NOTE: This text is in draft form only and does not represent our final report or approach to the Decadal Review.
Introduction
Optical and IR interferometry has, in the last few years, truly come of age as an important tool for stellar astrophysics. The science results of the current generation of arrays, based on very limited imaging capability, are substantial, with more than 100 refereed papers in the last two years. A large proportion of the publications report new and often unexpected results. Meanwhile, instrumental developments are rapidly moving optical interferometry towards more sophisticated imaging capability, with the first images of rotationally distorted stars now in the literature. With respect to infrastructure, the U.S. is in a position of strength, with multiple facilities, each offering a unique capability. There is great potential for enhancement of capabilities and productivity. However, these facilities are private or mission-driven. Community access is severely limited, and funding resources are in some cases marginal and/or fragile. An endorsement of interferometry by the Decadal Survey Committee will pave the way to the next generation of optical array capabilities while extending the utilization of very high angular resolution to the optical astronomy community.
Background
This effort to raise the profile of optical/IR interferometry is being lead by the United States Interferometry Consortium (USIC). USIC consists of representatives from many of the ground based interferometry groups in the US, including CHARA, ISI, MSC (representing KECK-I and PTI), NPOI and MRO. USIC came about as a result of the Future Directions in Interferometry Workshop, which took place in Tucson in November of 2006. This workshop included representatives from around the world and the text of the consensus report is available on line. The workshops highest priority recommendation to NOAO is for “competitive, peer-reviewed guest observer” access to optical interferometry. Opening the doors of our optical arrays to general competition would complete the initial development phase of optical interferometry, begun with the 1990 Decadal recommendation in this area. Without continued, and expanding, support for the current facilities the current US scientific capability will rapidly fall behind our main competition, for example the VLTI, and we will miss the great scientific opportunities this technique can deliver.
Science
With the existing facilities now producing real images of stellar surfaces (see for example the images of Altair produced by MIRC) , and with more imaging capability to come on line before the end of this decade, interferometry is poised to make major contributions to the study of stellar astrophysics. Such arrays of telescopes are especially well suited for a wide range of stellar science, including stellar surfaces, interiors and systems, as revealed by stellar shapes, surface light distributions, binary orbits, pulsations, shells, jets and other phenomena. Some studies of exoplanets and of AGN’s have already begun. With ongoing funding currently in doubt for a number of existing facilities it is vitally important to expand the user community of the existing instruments, as well as to raise awareness of the current capabilities and future potential of this technique.
(NOTE: We will live and die by the scientific case. Obviously we need much more here including a good coverage of what has been done to date, what is being worked on now and what other areas of astronomy to which we would hope to contribute in the next decade)
Future discoveries enabled by optical interferometry cannot be predicted easily, but one can identify certain areas in which discoveries are more likely than in others. No other existing or proposed facilities (JWST, ALMA, or single dish telescope like GMT, ELT, Gemini etc.) have diameters in excess of 100 meters and operate in the optical to mid-infrared. Interferometry with baselines up to 400 meter will always outperform any existing or planned facility for many decades to come from the point of view of spatial resolution. Furthermore, with the optical telescopes of 30 to 100 meter, technology will most likely reach its limit of aperture size within this time frame. The next generation after ELT, GMT, TMT seems to have no alternative other than interferometry.
Recommendations
Support of facilities
During the next decade, funding should be made available for significant upgrades in capabilities as well as observing efficiency and tools for the current generation of US based optical interferometers. This should include funding to provide more open access for the community to the existing instruments as well as training of students and post-doctoral research associates in the use of these facilities. Upgrades to the current facilities could include the addition of more apertures, upgrades to beam combiner technologies, efforts to increase sensitivity including the addition of Adaptive Optics, observation planing tools, data reduction tools, as well as funding for the staff required to support these activities. Expanded capability of the current facilities, and more awareness and use across the community of these facilities, will not only produce unique scientific results but will also aid in the study and design of the next generation arrays.
Astronomer base
Significant coordinated efforts should be made to train astronomers in using optical interferometers, and to create a pool of Ph.D. students and postdoctoral research associates working with the data obtained by optical interferometers.
Implementation
There are 6 optical interferometers based in the US. For US based optical interferometry to mature and continue to be a global player, activities at these 6 sites have to be coordinated and work towards optimizing individual capabilities (imaging, astrometry, spectroscopy etc.) with a vision that one day they will all be phased out, and a single km-based optical interferometer will be build by a US consortium. Between now (2007) and the operations of a km-based optical infrared interferometry (2020 to 2030 time frame), current generation optical interferometries provide the environment for technology development and maturing of the astronomer base.