Adaptive optics for large telescopes

The performance of large telescopes is determined both by their angular resolution and by their collection area. It is, therefore, important to achieve as high an angular resolution as possible by site selection, by avoiding image deterioration by the telescope and its environment, and by real time image restoration by adaptive optics. We summarize the principles of adaptive optics, their predicted performance and the current programs underway to implement adaptive optics for astronomical purposes.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

The light curve analysis of MM Herculis

The photoelectric light curves of MM Her obtained in 1983 and 1984 by Evren (1985, 1987) were analyzed by two different methods. Firsly, the effects of the wave-like distortions on the observations were removed from the observed magnitudes by obtaining its mathematical expression. The remaining light curves were analyzed by using Wood's approach. Later, the light curves of the same years were treated by the method of Wilson-Devinney and distortions seen in the light curves were thought to be explained by locating the spots on the surface of the cooler component. The results obtained by two different approaches are in good agreement.     

Determination and analysis of coronal hole radio spectra

Solar radio maps obtained by our group and others over a wide wavelength range (millimeter to meter) and over a considerable time span (1973–1978) have allowed us to compute the radio spectrum of an average coronal hole, i.e., the brightness temperature inside a coronal hole normalized by the brightness temperature of the quiet Sun outside the coronal hole measured at several different radio wavelengths. This radio spectrum can be used to obtain the changes of the quiet Sun atmosphere inside coronal holes and also as an additional check for coronal hole profiles obtained by other methods. Using a standard solar atmosphere and a computer program which included ray tracing, we have tried to reproduce the observed radio spectrum by computing brightness temperatures at many different wavelengths for a long series of modifications in the electron density, neutral particle density and temperature profiles of the standard solar atmosphere. This analysis indicates that inside an average coronal hole the following changes occur: the upper chromosphere expands by about 20% and its electron density and temperature decrease by about 10%. The transition zone experiences the largest change, expanding by a factor of about 6, its electron density decreases by a similar factor, and its temperature decreases by about 50%. Finally in the corona the electron density decreases by about 20% and the temperature by about 15%.     

Sunspot Observations Made by Hallaschka During the Dalton Minimum

We present and analyse the sunspot observations performed by Franz I.C. Hallaschka in 1814 and 1816. These solar observations were carried out during the so-called Dalton minimum, around the maximum phase of Solar Cycle 6. These records are very valuable because they allow us to complete observational gaps in the collection of sunspot group numbers, improving the coverage for this epoch. We have analysed and compared the observations made by Hallaschka with the records made by other contemporary observers. Unfortunately, the analysis of the sunspot areas and positions showed that they are too inaccurate for scientific use. We conclude, however, that the sunspot counts made by Hallaschka are similar to those made by other astronomers of that time. The observations by Hallaschka confirm a low level of solar activity during the Dalton minimum.     

X-ray plateaus followed by sharp drops in GRBs 060413, 060522, 060607A and 080330： Further evidences for central engine afterglow from gamma-ray bursts

The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are characterized by a plateau followed by a very sharp drop. The plateau could be explained within the framework of the external forward shock model but the sharp drop can not.We interpret the plateau as the afterglows of magnetized central engines, plausibly magnetars. In this model, the X-ray afterglows are powered by the internal magnetic energy dissipation and the sudden drop is caused by the collapse of the magnetar. Accordingly,the X-ray plateau photons should have a high linear polarization, which can be tested by future X-ray polarimetry.     

Distribution of iron on Vesta

We have completed a mapping study of 7.6 MeV gamma rays produced by neutron capture by Fe at the surface of the main belt asteroid 4 Vesta as measured by the bismuth germanate scintillator of the Gamma Ray and Neutron Detector (GRaND) on the Dawn spacecraft. The procedures used to determine Fe counting rates are presented, along with a global map, constituting the necessary initial step to quantify Fe abundances. While the final calibration of orbital data to absolute concentrations has not been determined, the range of fully corrected Fe counting rates is compared with that of Fe in howardites. We find that the global distribution of corrected Fe counting rates is generally consistent with mineralogy and composition determined independently by other instruments on the Dawn spacecraft, including measurements of pyroxene absorption bands by the Visible and Infrared Spectrometer and Framing Camera, and an index of diogenitic materials provided by neutron absorption measurements by GRaND. In addition, there is a distinctive low Fe region in the western hemisphere that was not reported by reflectance or optical observations, possibly indicating the presence of a cumulate eucrite component in Vesta's regolith.     

Book and Multimedia Reviews

Book reviewed in this article: Blind Watchers of the Sky: The People and Ideas that Shaped Our View of the Universe by Rocky Kolb Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets by John S. Lewis The Sun as a Star by Roger J. Tayler Shadow of a Star: The Neutrino Story of Supernova 1987A by Alfred K. Mann. W. H. Freeman Worlds Unnumbered: The Search for Extrasolar Planets by Donald Goldsmith     

Laboratory simulations of sulfur depletion at Eros

The X-ray spectrometer of the Near-Earth Asteroid Rendezvous (NEAR) mission discovered a low abundance of sulfur on the surface of asteroid Eros, which is seemingly inconsistent with the match of the overall surface composition to that of ordinary chondrites. Since troilite, FeS, is the primary sulfur-bearing mineral in ordinary chondrites, we investigated the hypothesis that sulfur loss from surface FeS could result from ‘space weathering’ by impact of solar wind ions and micrometeorites. We performed laboratory studies on the chemical alteration of FeS by 4 keV ions simulating exposure to the solar wind and by nanosecond laser pulses simulating pulsed heating by micrometeorite impact. We found that the combination of laser irradiation followed by ion impact lowers the S:Fe atomic ratio on the surface by a factor of up to 2.5, which is consistent with the value of at least 1.5 deduced from the NEAR measurements. Thus, our results support the hypothesis that the low abundance of sulfur at the surface of Eros is caused by space weathering.     

On the perturbations of the five outer planets by the four inner ones

The expressions given by Clemence were checked by the comparison of numerical values of their second differentials with the numerical values of the perturbing forces. Agreement was good in most cases, save that the use of the second differentials unduly magnified some terms rightly neglected by Clemence. In the appendix the work of Clemence is compared with that of Carpenter and the differences are found to be all smaller than the contributions from the secular variation of the eccentricity, which are so small that they were neglected by Clemence. The differences can be removed by considering the secular variation of the eccentricity and by making a small adjustment in the integration constants.     

Comments on the next generation of ground-based solar telescopes

The development of telescope capabilities tends to go in spurts. These are triggered by the availability of new techniques in optics, mechanics and/or instrumentation. So has nighttime telescope technology developed since the construction in the nineteen-forties of the 5-m Hale telescope, first by the introduction in the sixties of high efficiency electronic detectors, followed recently by the production of large 8- to 10-m mirrors and now by the implementation of adaptive optics. In solar astronomy, major steps were the introduction of the coronagraph by Lyot in the nineteen-thirties and the vacuum telescope concept by Dunn in the sixties. In the last thirty years, telescope developments in solar astronomy have relied primarily on improved instrumental capabilities. As in nighttime astronomy, these instruments and their detectors are reaching their limits set by the quantum nature of light and the telescope diffraction. Larger telescopes are needed to increase sensitivity and angular resolution of the observations. In this paper, I will review recent efforts to increase substantially the telescope capabilities themselves. I will emphasize the concept of a large all-wavelength, coronagraphic telescope (CLEAR) which is presently being developed.Dedicated to Cornelis de Jager     

Book and Multimedia Reviews

Book reviewed in this article: The Three Big Bangs: Comet Crashes, Exploding Stars, and the Creation of the Universe by Philip M. Dauber and Richard A. Muller Stars and Their Spectra: An Introduction to the Spectral Sequence by James B. Kaler Impact!: The Threat of Comets and Asteroids by Gerrit L. Verschuur Cosmic Clouds: Birth, Death, and Recycling in the Galaxy by James B. Kaler     

The metallic-line star HD 153286

The spectrum of the metallic-line and magnetic star HD 153286 has been studied. The line intensity is constant within the limit of the errors, while the radial velocity is slightly variable. A differential analysis by means of the curve-of-growth method has been made, using 5 And, F5 V as standard star. Moreover, an atmospheric model has been computed; the results of the fine analysis indicate a defect of Sc by a factor of 30 and a defect of Ca by a factor included between 4 and 25; Si, Ti, V, Cr and Fe are normal, Mn, Co, Ni in slight excess by factors included between 2 and 5. Sr, Y, Zr, Ba and La are in excess by factors lower than 10; Ce, Nd and Sm are almost normal; Eu and Gd are in excess by factors of 250 and 4, respectively. These results are compared with the predictions of the diffusion theory.     

The Effect of Method of Constructing the Mass Distribution on Single Stellar Populations

We use two methods of constructing the initial mass distribution, the traditional way and Monte Carlo simulation, to obtain integrated U - B, B - V, V-R and V-I colours and absorption-line indices denned by the Lick Observatory image dissector scanner (referred to as Lick/IDS), for instantaneous burst solarmetallicity single stellar populations with ages in the range 1-15 Gyr. We find that the evolutionary curves of all colours obtained by the traditional method are smoother than those by Monte Carlo simulation, that the U - B and B - V colours obtained by the two methods agree with one another, while the V - R and V - I colours by the traditional method are bluer than those by Monte Carlo simulation. A comparison of the Lick/IDS absorption-line indices shows that the variations in all the indices by the traditional method are smoother than that for the Monte Carlo simulation, and that all the indices except for TiO1 and TiO2 are consistent with those for the Monte Carlo simulation.     

Magnetic reconnection model for X-ray flare loop interaction

The subtle interactions between the magnetohydrodynamics (MHD) and transverse plasmons are investigated. It is shown that there is a resistive instability by the plasmon's soliton in a current sheet, which eventually turns into an eruptive instability at the magnetic field reconnection. In the case of ion-acoustic turbulence, the high temperature current sheet model must adopt the aromalous conductivity instead of the Coulomb conductivity. The numerical results are consistent with the observations obtained by Hanaoka (1994). Thus the flare caused by X-ray loop coalescence can be basically interpreted by this model of magnetic field reconnection driven by ponderomotive force.     

Book Reviews

Book reviewed in this article: Annual Review of Astronomy and Astrophysics, Volume 32 edited by G. Burbidge, D. Layzer and A. Sandage. Properties and Growth of Diamond edited by G. Davies. INSPEC, the Institution of Electrical Engineers, 1994 Noble Gas Geochemistry and Cosmochemistry edited by J. Matsuda. Explore the Planets by G. Jeffrey Taylor.     

BOOK REVIEWS

Book reviewed in this article: Meteorite, Urmaterie aus dem interplanetaren Raum by Rolf W. Bühler, Birkhäuser Verlag, Basel, Boston, Berlin, 1988, 192 pp. Kleine Meteoritenkunde by F. Heide. 3rd, totally revised edition by F. Wlotzka Meteorites and the Early Solar System edited by John F. Kerridge and Mildred S. Matthews.     

Momentum loss for antimatter meteors

The momentum loss for a possible antimatter meteor entrance can be described by the combination of two terms. One which can be characterized by the mechanism of annihilation and a second one, the well known mechanism, which is common for all koinomatter (ordinary) meteors. That is, the momentum loss caused by the air molecules swept up by the moving object. We discuss, in this paper, the contribution of the rocket effect caused by the action of the secondaries which can be produced by the annihilation interactions of the antiatoms with the air molecules. The momentum loss of an iron type meteor made of antimatter, as a function of its equivalent radius R, can be described by the formula, J (MeV/c) = 8R (cm), for values of R within the range 1 cm < R < 5 cm and can be resulted by a single annihilation interaction of a nucleon-antinucleon pair.     

Energetic particle fluxes at heliospheric shocks: Evidences of superdiffusion and comparison between analytical and numerical modeling

Energetic particles fluxes measured by spacecraft in the heliosphere are frequently observed to peak during interplanetary shock crossings, suggesting the shock to be the source of acceleration. It has been shown that the shape of the energetic particle fluxes, upstream and downstream of the shock, is affected by energetic particle transport properties. In this study we make a comparison among a number of shock crossings observed by the ACE spacecraft, and the energetic particle fluxes derived by a test-particle numerical model in the vicinity of a planar shock. We find that observations are in good agreement with a particle density profile obtained in the simulation by assuming superdiffusive transport both upstream and downstream of the shock region.     

Books and Multimedia Review

Book reviewed in this article: Turn Right at Orion: Travels Through the Cosmos by Mitchell Begelman. Satellites of the Outer Planets: Worlds in their Own Right, Second Edition by David A. Rothery. Our Worlds: The Magnetism and Thrill of Planetary Exploration by S. Alan Stern. A Dictionary of Earth Sciences, Second Edition by Ailsa Allaby and Michael Allaby.