Report of the Millimeter Array Science Workshop
Tucson, Arizona
October 5--7, 1995

The Millimeter Array Advisory Committee

The Science Working Groups

SUMMARY OF THE MMA SCIENCE WORKSHOP

On October 5-7, 1995, nearly 100 scientists and engineers gathered in Tucson for a workshop on the topic of the Millimeter Array (MMA). The twin objectives of the meeting were to update the scientific goals for the MMA and to compare those goals to the strawman design that had been developed by the technical working groups. To achieve the first goal, five scientific working groups and a technical group held separate meetings. To achieve the second goal, joint meetings were held. The scientific working groups summarized their conclusions in reports, which are appended to this document. Some of the most exciting scientific goals have been abstracted and listed as Section 2. of this report. Details may be found in the full reports. Each report also contains recommendations on technical requirements pertinent to the goals of that group. Many of the technical requirements are a result of a very productive give-and-take between the scientific and technical working groups, which characterized the spirit of the workshop.

The Millimeter Array Advisory Committee (MAC) met at the conclusion of the meeting and agreed on a set of recommendations for the future development of the MMA. These recommendations were subsequently refined, based on the working group reports, and are contained as Section 3. of this report. The most salient recommendation of the MAC is that development of the MMA should proceed as soon as possible. The scientific case for the MMA, already strong when the proposal was written, has become much stronger. The pioneering work of the existing arrays has opened new areas for research that can only be properly developed with the MMA. The excellent atmospheric properties of the sites now under consideration and the advances in technology since the MMA proposal also led the MAC to recommend initial operation with a submillimeter band and outrigger stations to allow resolution significantly better than 0.1 arcseconds. The MAC also identified capabilities in the original proposal which are now of less importance.

The scientific goals summarized in Section 2. demonstrate the broad scope of MMA science. From the largest structures in the Universe to the smallest, near-Earth objects, the MMA will revolutionize our understanding. The origin and evolution of galaxies, stars, and planetary systems will be studied by the MMA. The scientific and technological foundations for the MMA are sound. We believe it is time to move forward with this revolutionary scientific instrument.

HIGHLIGHTS OF MMA SCIENCE

• The MMA will image anisotropies in the Cosmic Background Radiation at angular scales up to 2 with sensitivities of 2K. No other instrument can measure accurately these seeds of galaxies in the early () Universe.

• The MMA will provide detailed imaging and polarimetry of the Sunyaev-Zel'dovich effect in distant () clusters. Combined with X-ray maps, the resulting information will lead to estimates of and , independent of the usual distance scale ladder.

• The MMA will image dust emission from galaxies with L at , probing the epoch of galaxy formation.

• The MMA will detect CO from ultraluminous galaxies at and from normal galaxies at , allowing study of early galaxy evolution and extending Tully-Fisher studies to much larger redshifts than can be reached with HI studies.

• At a southern hemisphere site, the MMA will obtain 0.25 pc resolution on the Magellanic Clouds, laboratories for understanding the role of metallicity, radiation field, and external pressure on cloud structure, star formation, and chemistry.

• The MMA will be able to detect many distant continuum sources. By observing absorption lines against these sources, the MMA will study the chemical evolution in damped L systems and intervening galaxies. In more local galactic clouds, such observations are the only way to search for the coldest molecular gas in the outer Galaxy and probe the chemistry of polyatomic molecules in diffuse and translucent clouds.

• The MMA will image with unrivalled clarity galactic molecular clouds to address the most fundamental questions concerning their structure: the nature and origin of the apparent filamentary and turbulent structure; and how this dynamic structure evolves into gravitationally bound cores that are destined to form stars.

• The MMA will map the magnetic field in molecular clouds: the transverse field will be mapped with linear polarization of dust emission; the line-of-sight field will be mapped using the Zeeman effect in molecules like CN and CCS.

• The MMA will trace the physical, dynamical, and chemical evolution of star-forming regions over an unprecedented scale from cloud cores (0.1 pc) to the inner circumstellar disk (5 AU); it will determine the nature of infall, rotation, and outflow on all these scales.

• The MMA will study the nature of dust-gas interactions in circumstellar envelopes and disks and determine the extent of the resulting chemical complexity as a function of evolution.

• With outrigger stations at 10 km baselines, the MMA will detect gaps in disks caused by planet formation, and probe the chemistry of the inner disks, constraining the major reservoirs of the biogenic elements.

• With outrigger stations at 10 km baselines, the MMA will allow us to study the effects of binaries on disks, testing theories of binary formation and leading to an understanding of whether planetary systems can form in binaries.

• With outrigger stations at 10 km baselines, the MMA will be able to detect Jovian planets in formation, shedding light on the frequency of planetary systems and their formation processes.

• The MMA will observe stars in every part of the H--R diagram, detecting photospheric or chromospheric emission from several thousand stars in under 10 minutes each. With outrigger stations on baselines of 10 to 30 km, the MMA can directly determine diameters and thus effective temperatures of the photospheres or chromospheres of several hundred stars.

• The MMA will study for the first time dust formation at distances of a few stellar radii around evolved stars and observe the levitation of material from the stellar surface. Together with measurements of the abundance profiles of molecules as functions of distance from the star, it will be possible to directly observe the precipitous drop in abundances of refractory molecules when they condense into dust grains.

• The MMA can address questions about the nature and origin of comets, both by detection of new molecular species and by studying their location with unprecedented spatial resolution; comets link the chemical processes in clouds and disks to those in the inner solar system and extend the range of our knowledge of the biogenic elements.

• The MMA can locate and provide orbits for near Earth objects with greater precision than can studies at visible wavelengths, allowing better predictions of subsequent apparitions and possible Earth impacts.

• With adaptations for solar observing, the MMA can study solar flares and associated phenomena -- in particular, the impulsive phase wherein electrons and ions are promptly accelerated to MeV energies.

• With adaptations for solar observing, the MMA can study the structure and dynamics of the solar chromosphere through multiband imaging, and through studies of solar oscillations in the chromosphere.

RECOMMENDATIONS OF THE MAC

1. We enthusiastically endorse the basic plan of the MMA, as outlined in the original proposal, further developed by the MDC technical working groups, and updated as described below. We believe it is crucial to proceed with development of the MMA as soon as possible.

2. Both sites under current consideration (Mauna Kea and Chile) look extremely attractive, especially from the point of view that submm observations are possible. The site in Chile has some advantages (even better atmospheric transmission, a larger area for the long baselines, and access to the southern sky, including the center of our Galaxy and the Magellanic Clouds). Since we have only half a year of testing in Chile, we recommend continued testing and consideration of both sites. We also recommend that medical advice on the effects of working at the altitude of the Chile site be solicited.

3. The excellent atmospheric properties of both sites, along with the scientific considerations of the science working groups, point to the need for the array to include a submillimeter capability, at the outset. We recommend that the dewar be designed with windows for the three submillimeter bands, and that the INITIAL receiver complement include a receiver for the 650 GHz window. This receiver need not include dual polarization. We believe that the submillimeter capability should be considered in the other specifications, but our current understanding is that the basic design suggested in the last report of the technical working group, including the CFRP backup structure, is adequate for submillimeter work. We are not asking for further enhancement in the specifications. Any further effects on the specs should be reported to the MAC for evaluation. Estimates for the cost of submillimeter receivers should be developed by the receivers group in the near future.

4. The new sensitivity estimates open up new possibilities for even higher spatial resolution. We recommend that the array be designed for INITIAL operation with at least 2 outrigger stations at a 5 km radius. We recommend studies of the optimum number and placement of outrigger stations, and the limitations that fiber non-linearities might place on very long baselines. The possibility of extension to even longer baselines (up to 30 km was suggested) should be considered in site evaluations, cost considerations and uv coverage analysis. Baselines longer than 10 km would be designed for resolution of very compact objects, such as stellar photospheres and possible gas-giant protoplanets; the imaging requirements should be viewed in the light of those goals.

5. The correlator should have eight, rather than four, independent spectral channels, but not more lags. In general, flexibility in the correlator should be enhanced, rather than the number of channels. We recommend including the option of very high resolution (10 Hz) for planetary radar as an option that does not drive the design. In particular, F-X designs should not be ruled out by this option if they prove to be superior in other ways.

6. Items 3-5 reflect enhancements to the basic design and are roughly in order of importance as perceived by the MAC, interpreting the reports of the working groups at the meeting. In particular, we noted that the correlator enhancement should not be done at the cost of items 3 and 4. We also identified some negative enhancements, items which could be eliminated. The MAC voted strongly in favor of eliminating all these, if necessary for cost or other reasons.
   • nutating subreflectors (pending up-coming tests of alternatives)
   • the 36-50 GHz band (but keep the 26-36 GHz band)
   • capability for simultaneous, dual-band observing (with a possible exception for calibration of the submm bands)
   • special adaptations for solar observing, which drive either the design or cost.

7. We recommend a reconsideration of the optimization between number and size of dishes. The reoptimization should include the changes listed in items 3-6 above. The recommendations of the working groups were not uniform on the effect of the science goals in this reoptimization. The MAC was evenly split on whether the science goals favored more or larger dishes.

8. We recommend activation of the working group on Software and Data Management, which is rumored to exist.

9. We recommend that a prototype antenna be built for testing purposes. We do not recommend a separate test array. Instead we recommend that NRAO staff spend time at the existing arrays to gain experience and test ideas cooperatively with the other MDC members. If tests with the MMA antennas are necessary, NRAO should consider doing these with the University arrays.

10. We recommend that NRAO hire one or more scientific staff members who have experience with the existing millimeter arrays, as soon as possible.

11. The efforts of the MDC working groups have improved the design significantly and we recommend continuation of this collaboration. Similar collaboration with existing sub-millimeter groups should be considered for the optimum development of sub-millimeter capability.

12. We recommend that each Technical Working Group include a member of the MAC. Specifically, we offer the following volunteers:

    Systems:

    John Carlstrom

    Receivers:

    Neal Erickson

    Antennas:

    John Bieging

    Phase Cal:

    Colin Masson

    Software:

    Jill Knapp