Theoretical modelling of Algol disks

A brief review of various theoretical approaches to model accretion disks is presented. Emphasis is given to models that determine self-consistently the structure of a disk together with the radiation field. It is argued that a proper treatment of the vertical structure is essential for calculating theoretical spectra to be compared with observations. In particular, it is shown that hot layers above an accretion disk (sometimes called disk chromospheres or coronae), whose presence is indicated by recent UV observations of strong emission lines of highly ionized species, may be explained using simple energy balance arguments.1987–88 JILA Visiting Fellow.This work was in part supported by a NASA grant ADP U-003-88 (Plavec and Hubeny). I also wish to thank the organizers of the IAU Colloquium 107 for the travel grant which enabled me to attend the meeting.     

A New Mechanism of Coronal Heating

The interaction between network magnetic fields and emerging intranetwork fields may lead to magnetic reconnection and microflares, which generate fast shocks with an Alfvén Mach number M _A<2. Protons and less abundant ions in the solar corona are then heated and accelerated by fast shocks. Our study of shock heating shows that (a) the nearly nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (v _th), which is an increasing function of the mass/charge ratio; (b) the heating by subcritical shocks with 1.1 M_A 1.5 leads to a large temperature anisotropy with T/T 50 for O⁵⁺ ions and a mild anisotropy with T_/T 1.2 for protons; (c) the large perpendicular thermal velocity of He⁺⁺ and O⁵⁺ ions can be converted to the radial outflow velocity (u) in the divergent coronal field lines; and (d) the heating and acceleration by shocks with 1.1 M_A 1.5 can lead to u(O⁵⁺) v _th(O⁵⁺) 460 km s^–1 for O⁵⁺ ions, u(He⁺⁺) v _th(He⁺⁺) 360 km s^–1 for He⁺⁺ ions, and u(H⁺) v _th(H⁺) 240 km s^–1 for protons at r=3–4 R . Our results can explain recent SOHO observations of the heating and acceleration of protons and heavier ions in the solar corona.     

Physical and chemical characteristics of the ISM inside and outside the heliosphere

This paper summarizes some of the discussions of working group 8–9 during the ISSI Conference on The Heliosphere in the Local Interstellar Medium. Because the subject of these working groups has become significantly broader during the last ten years, we have selected three topics for which recent observations have modified and improved our knowledge of the heliosphere and the surrounding interstellar medium. These topics are the number densities and ISM ionization states of hydrogen and helium, the newly discovered hot gas from the H wall seen in absorption, and the comparison between ISM and heliospheric minor element abundances. Papers from this volume in which more details on these topics can be found are quoted throughout the report.     

Are there different types of variability among B stars?

Conclusion It now appears difficult to write that the Cephei stars occupy a well defined instability strip in the HR diagram (Lesh and Aizenman, 1978): the Cep phenomenon — from any observational definition we could give — cannot be restricted to the small group of variables inside the box limited by the B 0.5 - B 2 spectral types and by the (II–III) – IV luminosity classes.So the pulsation mechanism is probably not strictly related to a narrow evolutionary stage. From this point of view, a mechanism related to envelope properties could better allow for variability in a wider region of the HR diagram. Which should be the new boundaries for such a phenomenon ? Should we expect an amplification of the phenomenon towards the center of its HR domain?     

Fine scale temporal structures in solar hard X-ray bursts observed by PHEBUS

We report here on preliminary results of a systematic study of fast temporal fluctuations in impulsive and extended solar X-ray bursts observed by PHEBUS at energies around 100 keV. Subsecond timescales are quite common in the impulsive events and are not observed in extended ones.     

SOHO: The Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO), together with the Cluster mission, constitutes ESA's Solar Terrestrial Science Programme (STSP), the first Cornerstone of the Agency's long-term programme Space Science — Horizon 2000. STSP, which is being developed in a strong collaborative effort with NASA, will allow comprehensive studies to be made of the both the Sun's interior and its outer atmosphere, the acceleration and propagation of the solar wind and its interaction with the Earth. This paper gives a brief overview of one part of STSP, the SOHO mission.     

The Edison infrared space observatory

For five years, theEdison program has had the goal of developing new designs for infrared space observatories which will break the cost curve by permitting more capable missions at lower cost. Most notably, this has produced a series of models for purely radiative and radiative/mechanical (hybrid) cooling which do not use cryogens and optical designs which are not constrained by the coolant tanks. Purely radiatively-cooled models achieve equilibrium temperatures as low as about 20 K at a distance of 1 AU from the sun. More advancedEdison designs include mechanical cooling systems attached to the telescope assembly which lower the optical system temperature to 5 K or less. Via these designs, near-cryogenic temperatures appear achievable without the limitations of cryogenic cooling. OneEdison model has been proposed to the European Space Agency as the next generation infrared space observatory and is presently under consideration as a candidate ESA Cornerstone mission. The basic design is also the starting point for elements of future infrared space interferometers.     

Field-aligned (Birkeland) currents

Following earlier suggestions of Edmond Halley and Anders Celsius for the magnetic behavior of auroral phenomena, Kristian Birkeland discovered in his polar expeditions of 1902–03 that large-scale electric currents were associated with the aurora. He was also the first to suggest that these currents originated far from earth and that they flowed into the upper polar atmosphere and out of it along magnetic field lines; the existence of such field-aligned currents was widely disputed until satellite and rocket-borne instruments confirmed their permanent existence. The importance of these Birkeland currents to the coupling between the magnetosphere and the polar ionosphere is emphasized by their intensity, which ranges between 10⁶ and 10⁷ amperes, and by the energy which they dissipate in the upper atmosphere, which can exceed by a considerable factor the energy dissipated there by auroral particles. The large- and small-scale average properties of field-aligned currents, determined from spacecraft observations, are reviewed here.     

Some crucial X-ray observations — One or two SNR(s) in Crux?

A new EXOSAT (LE/CMA) observation of the region in Crux (R.A. 11h 45m, Dec. -62°) where Markert et al. (1981) reported the existence of two x-ray SNR's is presented. After cleaning the CMA field from the point source component, due to the UV emission of the numerous stars in the field, the smoothed x-ray contours are compared to the 408 MHz radio map of Caswell et al. (1983). The existence of two, well-separate x-ray emission regions is confirmed by EXOSAT, and the current x-ray/radio picture is not sufficent to distinguish clearly between the assumption of one or two (possibly interacting) SNR's in the region.     

The scientific payload of the space-based Solar and Heliospheric Observatory (SOHO)

The space-based Solar and Heliospheric Observatory (SOHO) is a joint venture of ESA and NASA within the frame of the Solar Terrestrial Science Programme (STSP), the first Cornerstone of ESA's long-term programme Space Science — Horizon 2000. The principal scientific objectives of the SOHO mission are: a) a better understanding of the structure and dynamics of the solar interior using techniques of helioseismology, and b) a better insight into the physical processes that form and heat the Sun's corona, maintain it and give rise to its acceleration into the solar wind. To achieve these goals, SOHO carries a payload consisting of 12 sets of complementary instruments which are briefly described here.     

Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks: 1977–1990

The Voyager 1 and 2 spacecraft include instrumentation that makes comprehensive ion (E 28 keV) and electron (E 22 keV) measurements in several energy channels with good temporal, energy, and compositional resolution. Data collected over the past decade (1977–1988), including observations upstream and downstream of four planetary bow shocks (Earth, Jupiter, Saturn, Uranus) and numerous interplanetary shocks to 30 AU, are reviewed and analyzed in the context of the Fermi and shock drift acceleration (SDA) models. Principal findings upstream of planetary bow shocks include the simultaneous presence of ions and electrons, detection of tracer ions characteristic of the parent magnetosphere (O, S, O⁺), power-law energy spectra extending to 5 MeV, and large (up to 100:1) anisotropies. Results from interplanetary shocks include observation of acceleration to the highest energies ever seen in a shock ( 22 MeV for protons, 220 MeV for oxygen), the saturation in energy gain to 300 keV at quasi-parallel shocks, the observation of shock-accelerated relativistic electrons, and separation of high-energy (upstream) from low-energy (downstream) populations to within 1 particle gyroradius in a near-perpendicular shock. The overall results suggest that ions and electrons observed upstream of planetary bow shocks have their source inside the parent magnetosphere, with first order Fermi acceleration playing a secondary role at best. Further, that quasi-perpendicular interplanetary shocks accelerate ions and electrons most efficiently to high energies through the shock-drift process. These findings suggest that great care must be exercised in the application of concepts developed for heliosphere shocks to cosmic ray acceleration through shocks at supernova remnants.     

Closing address

Impressions from the workshop are summarized. Difficulties within the present observational and theoretical context are outlined and directions for fruitful future work are indicated. The workshop subtitle, the interface between MHD and plasma physics, still appears to be a goal for the future, though some encouraging results have been reported at this workshop.     

The structure of low-mass X-ray binaries

The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity.     

Electron coronal density irregularity\overline {n^2 } /(\bar n)^2 from measurements of K-coronal and lyman lines brightnesses

We propose a technique to derive the coronal density irregularity factor , wheren is the electron density. The absolute photometric comparison between the intensity of UV lines and the white-light K-coronal polarized brightness (pB) provides an unique constraint on the inhomogeneity of the corona. The ratio of the measured H I Lyman (Ly-) line intensity to the resonant-scattering dominated H I Lyman (Ly-) intensity can be used to extract the collisonal component of the Ly-. This component yields an estimate of . The quantity is then obtained from white-light K-coronal measurements. The use of lines of the same atomic species minimizes the effects due to outflow velocities (i.e., Doppler dimming), and reduces the errors introduced by the uncertainties in the ionization balance, the atomic parameters, and the solar abundances. The UVCS/SOHO unique capability of performing cotemporal and cospatial measurements of the Ly- and Ly- lines, and ofpB makes this instrument ideal for implementing this technique.     

Pulsars as gamma ray sources: Nebular shocks and magnetospheric gaps

I summarize the results of recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is primarily electrons and positrons with an admixture of heavy ions. Shocks which contain heavy ions that are a minority constituent by number but which carry most of the energy density in the upstream medium put 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) E ^-2, where N(E)dE is the number of particles with energy between E and E+dE. Synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front, provides the mechanism of thermalization and non-thermal particle acceleration. The maximum energy achievable by the pairs is _± m _± c ² = m _i c ² ₁/Z _i, where ₁ is the Lorentz factor of the upstream flow and Z _i is the atomic number of the ions. The shock's spatial structure contains a series of overshoots in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. These overshoots provide a new interpretation of the structure of the inner regions of the Crab Nebula, in particular of the wisps, surface brightness enhancements near the pulsar. The wisps appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar's wind in the Crab Nebula is spatially resolved, and allows one to measure ₁ 4 × 10⁶, the upstream magnetic field B ₁ to be 3 × 10^-5 Gauss, as well as to show that the total ion flow is 3 × 10³⁴ elementary charges/sec, in good agreement with the total current flow predicted by the early Goldreich and Julian (1969) model. The total pair outflow is shown to be about 5 × 10³⁷ pairs per second, in good agreement with the particle flux required to explain the nebular X—ray source.The energetics of particle acceleration within the magnetospheres of rotation powered pulsars and the consequences for pulsed gamma ray emission are also briefly discussed. The gamma ray luminosity above 100 MeV is shown to scale in proportion to _R ^1/2 , as is in accord with some of the simplest ideas about polar cap models. Models based on acceleration in the outer magnetosphere are also briefly discussed.     

Pulsations of the R Coronae Borealis stars

The radial pulsations of very luminous, low-mass models (L/M 10⁴, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars.     

Relations between ISM inside and outside the heliosphere

Thanks to remarkable new tools, such as the Goddard High Resolution Spectrograph (GHRS) on board the HST and the EUVE spectrometer on the interstellar side, and Ulysses particle detectors on the heliospheric side, it is possible now to begin to compare abundances and physical properties of the interstellar matter outside the heliosphere (from absorption features in the stellar spectra), and inside the heliosphere (from in situ or remote detection of the interstellar neutrals or their derivatives, the pick-up ions or the Anomalous Cosmic Rays detected by the two Voyager spacecraft).Ground-based and UV spectra of nearby stars show that the Sun is located between two volumes of gas of different heliocentric velocities V and temperatures T (see also Linsky et al, this issue). One of these clouds has the same velocity (V= 25.6 km s^–1 from = 255 and =8) and temperature (6700 K) as the heliospheric helium of interstellar origin probed by Ulysses, and is certainly surrounding our star (and then the Local Interstellar Cloud or LIC). This Identification allows comparisons between interstellar constituents on both sides of the heliospheric interface.Ly-alpha background data (absorption cell and recent HST-GHRS spectra) suggest that the heliospheric neutral H velocity is smaller by 5–6 km s^–1 than the local cloud velocity, and therefore that H is decelerated at its entrance into the heliosphere, in agreement with interaction models between the heliosphere and the ISM which include the coupling with the plasma. This is in favor of a non negligible electron density (at least 0.05 cm³). There are other indications of a rather large ionization of the ambient ISM, such as the ionization equilibrium of interstellar magnesium and of sodium. However the resulting range for the plasma density is still broad.The heliospheric neutral hydrogen number density (0.08–0.16 cm^–3) is now less precisely determined than the helium density (0.013–0.017 cm^–3, see Gloeckler, Witte et al, Mobius, this issue). The comparison between the neutral hydrogen to neutral helium ratios in the ISM (recent EUVE findings) and in the heliosphere, suggests that 15 to 70% of H does not enter the heliosphere. The comparison between the interstellar oxygen relative abundance (with respect to H and He) in the ISM and the heliospheric abundance deduced from pick-up ions is also in favor of some filtration, and thus of a non-negligible ionization.For a significant ISM plasma density, one expects a Hydrogen wall to be present as an intermediate state of the interstellar H around the interface between inside and outside. Since 1993, the two UVS instruments on board Voyager 1 and 2 indeed reveal clearly the existence of an additional Ly-alpha emission, probably due to a combination of light from the compressed H wall, and from a galactic source. On the other hand, the decelerated and heated neutral hydrogen of this H wall has recently been detected in absorption in the spectra of nearby stars (see Linsky, this issue).     

Geomagnetic storms,auroras and associated effects

Our knowledge of the interplanetary medium is outlined and its frictionless interaction with the geomagnetic cavity, first discussed by Chapman and Ferraro, is described. An important feature of this interaction is the interplanetary field which is compressed and may possibly lead to the formation of a shock wave.The possibility of frictional interaction between the solar wind and the cavity is discussed; an effect which appears to cause friction is the instability of interpenetrating ion-electron streams. This effect will also cause strong heating and trapping of ions and the generation of electromagnetic waves.The theory of propagation of geomagnetic disturbances in the magnetosphere and ionosphere is reviewed, first in general terms and than for some of the various components of a geomagnetic storm.Sea-level disturbances are divided into stormtime (Dst) and other (DS) components and also into different phases and the experimental data is reviewed. Theories of Dst, including the ringcurrent theory and magnetic tail theory are discussed and compared. Attempts to explain the complex DS field comprise the magnetospheric dynamo theory and the asymmetrical ring-current theory; these are compared in the light of experimental evidence.Motions of plasma and field lines in the magnetosphere are discussed in general terms: there are motions which deform the field and there are interchange motions. The former are opposed by Earth currents; the latter are not. The two types of motion are coupled through ionospheric Hall conductivity. Theories of the DS field in terms of the two types of motion are described; in particular motions caused by frictional interaction with the solar wind are discussed. These motions cause a helical twist in the field lines which propagates into the polar ionosphere as a hydromagnetic wave. In the ionosphere the motions of the field lines drive currents (moving-field dynamo) which cause the DS field.Drifts of neutral ionization in the lower ionosphere lead to localized accumulations which play a vital part in storm and auroral theory: they cause polarization fields which change the DS current system; they react on the magnetospheric motions to cause particle acceleration and precipitation.Auroral morphology and theories are briefly reviewed; the solar wind friction theory, although far from complete may provide a start. Further development should take the form of determining ionospheric drifts, polarization electric fields and consequent magnetospheric effects.A brief discussion is given of some associated effects: growth and decay of belts of geomagnetically trapped corpuscules; increase in ionospheric absorption of radio waves and lower-level X-ray production, ionospheric storm and high-latitude irregularities, micropulsations, VLF and ELF radio emissions from the magnetosphere, atmospheric heating and wave generation.     

Determination of the characteristics of the gamma-ray telescope Gamma-1

The Gamma-1 telescope has been developed through a collaboration of scientists in the USSR and France in order to conduct -ray astronomical observations within the energy range from 50 to 5000 MeV. The major characteristics of the telescope were established by Monte-Carlo simulations and calibrations made with the aid of electron and tagged -ray beams produced by an accelerator, and these have been found to be as follows: the effective area for photons coming along the instrument's axis varies from about 50 cm² at E = 50 MeV to approximately 230 cm² at E 300 MeV; the angular resolution (half opening of the cone embracing 68% events) is equal to 2.7° at E = 100 MeV, and 1.8° at E = 300 MeV; the energy resolution (FWHM) varies from 70% to 35% as the energy of the detected photons increases from 100 to 550 MeV; the telescope's field-of-view at the half-sensitivity level is 300–450 square degrees depending upon the spectrum of the detected radiation, and the event selection logic. Proceeding from the thus obtained characteristics it is demonstrated that a point source producing a photon flux J (E 100 MeV) = 3 × 10^-7 cm^-2 s^-1, can be detected with a 5 significance by observing it during 10⁶ s at the level of the Cygnus background, and a source having intensity J (E 100 MeV) = 10^-6 cm^-2 s^-1 can be detected to within a mean square positional accuracy of about 15.     

The effect of rotation on the hydrodynamics of stellar collapse

Summary Using values of d, _min, and _max that Van Riper (1978) has found most promising for a hydrodynamic envelope ejection, we have shown that even a small amount of rotation in the initial core can stop its collapse before nuclear densities are reached. We expected _i > 0.02 to produce significant deviations from a spherically symmetric collapse, but have found that _i as much as ten times smaller than this will not allow the core to reach densities as high as in the spherical collapse. In no case, however, does the core flatten very much, nor does the value of become very large. Low final 's preclude the formation of an axisymmetric torus. They also indicate that deformation of an iron core into a triaxial configuration or fragmentation of the core during its collapse is an extremely unlikely event. (Note: Classically, must exceed 0.27 before a dynamic instability to non-axisymmetric perturbations is encountered.)The small degree of flattening of the core also suggests that the reduced moment of inertia I of the core will always be relatively small in magnitude and hence that the third time derivative of I, which is proportional to the energy emitted in gravity wave radiation, will not be very significant. Numerically calculated estimates of I- during some of these model evolutions supports this suspicion. If the _min and used here are found to be realistic values after the detailed physics of the core collapse is well understood, it is clear that gravitational radiation from a core collapse will be difficult to measure.Finally, we should point out that it is the relatively large values of Y_min (near 4/3) combined with values of d near unity that (a) prevented the core from flattening significantly in these models and (b) prevented the core from reaching high configurations. If realistic values of either one (or both) of these parameters are found to be much smaller in more complete models of the core collapse, then the core will have to become flatter (and denser) before pressure gradients will support it along the rotation axis. All of the conclusions drawn here would be modified accordingly under those circumstances. It should also be noted that in general relativistic models, the critical for spherical collapse is somewhat larger than 4/3 (Van Riper, 1979). Therefore, we predict that when fully general relativistic core collapses are performed including rotation, a given choice of _min and _i will produce a slightly flatter and slightly denser core than the corresponding model that has been presented here.