Prior to the 1986 apparition of comet Halley, all attempts to determine the physical properties of cometary dust were limited to remote observations and the analysis of various particles captured by the Earth’s atmosphere. The in situ measurements made by the three spacecraft that passed within 10,000 km of the nucleus provided the first opportunity to investigate both the full size-range of particles and the complete process of dust production. Information on composition is derived through mass spectra and the scattering and emission of light from the grains, whilst the dynamics of the dust coma can be modeled from the three separate sets of measurements made over a period of eight days.

Zeeman spectroheliograms of photospheric magnetic fields (longitudinal component) in the CaI 6102.7 Å line are being obtained with the new 61-cm vacuum solar telescope and spectroheliograph, using the Leighton technique. The structure of the magnetic field network appears identical to the bright photospheric network visible in the cores of many Fraunhofer lines and in CN spectroheliograms, with the exception that polarities are distinguished. This supports the evolving concept that solar magnetic fields outside of sunspots exist in small concentrations of essentially vertically oriented field, roughly clumped to form a network imbedded in the otherwise field-free photosphere. A timelapse spectroheliogram movie sequence spanning 6 hr revealed changes in the magnetic fields, including a systematic outward streaming of small magnetic knots of both polarities within annular areas surrounding several sunspots. The photospheric magnetic fields and a series of filtergrams taken at various wavelengths in the Hα profile starting in the far wing are intercompared in an effort to demonstrate that the dark strands of arch filament systems (AFS) and fibrils map magnetic field lines in the chromosphere. An example of an active region in which the magnetic fields assume a distinct spiral structure is presented.

This describes our integrator RADAU, which has been used by several groups in the U.S.A., in Italy, and in the U.S.S.R. over the past 10 years in the numerical integration of orbits and other problems involving numerical solution of systems of ordinary differential equations. First- and second-order equations are solved directly, including the general second-order case. A self-starting integrator, RADAU proceeds by sequences within which the substeps are taken at Gauss-Radau spacings. This allows rather high orders of accuracy with relatively few function evaluations. After the first sequence the information from previous sequences is used to improve the accuracy. The integrator itself chooses the next sequence size. When a 64-bit double word is available in double precision, a 15th-order version is often appropriate, and the FORTRAN code for this case is included here. RADAU is at least comparable with the best of other integrators in speed and accuracy, and it is often superior, particularly at high accuracies.

We summarise recent progress in the understanding of the rotational evolution of low-mass stars (here defined as solar mass down to the hydrogen burning limit) both in terms of observations and modelling. Wide-field imaging surveys on moderate-size telescopes can now efficiently derive rotation periods for hundreds to thousands of open cluster members, providing unprecedented sample sizes which are ripe for exploration. We summarise the available measurements, and provide simple phenomenological and model-based interpretations of the presently-available data, while highlighting regions of parameter space where more observations are required, particularly at the lowest masses and ages ≳500 Myr.

Orbit determination for Near-Earth Asteroids presents unique technical challenges due to the imperative of early detection and careful assessment of the risk posed by specific Earth close approaches. This article presents a case study of asteroid 99942 Apophis, a 300-400 meter object that, for a short period in December 2004, held an impact probability of more than 2% in 2029. Now, with an orbit based on radar ranging and more than a year of optical observations, we can confidently say that it will pass safely by the Earth in 2029, although at a distance of only about six Earth radii from the geocenter. However, the extremely close nature of this encounter acts to obscure the trajectory in subsequent years, when resonant returns to the vicinity of the Earth are possible. In particular, an impact possibility in the year 2036 has a roughly 5% probability of persisting through the very favorable 2013 radar and optical observing apparition. In the event that the 2036 potential impact has not been eliminated by 2013, a precise characterization of the Yarkovsky accelerations acting on the asteroid may become an important part of the orbit estimation and impact prediction problem. Even so, the sixteen years available to effect a deflection from 2013 until 2029, after which the problem would become intractable, are sufficient to respond to the threat should a deflection effort become warranted.

The Kepler Mission is a space-based mission whose primary goal is to determine the frequency of Earth-size and larger planets in the habitable zone of solar-like stars. The mission will monitor more than 100,000 stars for patterns of transits with a differential photometric precision of 20 ppm at V = 12 for a 6.5 hour transit. It will also provide asteroseismic results on several thousand dwarf stars. It is specifically designed to continuously observe a single field of view of greater than 100 square degrees for 3.5 or more years.

This paper provides a short overview of the mission, a brief history of the mission development, expected results, new investigations by the recently chosen Participating Scientists, and the plans for the Guest Observer and Astrophysical Data Programs.

Key observational constraints for jet models in T Tauri stars are outlined, including the jet collimation scale, kinematic structure, and ejection/accretion ratio. It is shown that MHD self-collimation is most likely required. The four possible MHD ejection sites (stellar surface, inner disk edge, extended disk region, magnetosphere-disk reconnexion line) are then critically examined against observational constraints, and open issues are discussed.

This paper presents a short review on the current state of SN Ia progenitor origin. Type Ia supernova explosions (meaning thermonuclear disruption of a white dwarf) are observed to be widely diverse in peak luminosity, lightcurve width and shape, spectral features, and host stellar population environment. In the last decade alone, theoretical simulations and observational data have come together to seriously challenge the long-standing paradigm that all SNe Ia arise from explosions of Chandrasekhar mass white dwarfs. In this review I highlight some of the major developments (and changing views) of our understanding of the nature of SN Ia progenitor systems. I give a brief overview of binary star configurations and their plausible explosion mechanisms, and infer links between some of the various (observationally-categorized) SN Ia sub-classes and their progenitor origins from a theoretical standpoint.

PG1159-035 was originally detected as a 14.5 mag. blue object in a survey for QSO candidates (Green 1977), An SIT spectrogram obtained at the Hale 5m telescope at 6Å resolution showed this star to have a very blue continuum with absorption features near λ4686 and λ4650 which were tentatively attributed to Hell and the CIII/CIV complex, respectively. Possible narrow emission components to these lines appear in Figure 1 which shows confirming spectra obtained with the IIDS (at 2.5Å resolution) on the Kitt Peak 2.1m telescope. The blue continuum and possible presence of Hell in emission suggested that this star was perhaps similar to the helium mass-transfer binary AM CVn (HZ 29). For this reason, we put PG1159-035 on our program of high-speed photometry, expecting the star to show “flickering” associated with mass transfer into an accretion disk and possible orbital modulation or eclipses in the period range 10-20 minutes—photometric characteristics similar to those observed in AM CVn.

Thermochemical equilibrium calculations are successful in predicting the mineralogy as well as the major and trace element chemistry of circumstellar grains found in meteorites. The calculations also explain observations of dust close to AGB stars (within 1–3 stellar radii). The trace element chemistry in circumstellar graphite, SiC, and other refractory carbide grains agrees with equilibrium condensation calculations for circumstellar shells of carbon stars. Observed trace element abundance patterns in N stars are complementary to those found in SiC grains indicating fractional condensation in circumstellar shells. Condensation temperatures depend upon total pressure, C/O ratio, nitrogen abundances, and overall metallicity. Therefore for condensation temperatures to be meaningful, the total pressure and elemental abundances (i.e., C/O ratio, metallicity) must be specified.

The purpose of the present paper is to give an introduction to the nomenclature, and a few of the results, of helio- and asteroseismology. It is hoped that this may provide a useful background for the more specialized reviews, and the contributed papers, in these proceedings. Other recent, general reviews are, e.g., Deubner & Gough (1984), Leibacher et al (1985), and Christensen-Dalsgaard et al (1985a).

The Report of the Sub-Group on Numerical Standards of the IAU Working Group on Astronomical Standards (WGAS) is presented. The report is intended to incorporate the majority of the responses received from the e-mail recipients of the series of WGAS Circulars. The report proposes to retain the present (1976) IAU System of Astronomical Constants and also proposes to establish a IAU File of Current Best Estimates. Further, the report proposes the establishment of a Maintenance Committee, governed by a set of proposed by-laws, in order to oversee the maintenance of the new file.

This review discusses the complex morphology and perturbed kinematics, as well as the outstanding physical characteristics of the interstellar medium within a few 100 pc of the galactic center. A total of ~107.9 Mo of dense molecular gas, representing ~10% of the Galaxy's neutral mass content, has settled in a thin layer [size: 450 × 40 pc], composed of giant molecular cloud complexes. The spatial distribution is highly asymmetric with respect to the center, and motions differ considerably from equilibrium conditions. The dynamical situation is still obscure (explosive event vs. response to distorted gravitational potential), but any disturbance must have occurred quite recently (τdy ~106 yr). Evidence for large–scale star–forming activity is reviewed, and for a standard IMF, a total star formation rate Φ ~0.5 M0 yr−1 is inferred.

The galactic center clouds differ considerably in their physical and chemical characteristics from GMC's in the outer Galaxy. Pervasive high bulk gas temperatures (Tkin ~70 K), mean H2 densities of n ~104 cm−3, and linewidths ≥10–20 kms−1 are probably a consequence of the clouds' location in the steep gravitational potential of the central bulge.

The current status and remaining questions of MgII absorbers are reviewed with an eye toward new results incorporating high quality Hubble Space Telescope images of the absorbing galaxies. In the end, we find that our current picture of extended gaseous regions around galaxies at earlier epochs is in need of some revision; MgII absorbing “halos” appear to be patchier and their geometry less regular than previously inferred. We also find that the so-called “weak” MgII absorbers are associated with normal galaxies over a wide range of impact parameters, suggesting that this class of absorber does not strictly select low surface brightness, dwarf galaxies, or IGM material. We emphasise the need for a complete survey of the galaxies in quasar fields, and the importance of obtaining rotation curves of confirmed absorbing galaxies.

A new high-contrast imaging subtraction algorithm (TLOCI) is presented to maximize a planet signal-to-noise ratio. The technique uses an input spectrum and template PSFs to optimize the reference image coefficient determination to minimize the flux contamination via self-subtraction (thus maximizing its throughput wavelength per wavelength) of any planet that have a similar spectrum to the template spectrum in the image, while trying, at the same time, to maximize the speckle noise subtraction. The optimization is performed by a correlation matrix conditioning. Using laboratory Gemini Planet Imager data, the new algorithm is shown to be superior to the simple/double difference, polynomial fit and original LOCI algorithm.

The outward transfer of angular momentum of accreting matter can lead to the formation of a disk around the black hole. The structure and radiation spectrum of the disk depends, in the main, on the rate of matter inflow Ṁ into the disk at its external boundary. Dependence on the efficiency of mechanisms of angular momentum transport (connected with the magnetic field and turbulence) is weaker. If Ṁ = 10−9–3 × 10−8 M⊙/yr, the disk around the black hole is a powerful source of X-radiation with hv ∼ 1–10 keV and luminosity L ∼ 1037–1038 erg s−1. If the flux of the accreting matter decreases, the effective temperature of radiation and the luminosity will drop. At the same time when Ṁ > 10−9M⊙ yr−1, the optical luminosity of the disk exceeds the solar one. The main contribution to the optical luminosity of the black hole is due to the re-radiation of that part of the X-ray and ultraviolet energy which is initially produced in the central high temperature regions of the disk and which is then absorbed by the low temperature outer regions. The optical radiation spectrum of such objects must be saturated by the broad emission recombination and resonance lines. Variability is connected with the character of the motion of the black hole and the gas flow in binary systems and possibly with eclipses. For well defined conditions, the hard radiation can evaporate the gas. This can counteract the matter inflow into the disk and lead to autoregulation of the accretion.

If M ≫ 3 × 10−8 (M/M⊙) M⊙ yr−1, the luminosity of the disk around the black hole is stabilized at the critical level of Lcr = 1038 (M/M⊙) erg s−1. A small fraction of the accreting matter falls under the gravitational radius whereas the major part of it flows out with high velocities from the central regions of the disk. The outflowing matter is opaque to the disk radiation and completely transforms its spectrum. As a consequence, a black hole in a supercritical regime of accretion can appear as a bright star with a strong outflow of matter.

A sample of Large Magellanic Clouds (LMC) and Small Magellanic Clouds (SMC) clusters for which ages have been directly determined from main sequence turnoff photometry has been compiled. According to this sample, the LMC and SMC cluster age distributions are very different. The LMC contains a large population of 1 to 3 Gyr old clusters as well as a small number of clusters that appear to be as old as the Galactic halo globular clusters. Surprisingly however, only a single cluster is known with an age in the interval between 3 Gyr and the age, taken as 15 Gyr, of the oldest clusters. The SMC age distribution, on the other hand, is much broader. It extends back to approximately 12 Gyr but there appear to be no SMC clusters as old as the oldest in the LMC. The sample of clusters is also used to discuss the age-abundance relations for both galaxies. Little can be learned for the LMC but it appears that the chemical evolution of the SMC differed in form from that experienced in the solar neighbourhood. The first results of an imaging survey designed to find LMC clusters with ages exceeding 3 Gyr are also described.

We present a grid of stellar and substellar atmosphere models covering the range from solar-mass stars to the latest-type T and Y dwarfs with a single setup. For the first time our synthetic spectra and photometry reproduce the formation of clouds and in particular their clearing at the L/T transition. The BT-Settl models also naturally explain the dustier infrared properties of planets as an effect of low surface gravity.

I want to make two points:

1. 1. Observations suggest that the hot-spots move about either because the beam precesses (e.g. Ekers et al. 1978; Lonsdale & Morrison 1980) or more discontinuously, as in sources like 3C351 that have multiple hot-spots (Laing 1981). The natural interpretation is that the hot-spot at the end of the beam slides over the inner surface of a ‘cavity’ filled with very hot dilute ex-hot-spot material (Scheuer 1974), extending the cavity at various places at different times. Such a ‘dentist's drill’ model has various consequences:

   1. (i) The mean speed (length/age) at which the cavity elongates may be considerably smaller than the instantaneous speed V of the hot-spot, estimated in the customary way from pressure balance:

   2. (ii) Remark (i) raises the possibility that giant radio sources have become so long because their jets are unusually constant in direction. NGC6251, which is over 2 Mpc long, has a jet whose direction has not wavered by more than a few degrees in ∼ 107 years (Saunders et al. 1981).

   3. (iii) We should not be surprised by sources whose hot-spots lie some way behind the extreme ends of the source; this may merely mean that the beam at present impinges on one side of the ‘cavity’. 3C285 is a typical example; a more persuasive case is the Np component of 3C219 (Perley et al. 1980).

2. 2. Most of the radio emission of most really powerful radio sources comes from their hot spots. Contrariwise, straightforward equipartition calculations on models lead to at least as much emission from the ‘cavity’ as from the hot-spots (Scheuer 1974). While there are several possible explanations, one in particular is suggested by Laing's (1981) extensive data on 40 sources. He lists the synchrotron lifetimes in the hot-spots for electrons radiating at 15 GHz, and while these exceed the light crossing times, they do so by only one or two orders of magnitude. One expects flow through the hot spots at some fairly small fraction of the speed of light; the synchrotron loss cutoff in the hot-spot is then expected to appear in the mm or infra-red region. Expansion of the hot-spot material into the cavity shifts the loss cutoff to a much lower frequency. If the expansion is adiabatic (certainly an oversimplification) the ratio of loss cutoff frequencies is equal to the ratio of energy densities. We cannot use this relationship directly in the more interesting cases, as we cannot estimate the energy density (or even the size) of a ‘cavity’ that we cannot observe. However, we can estimate the cutoff frequency within the hot-spots (for some assumed streaming speed), and Figure 1 shows that the most powerful hot-spots have loss cutoff frequencies in the 10–100 GHz range, implying cutoff frequencies in the ‘cavity’ that could well be below 1 GHz.

The physical parameters of HII regions span orders of magnitude in scale. The classes most closely linked to star formation are the smallest, densest, and, presumably, youngest stages: compact, ultracompact, and hypercompact HII regions.

Although hypercompact HII regions have been known for over ten years, until recently only a very small number of these regions were known. Moreover, it is only in the past several years that these regions have come to be recognized as a distinct class of HII region and that attempts have been made to understand their place in the scheme of massive star formation.

Here we present a summary of the observational studies to date. We give special emphasis to radio continuum studies, which indicate density gradients within the ionized gas, and to radio spectral line studies, which show unusually broad recombination line profiles. Possible interpretations of these aspects of hypercompact HII regions are discussed, and their implications for the interpretation of hypercompact HII regions as an evolutionary stage in the high-mass star formation process.