Christophe Lovis

An Earth-mass planet orbiting α Centauri B

Exoplanets down to the size of Earth have been found, but not in the habitable zone—that is, at a distance from the parent star at which water, if present, would be liquid. There are planets in the habitable zone of stars cooler than our Sun, but for reasons such as tidal locking and strong stellar activity, they are unlikely to harbour water–carbon life as we know it. The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a signal is overwhelmed by stellar perturbations. Here we report the detection of an Earth-mass planet orbiting our neighbour star α Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of 3.236 days and is about 0.04 astronomical units from the star (one astronomical unit is the Earth–Sun distance).

A new list of thorium and argon spectral lines in the visible

Aims. We present a new list of thorium and argon emission lines in the visible obtained by analyzing high-resolution (R = 110 000) spectra of a ThAr hollow cathode lamp. The aim of this new line list is to allow significant improvements in the quality of wavelength calibration for medium- to high-resolution astronomical spectrographs. Methods. We use a series of ThAr lamp exposures obtained with the HARPS instrument (High Accuracy Radial-velocity Planet Searcher) to detect previously unknown lines, perform a systematic search for blended lines and correct individual wavelengths by determining the systematic offset of each line relative to the average wavelength solution. Results. We give updated wavelengths for more than 8400 lines over the spectral range 3785–6915 A. The typical internal uncertainty on the line positions is estimated to be ∼10 m s −1 (3.3 parts in 10 8 or 0.18 mA), which is a factor of 2–10 better than the widely used Los Alamos Atlas of the Thorium Spectrum (Palmer & Engleman 1983). The absolute accuracy of the global wavelength scale is the same as in the Los Alamos Atlas. Using this new line list on HARPS ThAr spectra, we are able to obtain a global wavelength calibration which is precise at the 20 cm s −1 level (6.7 parts in 10 10 or 0.0037 mA). Conclusions. Several research fields in astronomy requiring high-precision wavelength calibration in the visible (e.g. radial velocity planet searches, variability of fundamental constants) should benefit from using the new line list.

Cosmic dynamics in the era of Extremely Large Telescopes

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universes expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.

ESPRESSO: A High Resolution Spectrograph for the Combined Coudé Focus of the VLT

Luca Pasquini, A. Manescau, G. Avila, B. Delabre, H. Dekker, J. Liske, S. D’Odorico, F. Pepe, M. Dessauges, C. Lovis, D. Megevand, D. Queloz, S. Udry, S. Cristiani, P. Bonifacio, P. Dimarcantonio, V. D’Odorico, P. Molaro, E. Vanzella, M. Viel, M. Haehnelt, B. Carswell, M. Murphy, R. Garcia-Lopez, J.M. Herreros, J. Perez, M.R. Zapatero, R. Rebolo, G. Israelian, E. Martin, F. Zerbi, P. Spano, S. Levshakov, N. Santos and S. Zucker

High-precision calibration of spectrographs using laser frequency combs

We present the first stringent tests of a novel calibration system based on a laser frequency comb (LFC) for radial velocity measurements. The tests were obtained with the high resolution, optical HARPS spectrograph. Photon noise limited repeatability of 9 cm s-1 was obtained, using only little more than one of 72 echelle orders. In the calibration curve CCD inhomogeneities showed up and could be calibrated, which were undetectable with previous Th-Ar calibrations. To obtain an even higher repeatability and lower residuals, a larger spectral bandwidth is necessary. An improved version of the LFC is currently under development. The results of the latest tests will be presented.

High Resolution and High Precision-Spectroscopy with HARPS

Extra-solar planet search at a level of precision below 1 ms

Along the path towards extremely precise radial velocity measurements

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Hunting for the lowest-mass exoplanets

sets strong requirements to the quality and stability of the wavelength solution. It also forces us to understand the effects of instrumental stability, on the one hand, and the quality of the wavelength reference, on the other hand, since both will have an impact, although in a different way, on the short- and long-term precision of the instrument. This chapter presents the calibration principles of HARPS, which lead to its extra-ordinary wavelength solution and, as a direct consequence, to its unique radial-velocity precision. In particular it will focus on the improvements of the thorium-lamp calibrations we made during the past three years, but it willl also discuss the present limitations. Finally, we give an outlook on further possible improvements which can be made in view of the extreme precision required by instruments like CODEX@ELT.

High-precision calibration of spectrographs

In the last six years, thanks to the very high radial velocity precision of the HARPS spectrograph, it was possible to detect 21 out of the 30 super-Earth (extrasolar planets masses below 20 times the mass of the Earth) discovered up to date. The radial velocity precision of the instrument is estimated around 80 cm/s on a single measurement. The main instrumental limitations are the wavelength calibration and the stability of the light injection. We address both factors and present the results of recent tests on the HARPS spectrograph. We have identified the laser frequency comb as the ideal wavelength calibrator, due to the width, density and flux of the lines, and to its intrinsic stability. The results from the recent tests that we performed on HARPS are encouraging. The accurate guiding of the telescope is critical to maintain a stable light distribution at the injection stage, where the light is sent into the spectrograph entrance fiber. To pursue this goal we are testing a secondary guiding system which is able to apply the guiding corrections twenty times faster than the primary guiding system.

Setting New Standards with HARPS

In order to understand general planet characteristics and constrain formation models it is necessary to scan over the widest possible parameter range of discovered systems. Due to detection biases, the domain of very-low mass planets had remained poorly explored. Only with improving measurement precision it has been possible to enter in the sub-Neptune mass range. The HARPS planet search program has been particularly efficient in detecting such ice giants and super earths. The present talk will summarize the obtained results and the characteristics of the low-mass population of exoplanets.