Measuring precise radial velocities on individual spectral lines

Abstract

Context. Stellar activity is the main limitation to the detection of an Earth-twin using the radial-velocity (RV) technique. Despite many efforts in trying to mitigate the effect of stellar activity using empirical and statistical techniques, it seems that we are facing an obstacle that will be extremely difficult to overcome using current techniques. Aims. In this paper, we investigate a novel approach to derive precise RVs considering the wealth of information present in high-resolution spectra. Methods. This new method consists of building a master spectrum from all available observations and measure the RVs of each individual spectral line in a spectrum relative to this master. When analysing several spectra, the final product of this approach is the RVs of each individual line as a function of time. Results. We demonstrate on three stars intensively observed with HARPS that our new method gives RVs that are extremely similar to the one derived from the HARPS data reduction software. Our new approach to derive RVs demonstrates that the non-stability of daily HARPS wavelength solution induces night-to-night RV offsets with an standard deviation of 0.4 m s−1, and we propose a solution to correct for this systematic. Finally, and this is probably the most astrophysically relevant result of this paper, we demonstrate that some spectral lines are strongly affected by stellar activity while others are not. By measuring the RVs on two carefully selected subsample of spectral lines, we demonstrate that we can boost by a factor of two or mitigate by a factor of 1.6 the red noise induced by stellar activity in the 2010 RV measurements of α Cen B. Conclusions. By measuring the RVs of each spectral line, we are able to reach the same RV precision as other approved techniques. In addition, this new approach allows us to demonstrate that each spectral line is differently affected by stellar activity. Preliminary results show that studying in details the behaviour of each spectral line is probably the key to overcome the obstacle of stellar activity.

Testing the Spectroscopic Extraction of Suppression of Convective Blueshift

“…Additional results in the literature have shown that other processes besides RV conv may indeed play a dominant role near the solar minimum. For example, recent techniques demonstrate the ability to remove most power at the rotation period, but leave 1 m s −1 RV variability in corrected timeseries: Dumusque (2018) and Cretingier et al (2019, in revision) identify spectral lines insensitive to the suppression of convective blueshift; by computing RVs using these specially-selected line lists, the authors are able to reduce the RV RMS by a factor of 2.2, down to 0.9 m s −1 . Independently, Milbourne et al (2019) use solar images from HMI/SDO to reproduce the activity-driven RVs.…”

Public HARPS radial velocity database corrected for systematic errors

“…Eventually, we have selected a total of ∼ 3 000 reliable targets of F, G, K, M and L spectral types, to use for our subsequent NZP analysis. Figure 1 shows a Hertzsprung-Russel (HR) diagram of the HARPS sample stars over-plotted on top of the known stars within 100 pc, retrieved from the Gaia DR2 catalog (Gaia Collaboration et al 2016, 2018. During its operational time, HARPS observed mainly bright nearby main-sequence stars and late-type G and K giant stars.…”

An Extreme-precision Radial-velocity Pipeline: First Radial Velocities from EXPRES

“…In addition to our CCF RV solution, we have developed a new forward-modeling technique by adapting algorithms developed for the iodine RV technique (Marcy & Butler 1992;Butler et al 1996) as well as ideas from the "line-by-line" method developed by Dumusque (2018). Forward-modelling from empirical stellar spectral templates is known to produce velocities with less statistical scatter than the CCF method, and typically measures relative rather than absolute radial velocities (Anglada-Escudé & Butler 2012).…”