Gaspare Lo Curto

A spectrograph for exoplanet observations calibrated at the centimetre-per-second level

The best spectrographs are limited in stability by their calibration light source. Laser frequency combs are the ideal calibrators for astronomical spectrographs. They emit a spectrum of lines that are equally spaced in frequency and that are as accurate and stable as the atomic clock relative to which the comb is stabilized. Absolute calibration provides the radial velocity of an astronomical object relative to the observer (on Earth). For the detection of Earth-mass exoplanets in Earth-like orbits around solar-type stars, or of cosmic acceleration, the observable is a tiny velocity change of less than 10 cm s−1, where the repeatability of the calibration—the variation in stability across observations—is important. Hitherto, only laboratory systems or spectrograph calibrations of limited performance have been demonstrated. Here we report the calibration of an astronomical spectrograph with a short-term Doppler shift repeatability of 2.5 cm s−1, and use it to monitor the star HD 75289 and recompute the orbit of its planet. This repeatability should make it possible to detect Earth-like planets in the habitable zone of star or even to measure the cosmic acceleration directly.

ESPRESSO: the Echelle spectrograph for rocky exoplanets and stable spectroscopic observations

ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will combine the efficiency of modern echelle spectrograph design with extreme radial-velocity precision. It will be installed on ESOs VLT in order to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radialvelocity precision will be improved to reach cm/s level. Thanks to its characteristics and the ability of combining incoherently the light of 4 large telescopes, ESPRESSO will offer new possibilities in various fields of astronomy. The main scientific objectives will be the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs, and the analysis of the variability of fundamental physical constants. We will present the ambitious scientific objectives, the capabilities of ESPRESSO, and the technical solutions of this challenging project.

Discovery and monitoring of the likely IR counterpart of SGR 1806–20 during the 2004 γ-ray burst-active state

The sky region including the Chandra position of SGR 1806-20 was monitored in the IR band during 2004, following its increased high energy bursting activity. Observations were performed using NAOS-CONICA, the adaptive optics IR camera mounted on Yepun VLT, which provided images of unprecedented quality (FWHM better than 0.1). After the 2004 December 27th giant flare, the source position has been nailed by VLA observations of its radio counterpart, reducing the positional uncertainty to 0.04. Using IR data from our monitoring campaign, we discovered the likely IR counterpart to SGR 1806-20 based on positional coincidence with the Chandra and VLA uncertainty regions and flux variability of a factor of about 2 correlated with that at higher energies. We compare our findings with other isolated neutron star classes thought to be related, at some level, with SGRs.

The exoplanet hunter HARPS: unequalled accuracy and perspectives toward 1 cm s -1 precision

We present results from the first two years of operations of the HARPS spectrograph installed on the ESO 3.6m telescope at La Silla Observatory, Chile. This instrument, primarily built to detect extrasolar planetary systems, was designed to achieve the highest radial velocity precision ever, thanks to high mechanical and environmental stability, stable illumination, accurate wavelength calibration and tracking of instrumental drifts. HARPS has demonstrated a long-term accuracy at the 1 m s-1 level and below, exploring a new regime in RV precision. We present recent improvements in the wavelength calibration process, including the creation of a new ThAr reference atlas and the use of a much larger number of lines to fit the wavelength solution. We have also investigated the intrinsic stability of ThAr calibration lamps and show that they are able to provide a long-term wavelength reference at or below the 1 m s-1 level. Other instrumental error sources such as guiding accuracy and photon noise are discussed and a global error budget is presented. These efforts to further improve the RV precision are also part of a broader study to build a ultra-high accuracy spectrovelocimeter for the ESO OWL telescope, the CODEX project. The aim of this instrument is to reach an accuracy of 1 cm s-1 over timescales of at least ten years. This requires to push down the limits of present-day calibration techniques and to explore new technologies able to provide ultra-precise Doppler measurements.

The planet search programme at the ESO CES and HARPS - IV. The search for Jupiter analogues around solar-like stars

Context. In 1992 we began a precision radial velocity survey for planets around solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES LC) at the 1.4m telescope in La Silla (Chile) resulting in the discovery of the planet iota Hor b. We have continued the survey with the upgraded CES Very Long Camera (VLC) and the HARPS spectrographs, both at the 3.6m telescope, until 2007. Aims. In this paper we present additional radial velocities for 31 stars of the original sample with higher precision. The observations cover a time span of up to 15 years and permit a search for Jupiter analogues. Methods. The survey was carried out with three different instruments/instrument configurations using the iodine absorption cell and the ThAr methods for wavelength calibration. We combine the data sets and perform a joint analysis for variability, trends, and periodicities. We compute Keplerian orbits for companions and detection limits in case of non-detections. Moreover, the HARPS radial velocities are analysed for correlations with activity indicators (CaII H&K and cross-correlation function shape). Results. We achieve a long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC, 1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), respectively. This enables us to confirm the known planetary signals in iota Hor and HR 506 as well as the three known planets around HR 3259. A steady RV trend for epsilon Ind A can be explained by a planetary companion and calls for direct imaging campaigns. On the other hand, we find previously reported trends to be smaller for beta Hyi and not present for alpha Men. The candidate planet epsilon Eri b was not detected despite our better precision. Also the planet announced for HR 4523 cannot be confirmed. Long-term trends in several of our stars are compatible with known stellar companions. We provide a spectroscopic orbital solution for the binary HR 2400 and refined solutions for the planets around HR 506 and iota Hor. For some other stars the variations could be attributed to stellar activity, as e.g. the magnetic cycle in the case of HR 8323. Conclusions. The occurrence of two Jupiter-mass planets in our sample is in line with the estimate of 10% for the frequency of giant planets with periods smaller than 10 yr around solar-like stars. We have not detected a Jupiter analogue, while the detections limits for circular orbits indicate at 5 AU a sensitivity for minimum mass of at least 1M(Jup) (2M(Jup)) for 13% (61%) of the stars. (Less)

VLT/NACO observations of the high-magnetic field radio pulsar PSR J1119-6127

Context. Recent radio observations have unveiled the existence of a number of radio pulsars with spin-down derived magnetic fields in the magnetar range. However, their observational properties appear to be more similar to those of the classical radio pulsars than to the magnetars’s ones. Aims. To shed light on this puzzle we first have to determine whether the spin-down derived magnetic field values for these radio pulsars are indeed representative of the actual neutron star magnetic field or if they are polluted, e.g. by the effects of a torque from a fallback disk. Methods. To investigate this possibility, we have performed deep IR (J,H,Ks bands) observations of one of these high magnetic field radio pulsars (PSR J1119–6127) with the ESO VLT to search for IR emission which can be associated with a disk. Results. No IR emission is detected from the pulsar position down to J ∼ 24, H ∼ 23 and Ks ∼ 22. Conclusions. By comparing our flux upper limits with the predictions of fallback disk models, we have found that we can only exclude the presence of a disk with accretion rate u M > 3 × 10 16 gs −1 . This lower limit cannot rule out the presence of a substantial disk torque on the pulsar, which would then lead to overestimate the value of the magnetic field inferred from P and u P .W e have also compared the upper limit on the IR luminosity of PSR J1119–6127 with the IR luminosities of rotation-powered pulsars and magnetars. We found that, while magnetars are intrinsically more efficient IR emitters than rotation-powered pulsars, possibly because of their higher magnetic field, the relatively low IR emission efficiency of PSR J1119–6127 suggests that it is more similar to the latters than to the former.

A Fabry-Perot calibrator of the HARPS radial velocity spectrograph: performance report

The radial velocity (RV) technique has pushed the planet detection limits down to super-earths. To reach the precision required to detect earth-like planets it is necessary to reach a precision around 1cm.s-1. Part of the error budget is due to noise in the wavelength calibration of the spectrograph. The Observatory of Geneva has designed, built and tested in collaboration with ESO a calibrator system based on a Fabry-Perot interferometer to explore its potential to improve the wavelength calibration of RV spectrographs. We have obtained exciting results with the calibrator system demonstrated 10 cm s-1 stability over one night. By further improving the injection system we are aiming at a 1 m s-1 repeatability over the long term.

Achieving a few cm/sec calibration repeatability for high resolution spectrographs: the laser frequency comb on HARPS

The laser frequency comb, with its extreme precision, opens a new window for high precision spectroscopy for current facilities, as well as for the ELTs. We report on the latest performance of the laser frequency comb obtained in combination with the HARPS spectrograph, which allowed calibration with cm/sec repeatability. The laser frequency comb system developed is described. Details of its laboratory set-up, characterization and integration with HARPS are shown. The results of the recent test campaigns are presented, showing excellent performance in terms of repeatability as well as wavelength coverage. Preliminary on sky data and next activities to integrate such a system in HARPS are presented.

The performance of the new Fabry-Perot calibration system of the radial velocity spectrograph HARPS

The Observatory of Geneva has designed, built and tested in collaboration with ESO a calibrator system based on a Fabry-Perot (FP) interferometer to explore its potential in the calibration of radial velocity (RV) spectrographs. Today, the RV technique has pushed the planet detection limits down to super-earths but the reach the precision required to detect earth-like planets it is necessary to reach a precision around 1cm s-1. While a significant part of the error budget is the incompressible photon noise, another part is the noise in the wavelength calibration of the spectrograph. It is to address this problem that we have developed this new device. We have obtained exciting results with the calibrator system demonstrated 10 cm s-1 stability over one night and 1 m s-1 over 60 days. By further improving the injection system we are aiming at a 1 m s-1 repeatability over the long term.

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.