The CSO has been operational for about four years, during which time the telescope surface has been tuned-up, the various receivers and detectors have been brought on-line and the computer hardware and software debugged. The telescope is now operational to 700 GHz and is fully scheduled. Several discoveries have been made already, which have initiated new fields of research. One of the most striking is the detection of submillimeter water vapor masers. The famous 22 GHz line has been known for many years, but with just one line available it has proved very hard to understand the phenomenon of masing or how to use it to learn about the star-forming regions. Six new maser lines have been discovered at the CSO, and are seen strongly in both late-type stars and in HII regions. They will be extremely helpful in understanding masers and the physics of the masing regions and also for measuring the abundance of water, probably the most important interstellar molecule. Several other new molecular species have been found recently at the CSO, including HCl, NH2, and H2O+.

Also discovered recently is a new phase of stellar evolution, which occurs for red giant stars just before they completely lose their envelope to form planetary nebulae. It is manifest as a very fast molecular wind which is detected at the CSO as a wide spectral feature in carbon monoxide emission.

The CSO has been used for studies of young stellar objects and protostars where disks around optically visible T Tauri objects and imbedded objects such as L1551 have been studied. General mapping and spectroscopy studies of the structure of the interstellar medium with very high signal to noise and large dynamic range in the size of the maps, have shown that the velocity distribution in molecular clouds is non-Gaussian and this has been shown to be due to intermittency, a fundamental property of turbulence.

Many galaxies contain dust and molecular gas, which can be studied in the submillimeter. Nearby galaxies are being mapped to analyze the relationships between optical, HI, dust and molecular emissions. A particularly interesting case is the radio galaxy Centaurus A which is an elliptical galaxy with a dust lane and an active nucleus. The CSO has been able to map the molecular gas and show that the spectral profiles of the disk can be deduced from the gravitational potential of the elliptical galaxy. Spectra towards the nucleus show what may be infalling material to the central object. Molecular gas has been investigated in several other elliptical galaxies. A major goal for the CSO will be the detection of highly red-shifted infrared fine-structure lines from distant galaxies.

The scientific staff of the CSO consists of Professors Phillips (Director), Zmuidzinas and Carlstrom, and Drs. Lis, Keene, Schilke, Serabyn and Wang. Currently four students are working full time in the program both on instrumentation and observing projects. The telescope is also used by Professors Blake, Djorgovski, Muhleman and Scoville and other members of the astronomy staff at Caltech.

State of the art instrumentation is a strong point for the CSO, where considerable effort goes into research aimed at developing the best possible detectors for line and continuum work. For high resolution spectroscopy, superconducting tunnel junctions (SIS) receivers are built in the Downs Physics Laboratory to cover the frequency range from 200 to 1,000 GHz. These use our own designs of liquid helium cryostats, SIS devices (fabricated at JPL), cryogenic electronics and local oscillators etc.. We are currently planning an SIS array receiver and are developing a custom VLSI CMOS correlator with a custom GaAs digitizer, for the back-end spectrometer array. For continuum work we have high sensitivity bolometers cooled to 0.3 K by liquid 3He. We are constructing a 0.3 K bolometer array for continuum work and a Fabry-Perot spectrometer with 0.1 K bolometers for a high sensitivity, moderate resolution spectrometer for studies of molecular and atomic emission from distant galaxies.

A very attractive opportunity exists to perform pioneering investigations in submillimeter interferometery now that the CSO has been interferometrically coupled to the JCMT, at a separation of 150 m. This currently provides angular resolution of about 0.3''. A fixed baseline does not allow aperture synthesis mapping in the usual sense. However, measurements of a portion of the visibility function of molecular line emission is scientifically interesting. Analysis of the visibility function provides information on the distribution of the emission projected in the direction of the baseline vector between the two telescopes, for the CSO-JCMT baseline this is approximately 150 m east-west. For example, observations of a zero degree declination source at 345 GHz sample the east-west distribution of the emission with angular scales from 0.5 to 2.1''. For northern and southern sources, less angular information is available, but more is learned about the two dimensional distribution of the emission.

A few examples of the type of objects that we are studying with the single baseline heterodyne CSO-JCMT interferometer are:

• Protostellar collapse -- Although all models of star formation start with collapse of a molecular cloud core to form a protostar, the collapse has never been clearly observed. A submillimeter interferometer provides the best prospects for directly detecting the collapse of material onto a protostar. The infall may be detected by molecular line absorption against the dust continuum emission.

• Circumstellar disks -- There is considerable evidence for circumstellar disks around several protostars (i.e., L1551). The CSO-JCMT submillimeter interferometer extends the angular resolution of the millimeter arrays, so that the innermost regions of the disk can be probed.

• Ultraluminous galaxies -- The single CSO-JCMT baseline will enable us to probe the core regions of ultraluminous and active galaxies.