A.C. Cheung

Detection of Water in Interstellar Regions by its Microwave Radiation

A report of the detection of microwave radiation from water molecules in space, by the group which recently detected interstellar ammonia emission.

A pumping mechanism for anomalous microwave absorption in formaldehyde in interstellar space.

Molecular collision pumping mechanism for anomalous microwave absorption by formaldehyde rotational transition in Galaxy dust cloud

Distribution of ammonia density, velocity, and rotational excitation in the region of Sagittarius B2.

Ammonia inversion radiation in Sgr B2 region, observing distribution of density, velocity and rotational excitation

An interferometric and multitransitional study of the Orion methanol masers

The methanol source in OMC-1 has been interferometrically mapped in two masing transitions, the 6/sub 2/..-->..6/sub 1/(E) and 7/sub 2/reverse arrow7/sub 1/(E), which have wavelengths near 1 cm. The source was found to consist of 10 components with different Doppler velocities. Most of the components radiate in both of the mapped transitions, and have brightness temperatures or limits of 1000--4000 K. None of the components are resolved in declination, to a limit of 15. Six of the components are unresolved in right ascension, to a limit of 6, and the others have sizes of 6--8. The molecular excitation was studied with a series of single-dish measurements, and a new methanol transition, the 10/sub 2/..-->..10/sub 1/(E), was detected. The rotational temperature of the centimeter lines of methanol is approximately 110 K, close to that of the millimeter lines measured by Kutner et al. It is concluded that the methanol masers are being excited by infrared emission from other objects in the vicinity.

K-Band Traveling-Wave Maser Using Ruby

A K-band ruby traveling-wave maser (TWM) has been developed, which has provided a decrease in system noise temperature compared to other front ends presently used in radio telescopes at K-band by an order of magnitude. The maser uses a new type of photoetched slow-wave structure, integral with the ruby rod, especially suitable for millimeter-wave masers. It also employs a new type of built-in isolator configuration, which guarantees stable net gain of typically 30 dB over the tunable bandwidth, which is about 20 percent. Its phase stability, both short and long term, is excellent, making it highly suitable for use on an interferometer for radio astronomy.

Ammonia observations of DR 21, W51, NGC 1333, and other sources

The ammonia clouds in DR 21, W51, NGC 1333, NGC 2264, L134 N, and S140 have been mapped in several transitions. Ammonia has also been detected in several other sources we well. The DR 21 data reveal a long, narrow emission ridge and suggest a slight anomaly in the NH/sub 3/(1,1) satellite emission, as was previously predicted. The validity of the single-temperature local thermodynamic equilibrium (LTE) model is discussed, and curves describing the effect of a gradient in excitation temperature are presented.

VLA observations of DR 21 NH/sub 3/ (1,1) absorption: direct evidence for clumping

The (1,1) transition of NH/sub 3/ was observed in absorption against DR 21 with a synthesized beam of 2 size at the VLA. The DR 21 continuum is resolved by the VLA into components distributed within a range of about 30, but only the brightest (and most southerly) component is absorbed by NH/sub 3/. The indicated filling factor and optical depth of the absorption feature are consistent with a model suggested by emission results, ammonia existing in optically thick thermalized clumps. One of these would be producing the observed absorption.