Doppler Planet Search with a Fringing Spectrometer

David Erskine & Jian Ge, Lawrence Livermore Nat. Lab.

Prof. Gibor Basri, UC Berkeley collaborator

Mike Rushford, LLNL interferometry consultant

Bruce Macintosh and Charles Alcock, astronomer consultant

Abstract

Our extrasolar planet search differs from others in the use of a new kind of instrument for measuring stellar Doppler velocities, which we call a fringing spectrometer or spectrally dispersed interferometer.

The simple instrument response of the interferometer allows <1 m/s velocity resolution. Spectrally dispersing the fringes increases their fringe visibility and boosts signal to noise over traditional white light interferometers. The whole instrument can be optimized for high efficiency because the disperser is not used to resolve Doppler shifts and therefore the grating technology can be chosen to maximize throughput rather than to achieve tight point spread function. Other benefits are an etendue (field of view) which is two orders of magnitude greater than current high radial velocity spectrometers, low cost and compact size (TV-sized instead of kitchen-sized).

This is a pilot project funded by the Laboratory Directed Research and Developement program (98-ERD-054). It is an excellant example of cross disciplinary research as we are melding velocimetry and white light interferometry technology developed at H-division's two stage gas gun (David Ersine) together with astronomical spectroscopy (Jian Ge and Gibor Basri).

We have nearly finished benchtop testing on a prototype instrument, and are now engaged in adapting the instrument for use at an observatory on starlight. We anticipate that our cleaner instrument response will allow us to detect smaller mass planets and resolve photosphere dynamics (asteroseismology). An anticipated variation of the instrument which uses two interferometer cavity lengths in parallel can measure lineshape changes simultaneous to Doppler shifts. This is another avenue of probing the stellar photosphere dynamics.

Our immediate goal is to demonstrate on starlight the anticipated advantages of the instrument. Secondly, to begin an observational program to detect Saturn-like or smaller extrasolar planets (<3 m/s), or detect planetary companions to known extrasolar planets. Thirdly, to measure ringing of a stellar photosphere, analogous to the solar "5-minute" oscillations.

• Doppler technique for detecting planets showing typical velocity noise in current data.

• The difficulty with grating spectrometers.

• Here's our solution to that problem.

• Fringing spectra are moire fringes.

  Comparison of size to Keck spectrometer.

  Photo of prototype instrument.

  Noise floor tests.

Last modified on 09/22/99

UCRL-MI-134127