Opcical and X-ray observations have confirmed char the X- 

 rav source 1E0657— 56 is a cluster of galaxies at a redshifc z = 

 0.296. The measured temperature T = 17 keV = 2.0 ± 0.3 X 

 108 K makes this cluster a strong candidate for the hottest 

 known. The exiscence of such a cluster at the observed red- 

 shift is difficult to reconcile with current theoretical models 

 involving a universe with the cricical density. The tempera- 

 ture measures the depth of the gravitational-potential well 

 and, together with a measurement of the radial distribution of 

 the gas, the mass of the cluster. A knowledge of the number 

 of clusters as a function of temperature can constrain the 

 spectrum of density fluctuations in the early universe. The 

 standard normalization for density fluctuations that produce 

 clusters refers to temperatures of 5—7 keV. Differences be- 

 tween cosmological models are strongly amplified for a 

 temperature of the order of 17 keV. The discovery of this high- 

 temperature cluster seems to rule out models with a CDM 

 power spectrum of fluctuations and CO = I. 



Optical and X-ray observations of early-type galaxies, 

 groups, and clusters of galaxies show that a greater fraction of 

 the gravitating mass is observable in rich clusters than in in- 

 dividual galaxies. In elliptical galaxies, only a few percent of 

 the gravitacing mass is observed in stars or gas, while in rich 

 clusters, approximately 30% of the mass is contained in hot 

 gas and galaxies. If the recent estimate of the Massive Com- 

 pact Halo Objects (MACHO) abundance in our galaxy is rep- 

 resentative of all galaxies, then the "observable" content of 

 galaxies, groups, and rich clusters (in the form of MACHOs, 

 stars, and gas) is nearly constant at 50% of the total mass. Fur- 

 thermore, the MACHO detection rate in our galaxy implies 

 that the stellar progenitors of MACHOs could produce the 

 observed gas mass and abundance of heavy elements in rich 

 clusters. There is no need to assume that any of the cluster gas 

 is primordial. The observed trend of increasing gas mass frac- 

 tion becween groups and clusters can be explained within the 

 context of hierarchical clustering scenarios. 



Optical and Infrared Astronomy 



The Optical and Infrared Astronomy Division carries out its 

 astrophysical research with optical, infrared, and gamma-ray 

 telescopes. In the optical domain, the Second CfA Survey, the 

 Century Survey (CS), and the 15R Survey were all completed 

 this year with a total of more than 25,000 redshifts having 

 been measured. These surveys confirm the nature and ampli- 

 tude of the large-scale clustering of galaxies and the prior 

 determination that voids in the radial direction do not generally 

 exceed diameters of about 5,000 km/s. The galaxy luminosity 

 function has a characteristic luminosity, M* = —20.6, in excel- 

 lent agreement with the value derived from the larger Las 

 Campanas Redshift Survey (LCRS), but the CS faint-end 

 slope, O. = —1. 14, is substantially steeper than the (X = -0.70 

 for the LCRS. The steeper CS slope agrees with the results of 

 B-band surveys. The real surprise is that the amplitude of the 

 luminosity function appears to increase by about 30% from 

 zero redshift to z - 0.15. 



The 2-Micron All-Sky Survey (2MASS) is now under way. 

 The first (Northern Hemisphere) telescope is in operation at 

 the Fred L. Whipple Observatory, a little under 9 years after 

 the submission of the initial Small Explorer proposal for such 

 a survey. 2MASS will produce a catalog of 500,000-1,000,000 

 galaxies and 100,000,000 stars. The complementary galaxy 

 redshift survey will start later this fall. 



The long-term ground-based effort to monitor the radial 

 velocities of hundreds of stars in the Hyades Cluster paid a 

 handsome dividend this year through the determination of 

 new masses and orbital parallaxes for three Hyades binaries. 

 These parallaxes establish a new benchmark for the distance 

 to the Hyades Cluster, one that agrees remarkably well 

 (within IO) with the just-announced Hipparcos distance of 

 46.34 ± 0.27 pc. 



The monitoring of the 0957 + 561 gravitational-lens system 

 continued co a successful conclusion with all parties now 

 agreeing on the time delay. This result allows the Hubble 

 Constant to be determined from the lens more reliably; the 

 main uncertainties now devolve on those of the lens' mass dis- 

 tribution. These uncertainties are believed to be of the order 

 of 10%, with the current estimate of H being about 65 kms"' 

 Mpc"'. 



Planetary Sciences 



The Planetary Sciences Division carries out research mainly 

 on the dynamics and materials of solar-system bodies. 

 Planetary scientists at the CfA have built upon earlier thermo- 

 dynamic studies of the sequence of minerals that would con- 

 dense from cooling disk material, looking for correspondences 

 between this set of minerals and those actually observed in 

 chondritic meteorites. They carried out the first study formally 

 to take account of the fact that once mineral condensation 

 starts, a portion of the material condensed (i.e., the interiors 

 of grains) is no longer accessible for further reaction with the 

 gas phase. They found that this process has a profound effect 

 on both the mineralogy and the chemistry of the condensate. 

 One of the condensation/isolation models they tested 

 produces the minerals in the carbonaceous class of chondrites 

 quite faithfully; another model accounts for the properties of 

 the enstatite class of chondnces. The concept tested, in which 

 slow grain growth produces carbonaceous chondrites and 

 rapid growth enstatite chondrites, is the only internally con- 

 sistent model that has accounted for the differences between 

 these two disparate chondrite classes. 



Division members have used a microprobe/electron micro- 

 scope to determine the mineralogical siting of manganese and 

 chromium in a primitive chondrite, important for interpret- 

 ing the Mn 9 - Cr 53 radiometric ages of chondrites that have 

 been used to understand early solar-system chronology. They 

 have also used the microprobe to study the minerals that form 

 inclusions and line cavities in the iron meteorite Gibeon, as 

 one of a series of studies of unusual meteoritic mineral assem- 

 blages. They concluded from the mineralogy that the meteorite 

 formed in a body that experienced early igneous differentia- 



65 



