A robust, template-free approach to precise radial velocity extraction

Rajpaul, Vinesh; Aigrain, S.; Buchhave, Lars A.

doi:10.1093/mnras/stz3599

Cited by 42 publications

(46 citation statements)

References 79 publications

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“…A methodology for correcting for these features is to consider spectral time series instead of individual spectra, as the former contain a richness of information far more extended than the latter. For example, this methodology shows its potential in the WOBBLE code (Bedell et al 2019), which is designed to remove telluric lines by performing a principal component analysis (PCA), or in the Gaussian process (GP) framework developed by Rajpaul et al (2020).…”

Section: Introductionmentioning

confidence: 99%

YARARA: Significant improvement in RV precision through post-processing of spectral time series

Cretignier

Dumusque

Hara

et al. 2021

A&A

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Aims. Even the most precise radial-velocity instruments gather high-resolution spectra that present systematic errors that a data reduction pipeline cannot identify and correct for efficiently by simply analysing a set of calibrations and a single science frame. In this paper we aim at improving the radial-velocity precision of HARPS measurements by 'cleaning' individual extracted spectra using the wealth of information contained in spectral time series. Methods. We developed YARARA, a post-processing pipeline designed to clean high-resolution spectra of instrumental systematics and atmospheric contamination. Spectra are corrected for: tellurics, interference patterns, detector stitching, ghosts, and fibre B contaminations, as well as more advanced spectral line-by-line corrections. YARARA uses principal component analysis on spectral time series with prior information to disentangle contaminations from real Doppler shifts. We applied YARARA to three systems, HD10700, HD215152, and HD10180, and compared our results to the standard HARPS data reduction software and the SERVAL post-processing pipeline.Results. We ran YARARA on the radial-velocity dataset of three stars intensively observed with HARPS: HD10700, HD215152, and HD10180. For HD10700, we show that YARARA enables us to obtain radial-velocity measurements that present an rms smaller than 1 m s −1 over the 13 years of the HARPS observations, which is 20% and 10% better than the HARPS data reduction software and the SERVAL post-processing pipeline, respectively. We also injected simulated planets into the data of HD10700 and demonstrated that YARARA does not alter pure Doppler-shifted signals. For HD215152, we demonstrated that the 1-year signal visible in the periodogram becomes marginal after processing with YARARA and that the signals of the known planets become more significant. Finally, for HD10180, the six known exoplanets are well recovered, although different orbital parameters and planetary masses are provided by the new reduced spectra. Conclusions. The post-processing correction of spectra using spectral time series allows the radial-velocity precision of HARPS data to be significantly improved and demonstrates that for the extremely quiet star HD10700 a radial-velocity rms better than 1 m/s can be reached over the 13 years of HARPS observations. Since the processing proposed in this paper does not absorb planetary signals, its application to intensively followed systems is promising and will certainly result in advances in the detections of the lightest exoplanets.

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Section: Introductionmentioning

confidence: 99%

YARARA: Significant improvement in RV precision through post-processing of spectral time series

Cretignier

Dumusque

Hara

et al. 2021

A&A

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“…Different methods consisting in a least-squares matching of a full spectrum template to the observations (Anglada-Escudé & Butler 2012;Astudillo-Defru et al 2015;Zechmeister et al 2018), rather than only using a selected set of absorption lines, have proven to work better for M-dwarf stars since they use more Doppler information contained in the spectrum. Recently, different methods have been proposed with the goal of improving RV extraction, such as using individual absorption lines (Dumusque 2018) or approaches using Gaussian processes (Rajpaul et al 2020).…”

Section: Introductionmentioning

confidence: 99%

The CARMENES search for exoplanets around M dwarfs

Lafarga

Ribas

Lovis

et al. 2020

A&A

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Context. For years, the standard procedure to measure radial velocities (RVs) of spectral observations consisted in cross-correlating the spectra with a binary mask, that is, a simple stellar template that contains information on the position and strength of stellar absorption lines. The cross-correlation function (CCF) profiles also provide several indicators of stellar activity. Aims. We present a methodology to first build weighted binary masks and, second, to compute the CCF of spectral observations with these masks from which we derive radial velocities and activity indicators. These methods are implemented in a python code that is publicly available. Methods. To build the masks, we selected a large number of sharp absorption lines based on the profile of the minima present in high signal-to-noise ratio (S/N) spectrum templates built from observations of reference stars. We computed the CCFs of observed spectra and derived RVs and the following three standard activity indicators: full-width-at-half-maximum as well as contrast and bisector inverse slope. Results. We applied our methodology to CARMENES high-resolution spectra and obtain RV and activity indicator time series of more than 300 M dwarf stars observed for the main CARMENES survey. Compared with the standard CARMENES template matching pipeline, in general we obtain more precise RVs in the cases where the template used in the standard pipeline did not have enough S/N. We also show the behaviour of the three activity indicators for the active star YZ CMi and estimate the absolute RV of the M dwarfs analysed using the CCF RVs.

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“…The second method we used to extract RVs was the GP-based method presented in Rajpaul et al (2020). In brief, GPs are used to model and align all pairs of spectra with each other; the pairwise RVs thus obtained are combined to produce differential stellar RVs, without constructing any template.…”

Section: Pairwise Gp Extractionmentioning

confidence: 99%

“…To avoid confusion from assigning multiple meanings to the Greek letter sigma, in this table we use 'SD' to denote the standard deviation of a given set of measurements, and 'error' to refer to the estimated 1σ error bars on measurements. Rajpaul et al (2020) showed that even a relatively crude implementation of the PWGP method, applied to an inactive star (where only modest if any improvements may have been expected compared to the DRS), resulted in RVs with precisions comparable to and rms scatter about 30 per cent lower than RVs extracted by the DRS and two other commonly-used codes, viz. HARPS-TERRA (Anglada-Escudé & Butler 2012) and SERVAL (Zechmeister et al 2018).…”

Section: Pairwise Gp Extractionmentioning

confidence: 99%

“…More recently, though, there has been a rapidly growing number of efforts to develop improved ways of extracting RVs from stellar spectra, particularly with a view to mitigating stellar activity and/or telluric contamination from the final RVs (e.g. Anglada-Escudé & Butler 2012;Dumusque 2018;Zechmeister et al 2018;Bedell et al 2019;Collier Cameron et al 2020;de Beurs et al 2020;Rajpaul, Aigrain & Buchhave 2020;Zhao & Tinney 2020). These techniques are often data-driven, exploiting the vast quantity of wavelengthand/or time-dependent information in stellar spectra -each typically containing hundreds of thousands of flux/wavelength pairs -to extract 'cleaner' RVs (i.e.…”

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confidence: 99%

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A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals

Rajpaul

Buchhave

Lacedelli

et al. 2021

Monthly Notices of the Royal Astronomical Society

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To date, only 18 exoplanets with radial velocity (RV) semi-amplitude <2 m s−1 have had their masses directly constrained. The biggest obstacle to RV detection of such exoplanets is variability intrinsic to stars themselves, e.g. nuisance signals arising from surface magnetic activity such as rotating spots and plages, which can drown out or even mimic planetary RV signals. We use Kepler-37 – known to host three transiting planets, one of which, Kepler-37d, should be on the cusp of RV detectability with modern spectrographs – as a case study in disentangling planetary and stellar activity signals. We show how two different statistical techniques – one seeking to identify activity signals in stellar spectra, and another to model activity signals in extracted RVs and activity indicators – can each enable a detection of the hitherto elusive Kepler-37d. Moreover, we show that these two approaches can be complementary, and in combination, facilitate a definitive detection and precise characterisation of Kepler-37d. Its RV semi-amplitude of 1.22 ± 0.31 m s−1 (mass 5.4 ± 1.4 M⊕) is formally consistent with TOI-178b’s $1.05^{+0.25}_{-0.30}$ m s−1, the latter being the smallest detected RV signal of any transiting planet to date, though dynamical simulations suggest Kepler-37d’s mass may be on the lower end of our 1σ credible interval. Its consequent density is consistent with either a water-world or that of a gaseous envelope ($\sim 0.4\%$ by mass) surrounding a rocky core. Based on RV modelling and a re-analysis of Kepler-37 TTVs, we also suggest that the putative (non-transiting) planet Kepler-37e should be stripped of its ‘confirmed’ status.

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A robust, template-free approach to precise radial velocity extraction

Cited by 42 publications

References 79 publications

YARARA: Significant improvement in RV precision through post-processing of spectral time series

YARARA: Significant improvement in RV precision through post-processing of spectral time series

The CARMENES search for exoplanets around M dwarfs

A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals

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