Searching Habitable Exoplanets with the SPIRou Spectropolarimeter: Tackling Stellar Magnetic Activity
Creators: name: Bellotti, Stefano
Publisher: name: Zenodo
Year: 2023

description: The search and characterization of habitable Earth-like planets have been substantial drivers for both current (e.g. TESS, CHEOPS, JWST ) and mid-term (PLATO, Ariel) space missions. The consequent investigation of the chemical variety of exoplanet atmospheres will drive significant progress in our understanding of planet formation and evolution, as well as a baseline to assess potential signatures of life. At present, the output of transit photometry campaigns dominates the planet detection statistics, but this method conveys only partial knowledge on the planet (i.e. the radius), and requires complementary follow-up in order to constrain (an upper limit of) the planetary mass, mainly via radial velocity surveys. Once radius and mass are known, it is possible to infer the bulk density of the planet, which is a crucial, first-order ingredient to a precise bulk composition determination and atmospheric<br> characterisation. At the low mass end of the main-sequence, M dwarfs represent key targets toward this goal. They<br> outnumber the other spectral types in the solar neighbourhood, feature higher Earth-like planet occurrence rates, and possess habitable zones closer-in, owing to their lower temperatures. However, M dwarfs can be highly active, which hampers the success of radial velocity searches. Stellar magnetic activity is in fact responsible for surface inhomogeneities (e.g. dark spots and faculae) that generate spurious radial velocity signatures (so called “activity jitter”) which, in turn, drown the planetary signature completely or prevent a precise mass determination. In this sense, stellar magnetic activity produces a noise floor that limits our sensitivity to small planets. To tackle this issue and mitigate the stellar contamination, we have to develop physically-motivated, jitter-filtering techniques and understand the jitter sources at play thoroughly, which thus requires an accurate model of the underlying magnetic field. During my Ph.D. project, I used high-resolution spectropolarimetric data in optical and near-infrared collected with ESPaDOnS, Narval and SPIRou, which enable us to efficiently combine velocimetry and magnetometry.<br> <br> In the first part of the project, I designed an algorithm that performs a randomised selection of “stable” spectral lines. Using the output line list to compute radial velocity time series, we observed a reduction of at least 50% in dispersion, demonstrating its potential at cleaning spurious signals. The algorithm was tested extensively, to corroborate its portability over M dwarfs of different activity levels and spectral types, and its applicability to time series with injected synthetic planets, to verify that the associated signal would not be significantly affected. To understand further the physical reason as to why some spectral lines combinations lead to time series with smaller jitter than others, I reconstructed the brightness map of the stellar surface by means of the Doppler Imaging technique while using distinct line selections and attempted at filtering the associated radial velocity curves. The results of this analysis<br> are still preliminary, but we already observe differences in the brightness image of a star when selecting atomic rather than molecular lines. In the second part, I monitored the large-scale magnetic field at the surface of well-known, active M dwarfs on both sides of the fully-convective boundary, exploiting the spectropolarimetric capabilities of the aforementioned instruments. In particular, I analysed time series of longitudinal magnetic field, Full-Width at Half Maximum of unpolarised mean profiles, and magnetic fluxes obtained via Zeeman broadening modelling, and I reconstructed the magnetic field topology by means of tomographic inversion (i.e. Zeeman-Doppler Imaging) across different epochs. I sought for a secular evolution of these quantities, potentially revealing the presence of magnetic cycles. These are known to introduce spurious noise in radial velocity time series, and modulations of the energetic radiation (e.g. UV, X-rays) and stellar wind impinging on planets. I found a variety of long-term trends that have some complementarities with our understanding of the solar magnetic cycle. Ultimately, these findings provide practical feedback to dynamo theories, which are incomplete and still debated even for the Sun.