| The PRIMA DDL-AOS Project Introduction |
| The PRIMA DDL-AOS Project Introduction |
In less than 10 years after the first detection of a planet orbiting another star, more than 100 giant extra-solar planets have been discovered. This avalanche of results has opened a very exciting field of research: exploration of the characteristics of other planetary systems. The discoveries of the past few years have stimulated new planetary formation models leading to a new picture of planet formation.
So far, all extra-solar planets found around stars in the vicinity of the Sun have been detected by radial-velocity measurements. Large surveys are conducted in both hemispheres with about 3000 G, K and early M stars being monitored regularly. While these surveys are very successful, their intrinsic biases imply some limitations both on the detection sensitivity and the data interpretation. In particular, this method is restricted to certain types of stars and leaves the inclination angle of the orbit sin i undetermined, thus providing only a lower limit to the mass of the planets.
The development of state-of-the-art optical long-baseline interferometric instruments like the Very Large Telescope Interferometer on Cerro Paranal opens new perspectives in this research area. The European Southern Observatory (ESO) is currently acquiring the hardware for PRIMA, a facility that will enable astrometric observations with the Very Large Telescope Interferometer (VLTI). The PRIMA facility holds the promise to carry out an exciting program on the astrophysics of extra-solar planets through precise astrometry, which is a very complementary technique to the radial-velocity method. It has a different detection bias, favoring planets in large orbits versus the short-period orbits preferentially detected by the radial-velocity technique. Moreover, astrometry measures two components (right ascension and declination) of the stellar reflex motion versus the single radial component that is observable spectroscopically. However, to play a significant role, an astrometric accuracy of order 10 µarcsec is needed, which is beyond the performance of current instrumentation (including HST).
ESO is currently in the process of commissioning step by step the VLTI in Chile. In its final configuration, the VLTI shall consists of four 8 m telescopes and up to eight movable 1.8 m ``auxiliary telescopes'' (four of them funded to date), plus delay lines and a laboratory for beam combination. Originally conceived as a facility for high-resolution imaging, the VLTI will initially be equipped with instruments for observations at near- and mid-infrared wavelengths (see Glindemann 2001a and 2001b for references).
The infrastructure of the VLTI is also well-suited to be turned into a powerful facility for an astrometric planet search program (Quirrenbach 1995; von der Lhe et al. 1995). This idea gained momentum after the discovery of 51 Peg b, and was embraced by ESO's Interferometry Science Advisory Committee (Paresce et al. 1996). The technical concept for the delay lines was changed to accommodate a dual-star operating mode (Derie et al. 2000), and ESO commissioned a detailed study to assess the feasibility of an astrometric facility at the VLTI. The PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) report concluded that this was indeed possible, and provided a conceptual design of the most important components needed (Quirrenbach et al. 1998). After carrying out further design studies, ESO is now acquiring the main subsystems for PRIMA through industry contracts except for the Differential Delay Lines.
PRIMA consists of four main hardware elements: A star separator, fringe tracker, internal metrology system, and differential delay lines (DDL). For financial reasons ESO decided to start PRIMA without the DDLs. PRIMA is now in an advanced status, with all the components, except the DDLs, already being manufactured or procured. Without additional external contributions, PRIMA would for many years remain without its DDLs. Based on our current best estimates we are convinced that PRIMA would not have been able to comply with the original accuracy requirements and reach the accuracy required for astrometric observations of extra-solar planets. The European astronomical community would have lost an unique opportunity to perform outstanding science and strengthen its position in the field of extra-solar planet research.
The goal of this project is to contribute to the development of the PRIMA hard- and software, in order to speed up the implementation of this astrometric facility with respect to ESO's current plan and to enable an early start of high-precision astrometric observations. As compensation for this effort we have asked and will receive guaranteed observing time with the VLTI (on the Auxiliary Telescopes only), negotiated according to the ESO standard rules for any instrument provided by the community. The guaranteed time will be used by our Consortium to conduct a large astrometric survey for extra-solar planets.
Our core hardware contribution will be the design and construction of the Differential Delay Lines, consistent with an astrometric precision of 10 µarcsec. We will also implement the data reduction software, define the calibration plan, and carry out the system analysis required to conduct a large astrometric survey program with capability to detect extra-solar planets.
When PRIMA becomes operational, the first immediate objective of our survey will be to constrain all orbital parameters of planetary systems detected by radial-velocity surveys in order to precisely determine the planet masses and the relative orientation of planetary orbits in multiple systems. We will dramatically improve our understanding of the planetary mass function and put constrains on planetary formation models. The observations will help us to understand possible dynamical interactions in multiple planetary systems. The detection sensitivity for planets with larger orbital radii will be considerably improved, the astrometric technique being more efficient than radial-velocity measurements for these kind of objects. Our second main objective will be to conduct a large planet search program for stellar types that cannot be efficiently observed by the radial-velocity surveys. This will extend our knowledge about extra-solar planets through the time (with younger systems) and mass of stars. It means observing young and chromospherically active stars, as well as more massive early-type stars with broad spectral lines (early F and A stars). This will provide us with unique data to understand both the impact of the stellar type on planet formation, and the orbital evolution in young planetary systems.
Last changed: Friday, 11-Nov-2005 10:52:51 CET
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