THE HARPS-TERRA PROJECT. I. DESCRIPTION OF THE ALGORITHMS, PERFORMANCE, AND NEW MEASUREMENTS ON A FEW REMARKABLE STARS OBSERVED BY HARPS

Abstract

Doppler spectroscopy has uncovered or confirmed all the known planets\norbiting nearby stars. Two main techniques are used to obtain precision Doppler\nmeasurements at optical wavelengths. The first approach is the gas cell method,\nwhich consists on the least-squares matching of the spectrum of Iodine\nimprinted on the spectrum of the star. The second method relies on the\nconstruction of a stabilized spectrograph externally calibrated in wavelength.\nThe most precise stabilized spectrometer in operation is HARPS, operated by ESO\nin La Silla Observatory/Chile. The Doppler measurements obtained with HARPS are\ntypically obtained using the Cross-Correlation Function technique (CCF). It\nconsists of multiplying the stellar spectrum with a weighted binary mask and\nfinding the minimum of such product as a function of the Doppler shift. It is\nknown that CCF is suboptimal in exploiting the Doppler information in the\nstellar spectrum. Here, we describe an algorithm to obtain precision RV\nmeasurements using least-squares matching of each observed spectrum to a high\nsignal-to-noise ratio template derived from the same observations. Such\nalgorithm is implemented in our software called HARPS-TERRA (Template Enhanced\nRadial velocity Re-analysis Application). New radial velocity measurements on a\nrepresentative sample of stars observed by HARPS is used to illustrate the\nbenefits of the proposed method. We show that, compared to CCF, template\nmatching provides a significant improvement in accuracy, specially when applied\nto M dwarfs.\n