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Abstract:

We study the merging of massive stars inside a common envelope for binary systems consisting of a red supergiant with a mass of 15 - 20 and a main-sequence companion of 1 - 5 M.. We are particularly interested in the stage when the secondary, having overfilled its Roche lobe inside the common envelope, starts to transfer mass to the core of the primary at a very high mass-transfer rate and the subsequent nucleosynthesis in the core-impact region. Using a parametrized model for the structure of the envelope at this stage, we perform 2-dimensional hydrodynamical calculations with the Munich Prometheus code to calculate the dynamics of the stream emanating from the secondary and its impact on the core of the primary. We find that, for the lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich material can penetrate deep into the primary core where nucleosynthesis through the hot CNO cycle can take place and that the associated neutron exposure may be sufficiently high for significant s-processing. For mass-transfer rates at the high end of our estimated range and higher densities in the stream, the stream impact can lead to the dredge-up of helium, but the neutron production is too low for significant s-processing.

Abstract:

We discuss the various phases encountered during common-envelope (CE) evolution, starting with the criterion for dynamical mass transfer and the onset of a common-envelope phase, the main spiralin phase and ending with the final merging of the binary components (in cases where the envelope is not ejected earlier). We emphasize the different physical processes and uncertainties in these different phases and try to clarify the main issues involved in modeling these. Results of a systematic study of the quasi-static response of a common envelope to a spiralling-in binary shed some light on the various phases and the conditions under which CE ejection may be expected. It suggests that, when the CE is ejected, the process tends to be very efficient. However, a simple energy criterion for the ejection is generally not sufficient since in many cases the spiral-in process can be self-regulating and non-dynamic. The physics of the spiral-in is fundamentally different if the initial mass donor is a radiative star. In this case, it is possible that a CE system (contact system) may become semi-detached again without experiencing significant spiral-in, and the mass loss from the contact configuration is best described by a frictionally driven stellar wind. If the spiralling-in star is compact, CE ejection may also occur when it penetrates below the convective envelope. Finally we outline a scheme, combining both quasi-static and hydrodynamical approaches, by which it can be hoped that a detailed, semi-quantitative understanding of CE evolution may be achieved in the foreseeable future.

Abstract:

Graphs of old nova ultraviolet luminosities against orbital inclination show that there is an inverse correlation between the two quantities. DQ Her and GK Per, which are thought to be magnetic systems with disrupted inner disks, appear to have low ultraviolet luminosities. Instead, HR Del has a considerably higher luminosity than other novae with similar inclination and RR Pic is brighter also. The question arises whether these novae have maintained a form of activity possibly involving a low level of nuclear burning on a low mass white dwarf for many decades after the outburst.

Abstract:

We present mid-infrared photometric results obtained with ISOCAM and ISOPHOT on 3 millisecond pulsars and 3 ordinary radio pulsars. No detections have been obtained for the three ms pulsars nor the two more distant radio pulsars. A faint enhancement in the brightness map at 90 mum is seen at about 5 arcsec from the radio position of PSR J0108-1431, the nearest radio pulsar (Tauris et al, 1994), located at 85 pc from us. We conclude that this 90 mum emission, amounting to about 12 mJy, originates either from material orbiting the pulsar or from cirrus on the line of sight. We deduce the upper limits on mass of dust orbiting this pulsar and on the mean temperature of grains.

Abstract:

Re-examination of extensive photometric data of TV Col reveals evidence for a permanent positive superhump. Its period (6.4 hr) is 16 percent longer than the orbital period and obeys the well known relation between superhump period excess and binary period. At 5.5-hr, TV Col has an orbital period longer than any known superhumping cataclysmic variable and, therefore, a mass ratio which might be outside the range at which superhumps can occur according to the current theory. We suggest several solutions for this problem.