Fundamental physics with ESPRESSO: Towards an accurate wavelength calibration for a precision test of the fine-structure constant

Abstract

Observations of metal absorption systems in the spectra of distant quasars allow one to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable highresolution spectrograph Espresso, which was recently installed at the VLT. In preparation of the fundamental physics related part of the Espresso GTO program, we present a thorough assessment of the Espresso wavelength accuracy and identify possible systematics at each of the different steps involved in the wavelength calibration process. Most importantly, we compare the default wavelength solution, which is based on the combination of Thorium-Argon arc lamp spectra and a Fabry-Pérot interferometer, to the fully independent calibration obtained from a laser frequency comb. We find wavelength-dependent discrepancies of up to 24 m/s. This substantially exceeds the photon noise and highlights the presence of different sources of systematics, which we characterize in detail as part of this study. Nevertheless, our study demonstrates the outstanding accuracy of Espresso with respect to previously used spectrographs and we show that constraints of a relative change of the finestructure constant at the 10 −6 level can be obtained with Espresso without being limited by wavelength calibration systematics.

Non-detection of $^6$Li in Spite plateau stars with ESPRESSO

“…A similarly larger error than expected is found on radial velocities by González Hernández et al (2020). On the other hand, this uncertainty is only 0.06 m s −1 for K. In addition to the uncertainties involved in determining the transition wavelengths for Fe, the wavelength solution exhibits systematic errors of the order of 20 m s −1 (Schmidt et al 2021) which is comparable to our fitting error. As the uncertainties in radial velocity are driving the uncertainties in 6 Li/ 7 Li, we strongly encourage an improvement in the reference Fe lines to enable better radial velocity measurements using Fe lines.…”

“…The photon-limited radial velocity precision for stellar spectra that are rich in sharp spectral lines approaches 0.1 m s −1 thanks to the combined use of a ThAr lamp and Fabry-Perót interferometer that cover the entire optical wavelength range, and the temperature and pressure stabilisation of the instrument (Pepe et al 2021). We stress however that this internal precision is different from the accuracy of the instrument, which is better characterised by the accuracy of the wavelength solution at the level of 20 m s −1 with significant dependence on wavelength (Schmidt et al 2021).…”

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

“…Considerable effort has gone into wavelength calibration of the new ESPRESSO spectrograph (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) on the VLT [1591][1592][1593]. In particular, laser frequency combs (LFCs) and Fabry-Pérot devices produce ESPRESSO calibrations down to an accuracy of 24 m/s [1594]. In principle the accuracy should be somewhat better than this value (and indeed 1 limit of 1-2 m/s has been reached for the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument).…”

Varying fundamental constants and dark energy in the ESPRESSO era

“…As for measurements in the optical, the arrival of the ESPRESSO spectrograph 8 , operating at the combined Coudé focus of the VLT, enables new and more stringent tests. Preliminary analyses already demonstrate that the dominant sources of systematics limiting previous high resolution spectrographs are not present 9 , and the first results of its measurements of the fine-structure constant should be reported soon.…”