Guenter Wiedemann

A Sensitive Search for Methane in the Infrared Spectrum of τ Bootis

We have searched for a methane signature in the infrared spectrum of τ Bootis, produced by the planetary companion. The observations comprise 598 low-noise (signal-to-noise ratio ~100), high-resolution (λ/δλ = 4 × 104) spectra near 3044 cm-1, which we analyze by cross-correlating with a modeled planetary spectrum based on the work of Burrows & Sharp and Sudarsky, Burrows, & Pinto. The 3 σ random noise level of our analysis is ~ 5 × 10-5 stellar continuum flux units, and the confusion noise limit—measuring the resemblance of a cross-correlation feature to the spectrum of methane—is ~ 2.5 × 10-4. We find a significant cross-correlation amplitude of ~ 3.3 × 10-4 continuum units at a velocity near that of the star. This is likely due to methane from a low-mass companion in a long-period orbit. Fischer, Butler, & Marcy report a long-term velocity drift indicative of such a companion. But the system is known to be a visual binary with an eccentric orbit and is rapidly approaching periastron. Whether the visual companion can account for our observations and the Fischer et al. velocity drift depends on knowing the orbit more precisely. The stability of planetary orbits in this system also depends crucially on the properties of the binary orbit. A second cross-correlation feature, weaker and much more diffuse, has intensity amplitude ~ 2 × 10-4 continuum units and occurs at a velocity amplitude of 71(±10) km s-1, in agreement with the orbit claimed for the planet by Cameron et al. Like the first feature, it has passed several tests designed to reject systematic errors. We discuss the possibility that this second feature is due to the planet.

THE PROPOSED GIANT PLANET ORBITING VB 10 DOES NOT EXIST

We present high-precision relative radial velocities of the very low mass star VB 10 that were obtained over a time span of 0.61 years as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the Very Large Telescope with an ammonia gas cell. The typical internal precision of the measurements is 10 m s(-1). These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB 10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo & Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the similar to 1 km s(-1) radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M-p > 3 M-Jup and the orbital period and inclination suggested by Pravdo & Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo & Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low mass stars.

CO fundamental bands in late-type stars. II, Spectrum simulations for F-K stars

The procedure of Ayres and Wiedemann (1989) was applied to a range of stellar-atmosphere models to study the CO Delta-v = 1 spectrum and to establish its use as a remote sensor of thermal conditions in late-type stars. Spectra were computed to examine the sensitivity of the CO Delta-v = 1 to fundamental stellar parameters and to assess the errors introduced into the spectrum intepretation by uncertain input parameters and non-LTE effects. Results of the sensitivity study demonstrate that CO fundamental spectra are useful probes for the temperature structure of the outer layers of cool stellar atmospheres, but that their value is limited by the uncertainties introduced by non-LTE effects. However, in stars with surface gravities of log g of about 1.5 and greater, the values of these uncertainties are reasonably small. 37 refs.