Search for exoplanets with the radial-velocity technique: quantitative diagnostics of stellar activity

AbstractA new apodized Keplerian model is proposed for the analysis of precision radial velocity (RV) data to model both planetary and stellar activity (SA) induced RV signals. A symmetrical Gaussian apodization function with unknown width and center can distinguish planetary signals from SA signals on the basis of the width of the apodization function. The general model for m apodized Keplerian signals also includes a linear regression term between RV and the stellar activity diagnostic In (R'hk), as well as an extra Gaussian noise term with unknown standard deviation. The model parameters are explored using a Bayesian fusion MCMC code. A differential version of the Generalized Lomb-Scargle periodogram provides an additional way of distinguishing SA signals and helps guide the choice of new periods. Sample results are reported for a recent international RV blind challenge which included multiple state of the art simulated data sets supported by a variety of stellar activity diagnostics.

Download Full-text

Dealing with activity in RV planet searches

Proceedings of the International Astronomical Union ◽

10.1017/s174392131600274x ◽

2015 ◽

Vol 11 (A29A) ◽

pp. 193-195

Author(s):

Isabelle Boisse

Keyword(s):

Time Series ◽

Radial Velocity ◽

Radial Velocities ◽

Velocity Measurements ◽

Stellar Activity ◽

The Future

AbstractPrecise radial velocity measurements of a star allow to search for planets. But this method has to face with irregularly time series. Stellar variabilities: pulsation, granulation, stellar activity on a short and long timescale, also modify the measure of the radial velocities. There is indeed a growing literature of controversies on how a signal is interpreted as a planet or due to stellar activity. I present how the star variations change the measured RVs, which techniques and indices are used by several teams to disentangle activity and planets, and the future options that are being studied.

Download Full-text

The GAPS Programme with HARPS-N at TNG

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731010 ◽

2018 ◽

Vol 616 ◽

pp. A155 ◽

Cited By ~ 12

Author(s):

A. F. Lanza ◽

L. Malavolta ◽

S. Benatti ◽

S. Desidera ◽

A. Bignamini ◽

...

Keyword(s):

Radial Velocity ◽

Cross Correlation ◽

Main Sequence ◽

Activity Level ◽

P Value ◽

Main Sequence Stars ◽

Stellar Spectrum ◽

Stellar Activity ◽

Binary Weighted ◽

Low Activity

Aims. Stellar activity is the ultimate source of radial-velocity (hereinafter RV) noise in the search for Earth-mass planets orbiting late-type main-sequence stars. We analyse the performance of four different indicators and the chromospheric index log R′HK in detecting RV variations induced by stellar activity in 15 slowly rotating (υ sin i ≤ 5 km s−1), weakly active (log R′HK ≤ −4.95) solar-like stars observed with the high-resolution spectrograph High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N). Methods. We consider indicators of the asymmetry of the cross-correlation function (CCF) between the stellar spectrum and the binary weighted line mask used to compute the RV, that is the bisector inverse span (BIS), ΔV, and a new indicator Vasy(mod) together with the full width at half maximum (FWHM) of the CCF. We present methods to evaluate the uncertainties of the CCF indicators and apply a kernel regression (KR) between the RV, the time, and each of the indicators to study their capability of reproducing the RV variations induced by stellar activity. Results. The considered indicators together with the KR prove to be useful to detect activity-induced RV variations in ~47 ± 18 percent of the stars over a two-year time span when a significance (two-sided p-value) threshold of one percent is adopted. In those cases, KR reduces the standard deviation of the RV time series by a factor of approximately two. The BIS, the FWHM, and the newly introduced Vasy(mod) are the best indicators, being useful in 27 ± 13, 13 ± 9, and 13 ± 9 percent of the cases, respectively. The relatively limited performances of the activity indicators are related to the very low activity level and υ sin i of the considered stars. For the application of our approach to sun-like stars, a spectral resolution allowing λ/Δλ ≥ 105 and highly stabilized spectrographs are recommended.

Download Full-text

Disentangling planetary orbits from stellar activity in radial-velocity surveys

International Journal of Astrobiology ◽

10.1017/s147355041300044x ◽

2014 ◽

Vol 13 (2) ◽

pp. 155-157 ◽

Cited By ~ 6

Author(s):

R. D. Haywood ◽

A. Collier Cameron ◽

D. Queloz ◽

S.C.C. Barros ◽

M. Deleuil ◽

...

Keyword(s):

Radial Velocity ◽

High Accuracy ◽

Stellar Surface ◽

Stellar Activity ◽

The Earth ◽

Planetary Orbits ◽

Low Mass ◽

Do So

AbstractThe majority of extra-solar planets have been discovered (or confirmed after follow-up) through radial-velocity (RV) surveys. Using ground-based spectrographs such as High Accuracy Radial Velocity Planetary Search (HARPS) and HARPS-North, it is now possible to detect planets that are only a few times the mass of the Earth. However, the presence of dark spots on the stellar surface produces RV signals that are very similar in amplitude to those caused by orbiting low-mass planets. Disentangling these signals has thus become the biggest challenge in the detection of Earth-mass planets using RV surveys. To do so, we use the star's lightcurve to model the RV variations produced by spots. Here we present this method and show the results of its application to CoRoT-7.

Download Full-text

The GAPS Programme at TNG

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937369 ◽

2020 ◽

Vol 638 ◽

pp. A5 ◽

Cited By ~ 6

Author(s):

I. Carleo ◽

L. Malavolta ◽

A. F. Lanza ◽

M. Damasso ◽

S. Desidera ◽

...

Keyword(s):

Radial Velocity ◽

Near Infrared ◽

Semimajor Axis ◽

Young Stars ◽

Homogeneous Sample ◽

Stellar Activity ◽

Orbital Parameters ◽

Hot Jupiters ◽

Multi Band ◽

Hot Jupiter

Context. The existence of hot Jupiters is still not well understood. Two main channels are thought to be responsible for their current location: a smooth planet migration through the protoplanetary disk or the circularization of an initial highly eccentric orbit by tidal dissipation leading to a strong decrease in the semimajor axis. Different formation scenarios result in different observable effects, such as orbital parameters (obliquity and eccentricity) or frequency of planets at different stellar ages. Aims. In the context of the GAPS Young Objects project, we are carrying out a radial velocity survey with the aim of searching and characterizing young hot-Jupiter planets. Our purpose is to put constraints on evolutionary models and establish statistical properties, such as the frequency of these planets from a homogeneous sample. Methods. Since young stars are in general magnetically very active, we performed multi-band (visible and near-infrared) spectroscopy with simultaneous GIANO-B + HARPS-N (GIARPS) observing mode at TNG. This helps in dealing with stellar activity and distinguishing the nature of radial velocity variations: stellar activity will introduce a wavelength-dependent radial velocity amplitude, whereas a Keplerian signal is achromatic. As a pilot study, we present here the cases of two known hot Jupiters orbiting young stars: HD 285507 b and AD Leo b. Results. Our analysis of simultaneous high-precision GIARPS spectroscopic data confirms the Keplerian nature of the variation in the HD 285507 radial velocities and refines the orbital parameters of the hot Jupiter, obtaining an eccentricity consistent with a circular orbit. Instead, our analysis does not confirm the signal previously attributed to a planet orbiting AD Leo. This demonstrates the power of the multi-band spectroscopic technique when observing active stars.

Download Full-text

EXPRES. III. Revealing the Stellar Activity Radial Velocity Signature of ϵ Eridani with Photometry and Interferometry

The Astronomical Journal ◽

10.3847/1538-3881/ac3235 ◽

2021 ◽

Vol 163 (1) ◽

pp. 19

Author(s):

Rachael M. Roettenbacher ◽

Samuel H. C. Cabot ◽

Debra A. Fischer ◽

John D. Monnier ◽

Gregory W. Henry ◽

...

Keyword(s):

Radial Velocity ◽

Current Data ◽

Stellar Surface ◽

Inversion Algorithm ◽

Stellar Activity ◽

Activity Component ◽

Beam Combiner ◽

Debris Disk ◽

Flexible Models ◽

High Cadence

Abstract The distortions of absorption line profiles caused by photospheric brightness variations on the surfaces of cool, main-sequence stars can mimic or overwhelm radial velocity (RV) shifts due to the presence of exoplanets. The latest generation of precision RV spectrographs aims to detect velocity amplitudes ≲ 10 cm s−1, but requires mitigation of stellar signals. Statistical techniques are being developed to differentiate between Keplerian and activity-related velocity perturbations. Two important challenges, however, are the interpretability of the stellar activity component as RV models become more sophisticated, and ensuring the lowest-amplitude Keplerian signatures are not inadvertently accounted for in flexible models of stellar activity. For the K2V exoplanet host ϵ Eridani, we separately used ground-based photometry to constrain Gaussian processes for modeling RVs and TESS photometry with a light-curve inversion algorithm to reconstruct the stellar surface. From the reconstructions of TESS photometry, we produced an activity model that reduced the rms scatter in RVs obtained with EXPRES from 4.72 to 1.98 m s−1. We present a pilot study using the CHARA Array and MIRC-X beam combiner to directly image the starspots seen in the TESS photometry. With the limited phase coverage, our spot detections are marginal with current data but a future dedicated observing campaign should allow for imaging, as well as allow the stellar inclination and orientation with respect to the debris disk to be definitively determined. This work shows that stellar surface maps obtained with high-cadence, time-series photometric and interferometric data can provide the constraints needed to accurately reduce RV scatter.

Download Full-text

Revisiting Proxima with ESPRESSO

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037745 ◽

2020 ◽

Vol 639 ◽

pp. A77 ◽

Cited By ~ 6

Author(s):

A. Suárez Mascareño ◽

J. P. Faria ◽

P. Figueira ◽

C. Lovis ◽

M. Damasso ◽

...

Keyword(s):

Correlation Function ◽

Radial Velocity ◽

Cross Correlation ◽

Full Width Half Maximum ◽

Cross Correlation Function ◽

Minimum Mass ◽

Full Width ◽

Half Maximum ◽

Stellar Activity ◽

The Cross

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.

Download Full-text

Exoplanet Imitators: A Test of Stellar Activity Behavior in Radial Velocity Signals

The Astronomical Journal ◽

10.3847/1538-3881/ab53ec ◽

2019 ◽

Vol 159 (1) ◽

pp. 23 ◽

Cited By ~ 11

Author(s):

Chantanelle Nava ◽

Mercedes López-Morales ◽

Raphaëlle D. Haywood ◽

Helen A. C. Giles

Keyword(s):

Radial Velocity ◽

Stellar Activity ◽

Activity Behavior

Download Full-text

Understanding stellar activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731139 ◽

2017 ◽

Vol 606 ◽

pp. A107 ◽

Cited By ~ 13

Author(s):

M. Oshagh ◽

N. C. Santos ◽

P. Figueira ◽

S. C. C. Barros ◽

J.-F. Donati ◽

...

Keyword(s):

Radial Velocity ◽

Stellar Activity

Download Full-text

Multi-band high resolution spectroscopy rules out the hot Jupiter BD+20 1790b

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732350 ◽

2018 ◽

Vol 613 ◽

pp. A50 ◽

Cited By ~ 12

Author(s):

I. Carleo ◽

S. Benatti ◽

A. F. Lanza ◽

R. Gratton ◽

R. Claudi ◽

...

Keyword(s):

Radial Velocity ◽

Hot Spots ◽

Near Infrared ◽

High Resolution Spectroscopy ◽

Velocity Measurements ◽

Stellar Activity ◽

Multi Wavelength ◽

First Time ◽

Multi Band

Context. Stellar activity is currently challenging the detection of young planets via the radial velocity (RV) technique. Aims. We attempt to definitively discriminate the nature of the RV variations for the young active K5 star BD+20 1790, for which visible (VIS) RV measurements show divergent results on the existence of a substellar companion. Methods. We compare VIS data with high precision RVs in the near-infrared (NIR) range by using the GIANO–B and IGRINS spectrographs. In addition, we present for the first time simultaneous VIS-NIR observations obtained with GIARPS (GIANO–B and HARPS–N) at Telescopio Nazionale Galileo (TNG). Orbital RVs are achromatic, so the RV amplitude does not change at different wavelengths, while stellar activity induces wavelength-dependent RV variations, which are significantly reduced in the NIR range with respect to the VIS. Results. The NIR radial velocity measurements from GIANO–B and IGRINS show an average amplitude of about one quarter with respect to previously published VIS data, as expected when the RV jitter is due to stellar activity. Coeval multi-band photometry surprisingly shows larger amplitudes in the NIR range, explainable with a mixture of cool and hot spots in the same active region. Conclusions. In this work, the claimed massive planet around BD+20 1790 is ruled out by our data. We exploited the crucial role of multi-wavelength spectroscopy when observing young active stars: thanks to facilities like GIARPS that provide simultaneous observations, this method can reach its maximum potential.

Download Full-text