One of the most effective means employed by astronomers to search for planets around stars other than our sun is to try to detect the velocity shift of the star as the planet moves about the star in its orbit. This well proven technique employs an instrument called a spectrograph to measure the small shifts in wavelength of features in a star's spectrum. The shifts are due to the familiar Doppler effect and can be directly related to the changes in the star's velocity. But these velocity shifts are very small, and in order to measure them with great accuracy it is necessary to employ some scheme to calibrate the wavelength of the light in the spectrograph. One relatively new technique is to use interferometry, measuring the "interference" of two beams of light when they come together. At a particular wavelength, the two beams of light will cause fringes to appear which are spaced according to their wavelengths. By measuring the spacing of these fringes, wavelengths can be determined very accurately.
Dr. Jerry Edelstein of the University of California-Berkeley and Dr. James Lloyd of Cornell University are applying this technique to a spectrograph that works not in visual light, but in the infrared. Infrared light is very useful in the study of cooler and smaller stars which are more numerous than hot stars. Finding planets around these stars will give us a much clearer picture than we currently have of how planets are actually formed and evolve. Since these stars are cooler than the massive hot stars that have been most frequently studied, their habitable zones, the region at the distance from the star where conditions may be suitable for life to begin, are much closer to the parent star. Most of the planets discovered so far have been "gas giants" like Jupiter and are located too far from their star for biology as we know it to exist. But with this new instrument, smaller earth-like planets may be detected in the habitable zone of cool stars. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
This project developed TEDI, the TripleSpec Exoplanet Discovery Instrument, an instrument designed specifically for the radial-velocity study of low-mass stars. TEDI uses an Externally Dispersed Interferometer (EDI) as a novels means of achieving high radial velocity precision of 15 m/s across the near IR (0.9-2.4 um) range. The instrument’s development has been funded on a previous NSF ATI grant in a collaborative venture between UC Berkeley and Cornell. The instrument was installed on the Palomar 200" telescope, software for processing these unconventional data developed, and the instrument stability and radial velocity precision achievable characterized and validated at the 15 m/s systematic error level, more than an order of magnitude improvement over the 200 m/s precision previously achieved. Radial-velocity planet searches, a prolific and reliable planet detection method, have been almost entirely in the optical band and of solar-type stars. Consequently, knowledge about planetary companions to the populous but visibly-faint low mass stars is limited. Studying exoplanets about low mass stars is important because their distribution discriminates among planet formation theories, and because low-mass stars can more readily reveal very low-mass planets through measurable Doppler shifts. The importance of the M dwarf frontier, and the necessity of IR radial velocity (RV) capabilities in the landscape of exoplanet search capabilities are becoming increasingly apparent. The Astronomy and Astrophysics Advisory Committee (AAAC) was tasked by NSF and NASA to create an "ExoPlanet Task Force" (ExoPTF) to "recommend a 15-year strategy to detect and characterize exoplanets". The highest priority recommendation of the ExoPTF was to intensify searches for planets around M dwarfs, with the ultimate payoff being the discovery of transiting systems for study with Spitzer/JWST. This project has very nearly met the ExoPTF intermediate milestone of 10 m/s. Intellectual Merit: EDI is an enabling technology that increases a conventional spectrograph’s radial velocity or spectral resolution many fold. EDI’s added spectral response and internal fiducial can defeat systematic instrumental noise and directly improve measurements otherwise limited by seeing, telescopes, or optics. These advantages of EDI are essential to obtaining our unique planetary-science program based on an IR Doppler survey of the low mass dwarf stars. The capability for IR Doppler detection of M dwarf planet candidates forms a critical piece of exoplanet detection complementary to other ground and space based methods of planet detection and has been strongly endorsed by the ExoPlanet Task Force. Broader Impact: The program intends to develop a new tool for high velocity precision spectroscopy, enabling extrasolar planet and other fundamental stellar astrophysics. Our work contributes to the finding of terrestrial planets in habitable zones, a discovery that will have a broad impact on public science. The fact that EDI has the potential for reaching few m/s measurement accuracies using spectrographs of only moderate resolution (small size) gives it a broader potential to bring RV measurement capabilities to modest aperture telescopes accessible to a large community of students as well as to provide high resolution RV for the next generation very large aperture telescopes using spectrographs of more limited size. This TEDI instrument could be directly applied to two copies of the TripleSpec spectrograph, one at Apache Point and ‘NIRES’ in fabrication for Keck. Large, fast telescopes that are constrained by pupil variation (e.g. HET) can benefit from EDI’s high input-function tolerance. In the course of this proposal, several undergraduates, 2 PhD students and 3 post-doctoral scholars were educated.
