Context. Stellar signals are the main limitation for precise radial-velocity (RV) measurements. These signals arise from the photosphere of the stars. The ms super(-1)perturbation created by these ...signals prevents the detection and mass characterization of small-mass planetary candidates such as Earth-twins. Several methods have been proposed to mitigate stellar signals in RV measurements. However, without precisely knowing the stellar and planetary signals in real observations, it is extremely difficult to test the efficiency of these methods. Aims. The goal of the RV fitting challenge is to generate simulated RV data including stellar and planetary signals and to perform a blind test within the community to test the efficiency of the different methods proposed to recover planetary signals despite stellar signals. Methods. In this first paper, we describe the simulation used to model the measurements of the RV fitting challenge. Each simulated planetary system includes the signals from instrumental noise, stellar oscillations, granulation, supergranulation, stellar activity, and observed and simulated planetary systems. In addition to RV variations, this simulation also models the effects of instrumental noise and stellar signals on activity observables obtained by HARPS-type high-resolution spectrographs, that is, the calcium activity index log(R'HK)and the bisector span and full width at half maximum of the cross-correlation function. Results. We publish the 15 systems used for the RV fitting challenge including the details about the planetary systems that were injected into each of them.
Stellar inclination is an important parameter for many astrophysical studies. Although different techniques allow us to estimate stellar inclination for fast rotators, it becomes much more difficult ...when stars are rotating slower than ~2-2.5 km s super(-1). By using the new activity simulation SOAP 2.0 which can reproduce the photometric and spectroscopic variations induced by stellar activity, we are able to fit observations of solar-type stars and derive their inclination. For HD 189733, we estimate the stellar inclination to be i = 84 super(+6) sub(-20) deg, which implies a star-planet obliquity of psi = 4+ super(18) sub(-4) considering previous measurements of the spin-orbit angle. For alpha Cen B, we derive an inclination of i = 45 super(+9) sub(-19), which implies that the rotational spin of the star is not aligned with the orbital spin of the alpha Cen binary system. In addition, assuming that alpha Cen Bb is aligned with its host star, no transit would occur. The inclination of alpha Cen B can be measured using 40 radial-velocity measurements, which is remarkable given that the projected rotational velocity of the star is smaller than 1.15 km s super(-1).
This paper presents SOAP 2.0, a new version of the Spot Oscillation And Planet (SOAP) code that estimates in a simple way the photometric and radial velocity (RV) variations induced by active ...regions. The inhibition of the convective blueshift (CB) inside active regions is considered, as well as the limb brightening effect of plages, a quadratic limb darkening law, and a realistic spot and plage contrast ratio. SOAP 2.0 shows that the activity-induced variation of plages is dominated by the inhibition of the CB effect. For spots, this effect becomes significant only for slow rotators. In addition, in the case of a major active region dominating the activity-induced signal, the ratio between the FWHM and the RV peak-to-peak amplitudes of the cross correlation function can be used to infer the type of active region responsible for the signal for stars with v sin i < or =, slant 8 kms super(-1). A ratio smaller than three implies a spot, while a larger ratio implies a plage. Using the observation of HD 189733, we show that SOAP 2.0 manages to reproduce the activity variation as well as previous simulations when a spot is dominating the activity-induced variation. In addition, SOAP 2.0 also reproduces the activity variation induced by a plage on the slowly rotating star alpha Cen B, which is not possible using previous simulations. Following these results, SOAP 2.0 can be used to estimate the signal induced by spots and plages, but also to correct for it when a major active region is dominating the RV variation.
Context.
Stellar activity and instrumental signals are the main limitations to the detection of Earth-like planets using the radial-velocity (RV) technique. Recent studies show that the key to ...mitigating those perturbing effects might reside in analysing the spectra themselves, rather than the RV time series and a few activity proxies.
Aims.
The goal of this paper is to demonstrate that we can reach further improvement in RV precision by performing a principal component analysis (PCA) decomposition of the shell time series, with the shell as the projection of a spectrum onto the space-normalised flux versus flux gradient.
Methods.
By performing a PCA decomposition of shell time series, it is possible to obtain a basis of first-order spectral variations that are not related to Keplerian motion. The time coefficients associated with this basis can then be used to correct for non-Dopplerian signatures in RVs.
Results.
We applied this new method on the YARARA post-processed spectra time series of HD 10700 (
τ
Ceti) and HD 128621 (
α
Cen B). On HD 10700, we demonstrate, thanks to planetary signal injections, that this new approach can successfully disentangle real Dopplerian signals from instrumental systematics. The application of this new methodology on HD 128621 shows that the strong stellar activity signal seen at the stellar rotational period and one-year aliases becomes insignificant in a periodogram analysis. The RV root mean square on the 5-yr data is reduced from 2.44 m s
−1
down to 1.73 m s
−1
. This new approach allows us to strongly mitigate stellar activity, however, noise injections tests indicate that rather high signal-to-noise ratio (S/N > 250) is required to correct for the observed activity signal on HD 128621.
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, HD 10700, HD 215152, and HD 10180, 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: HD 10700, HD 215152, and HD 10180. For HD 10700, 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 HD 10700 and demonstrated that YARARA does not alter pure Doppler-shifted signals. For HD 215152, 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 HD 10180, 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 HD 10700 a radial-velocity rms better than 1 m s
−1
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.
The 55 Cancri system reassessed Bourrier, V.; Dumusque, X.; Dorn, C. ...
Astronomy and astrophysics (Berlin),
11/2018, Letnik:
619
Journal Article
Recenzirano
Odprti dostop
Orbiting a bright, nearby star the 55 Cnc system offers a rare opportunity to study a multiplanet system that has a wide range of planetary masses and orbital distances. Using two decades of ...photometry and spectroscopy data, we have measured the rotation of the host star and its solar-like magnetic cycle. Accounting for this cycle in our velocimetric analysis of the system allows us to revise the properties of the outermost giant planet and its four planetary companions. The innermost planet 55 Cnc e is an unusually close-in super-Earth, whose transits have allowed for detailed follow-up studies. Recent observations favor the presence of a substantial atmosphere yet its composition, and the nature of the planet, remain unknown. We combined our derived planet mass (Mp = 8.0 ± 0.3 MEarth) with refined measurement of its optical radius derived from HST/STIS observations (Rp = 1.88 ± 0.03 REarth over 530–750 nm) to revise the density of 55 Cnc e (ρ = 6.7 ± 0.4 g cm−3). Based on these revised properties we have characterized possible interiors of 55 Cnc e using a generalized Bayesian model. We confirm that the planet is likely surrounded by a heavyweight atmosphere, contributing a few percents of the planet radius. While we cannot exclude the presence of a water layer underneath the atmosphere, this scenario is unlikely given the observations of the planet across the entire spectrum and its strong irradiation. Follow-up observations of the system in photometry and in spectroscopy over different time-scales are needed to further investigate the nature and origin of this iconic super-Earth.
Context. Doppler spectroscopy has been used in astronomy for more than 150 yr. In particular, it has permitted us to detect hundreds of exoplanets over the past 20 yr, and the goal today of detecting ...Earth-like planets requires a precision around 0.1 m s−1 or better. Doppler spectroscopy has also been and will be of major importance for other studies such as the variability of fundamental constants and cosmological studies. For all these applications, it is crucial to have the best possible wavelength calibration. Despite the fact that the HARPS spectrograph has been operational at the 3.6-m ESO telescope for more than 15 yr, and that it provides among the most precise Doppler measurements, improvements are still possible. One known problem, for instance, is the non-fully regular block-stitching of the charge-coupled devices (CCDs), which in some cases introduces one-year period parasitic signals in the measured radial velocity. Aims. The aim of the presented work is to improve the wavelength calibration of the HARPS spectrograph to push further its planet-detection capabilities. Methods. The properties of the CCD stitching-induced pixel-size anomalies were determined with light-emitting-diode (LED) flat-field frames, and then a physical, gap-corrected map of the CCDs is used for the fitting model of the spectral orders. We also used a new thorium line list, based on much higher-accuracy measurements than the one used up to now. We derive new wavelength solutions for the 15 yr of HARPS data, both before and after the fibre upgrade that took place in 2015. Results. We demonstrate that we do indeed correct the gap anomalies by computing the wavelength solutions of laser frequency comb exposures, known to have a very low dispersion, both with and without taking the gap correction into account. By comparing the rms of the most stable stars of the HARPS sample, we show that we globally decrease the radial velocity (RV) dispersion of the data, especially for the data acquired after the change of fibres of 2015. Finally, the comparative analysis of several individual systems shows that we manage to attenuate the periodogram power at one year in most cases. The analysis of the RVs derived from individual stellar lines also shows that we indeed correct the stitching-induced RV variation. Conclusions. This improved calibration of the HARPS spectrograph allows to go deeper in the search for low-amplitude radial-velocity signals. This new calibration process will be further improved by combining the thorium calibration spectra with laser frequency comb and Fabry–Perot calibration spectra, and this will not only be used for HARPS but notably also for HARPS-N and the new ESPRESSO spectrograph.
Context.
The detection of Earth-like planets with the radial velocity (RV) method is extremely challenging today due to the presence of non-Doppler signatures such as stellar activity and ...instrumental signals that mimic and hide the signals of exoplanets. In a previous paper, we presented the YARARA pipeline, which implements corrections for telluric absorption, stellar activity, and instrumental systematics at the spectral level, and then it extracts line-by-line (LBL) RVs with a significantly better precision than standard pipelines.
Aims.
In this paper, we demonstrate that further gains in RV precision can be achieved by performing principal component analysis (PCA) decomposition on the LBL RVs.
Methods.
The mean-insensitive nature of PCA means that it is unaffected by true Doppler shifts, and thus can be used to isolate and correct nuisance signals other than planets.
Results.
We analysed the data of 20 intensively observed HARPS targets by applying our PCA approach on the LBL RVs obtained by YARARA. The first principal components show similarities across most of the stars and correspond to newly identified instrumental systematics for which we can now correct. For several targets, this results in an unprecedented RV root-mean-square of around 90 cm s
−1
over the full lifetime of HARPS. We used the corrected RVs to confirm a previously published 120-day signal around 61 Vir, and to detect a super-Earth candidate (
K
~ 60 ± 6 cm s
−1
,
m
sin
i
= 6.6 ± 0.7
M
⊕
) around the G6V star HD 20794, which spends part of its 600-day orbit within the habitable zone of the host star.
Conclusions.
This study highlights the potential of LBL PCA to identify and correct hitherto unknown, long-term instrumental effects and thereby extend the sensitivity of existing and future instruments towards the Earth analogue regime.
Abstract
The time-variable velocity fields of solar-type stars limit the precision of radial-velocity determinations of their planets’ masses, obstructing detection of Earth twins. Since 2015 July, ...we have been monitoring disc-integrated sunlight in daytime using a purpose-built solar telescope and fibre feed to the HARPS-N stellar radial-velocity spectrometer. We present and analyse the solar radial-velocity measurements and cross-correlation function (CCF) parameters obtained in the first 3 yr of observation, interpreting them in the context of spatially resolved solar observations. We describe a Bayesian mixture-model approach to automated data-quality monitoring. We provide dynamical and daily differential-extinction corrections to place the radial velocities in the heliocentric reference frame, and the CCF shape parameters in the sidereal frame. We achieve a photon-noise-limited radial-velocity precision better than 0.43 m s−1 per 5-min observation. The day-to-day precision is limited by zero-point calibration uncertainty with an RMS scatter of about 0.4 m s−1. We find significant signals from granulation and solar activity. Within a day, granulation noise dominates, with an amplitude of about 0.4 m s−1 and an autocorrelation half-life of 15 min. On longer time-scales, activity dominates. Sunspot groups broaden the CCF as they cross the solar disc. Facular regions temporarily reduce the intrinsic asymmetry of the CCF. The radial-velocity increase that accompanies an active-region passage has a typical amplitude of 5 m s−1 and is correlated with the line asymmetry, but leads it by 3 d. Spectral line-shape variability thus shows promise as a proxy for recovering the true radial velocity.
ABSTRACT
Stellar magnetic activity produces time-varying distortions in the photospheric line profiles of solar-type stars. These lead to systematic errors in high-precision radial-velocity ...measurements, which limit efforts to discover and measure the masses of low-mass exoplanets with orbital periods of more than a few tens of days. We present a new data-driven method for separating Doppler shifts of dynamical origin from apparent velocity variations arising from variability-induced changes in the stellar spectrum. We show that the autocorrelation function (ACF) of the cross-correlation function used to measure radial velocities is effectively invariant to translation. By projecting the radial velocities on to a subspace labelled by the observation identifiers and spanned by the amplitude coefficients of the ACF’s principal components, we can isolate and subtract velocity perturbations caused by stellar magnetic activity. We test the method on a 5-yr time sequence of 853 daily 15-min observations of the solar spectrum from the HARPS-N instrument and solar-telescope feed on the 3.58-m Telescopio Nazionale Galileo. After removal of the activity signals, the heliocentric solar velocity residuals are found to be Gaussian and nearly uncorrelated. We inject synthetic low-mass planet signals with amplitude K = 40 cm s−1 into the solar observations at a wide range of orbital periods. Projection into the orthogonal complement of the ACF subspace isolates these signals effectively from solar activity signals. Their semi-amplitudes are recovered with a precision of ∼ 6.6 cm s−1, opening the door to Doppler detection and characterization of terrestrial-mass planets around well-observed, bright main-sequence stars across a wide range of orbital periods.
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