densities, the radius of the star diverges as the critical luminosity is approached. In real stars the radiative envelope expands as the luminosity increases until the star intersects the Hayashi track. Once on the Hayashi track, luminosities in excess of the critical luminosity can be accommodated by forcing most of the mass of the envelope into the convection zone. 17 references.

Neutron diffusion, primordial nucleosynthesis, and the r-process

Abstract The application of standard particle physics to big bang cosmology predicts that the temperature interval between roughly 1 TeV and the onset of primordial nucleosynthesis at 100 keV should be rich in physical phenomena; electroweak symmetry breaking, chiral symmetry breaking, and quark confinement should all occur in this interval. First-order phase transitions can produce entropy inhomogeneities because of the uneven release of latent heat, and by shocks and detonations if significant supercooling occurs. In addition, stable objects such as black holes or soliton stars can be produced, and these objects can generate entropy by accretion. These possibilities suggest that the assumption of homogeneity in the standard model of big bang nucleosynthesis could be seriously wrong. In this article I analyse nucleosynthesis in inhomogeneous cosmologies, and I conclude that if baryon density contrasts of order 10 can be created on the proper scales, then the success of the standard model in predicting light-element abundances can be matched by a model containing a closure density of baryons. I argue that the prediction of the cosmological production of r -process elements by the inhomogeneous models is the most promising means of discriminating between these models and the standard model.

On the explosion in a common envelope scenario for type Ia supernovae

Common envelope binaries in which a white dwarf stably burns hydrogen accreted from an evolved companion have recently been proposed as the progenitors of type Ia supernovae. The paper examines this scenario and finds difficulties with it. The supernova explosion itself will strip the hydrogen-rich envelope off the evolved companion and contaminate the ejecta with a mass fraction X(H) greater than about 0.1 of hydrogen, which is at least a factor of 5 larger than the upper limit for SN 1981 B. The luminosity, contact binary Roche geometry, and supernova rate required in the scenario demand that greater than about 10 percent of all stars more luminous than 10 to the 38th ergs/s be unidentified eclipsing variables with periods in the range 3-1000 days. The Yale Bright Star Catalog should contain 2-15 such objects, and the Henry Draper Catalog should contain 6-40 of them. The fact that no object with the predicted properties is known while dozens are predicted to exist in well-studied catalogs of bright stars is a serious problem for the scenario. 31 refs.

Hydrogen as a test of type IA supernova progenitor models

If type Ia supernovae are produced by the explosion of a white dwarf accreting matter from a hydrogen-rich companion, the material stripped and ablated from the companion during the explosion will contaminate the supernova spectrum with hydrogen. The amount of material stripped and ablated from the companion is calculated, showing that a hydrogen mass fraction of at least X(H) greater than 0.01 should exist in type Ia supernovae if their progenitors are cataclysmic variable-like systems. This amount of hydrogen is barely consistent with current upper limits, but the inhomogeneous distribution of the hydrogenin the ejecta must be taken into account before detailed comparisons can be made. White dwarfs accreting from Roche lobe-filling red giants can probably be ruled out as SN Ia progenitors because they contaminate the spectrum with too much hydrogen. 11 refs.