Characteristics of nearby interstellar matter

There is a warm tenuous partially ionized cloud (T10⁴ K,n(HI)0.1 cm^–3,n(Hii 0.22–0.44 cm^–3) surrounding the solar system which regulates the environment of the solar system, determines the structure of the heliopause region, and feeds neutral interstellar gas into the inner solar system. The velocity (V–20 km s^–1 froml335°,b0° in the local standard of rest) and enhanced Caii and Feii abundances of this cloud suggest an origin as evaporated gas from cloud surfaces in the Scorpius-Centaurus Association. Although the soft X-ray emission attributed to the Local Bubble is enigmatic, optical and ultraviolet data are consistent with bubble formation caused by star formation epochs in the Scorpius-Centaurus Association as regulated by the nearby spiral arm configuration. The cloud surrounding the solar system (the local fluff) appears to be the leading region of an expanding interstellar structure (the squall line) which contains a magnetic field causing polarization of the light of nearby stars, and also absorption features in nearby upwind stars. The velocity vectors of the solar system and local fluff are perpendicular in the local standard of rest. Combining this information with the low column densities seen towards Sirius in the anti-apex direction, and the assumption that the cloud velocity vector is parallel to the surface normal, suggests that the Sun entered the local fluff within the historical past (less than 10 000 years ago) and is skimming the surface of the cloud. Comparison of magnesium absorption lines towards Sirius and anomalous cosmic-ray data suggest the local fluff is in ionization equilibrium.Reason has moons, but moons not hers, Lie mirror'd on her sea, Confounding her astronomers, But, O! delighting me.Ralph Hodgson     

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?     

Detection limits in the far infrared

The advent of far infrared arrays will change fundamentally the means of analyzing observations in this spectral region. Sources much fainter than traditional confusion limits will be extracted from images by using computer algorithms similar to CLEAN or DAOPHOT. We have conducted numerical experiments to evaluate these techniques and show that they will permit long integrations (10,000 sec at 60 m, 200 sec at 100 m) to achieve nearly photon-background-limited performance and hence very deep detection limits. The dominant noise sources—photon noise, confusion by distant galaxies, and confusion by IR cirrus — scale with nearly the same power of the telescope aperture. As a result, the integration times required to reach confusion limits are nearly aperture-independent.     

Initial observations of low energy charged particles near the Earth's bow shock on ISEE-1

We report initial measurements from the ULECA sensor of the Max-Planck-Institut/University of Maryland experiment on ISEE-1. ULECA is an electrostatic deflection — total energy sensor consisting of a collimator, deflection analyzer and an array of solid state detectors. The position of a given detector, which determines the energy per charge of an incident particle, together with the measured energy determine the particle's charge state. We find that a rich variety of phenomena are operative in the transthermal energy regime (10 keV/Q to 100 keV/Q) covered by ULECA. Specifically, we present observations of locally accelerated protons, alpha particles, and heavier ions in the magnetosheath and upstream of the Earth's bow shock. Preliminary analysis indicates that the behavior of these locally accelerated particles is most similar at the same energy per charge.     

Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment

Velocity and direction of the flow of the interstellar helium and its temperature and density have been determined from the measurements of the ULYSSES/GAS experiment for two different epochs: during the in-ecliptic path of ULYSSES, representing solar maximum conditions, and during the south to the north pole transition (11/94-6/95), close to the solar minimum conditions. Within the improved error bars the values are consistent with results published earlier.The determination of the density n of the interstellar helium at the heliospheric boundary from observations in the inner solar system requires knowledge about the loss processes experienced by the particles on their way to the observer. The simultaneous observation of the helium particles arriving on direct and indirect orbits at the observer provides a tool to directly determine the effects of the loss processes assumed to be predominantly photoionization and — for particles travelling close to the Sun — electron impact ionization by high-energy solar wind electrons.Such observations were obtained with the ULYSSES/GAS instrument in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar density could be derived. Assuming photoionization to be the only loss process reasonable fits to the observations were obtained for an ionization rate = 1.1 · 10^–7 s^–1 and a density n 1.7 · 10^–2 cm^–3. Including, in addition, electron impact ionization, a photoionization = 0.6 · 10^–7 s^–1 was sufficient to fit both observations, resulting in a density n 1.4 · 10^–2 cm^–3.On leave from Space Research Centre, Warsaw, Poland.     

Identifications of emission lines in the EUV solar spectrum

Conclusions During the past three years there have been significant extensions of the solar data available. Over most of the solar spectrum between 1 – 2200 the new or improved observations have led to interesting problems in line identifications. The identifications have in turn led to new methods of determining the physical conditions in the solar atmosphere, eg electron density determinations from the Hei like ion intercombination line to forbidden line ratio (Gabriel and Jordan, 1969b). The majority of the strong lines have now been identified, either by theoretical considerations or from the extensive laboratory data which have recently become available. However, weak lines may also aid the understanding of the chromosphere and corona and work on the identifications of all remaining features observed must continue.     

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.     

A synthesis of our present knowledge of interstellar dust

The present state of knowledge as regards interstellar dust is reviewed in Section 1 (Introduction); Section 2 (Composition of Dust Grains: graphite, silicate, dirty-ice, diamond); Section 3 (Size of Grains: mainly r 10^–6 cm); Section 4 (Charge and Temperature of Grains: charge varies from 1–10 electrons (H i clouds) to 500 electrons (H ii clouds); temperature of grain material is about 10–20 K); Section 5 (Distribution and Origin of Grains: confined mainly to discs and arms of spiral galaxies, having had a passive origin by efflux from late-type stars or carbon-stars); Section 6 (Cosmogonical and Cosmological Aspects of Interstellar Grains: accretion by electrical-image forces of one dust grain onto a similarly-charged grain links up the absence of dust and gas in elliptical galaxies with the absence of a magnetic field of the type found in spirals. The origin of the 3 K background radiation field could be produced by a population of rotating silicate grains of r 10^–7 cm); Section 7 (Conclusion).     

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.     

Collapse,equilibrium, and fragmentation of rotating,adiabatic clouds

Numerical calculations of the collapse of adiabatic clouds from uniform density and rotation initial conditions show that when restricted to axisymmetry, the clouds form either near-equilibrium spheroids or rings. Rings form in the collapse of low thermal energy clouds and have = T/ ¦W¦ 0.43. When the axisymmetric constraint is removed and an initial m=2 density variation is introduced, clouds either collapse to form near-equilibrium ellipsoids or else fragment into binary systems through a bar phase. Ellipsoids form in the collapse of high thermal energy clouds and have 3 0.27. The results are consistent with the critical values of for instabilities in Maclaurin spheroids, and suggest that protostellar clouds may undergo a dynamic fragmentation in the nonisothermal collapse regime.National Academy of Sciences — National Research Council Resident Research Associate.     

X-ray binaries and stellar evolution

Observational evidence suggests that most — if not all — binary X-ray sources are neutron stars. The evolutionary status and possible formation mechanisms of the type I (massive) and type II (low-mass) X-ray binaries are discussed. The difference between the standard massive X-ray binaries and the Be/X-ray binaries is ascribed to a somewhat different evolutionary history and status, and possible reasons for the existence of short- and long — period X-ray pulsars are discussed. Type II X-ray sources in globular clusters were most probably formed by capture processes; their formation rate inferred from the observations indicates that only a small fraction ( 1 to 10 percent) of the originally formed neutron stars have remained in their clusters. Type II sources in the galactic bulge may also have formed from cataclysmic binaries in which a white dwarf was driven over the Chandrasekhar limit by accretion.     

High-resolution ultraviolet solar observations from sounding rockets and spacelab

High spatial (1) and temporal (20 s) resolution UV spectroscopy of the Sun has been carried out with a new instrument flown on sounding rockets. These observations reveal a multitude of new highly energetic phenomena in the outer solar atmosphere which may play a decisive rôle in the mechanical energy balance of the chromosphere, transition zone and corona.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Concerning the interpretation of the ultraviolet radiation from B stars

Measurements of the shape of the ultraviolet spectrum from B stars are compared with the theoretical spectra predicted from a homogeneous series of eight model atmospheres which are known to be close to a state of radiative equilibrium and to give a good representation of the ordinarily observed spectral region. The broad-band photometer measurements of Byram, Chubb, and Friedman in the region 1314 indicate that the stars become brighter in the ultraviolet as their temperature increases. The theoretical spectra reproduce this trend. However, the theoretical spectra are about three times as bright at 1314 relative to their brightness at 5560 as is observed.The spectral observations at 50Å resolution of Stecher and Milligan of six absorption-line stars are compared in detail with theoretical spectra. The observed shape of the spectrum is reproduced well by the models from 2600 to longer wavelengths. At wavelengths shorter than 2600 Å, the observed fluxes from B stars are less than the predicted fluxes. At 2000 the deficiency is between a factor two and a factor four. The spectrum of Canis Majoris is observed to have a different shape from that found for four other early-type stars. In the case of Canis Majoris the deficiency at 2000 is about a factor 13.The proper manner in which to compare theory and observation is discussed and some astrophysical terminology is explained. Theoretical fluxes, , are given in Table 1 for eight early B type model atmospheres at wavelengths between the Lyman limit and 6251. These fluxes have been computed without consideration of the opacity due to line blanketing. It is shown that line blanketing can probably account for the differences noted between predicted and observed ultra-violet spectra of B stars. It is not necessary at present to invoke unusual sources of opacity in the stellar atmosphere or in the space between the star and the earth in order to explain the observations. Spectra of B stars in the 2000 region at sufficient resolution to show the line spectrum would clarify the problem.     

The evolution of massive stars to explosion

We review the possible evolutionary paths from massive stars to explosive endpoints as various types of supernovae associated with Population I and hence with massive stars: Type II-P, Type II-L, Type Ib, Type Ic, and the hybrid events SN 1987K and SN 1993J. We identify SN 1954A as another hybrid event from the evidence for both H and He in its spectrum with velocities nearly the same as SN 1983J. Evidence for ejected⁵⁶Ni mass of 0.07 M suggests that SN II-P underwent standard iron core collapse, not collapse of an O–Ne–Mg core nor thermonuclear explosion of a C–O core. Most SN II-P presumably arise in single stars or wide binaries of 10–20 M. There may be indirect evidence for duplicity in some cases in the form of strong Ba II lines, such as characterized SN 1987A. SN II-L are recognizably distinct from typical SN II-P and must undergo a significantly different evolution. Despite indications that SN II-L have small envelopes that may be helium enriched, they are also distinct from events like SN 1993J that must have yet again a different evolution. The SN II-L that share a common Luminosity seem to have ejected a small nickel mass and hence may come from stars with O–Ne–Mg cores. The amount of nickel ejected by the exceptionally bright events, SN 1980K and SN 1979C, remains controversial. SN Ib require the complete loss of the H envelope, either to a binary companion or to a wind. The few identified have relatively large ejecta masses. It is not clear what evolutionary processes distinguish SN Ib's evolving in binary systems from hybrid events that retain some H in the envelope. SN Ic events are both H and He deficient. Binary models that can account for transfer of an extended helium envelope from low mass helium cores, 2 to 4 M, imply C–O core masses that are roughly consistent with that deduced from the ejecta mass plus a neutron star, 2 to 3 M. It is possible that the hybrid events are the result of Roche lobe overflow and that the pure events, SN Ib or SN Ic, result from common envelope evolution.     

LISM structure — Fragmented superbubble shell?

Small scale structure in local interstellar matter (LISM) is considered. Overall morphology of the local cloud complex is inferred from Ca II absorption lines and observations of H I in white dwarf stars. Clouds with column densities ranging from 2–100 × 10¹⁷ cm^–2 are found within 20 pc of the Sun. Cold (50 K) dense (10⁵ cm^–3) small (5–10 au) clouds could be embedded and currently undetected in the upwind gas. The Sun appears to be embedded in a filament of gas with thickness 0.7 pc, and cross-wise column density 2 × 10¹⁷ cm^–2. The local magnetic field direction is parallel to the filament, suggesting that the physical process causing the filamentation is MHD related. Enhanced abundances of refractory elements and LISM kinematics indicate outflowing gas from the Scorpius-Centaurus Association. The local flow vector and Sco data are consistent with a 4,000,000 year old superbubble shell at –22 km s^–1, which is a shock front passing through preshock gas at –12 km s^–1, and yielding cooled postshock gas at –26 km s^–1in the upwind direction. A preshock magnetic field strength of 1.6 G, and postshock field strength of 5.2 G embedded in the superbubble shell, are consistent with the data.Abbreviations LISM Local ISM - SIC Surrounding Interstellar Cloud - LIC Local Interstellar Cloud     

Hydromagnetics of the magnetosphere

Different models of the magnetosphere are discussed critically. It is pointed out that there is a principal difference between the case when the impinging interplanetary plasma has no initial magnetization, B ₀ = 0, (as in the Chapman-Ferraro theory), and the case when the plasma is initially magnetized, B ₀ 0, even if B ₀ is very small.In the former case the plasma remains unmagnetized (like a superconductor) and cannot penetrate into the magnetosphere. Therefore the plasma is separated by a sharp boundary from the magnetosphere, (closed magnetosphere model).In the latter case when the plasma is magnetized (which is more realistic) there is a possibility that field lines run from the earth to infinity (open magnetosphere model). Particles from the interplanetary space may penetrate into the magnetosphere. At the same time there may be a number of discontinuity surfaces of different character, such as the Cahill discontinuity.It is important to make terrella experiments in order to study the complicated phenomena occurring when a magnetized plasma penetrates into a dipole field.     

Anomalous Cosmic Rays and Solar Modulation

We review aspects of anomalous cosmic rays (ACRs) that bear on the solar modulation of energetic particles in the heliosphere. We show that the latitudinal and radial gradients of these particles exhibit a 22-year periodicity in concert with the reversal of the Sun's magnetic field. The power-law index of the low energy portion of the energy spectrum of ACRs at the shock in 1996 appears to be -1.3, suggesting that the strength of the solar wind termination shock at the helioequatorial plane is relatively weak, with s 2.8. The rigidity dependence of the perpendicular interplanetary mean free path in the outer heliosphere for particles with rigidities between 0.2 and 0.7 GV varies approximately as R2, where R is particle rigidity. There is evidence that ACR oxygen is primarily multiply charged above 20 MeV/nuc and primarily singly-charged below 16 MeV/nuc. The location of the termination shock was at 65 AU in 1987 and 85 AU in 1994.     

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).     

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.