The `Ohana concept considers interferometric coupling of 3 to 10 meter class telescopes of Mauna Kea taking advantage of two technological breakthroughs: adaptive optics and coherent transport of light with single mode optical fibers. This concept, proposed in 1996 (Mariotti et al, AASS, 116, 381-393, SPIE 3350, 785-792), was informally discussed at a first meeting in March 2000. It was considered worth pursuing by the participants and a number of preliminary actions was undertaken.
A plan was drafted comprising three phases:Members of IRTF, UKIRT and Subaru were invited by the `Ohana committee and largely contributed to the progress of the meeting. They have been invited to join the `Ohana committee.
The meeting was divided into two parts. The first day addressed technical issues and the second day was dedicated to science cases and to definition of action items.
The use of single mode fibers for beam transport over hectometric distances in the visible and near infrared has been demonstrated in laboratory experiments in Japan (MIRA) as well as in France (IRCOM), and will be pursued at Observatoire de Paris for K band fibers.
It is believed that the existence of an empty 4 inch duct linking telescopes could be available for a permanent link, but requires confirmation from UH. It appears that for a temporary connection of close observatories on the Eastern ridge, an overground solution is feasible and indeed preferable. However, for the western side, the wikaiu bug habitat may prevent overground routing of the fibers in which case an underground solution readily exists.
Several solutions have been discussed. For a baseline involving Keck telscopes the fringe tracker camera may be available, as long as the dedicated optics are provided for fiber focusing. The Meudon group is considering building a Nicmos array. The availability for an innovative detector at University of Hawaii should be investigated.
Four illustrative science cases were presented and they already proved to be strong science drivers for phase II of the instrument.
In AGNs, the topics of interest are the size of the Broad Line Region and the strucrure of the base of jets in the infrared. This will require baseline on the order of or greater than 200 meters. However, the sensitivity required will imply the need for the larger apertures. Significant results can be obtained by obtaining visibilities in H band and Pa{beta}; a moderate spectral resolution is needed for the determination of the size of the BLR. For the study of jets, calibrated fluxes and polarimetric measurements will be needed. A deeper study is necessary in order to examine the significance of a few visibility measurements for constraining models.
`Ohana will be able to probe the inner central AUs of the circumstellar environment, including the inner accretion disk, and star/disk interface region, which hold important clues on the accretion/ejection mechanism. Thermal emission from dust heated by the viscous accretion process dominates the near-infrared continuum. It will be possible to constrain the size of the disk truncation radius (expected to lie close to the corotation radius - 0.07 au) and discriminate between different disk models (magnetized versus purely hydro). Brgamma observations with moderate spectral resolution (a few 100) would constrain the geometry of the star/disk interface region and the wind formation regions. Baseline greater than 200 meters will be needed to answer these questions. However, the sensitivity is not an issue, and the smaller telescopes can be used for this program. Even a very limited number of visibilities constrain models. About 100 sources are accessible. K band is the most suited for this program.
The distance ladder can be improved in two ways. First by indirect measurements: angular diameter measurement coupled with linear diameter derived from models. Second, by direct measurement: angular diameter variation coupled with spectroscopic photosphere velocity determination. The sample of sources for `Ohana comprises at least 100 targets for the indirect measurement and from 60 up to more than 100 for the direct measurement. Besides, the resolving power of `Ohana will allow to study the atmosphere of these stars and compare with current models which are a source of bias in the final distance ladder calibration. A large fraction of this program can be achieved with moderate baselines (~200 meters) and pupils (4 meters).
The modeled diameter is on the order of 100 micro-as. However a measure of this diameter will strongly constrain the equation of state for cooling models. This is unique science as the full resolution of `Ohana is required as well as the largest pupils: the resolution of `Ohana is 425 micro-as in H band and so a diameter measurement translates into a 95% visibility measurement. The required accuracy on the visibility is 1% and there are half a dozen candidates within 10 pc.
A modular injection device, the interfaces of which will have to be adapted to the f-ratio of each input beam, will be designed for Keck, CFHT and Gemini telescopes and possibly Subaru pending steric compatibility. It will verify the injection, measure the efficiency of AO to fiber coupling, investigate polarisation and fiber stability during the transport from focus to ground and give clues to the final sensitivity of the array. The tests, to begin on the telescopes in the fall of 2001, are expected to be completed by the end of that year. A large fraction of the tests will be done during daytime, while technical and/or discretionnary time could be considered for the sky validation.
Both silica and fluoride glass fibers should be used for these tests and the injection modules should preserve compatibility for both types of fibers.
Cost for phase I includes the construction of 2 injection modules (provision for phase II), a length of 50 meters of infrared fibers (and also silica fibers), laboratory measurement of beam transport in it and the assembly of an available IR detector. These costs can be beared by Meudon. New visits of Woillez to Hawaii will be necessary and it is wished to continue the arrangement which supported Woillez visit in November to December 2000.
Phase I also includes the design and cost estimate of phase II for which the following goals and requirements were established. A requirement is the minimum achievement at which a task is said to be completed, while a goal is the ideal objective one would like to reach.
Phase II should not only demonstrate the technical feasability of telescopes' coupling on Mauna Kea summit, but also lead to some unique science results beyond the VLTI and Keck Interferometer capabilities. Main parameters for the choice of telescopes are: telescope aperture, baseline length, baseline orientation (North/South orientation simplifies delay line problem).
The science discussion led to the following potential objectives, ordered by probable increasing difficulty:
Extending the VLTI and KI resolutions, any of those programs can bring unique science.
Three baseline configurations satisfy the phase II requirements, offer the technical demonstration capability and contribute to these science goals. Among all possible configurations, this choice appears to select the simplest ones, which can be ordered by increasing difficulty:
For any of these baselines, the same pair of matched infrared and/or silica fibers ca. 450 meters in length can be used.
Recombining equipment can be made available from existing systems for both infrared and silica fibers.
Cost estimates for phase II will be consolidated in 2001 and discussed among partners. Yet, a significant fraction of the cost (beam combiner and possibly fibers) may be covered as the french contribution to the project.
Phase II is likely to extend over 2002 and 2003. Its precise definition requires additional studies and will be the subject of the third `Ohana meeting to be held in the fall of 2001.