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Doppler spectroscopy has uncovered or confirmed all the known planetsorbiting nearby stars. Two main techniques are used to obtain precision Dopplermeasurements at optical wavelengths. The first approach is the gas cell method,which consists on the least-squares matching of the spectrum of Iodineimprinted on the spectrum of the star. The second method relies on theconstruction of a stabilized spectrograph externally calibrated in wavelength.The most precise stabilized spectrometer in operation is HARPS, operated by ESOin La Silla Observatory/Chile. The Doppler measurements obtained with HARPS aretypically obtained using the Cross-Correlation Function technique (CCF). Itconsists of multiplying the stellar spectrum with a weighted binary mask andfinding the minimum of such product as a function of the Doppler shift. It isknown that CCF is suboptimal in exploiting the Doppler information in thestellar spectrum. Here, we describe an algorithm to obtain precision RVmeasurements using least-squares matching of each observed spectrum to a highsignal-to-noise ratio template derived from the same observations. Suchalgorithm is implemented in our software called HARPS-TERRA (Template EnhancedRadial velocity Re-analysis Application). New radial velocity measurements on arepresentative sample of stars observed by HARPS is used to illustrate thebenefits of the proposed method. We show that, compared to CCF, templatematching provides a significant improvement in accuracy, specially when appliedto M dwarfs.

The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS