tion, followed by break-up and reassembly in orbit while its 

 metallic core material was still partially molten. 



During the past year. Division members moved closer to an 

 understanding of the evolutionary and dynamical processes — 

 often occurring in the distant past — that carved the asteroid 

 belt from a field of small uniformly distributed bodies into 

 their present spatial distribution. They found that effects oc- 

 curring naturally as a result of a diminishing residual solar 

 nebula, coupled with small concomitant changes in planetary 

 orbits, can produce the present distribution shown by some 

 13,000 minor planets to a remarkable degree. Some of the 

 properties that seem to be accounted for are (a) the gaps at 

 various mean motion resonances, (b) the high average values 

 of the asteroidal eccentricities and inclinations, and (c) the 

 scarcity of bodies with semimajor axes greater than 3.4 AU. 



In collaboration with others in the Solar and Stellar 

 Division, scientists in the Planetary Sciences Division have 

 studied members of rhe solar system at extremes of 

 heliocentric distance. Images of no fewer than 26 new comets 

 have been analyzed by using data from rhe Solar Heliosphenc 

 Observatory (SOHO); the likelihood that all but rwo are 

 members of rhe Kreutz group of related objects confirms the 

 extreme prevalence of that system of objects that pass within 

 less than two radii of the Sun's center. 



Observations with the FLWO 1.2-m reflector of several new- 

 candidate members of the Kuiper Belt will make that tele- 

 scope the third most prolific in the world with regard to the 

 number of different such objects observed. This follow-up is 

 essential lest preconceived, and possibly erroneous, ideas on 

 rhe Kuiper Belt's makeup — principally as a "main belt" of ob- 

 jects in near-circular, low-inclination orbits with mean distan- 

 ces in the range 42—46 AU and a set of objects in somewhat more 

 eccentric and inclined orbits in 2:3 resonance with Neprune — 

 prevail. 



Radio and Geoastronomy 



The Radio and Geoastronomy Division carries out a variety of 

 research projects, primarily involving use of radio techniques. 

 Part of this research involves astrochemistry, specifically carbon 

 chain and ring molecules, which are important constituents of 

 the interstellar medium. They include the largest known 

 interstellar molecules and have been suggested as carriers of 

 the diffuse interstellar bands, a long-standing unsolved prob- 

 lem in astrophysics. Development of a Fourier transform 

 microwave spectrometer of unprecedented sensitivity has led 

 in the last year to the unambiguous detection and identifica- 

 tion of 21 previously undetected carbon-bearing molecules, 

 including C,H, C„H, HC„N, and HC.jN. HC,;N has a 

 molecular weight of 171 amu, more than twice that of glycine, 

 the simplest amino acid. Several of these new molecules have 

 since been found in circumstellar envelopes and molecular 

 cloud cores. 



The long-standing problem of the registration of radio and 

 infrared images of the Galactic Center region was solved by 

 finding emission in the radio (SiO masers) and infrared (red 



giant stars) from the same objects. The regisrration accuracy is 

 30 mas, sufficient to rule out all currently detected infrared 

 point sources from collocation with Sgr A*, and to place strin- 

 gent upper limits on the 2.2 i flux density of Sgr A*. These 

 results strongly constrain models of the emission from mas- 

 sive black holes and in some cases require a significant reduc- 

 tion in the model mass accretion rate. 



Until now, very few candidate protostars have shown kine- 

 matic evidence of star-forming inward motions, based on the 

 blueshift of their spectral-line profiles. This long -sought 

 evidence of gravitational infall is considered much more con- 

 clusive than the redness of the continuum spectrum, used to 

 identify candidates. The most extensive survey to date of can- 

 didate protostars was carried out recently in three molecular 

 lines capable of showing blue-shifted "infall asymmetry." 

 Some 47 far-infrared and submillimeter sources within 400 pc 

 were observed, revealing 15 sources that pass strict criteria for 

 spectroscopic evidence of infall. The incidence of infall asym- 

 metry in this survey increases dramatically with the redness of 

 the continuum spectrum: The reddest "Class o" sources show 

 a much higher incidence of infall asymmetry than do the less 

 red "Class I" sources. 



Solar and Stellar Physics 



The Solar and Stellar Physics Division uses mostly optical and 

 infrared techniques to study properties of the Sun and stars. 

 For the past few years, scientists in this Division have been 

 developing an Advanced Fiber Optic Echelle (AFOE) 

 spectrograph and applying it to extrasolar planet research and 

 asrroseismology. (The High Altitude Observatory is also a 

 major collaborator.) Currently the AFOE is installed at the 

 1.5-m telescope at FLWO and is capable of measuring the 

 radial velociry of Sun-like stars with a short-term precision of 

 about I m/s (for asrroseismology) and long-rerm accuracy (for 

 exoplanet research) of about 8 m/s. SAO scientists recently 

 discovered a Jupirer-mass planet in a near-circular orbit about 

 the G2 V star n CrB, with a period of about 40 days and semi- 

 major axis of about 0.23 AU. This discovery is important be- 

 cause it helps fill the gap between solar-system-like giant 

 planets and very close "hot Jupiters" like 51 Peg. Its circular 

 orbir suggests that the planet was formed in a dissipative 

 proroplanetary disk like the solar-system planets, and pre- 

 sumably within the ice condensation zone beyond a few 

 astronomical units. If so, how it came to be located at 0.23 

 AU is a mystery. The AFOE team is continuing to monitor 

 about 100 Sun-like stars, and expects to find several additional 

 extrasolar planets in the next year or so. 



The recently discovered Kuiper Belt object 1996 TL66 

 provides strong evidence that the Kuiper Belt consists of two 

 dynamical components: objects in nearly circular orbits (the 

 "Classical" Kuiper Belt) and objects in large, highly eccentric 

 orbits (the "Scattered" Kuiper Belt). Thus far, known objects 

 in the classical Kuiper Belt have orbits restricted to inside 

 — 50 AU, including objects like Pluto that are in mean mo- 

 tion resonances with Neptune. In contrast, as inferred from 



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