Revisiting Proxima with ESPRESSO

Abstract

Context. The discovery of Proxima b marked one of the most important milestones in exoplanetary science in recent years. Yet the limited precision of the available radial velocity data and the difficulty in modelling the stellar activity calls for a confirmation of the Earth-mass planet. Aims. We aim to confirm the presence of Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refine the planetary parameters taking advantage of its improved precision. Methods. We analysed 63 spectroscopic ESPRESSO observations of Proxima (Gl 551) taken during 2019. We obtained radial velocity measurements with a typical radial velocity photon noise of 26 cm s−1. We combined these data with archival spectroscopic observations and newly obtained photometric measurements to model the stellar activity signals and disentangle them from planetary signals in the radial velocity (RV) data. We ran a joint Markov chain Monte Carlo analysis on the time series of the RV and full width half maximum of the cross-correlation function to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with the stellar activity signals. Results. We confirm the presence of Proxima b independently in the ESPRESSO data and in the combined ESPRESSO+ HARPS+UVES dataset. The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 M⊕. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 M⊕. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of only 40 cm s−1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕. We find that forthe case of Proxima, the full width half maximum of the cross-correlation function can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the RV data from part of the influence of stellar activity. The activity-induced RV signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot dominated. This could be used to create differential RVs that are activity dominated and can be used to disentangle activity-induced and planetary-induced signals. The data collected excludes the presence of extra companions with masses above 0.6 M⊕ at periods shorter than 50 days.

The HARPS search for southern extra-solar planets – XLV. Two Neptune mass planets orbiting HD 13808: a study of stellar activity modelling’s impact on planet detection

“…It has been argued that CCF FWHM and log HK measurements should both behave, to first order, as the convective blueshift suppression term in the method, which in turn is a close proxy for the integrated active region coverage on the visible stellar hemisphere (Rajpaul et al 2015); indeed, it has been shown in practice that the CCF FWHM can be a good tracer of photometric flux (e.g. Suárez Mascareño et al 2020). Thus, as we did not have photometric measurements of HD 13808 available, we chose to interpret the FWHM as a proxy for the flux Ψ( ).…”

Detection Limits of Low-mass, Long-period Exoplanets Using Gaussian Processes Applied to HARPS-N Solar Radial Velocities

“…Successful exo-Earth discovery therefore requires both more sophisticated models of stellar variability and the improved RV precision, long-term stability, and dense observational sampling from a next-generation spectrograph (Wright & Robertson 2017) such as the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO; Pepe et al 2013Pepe et al , 2021Damasso et al 2020;Suárez Mascareño et al 2020), NEID (Allen et al 2018), EXtreme PREcision Spectrometer (EXPRES; Jurgenson et al 2016;Blackman et al 2020;Brewer et al 2020;Petersburg et al 2020), HARPS3 (Thompson et al 2016), or GMT Consortium Large Earth Finder (G-CLEF; Szentgyorgyi et al 2014).…”

K2-111: an old system with two planets in near-resonance†

“…With radial velocity (RV) signatures of the smallest planets having semi-amplitudes of meters per second, or lower, high resolution stable spectrographs, such as HARPS (Mayor et al 2003) and HARPS-N (Cosentino et al 2012) have been essential to further characterise these systems, precisely determining planetary masses and measuring the physical and chemical properties of the host stars. Newer-generation spectrographs, like ESPRESSO (Pepe et al 2014(Pepe et al , 2020, will deliver long-term RV precision of 10 cm/s, a factor of at least 5 better than HARPS-N, and are thus expected to perform even better (as shown recently in Suárez Mascareño et al 2020).…”

“…similar wavelength ranges as the HARPS-N chromatic RVs (except a longer range for the red RVS thanks to the extended wavelength range of ESPRESSO). Details on the extraction can be found in Suárez Mascareño et al (2020). The HARPS-N and ESPRESSO 1 RV time series overlap between October 2018 and February 2019.…”