MRI: Development of a State-of-the-Art Multiband Receiver for Arizona Radio Observatory's New ALMA Antenna

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As part of the development phase of the Atacama Large Millimeter Array (ALMA), two 12 meter ALMA prototype antennas (APA) were constructed for evaluation and testing, and installed at the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) site in Socorro, New Mexico. After completion of the ALMA testing and evaluation phase, the Arizona Radio Observatory (ARO) at the University of Arizona (UA) acquired one of the two APA telescopes. After acquisition, the UA moved their APA to Kitt Peak, replacing the 40-year-old 12-m telescope, and rechristening the telescope as the new Arizona Radio Observatory (ARO) 12 m. The subject proposal funds a modern, multi-wavelength receiver system which will realize the full scientific potential of the new ARO 12 m antenna. The receiver will cover the 1-4 mm wavelength range in four separate bands, and will enable a broad range of astronomical research including spectroscopy of molecular clouds and planetary nebulae, astrobiological studies of disks and pre-biotic molecules, and the study of black holes. The new receiver and 12 m antenna will have a broad user base with U.S. astronomers and scientists from universities and national labs, the Event Horizon Telescope consortium, and the European Southern Observatory (ESO). As part of this program ARO will allocate 28 days of observing time per year to the general U.S. community to access ARO's unique northern hemisphere observing capability and to enable pathfinder science for ALMA. The program will incorporate significant student training at all levels, from high school through graduate school. This telescope, along with the ARI 10 m telescope on Mt Graham, are perhaps the only remaining general purpose radio telescopes operated by a US university. As such, they are among the few remaining opportunities for direct student involvement in radio telescope instrumentation and operations. The dual-polarization 4-band receiver will cover the astronomically important atmospheric windows at 1, 2, 3, and 4 mm wavelength regions well suited to the Kitt Peak site. The 3 (84-116 GHz), 2 (130-180 GHz) and 1 mm (211-275 GHz) bands will employ ALMA-type, sideband-separating (SBS), superconducting (SIS) mixers, which represent the world's best in sensitivity and stability. SBS 2 mm mixers will be developed in collaboration with the National Radio Astronomy Observatory (NRAO) Central Development Lab (CDL) to cover a band scientifically important and well matched to the Kitt Peak site. The mixer chip will be a series array of four SIS junctions, similar to ALMA Band 6, and will be fabricated by the University of Virginia Microfabrication Lab (UVML). The 4 mm band (67-90 GHz) will utilize NRAO HFET amplifiers, previously tested on the old ARO 12 m. This system will also be SBS through an E-band down-converter developed by ARO. The 3 mm mixers are already in ARO's possession and the 1 mm devices will be obtained from NRAO. The mixers/amplifiers will be mounted in the same dewar using a modular design with each band and associated optics positioned on a separate plate. The receiver system will bolt onto the flange of the antenna cabin at the secondary focus. The spectral-line control system is fully operational at the new 12 m and will accommodate the new receiver with minimal modification. The new receiver will be used mainly for spectroscopic observations of interstellar and circumstellar molecules, and will: i) cover important molecular transitions, such as the J=1 - 0 lines of CO, HCN, N2H+, H2CO, and HCO+, used widely as density and temperature tracers in interstellar gas; and ii) provide wider frequency coverage necessary for many astrochemical programs. A major focus will be the investigation of the extent of molecular matter in the Galaxy. The subject work will redefine translucent cloud boundaries using low-level CO emission, characterize dense cloud cores chemically and physically, probe the sky as seen by Planck, and investigate hybrid gaseous/debris disks. The surprisingly rich chemical content of planetary nebulae and its connection to the diffuse interstellar medium will be further investigated. Combined with laboratory spectroscopy, the molecular carriers of the biogenic element phosphorus will be sought. The ARO 12 m will also join the Event Horizon Telescope for 1 mm VLBI observations, improving overall sensitivity and adding an intermediate baselines for probing the larger scale structure around black holes.