Appendixes



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The Atacama Large Millimeter Array: Implications of a Potential Descope Appendixes

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The Atacama Large Millimeter Array: Implications of a Potential Descope Appendix A Letter of Request NATIONAL SCIENCE FOUNDATION 4201 WILSON BOULEVARD ARLINGTON, VIRGINIA 22230 1 February 2005 Dr. C. Megan Urry and Dr. Roger D. Blandford Co-Chairs, Committee on Astronomy and Astrophysics Board on Physics and Astronomy National Research Council National Academy of Sciences 2101 Constitution Avenue, NW Washington, DC 20418 Dear Dr. Urry: With this letter I am requesting that the Committee on Astronomy and Astrophysics (CAA) provide an assessment of the scientific impact of a possible change in the design of the Atacama Large Millimeter Array (ALMA). First recommended in the 1991 Decadal Survey as the Millimeter Array, and now conceived in an international partnership as the Atacama Large Millimeter Array, ALMA was reaffirmed as a high priority for completion in the 2000 Decadal Survey.

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The Atacama Large Millimeter Array: Implications of a Potential Descope ALMA is a global project being carried out between North America (the US and Canada), the European Southern Observatory (ESO, and Spain), and Japan. The core instrument has been planned as an array of 64 12-meter-diameter antennas spread over a region up to ~14 km in diameter, located on a large, high desert plateau in northern Chile, and equipped with dual-polarization receivers covering 4 millimeter- and submillimeter-wave atmospheric windows. The antennas will have an rms surface error of <25 μm and the array will operate at up to a frequency of at least 720 GHz. ALMA construction costs are estimated to be $552M (FY 2000 dollars), with construction spread over a period of 10 years. Japan will supply significant enhancements to the core instrument, notably a “compact array” of four 12 m and twelve 7 m antennas to significantly improve the quality of large-field images produced by the core array, and receivers covering 4 additional receiver bands. The ALMA project has three level-l science requirements: The ability to detect spectral line emission from CO or C+ in a normal galaxy like the Milky Way at a redshift of z = 3, in less than 24 hours of observation. The ability to image the gas kinematics in a solar-mass protostellar/protoplanetary disk at a distance of 150 pc (roughly, the distance of the star-forming clouds in Ophiuchus or Corona Australis), enabling one to study the physical, chemical, and magnetic field structure of the disk and to detect the tidal gaps created by planets undergoing formation. The ability to provide precise images at an angular resolution of 0.1". Here the term precise image means accurately representing the sky brightness at all points where the brightness is greater than 0.1% of the peak image brightness. This requirement applies to all sources visible to ALMA that transit at an elevation greater than 20 degrees. The Committee is requested to evaluate the following issues related to a possible descope of the array to a number of antennas in the range 36-56. Were such a descope to be carried out, what would be the impact on the attainability of the level-l science requirements? what would be the loss of speed, image quality, mosaicing ability and point-source sensitivity? (A parametric representation of these performance changes would be welcome.) would ALMA still be sufficiently transformational to warrant continued support by the United States? is there a particular threshold in the number of antennas, below which ALMA would suffer a significant degradation in its performance

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The Atacama Large Millimeter Array: Implications of a Potential Descope in the above or other relevant scientific areas sufficiently serious to warrant attention? The Astronomy Division would require the results of your evaluation by May 2005, as input to its recommendations to the National Science Board. If you have any questions regarding ALMA, please contact Dr. Robert Dickman at 703-292-4893 (rdickman@nsf.gov), the ALMA Staff Associate at NSF. Sincerely, G. Wayne Van Citters Director Division of Astronomical Sciences

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