Search

The TEDI (TripleSpec Exoplanet Discovery Instrument) is a dedicated instrument for the near-infrared radial velocity search for planetary companions to low-mass stars with the goal of achieving meters-per-second radial velocity precision. Heretofore, such planet searches have been limited almost entirely to the optical band and to stars that are bright in this band. Consequently, knowledge about planetary companions to the populous but visibly faint lowmass stars is limited. In addition to the opportunity afforded by precision radial velocity searches directly for planets around low mass stars, transits around the smallest M dwarfs offer a chance to detect the smallest possible planets in the habitable zones of the parent stars. As has been the the case with followup of planet candidates detected by the transit method requiring radial velocity confirmation, the capability to undertake efficient precision radial velocity measurements of midlate M dwarfs will be required. TEDI has been commissioned on the Palomar 200” telescope in December 2007, and is currently in a science verification phase. 1. Planets around M dwarfs The majority of the host stars of the planets detected to date lie in the range 0.7 to 1.4 M . Massive stars are generally not amenable to radial velocity studies due to featureless spectra from their hot atmospheres. Low-mass stars are cool and therefore faint in the green-visible where the Iodine cells provides absorption reference lines. Transit surveys similarly have been primarily sensitive to planets around solar-type stars, due to both the population probed by optical photometry over relatively narrow fields, and the requirement of radial velocity confirmation with optical spectrographs. Even with the difficulty of detecting planets around cool stars, some of the most interesting examples have been discovered around M type stars: GJ 876 (Marcy et al. 2001) is a multiple planet system exhibiting resonant interactions and GJ 436 (Butler et al. 2004) is the lowest mass planet yet to be detected with transits. Bond et al. (2004) argue that the unusual binary microlensing event OGLE 2003-BLG-235/MOA 2003BLG-53 is a 1.2 −1.2 Mjup planet orbiting a 0.36 +0.3 −0.28 M M2-M7 main sequence star. Perhaps the best candidate so far for an image of an extrasolar “planet” is the putative 5 Mjup companion to the ∼ 25 Mjup young M8 brown dwarf 2MASS J12073346-3932539 (Chauvin et al. 2004). There are multiple reasons to focus on planets around low-mass stars. Low-mass stars and brown dwarfs dominate the stellar population in both number and mass. As the mass of the primary decreases, the radial velocity signature increases. Similarly, the radius of M dwarfs is favorable for transits: an Earth-radius planet orbiting a 0.08 M star produces a transit of the same depth as a Jupiter-radius planet orbiting a sun like star (see Figure 1).