Alpha Centauri, the hunt for planets.

In May, we observed Alpha Cen B almost every night. The new tip-tilt system brought more photons into the FINDS fiber - on single stars we measured an efficiency improvement of about 8%. Unfortunately, even with the improved guiding, the contamination from Alpha Cen A was significant on several of our nights when there was poor seeing. I've spoken with Xavier Dumusque on the Geneva team and they have similar results with the HARPS data. We are now revising our analysis code to see if we can model out this contamination. Fingers crossed, we'll have some results soon.

The search for true Earth analogs faces several challenges and one of them is identification and removal of signals on the surface of the star. Stars are boiling cauldrons of plasma threaded with magnetic fields. The photospheres of the stars have spots, flares, granulation, pulsations and other flows that are imprinted in the stellar spectrum and obscure tiny signals from small rocky worlds.

To understand these sources of stellar noise, we are using CHIRON on other stars, including Tau Ceti and Epsilon Eridani, two stars that Frank Drake observed with Project Ozma. Tau Ceti is a favorite of Doppler planet hunters because the stellar velocities are so constant... or are they? A recent analysis by Tuomi et al. suggests that low amplitude signals may be embedded in the data. We are now able to check that result.

Epsilon Eridani is a more challenging star with spots that mimic Doppler velocity signals. We are writing up our results for that star now - by luck, we caught the star during a stretch when it had large persistent spots; this data set is a training ground for my students who are writing computer algorithms to identify and de-correlate stellar noise from Doppler data.

This work is part of an ongoing project which aims to detect terrestrial planets in our neighbouring star system α Centauri using the Doppler method. Owing to the small angular separation between the two components of the α Cen AB binary system, the observations will to some extent be contaminated with light coming from the other star. We are accurately determining the amount of contamination for every observation by measuring the relative strengths of the H-α and NaD lines. Furthermore, we have developed a modified version of a well established Doppler code that is modelling the observations using two stellar templates simultaneously. With this method we can significantly reduce the scatter of the radial velocity measurements due to spectral cross-contamination and hence increase our chances of detecting the tiny signature caused by potential Earth-mass planets. After correcting for the contamination we achieve radial velocity precision of ∼2.5ms −1 for a given night of observations. We have also applied this new Doppler code to four southern double-lined spectroscopic binary systems (HR159, HR913, HR7578, HD181958) and have successfully recovered radial velocities for both components simultaneously.