CRIRES search for planets around the lowest-mass stars

Abstract wrote:The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths. We investigate the precision that can be achieved in radial velocity measurements of low mass stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical. For early-M stars, radial velocity measurements in the nIR offer no or only marginal advantage in comparison to optical measurements. Although they emit more flux in the nIR, the richness of spectral features in the optical outweighs the flux difference. We find that nIR measurement can be more precise than optical measurements in stars of spectral type ~M3, and from there the nIR offers significant gains in precision towards cooler objects. We studied potential calibration strategies in the nIR finding that a stable spectrograph with a ThAr calibration probably offers the best choice with currently available technology. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on radial velocity measurements. Our spot simulations reveal that the radial velocity jitter does not decrease as dramatically towards longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. At low temperature contrast (~200K), the jitter shows a decrease towards the nIR up to a factor of ten, but it is substantially smaller with larger temperature contrast. Forthcoming nIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and nIR wavelengths can provide strong constraints on spot properties in active stars.

For example, our planet search project includes 31 objects with estimated masses below 0.2M(Sol), 22 of which have estimated masses below 0.15M(Sol). A previous high-precision radial velocity planet search utilizing a visible wavelength spectrograph on an identical telescope (UVES on the UT2 telescope of the VLT) was only able to target two objects with estimated masses below 0.2M(Sol), and only one of these two has an estimated mass below 0.15M(Sol) (Zechmeister et al. 2009).
In addition to enabling the search for planets around more low-mass stars, the gas cell and radial velocity measurement algorithm we have developed also opens up a new frontier on the search for potentially habitable planets. The orbital period range for a planet in the habitable zone around a star with a mass M = 0.10M(Sol) would be 3 – 21 days (Selsis et al. 2007). The 5ms−1 precision obtainable with our method corresponds to the velocity semi-amplitudes induced by a 2.5M(Earth) planet or a 4.6M(Earth) planet on the inner and outer edges of the habitable zone respectively. Therefore, it should be possible to detect Super-Earth type planets in the habitable zones of very low-mass stars with a reasonable expenditure of observing time on current facilities.

Lazarus wrote:Incidentally we should probably merge this thread with this one and perhaps this one too... keep Borislav's title, we should keep the CRIRES prominent.