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Black Hole

Lensing of space time around a black hole. At Oxford we study black holes observationally and theoretically on all size and time scales - it is some of our core work.

Credit: ALAIN RIAZUELO, IAP/UPMC/CNRS. CLICK HERE TO VIEW MORE IMAGES.

Michael Cretignier

Postdoctoral Research Assistant

Sub department

  • Astrophysics
  • About
  • Publications

Using Doppler Imaging to model stellar activity and search for planets around Sun-like stars

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025) staf1337

Authors:

Baptiste Klein, Suzanne Aigrain, Michael Cretignier, Xavier Dumusque, Khaled Al Moulla, Jean-François Donati, Niamh K O’Sullivan, Haochuan Yu, Andrew Collier Cameron, Oscar Barragán, Annelies Mortier, Alessandro Sozzetti

Abstract:

Abstract Doppler Imaging (DI) is a well-established technique to map a physical field at a stellar surface from a time series of high-resolution spectra. In this proof-of-concept study, we aim to show that traditional DI algorithms, originally designed for rapidly-rotating stars, have also the ability to model the activity of Sun-like stars, when observed with new-generation highly-stable spectrographs, and search for low-mass planets around them. We used DI to retrieve the relative brightness distribution at the surface of the Sun from radial velocity (RV) observations collected by HARPS-N between 2022 and 2024. The brightness maps obtained with DI have a typical angular resolution of ~36○ and are a good match to low-resolution disc-resolved Dopplergrams of the Sun at epochs when the absolute, disc-integrated RV exceeds ~2 m s−1. The RV residuals after DI correction exhibit a dispersion of about 0.6 m s−1, comparable with existing state-of-the-art activity correction techniques. Using planet injection-recovery tests, we also show that DI can be a powerful tool for blind planet searches, so long as the orbital period is larger than ~100 days (i.e. 3 to 4 stellar rotation periods), and that it yields planetary mass estimates with an accuracy comparable to, for example, multi-dimensional Gaussian process regression. Finally, we highlight some limitations of traditional DI algorithms, which should be addressed to make DI a reliable alternative to state-of-the-art RV-based planet search techniques.
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Studying the variability of the He triplet to understand the detection limits of evaporating exoplanet atmospheres

Astronomy & Astrophysics EDP Sciences 700 (2025) a8

Authors:

Samson J Mercier, Xavier Dumusque, Vincent Bourrier, Khaled Al Moulla, Michael Cretignier, William Dethier, Gaspare Lo Curto, Pedro Figueira, Christophe Lovis, Francesco Pepe, Nuno C Santos, Stéphane Udry, François Wildi, Romain Allart, Frédérique Baron, François Bouchy, Andres Carmona, Marion Cointepas, René Doyon, Yolanda GC Frensch, Nolan Grieves, Lucile Mignon, Louise D Nielsen

Abstract:

With more than a dozen significant detections, the helium triplet has emerged as a key tracer of evaporating exoplanet atmospheres. This near-infrared feature can be observed from the ground and holds great promise, especially with upcoming observations provided by new-generation instruments such as the Near Infrared Planet Searcher (NIRPS). However, as the helium triplet is also present in stellar spectra, careful removal of the average stellar contribution is necessary to accurately characterize the atmospheres of transiting exoplanets. In this study, we analyze multi-epoch observations of the Sun obtained with NIRPS to investigate the temporal variability of the helium triplet. Our findings reveal significant variability across different timescales, ranging from minutes to days. We identify telluric contamination and stellar activity as likely sources for the short-term and long-term variability, respectively. Importantly, we demonstrate that this variability has minimal impact on the retrieval of planetary parameters crucial to the study of atmospheric escape.
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Measuring the Sun’s radial velocity variability due to supergranulation over a magnetic cycle

Monthly Notices of the Royal Astronomical Society Oxford University Press 541:4 (2025) 3942-3962

Authors:

Niamh K O’Sullivan, Suzanne Aigrain, Michael Cretignier, Ben Lakeland, Baptiste Klein, Xavier Dumusque, Nadège Meunier, Sophia Sulis, Megan Bedell, Annelies Mortier, Andrew Collier Cameron, Heather M Cegla

Abstract:

In recent years, supergranulation has emerged as one of the biggest challenges for the detection of Earth-twins in radial velocity planet searches. We used eight years of Sun-as-a-star radial velocity observations from HARPS-N to measure the quiet-Sun’s granulation and supergranulation properties of most of its 11-yr activity cycle, after correcting for the effects of magnetically active regions using two independent methods. In both cases, we observe a clear, order of magnitude variation in the time-scale of the supergranulation component, which is largest at activity minimum and is strongly anticorrelated with the relative Sunspot number. We also explored a range of observational strategies which could be employed to characterize supergranulation in stars other than the Sun, showing that a comparatively long observing campaign of at least 23 nights is required, but that up to 10 stars can be monitored simultaneously in the process. We conclude by discussing plausible explanations for the ‘supergranulation’ cycle.
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In-depth characterization of the Kepler-10 three-planet system with HARPS-N radial velocities and Kepler transit timing variations

Astronomy & Astrophysics EDP Sciences 696 (2025) a233

Authors:

AS Bonomo, L Borsato, VM Rajpaul, L Zeng, M Damasso, NC Hara, M Cretignier, A Leleu, N Unger, X Dumusque, F Lienhard, A Mortier, L Naponiello, L Malavolta, A Sozzetti, DW Latham, K Rice, R Bongiolatti, L Buchhave, AC Cameron, AF Fiorenzano, A Ghedina, RD Haywood, G Lacedelli, A Massa, F Pepe, E Poretti, S Udry
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TESS and HARPS-N unveil two planets transiting TOI-1453

Astronomy & Astrophysics EDP Sciences 696 (2025) a86

Authors:

M Stalport, A Mortier, M Cretignier, JA Egger, L Malavolta, DW Latham, KA Collins, CN Watkins, F Murgas, LA Buchhave, M López-Morales, S Udry, SN Quinn, AM Silva, G Andreuzzi, D Baker, W Boschin, DR Ciardi, M Damasso, L Di Fabrizio, X Dumusque, A Fukui, R Haywood, SB Howell, JM Jenkins, A Leleu, P Lewin, J Lillo-Box, AF Martínez Fiorenzano, N Narita, M Pedani, M Pinamonti, E Poretti, RP Schwarz, S Seager, A Sozzetti, EB Ting, A Vanderburg, JN Winn, C Ziegler
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