Abstract: We have discovered five detached close binary stars out of seven white dwarfs chosen for their low mass (<0.45 Msun). The high success rate of our observations supports the notion that evolution within a binary star is needed to obtain white dwarfs with masses below 0.45 Msun. We have measured the orbital period and radial velocity amplitude of four of our discoveries. No late-type features are seen in any of our targets at a level which forces the mass of any main-sequence companion to be less than 0.1 Msun. This, together with our measured mass functions, implies that the companion stars are in fact white dwarfs. Our observations raise the number of detached, double-degenerate close binaries with known orbital periods from two to six. All of the orbits are circular, a consequence of past interaction. We find periods of 1.1, 3.3 and 4.8 d for 1713+332, 1241-010 and 1317+453. 2331+290 has a very short period, most likely 4h, but with the 1 cycle d-1 alias at 4.8 h also a possibility. Gravitational radiation will cause this star to merge within about 2 × 10^9 yr. Close double-degenerates go through one or more stages during which the two stars orbit inside a common envelope. The long orbital periods of 1241-010 and 1317+453 suggest that the ejection of the envelope during the common-envelope phase is very efficient. The spectrum of the companion is directly detectable in 1713+332 but in none of the other systems.

Abstract: We study the secular evolution and gravitational wave signature of a newly formed, rapidly rotating neutron star. The neutron star may arise from core collapse in a massive star or from the accretion-induced collapse of a white dwarf. After a brief dynamical phase, the nascent neutron star settles into an axisymmetric, secularly unstable state. Gravitational radiation drives the star to a nonaxisymmetric, stationary equilibrium configuration via the bar-mode instability. The emitted quasi-periodic gravitational waves have a unique signature: the wave frequency sweeps downward from a few hundred Hertz to zero, while the wave amplitude increase from zero to a maximum and then decays back to zero. Such a wave signal could detected by broadband gravitational wave interferometers currently being constructed. We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency (f greater than or approximately = 1000 Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.

Abstract: We have found that the white dwarf PG 1101+364 is a double-lined white dwarf/white dwarf binary with an orbital period of 3.47 hours. PG 1101+364 is the shortest period detached double-degenerate yet found and gravitational radiation will cause it to merge in 2.5 X 10**9 years. PG 1101+364 has a mass ratio of 0.87 +/- 0.03. From the relative strengths of the narrow core of Halpha, the lighter and therefore larger star appears to contribute more of the light by an amount that indicates almost equal temperatures for the two stars. We chose to observe PG 1101+364 on the basis of its low spectroscopic mass (0.31 Msun), and the discovery of its binary nature confirms the belief that binary evolution is required to produce such low mass white dwarfs. The radial velocity semi-amplitudes (69.7 +/- 1.7 km/s and 80.3 +/- 1.6 km/s) and the spectroscopic mass show that we see the orbit of PG 1101+364 at a low inclination of ~ 25 deg.

Abstract: Evolution of the coalescence rate of double neutron stars (NS) and neutron star -- black hole (BH) binaries are computed for model galaxies with different star formation rates. Assuming gamma-ray bursts (GRB) to originate from NS+NS or NS+BH merging in distant galaxies, theoretical logN--logS distributions and tests of gamma-ray bursts (GRB) are calculated for the first time taking the computed merging rates into account. We use a flat cosmological model (Omega=1) with different values of the cosmological constant Lambda and under various assumptions about the star formation history in galaxies. The calculated source evolution predicts a 5-10 times increase of the source statistics at count rates 3-10 times lower than the exising BATSE sensitivity limit. The most important parameter in fitting the 2nd BATSE catalogue is the initial redshift of star formation, which is found to be z_*=2-5 depending on a poorly determined average spectral index of GRB.

Abstract: A newly formed protoneutron star, prior to the loss of neutrinos, will consist of a charge-neutral mixture of neutrons, protons, electrons, muons and trapped neutrinos in the lowest energy state available under these circumstances. However, in a few seconds, the neutrinos will escape, enabling the interior of the star to find a lower energy. We study two possibilities: (1) A significant number of baryons will, through the weak interaction, convert to hyperons. (2) Hadronic matter may convert to quark matter in the interior where the pressure is high. In either case the resulting softer equation of state will not support as large a range of stars either in mass or baryon number. Therefore, beyond the baryon number of either of the above equilibrated sequences there is a significant range of protoneutron stars that possess no hydrostatically stable configurations after neutrino loss. They will subside into a black hole on the time-scale of neutrino loss, about ten seconds, and after the processed material of the presupernova star has been ejected. The uncertainty as to which of the two possibilities for the ground state actually holds is due mainly to the imperfect description available for the quark matter phase of cold charge-neutral baryonic matter. We discuss these mechanisms in connection with the apparent absence of a neutron star in SN1987 A and the possible deficit of neutron star supernova associations . tThis work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Division of Nuclear Physics, of the U.S. Department of Energy under Contract DE-AC03-76SF00098. Prompt Subsidence of a Protoneutron Star into a Black Hole

Abstract: It is commonly accepted that candidates for very high energy γ-ray sources are neutron stars, binary systems, black holes etc. Close binary systems containing a normal hot star and a neutron star (or a black hole) form an important class of very high energy γ-ray sources. Such systems are variable in any region of the electromagnetic spectrum and they enable us to study various stages of stellar evolution, accretion processes, mechanisms of particle acceleration, etc. Phenomena connected with this class of very high energy γ-ray sources are discussed. Particular emphasis has been placed on the TeV energy region.

Abstract: I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as spin-powered pulsars with relatively low magnetic fields and fast rotation I discuss how the new-born neutron star is spun down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron star becomes an accretion-powered X-ray pulsar Detailed observations of massive radio pulsar binaries like PSR 1259-63 will yield valuable information about this phase of initial spindown I indicate how the spin of the neutron star then evolves under accretion torques during the subsequent phase as an accretion-powered pulsar Finally, I describe how the neutron star is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar magnetic field, the origins of which are still not completely understood

Abstract: The X-ray spectrum and the rapid X-ray variability of accreting compact objects have a common origin, and their properties can therefore be expected to be coupled. Indeed, it turns out that the X-ray spectrum and the power spectrum show correlated variations, which occur as a function of variations in the mass fluxx Ṁ. Stellar mass black holes and neutron stars have similar mass and size, and therefore their accretion phenomena may be expected to show similarities. Indeed, similarities exist that indicate that a unified description may be possible. If a particular phenomenon is seen in both neutron star and black-hole candidate systems this shows immediately that it cannot be due to any property that is unique to either neutron stars or black holes, such as the presence or absence of a surface, or of a strong non-aligned magnetic field. The quest for characteristics that are unique to black holes continues - I shall mention a few candidates below.

Abstract: Observational evidence, and theoretical models of the magnetic field evolution of neutron stars is discussed. Observational data indicates that the magnetic field of a neutron star decays significantly only if it has been a member of a close interacting binary. Theoretically, the magnetic field evolution has been related to the processing of a neutron star in a binary system through the spin evolution of the neutron star, and also through the accretion of matter on the neutron star surface. I describe two specific models, one in which magnetic flux is expelled from the superconducting core during spin-down, via a copuling between Abrikosov fluxoids and Onsager-Feynman vortices; and another in which the compression and heating of the stellar crust by the accreted mass drastically reduces the ohmic decay time scale of a magnetic field configuration confined entirely to the crust. General remarks about the behaviour of the crustal field under ohmic diffusion are also made.

Abstract: We investigate the production of 26 Al during hydrogen burning and its ejection by massive single and binary stars. Effects of convection and rotation are studied. We discuss the importance of RSGs, LBVs and WR stars to the total Galactic 26 Al production, and the detection probability of the 26 Al decay in individual objects as P Cygni, γ Velorum and η Carinae.

Abstract: A newly formed neutron star in a supernova finds itself in a dense environment, in which the gravitational energy of accreting matter can be lost to neutrinos. For the conditions in SN 1987A, ≲0.1M ⊙ may have fallen back onto the central neutron star on a timescale of hours after the explosion, after which the accretion rate is expected to drop sharply. Radiation is trapped in the flow until the mass accretion rate drops to 2×10−4 M ⊙ yr−1 at which point radiation can begin to escape from the shocked envelope at an Eddington limit luminosity. Between this neutrino limit and the Eddington limit, 3×10−8 M ⊙ yr−1, there are no steady, spherical solutions for neutron star accretion. SN 1987A should have reached the neutrino limit within a year of the explosion; the current lack of an Eddington luminosity can be attributed to black hole formation or to a clearing of the neutron star envelope. There is no evidence for newly formed neutron stars in supernovae. Radio supernovae, which were initially interpreted as pulsar activity, probably involve circumstellar interaction; SN 1993J shows especially good evidence for outer shock phenomena.

Abstract: Binary formation is the dominant mode of star formation in the nearby Taurus and Ophiuchus star forming regions. I describe very briefly our present understanding of multiplicity among the youngest stars, the evolution and structure of their circumstellar disks, and their potential as sites of planetary system formation.

Abstract: Radio interferometric mea~urementsl-~ and observations of the speed of the interstellar scintillation patterns5i6 show that a large number of pulsars have proper motions between some ten and several hundred km/s. Reevaluation of older data based upon an improved distance scale for the pulsars together with new measurements that extended the sample of pulsars with known transverse velocities4 reveal a significant number of objects with velocities close to 1000 km/s or even higher'. The complete sample of 99 pulsars considered by Lyne and Lorimer7 includes 86 pulsars with interferometry data and another 13 with scintillation data. The sample has a mean transverse velocity of 300 f 30 km/s. The increase of this number compared with previous analyses reflects both the change of the adopted distance scale and the greater number of young and higher velocity pulsars in the recent astrometric surveys. Considering only the 29 youngest pulsars of the sample (younger than 3 Myr), one finds a mean transverse velocity of 345 f 70 km/s, implying an average space and birth velocity of pulsars of (wns) = 450 f 90 km/s. (The r.m.s. value is 535 km/s.) Associations between young supernova remnants and nearby pulsars'-'' yield consistent results and might indicate even faster motions. Taking 13 reasonably convincing associations, Lyne and Lorimer' deduce a mean transverse velocity of about 530 f 180 km/s, corresponding to an average space velocity of 690 f 230 km/s. The 10 oldest pulsars of the analysed sample have a mean transverse speed of only 105 f 25 km/s, which confirms a suggested strong selection effect6 that makes the observed velocities unrepresentative of those acquired at birth. Young pulsars appear to have higher velocities on average than