A. Mortier

University of Birmingham

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S. Udry

University of Geneva

A. Sozzetti

Osservatorio Astrofisico di Torino

A. Collier Cameron

University of St Andrews

X. Dumusque

University of Geneva

David W. Latham

Center for Astrophysics Harvard & Smithsonian

Mercedes López‐Morales

Space Telescope Science Institute

Lars A. Buchhave

Technical University of Denmark

F. Pepe

University of Geneva

R. D. Haywood

University of Exeter

N. C. Santos

Universidade do Porto

Cooperative Institutions

Centre National de la Recherche Scientifique

263

Max Planck Society

213

Max Planck Institute for Astronomy

176

National Institute for Astrophysics

156

University of Geneva

114

Center for Astrophysics Harvard & Smithsonian

109

Instituto de Astrofísica de Canarias

California Institute of Technology

Université Paris Cité

Institute of Astrophysics and Space Sciences

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Identifying Exoplanets with Deep Learning. IV. Removing Stellar Activity Signals from Radial Velocity Measurements Using Neural Networks

arXiv (Cornell University) (2020)

Zoë L. de Beurs Andrew Vanderburg Christopher J. Shallue X. Dumusque A. Collier Cameron

Exoplanet detection with precise radial velocity (RV) observations is currently limited by spurious RV signals introduced by stellar activity. We show that machine learning techniques such as linear regression and neural networks can effectively remove the activity signals (due to starspots/faculae) from RV observations. Previous efforts focused on carefully filtering out activity signals in time using modeling techniques like Gaussian Process regression (e.g. Haywood et al. 2014). Instead, we systematically remove activity signals using only changes to the average shape of spectral lines, and no information about when the observations were collected. We trained our machine learning models on both simulated data (generated with the SOAP 2.0 software; Dumusque et al. 2014) and observations of the Sun from the HARPS-N Solar Telescope (Dumusque et al. 2015; Phillips et al. 2016; Collier Cameron et al. 2019). We find that these techniques can predict and remove stellar activity from both simulated data (improving RV scatter from 82 cm/s to 3 cm/s) and from more than 600 real observations taken nearly daily over three years with the HARPS-N Solar Telescope (improving the RV scatter from 1.47 m/s to 0.78 m/s, a factor of ~ 1.9 improvement). In the future, these or similar techniques could remove activity signals from observations of stars outside our solar system and eventually help detect habitable-zone Earth-mass exoplanets around Sun-like stars.

Radial velocity

Spurious relationship

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HD152843 b & c: the masses and orbital periods of a sub-Neptune and a super-puff Neptune

Monthly Notices of the Royal Astronomical Society (2024)

Belinda Nicholson S. Aigrain Nora L. Eisner M. Cretignier Oscar Barragán

ABSTRACT We present the characterization of the two transiting planets around HD 152843 (TOI 2319, TIC 349488688) using an intensive campaign of HARPS-N radial velocities, and two sectors of TESS data. These data reveal a unique and fascinating system: HD 152843 b and c have near equal masses of around 9 $M_{\hbox{$\oplus $}}$ but differing radii of $3.05 \pm 0.11$$R_{\hbox{$\oplus $}}$ and $5.94 _{ - 0.16 } ^ { + 0.18 }$$R_{\hbox{$\oplus $}}$, respectively, and orbital periods of $11.62071 _{ - 0.000106 } ^ { + 9.6e-05 }$ and $19.502104 _{ - 8.5e-05 } ^ { + 7.4e-05 }$ d. This indicates that HD 152843 c is in the lowest fifth-percentile in density of the known exoplanet population, and has the longest orbital period among these low-density planets. Further, HD 152843 c’s radius places it in the ‘Saturn valley’, the observed lack of planets larger than Neptune, but smaller than Saturn. The orbital periods of these planets indicate they are near a $5:3$ mean motion resonance, indicating the possibility of transit timing variations, and hints at the possibility of interaction with a third planet at some point in the evolution of this system. Further, the brightness of the host star and the low density of HD 152843 c make it a key target for atmospheric characterization.

Neptune

Orbital period

10.1093/mnras/stae1821

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The magnetically quiet solar surface dominates HARPS-N solar RVs during low activity

arXiv (Cornell University) (2023)

Ben S Lakeland T. Naylor R. D. Haywood N. Meunier Federica Rescigno

Using images from the Helioseismic and Magnetic Imager aboard the \textit{Solar Dynamics Observatory} (SDO/HMI), we extract the radial-velocity (RV) signal arising from the suppression of convective blue-shift and from bright faculae and dark sunspots transiting the rotating solar disc. We remove these rotationally modulated magnetic-activity contributions from simultaneous radial velocities observed by the HARPS-N solar feed to produce a radial-velocity time series arising from the magnetically quiet solar surface (the 'inactive-region radial velocities'). We find that the level of variability in the inactive-region radial velocities remains constant over the almost 7 year baseline and shows no correlation with well-known activity indicators. With an RMS of roughly 1 m/s, the inactive-region radial-velocity time series dominates the total RV variability budget during the decline of solar cycle 24. Finally, we compare the variability amplitude and timescale of the inactive-region radial velocities with simulations of supergranulation. We find consistency between the inactive-region radial-velocity and simulated time series, indicating that supergranulation is a significant contribution to the overall solar radial velocity variability, and may be the main source of variability towards solar minimum. This work highlights supergranulation as a key barrier to detecting Earth twins.

Radial velocity

Sunspot

QUIET

10.48550/arxiv.2311.16076

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Tracking Magnetic Variability From High-Resolution Unpolarised Spectra

A. Mortier Gregory A. Feiden

Tracking (education)

10.5281/zenodo.59214

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Citations (2)

New and updated stellar parameters for 71 evolved planet hosts. On the metallicity-giant planet connection

Astronomy and Astrophysics (2013)

A. Mortier N. C. Santos S. G. Sousa V. Adibekyan E. Delgado Mena

Context. It is still being debated whether the well-known metallicity–giant planet correlation for dwarf stars is also valid for giant stars. For this reason, having precise metallicities is very important. Precise stellar parameters are also crucial to planetary research for several other reasons. Different methods can provide different results that lead to discrepancies in the analysis of planet hosts. Aims. To study the impact of different analyses on the metallicity scale for evolved stars, we compare different iron line lists to use in the atmospheric parameter derivation of evolved stars. Therefore, we use a sample of 71 evolved stars with planets. With these new homogeneous parameters, we revisit the metallicity–giant planet connection for evolved stars.Methods. A spectroscopic analysis based on Kurucz models in local thermodynamic equilibrium (LTE) was performed through the MOOG code to derive the atmospheric parameters. Two different iron line list sets were used, one built for cool FGK stars in general, and the other for giant FGK stars. Masses were calculated through isochrone fitting, using the Padova models. Kolmogorov-Smirnov tests (K-S tests) were then performed on the metallicity distributions of various different samples of evolved stars and red giants.Results. All parameters compare well using a line list set, designed specifically for cool and solar-like stars to provide more accurate temperatures. All parameters derived with this line list set are preferred and are thus adopted for future analysis. We find that evolved planet hosts are more metal-poor than dwarf stars with giant planets. However, a bias in giant stellar samples that are searched for planets is present. Because of a colour cut-off, metal-rich low-gravity stars are left out of the samples, making it hard to compare dwarf stars with giant stars. Furthermore, no metallicity enhancement is found for red giants with planets (log g < 3.0 dex) with respect to red giants without planets.

Giant planet

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Confronting compositional confusion through the characterisation of the sub-Neptune orbiting HD 77946

Monthly Notices of the Royal Astronomical Society (2024)

Larissa Palethorpe A. Anna John A. Mortier Jeanne Davoult T. G. Wilson

Abstract We report on the detailed characterization of the HD 77946 planetary system. HD 77946 is an F5 (M* = 1.17 M⊙, R* = 1.31 R⊙) star, which hosts a transiting planet recently discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and RV observations, and find that the planet, HD 77946 b, orbits with period Pb = $6.527282_{-0.000020}^{+0.000015}$ d, has a mass of Mb = 8.38 ± 1.32 M⊕, and a radius of $R_{\rm b} = 2.705_{-0.081}^{+0.086}$R⊕. From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a ∼1% H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with $T_{\rm eq}~= 1248^{+40}_{-38}~$K, water may be in a supercritical state. The characterisation of HD 77946 b, adding to the small sample of well-characterised sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways. Furthermore, HD 77946 b has one of the highest transmission spectroscopic metrics for small planets orbiting hot stars, thus transmission spectroscopy of this key planet could prove vital for constraining the compositional confusion that currently surrounds small exoplanets.

Neptune

Confusion

10.1093/mnras/stae707

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New and updated stellar parameters for 90 transit hosts

Astronomy and Astrophysics (2013)

A. Mortier N. C. Santos S. G. Sousa J. Fernandes V. Adibekyan

Context. Precise stellar parameters are crucial in exoplanet research for correctly determining of the planetary parameters. For stars hosting a transiting planet, determining of the planetary mass and radius depends on the stellar mass and radius, which in turn depend on the atmospheric stellar parameters. Different methods can provide different results, which leads to different planet characteristics.}%Spectroscopic surface gravities have shown to be poorly constrained, but the photometry of the transiting planet can provide an independent measurement of the surface gravity. Aims. In this paper, we use a uniform method to spectroscopically derive stellar atmospheric parameters, chemical abundances, stellar masses, and stellar radii for a sample of 90 transit hosts. Surface gravities are also derived photometrically using the stellar density as derived from the light curve. We study the effect of using these different surface gravities on the determination of the chemical abundances and the stellar mass and radius. Methods. A spectroscopic analysis based on Kurucz models in LTE was performed through the MOOG code to derive the atmospheric parameters and the chemical abundances. The photometric surface gravity was determined through isochrone fitting and the use of the stellar density, directly determined from the light curve. Stellar masses and radii are determined through calibration formulae. Results. Spectroscopic and photometric surface gravities differ, but this has very little effect on the precise determination of the stellar mass in our spectroscopic analysis. The stellar radius, and hence the planetary radius, is most affected by the surface gravity discrepancies. For the chemical abundances, the difference is, as expected, only noticable for the abundances derived from analyzing of lines of ionized species.

Stellar mass

10.1051/0004-6361/201322240

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Citations (73)

Correcting the spectroscopic surface gravity using transits and asteroseismology - No significant effect on temperatures or metallicities with ARES and MOOG in local thermodynamic equilibrium

Astronomy and Astrophysics (2014)

A. Mortier S. G. Sousa V. Adibekyan I. M. Brandão N. C. Santos

Context. Precise stellar parameters (effective temperature, surface gravity, metallicity, stellar mass, and radius) are crucial for several reasons, amongst which are the precise characterization of orbiting exoplanets and the correct determination of galactic chemical evolution. The atmospheric parameters are extremely important because all the other stellar parameters depend on them. Using our standard equivalent-width method on high-resolution spectroscopy, good precision can be obtained for the derived effective temperature and metallicity. The surface gravity, however, is usually not well constrained with spectroscopy. Aims. We use two different samples of FGK dwarfs to study the effect of the stellar surface gravity on the precise spectroscopic determination of the other atmospheric parameters. Furthermore, we present a straightforward formula for correcting the spectroscopic surface gravities derived by our method and with our linelists. Methods. Our spectroscopic analysis is based on Kurucz models in local thermodynamic equilibrium, performed with the MOOG code to derive the atmospheric parameters. The surface gravity was either left free or fixed to a predetermined value. The latter is either obtained through a photometric transit light curve or derived using asteroseismology. Results. We find first that, despite some minor trends, the effective temperatures and metallicities for FGK dwarfs derived with the described method and linelists are, in most cases, only affected within the errorbars by using different values for the surface gravity, even for very large differences in surface gravity, so they can be trusted. The temperatures derived with a fixed surface gravity continue to be compatible within 1 sigma with the accurate results of the infrared flux method (IRFM), as is the case for the unconstrained temperatures. Secondly, we find that the spectroscopic surface gravity can easily be corrected to a more accurate value using a linear function with the effective temperature.

Surface gravity

Asteroseismology

Effective temperature

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VizieR Online Data Catalog: HD175607 RV, logRHK and Halpha index (Mortier+, 2016)

VizieR Online Data Catalog (2015)

A. Mortier J. P. Faria N. C. Santos V. Rajpaul P. Figueira

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