Optical interferometric observations of a transient event of 1986 March 13 in the coma of Comet Halley

During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H₂ O⁺ emission was recorded in thecoma at ～ 0^h UT on 1986 March 13. Observations were carried out using a special IHW filter for H₂ O⁺ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24?39’N,72? 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ～ 35km s^?1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet     

The University of Durham Southern Hemisphere VHE gamma ray telescope

The design of very high energy (VHE) gamma ray telescopes is discussed and it is shown that a triple-mirror telescope operating in threefold fast coincidence provides an effective instrument. The design, construction and operation of a large VHE gamma ray telescope with an energy threshold of 300 GeV (at the zenith) and incorporating several novel features is described. The use of a coincidence guard ring technique is shown to result in the rejection of about 50% of the off-source (proton) events which form a high background and limit the sensitivity of such telescopes.     

Studies on post-flare loop prominence of 1981 April 27

By use of the H observations of the Astrophysical Observatory in Catania, Italy and the Purple Mountain Observatory in Nanking, China as well as hard X-ray and gamma-ray burst data from the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS), a major eruptive loop prominence was studied during the limb solar flare event of 1981 April 27.Our preliminary analysis shows that there seems to exist a second abrupt energy release for this event, almost 20 min after the end of the impulsive phase of the flare. This energy release is probably associated with the rapidity in upward motion or activation of the loop prominence.A possible candidate for such a process could be the reconnection of the old magnetic field with a newly emerging magnetic field.A theoretical gross estimate for the energy release and particle acceleration has also been made in this work. It appears that the proposed model for charged particle acceleration is very efficient.     

The U.S. National 8 m telescope project

The National Optical Astronomy Observatories (NOAO) are planning to buld two 8 m telescopes, one for Mauna Kea, Hawaii, the other for a site in Chile. Optical configurations, primary mirror systems, and the telescope mounting are discussed. A new optical testing method is outlined. The system imaging goal is 0.25 FWHM. Construction could begin in the early 1990's.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation.     

Shock chemistry in the molecular clouds associated with SNR IC 443

Several interstellar molecules have been detected toward the highly perturbed B and G clouds associated with the supernova remnant IC 443 via their 3 mm transitions, including N2H+, SiO, SO, CN, HNC, and H13CO+. The (J, K) = (1, 1) and (2, 2) inversion lines of metastable ammonia have also been observed, as well as the J = 3-2 transition of HCO+ at 1.2 mm. Analysis of the (1, 1) and (2, 2) inversion lines of NH3 indicates minimum gas kinetic temperatures of TK = 70 K toward cloud B, and TK = 33 K in cloud G. Modeling of the J = 1-0 and J = 3-2 transitions of HCO+ implies densities greater than 10(5) cm-3 toward both positions. These data clearly show that hot and dense material is present in IC 443, and they suggest the presence of shocks in both regions. A careful analysis of the HCO+ lines indicates that the HCO+ abundance is at most enhanced by factors of a few over that found in cold, quiescent gas. This conclusion contradicts past claims of HCO+ abundance enhancements of several orders of magnitude in the perturbed regions. The N2H+ abundance was also found to be similar to that in cold gas, suggesting that there is no increase in ionization in the clouds. The abundances of SO and CS, as well as CN and NH3, do not appear to differ significantly from those found in cold dark clouds, although chemistry models predict sulfur-containing species to undergo high-temperature enhancements. SiO, however, is found to have an abundance in the perturbed gas 100 times larger than the upper limits observed in the dark cloud TMC 1, a result in agreement with high temperature chemistry models. In addition, the HNC/HCN ratio in both IC 443 B and G was found to be approximately 0.1--far from the ratio of 1 predicted by low-temperature ion-molecule chemistry, but similar to the values observed in clouds where elevated temperatures are present.     

Radiometry of near-earth asteroids

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.     

External radiation field in Rayleigh-Cabannes atmospheres with constant and linear sources

The external field of radiation in Rayleigh-Cabannes atmospheres with constant and linear sources is found using the resolvent matrix approach. If the internal sources are constant the external field may be described by theX-, Y-, andH-matrices. For the case with linear sources we need the derivatives of these matrices with respect to angular variable. The respective scheme for their determination is given.A set of integro-differential equations for theX- andY-matrices is derived and solved numerically. Some relations between the moments of theH-matrix are given and a sample of results for external fields are provided.     

Radio observations of the M8.1 solar flare of 23 June, 1988: Evidence for energy transport by thermal processes

The Very Large Array (VLA) and the frequency agile interferometer at the Owens Valley Radio Observatory (OVRO) were used to observe the M8.1 flare of 23 June, 1988. The VLA obtained images prior to and during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency agile interferometer at Owens Valley obtained interferometer amplitude and total power spectra of the flare at 45 frequencies between 1 and 18 GHz. The observations were supplemented by radiometer measurements made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray observations, by USAF SOON H filtergrams, and by a KPNO photospheric magnetogram.The radio data reveal a wide variety of phenomena, including: (i) a multiply impulsive microwave burst that is essentially thermal in character; (ii) stationary discrete components at 1.5 GHz, associated temporally and spatially with distant brightenings in Ha; (iii) a dynamical component at 1.5 GHz associated with hot plasma moving subsonically into the corona; (iv) the appearance of intense, short-lived, decimetric burst activity near the lead sunspot in the active region at 1.5 GHz, indicative of a high degree of inhomogeneity in the source.The unusually complete radio coverage allows us to investigate the transport of energy from the initial site to sites of distant H brightenings. The transport of energy appears to be most consistent with slow, thermal processes, rather than rapid transport by nonthermal electron beams.     

Solution of the equation of transfer for coherent scattering in an exponential atmosphere by the method of discrete ordinates

A solution of the transfer equation for coherent scattering in stellar atmosphere with Planck's function as a nonlinear function of optical depth, viz., $$B_v (T) = b_0 + b_1 {\text{ }}e^{ - \beta \tau } $$ is obtained by the method of discrete ordinates originally due to Chandrasekhar.     

Impact-generated atmospheres over Titan, Ganymede, and Callisto

The competition between impact erosion and impact supply of volatiles to planetary atmospheres can determine whether a planet or satellite accumulates an atmosphere. In the absence of other processes (e.g., outgassing), we find either that a planetary atmosphere should be thick, or that there should be no atmosphere at all. The boundary between the two extreme cases is set by the mass and velocity distributions and intrinsic volatile content of the impactors. We apply our model specifically to Titan, Callisto, and Ganymede. The impacting population is identified with comets, either in the form of stray Uranus-Neptune planetesimals or as dislodged Kuiper belt comets. Systematically lower impact velocities on Titan allow it to retain a thick atmosphere, while Callisto and Ganymede get nothing. Titan's atmosphere may therefore be an expression of a late-accreting, volatile-rich veneer. An impact origin for Titan's atmosphere naturally accounts for the high D/H ratio it shares with Earth, the carbonaceous meteorites, and Halley. It also accounts for the general similarity of Titan's atmosphere to those of Triton and Pluto, which is otherwise puzzling in view of the radically different histories and bulk compositions of these objects.