Seminars and Colloquia at ESO Santiago

The characterisation of stellar magnetism of exoplanet hosts has increased momentum, since it is key to understand magnetic star-planet interactions. The magnetic field of a star in fact regulates the environment in which exoplanets are embedded and their irradiation, ultimately impacting the evolution of the planetary atmosphere and its habitability. We are conducting a spectropolarimetric campaign to unveil the magnetic field properties of known exoplanet hosting stars included in the current list of Ariel targets. In this talk I will describe spectropolarimetry, an observational technique sensitive to stellar magnetic fields, and I will focus on two of host stars: GJ 436 and HD 63433. The former is an M dwarf orbited by a warm-Neptune experiencing substantial atmospheric loss, modelled as a comet-like trail of hydrogen atoms. The latter is a young, solar-like star hosting two sub-Neptunes and an Earth-sized planet that have likely experienced different atmospheric evolution paths, since only the outer sub-Neptune planet has retained its atmosphere. I will conclude by describing another exoplanet host, DS Tuc, and the dedicated spectropolarimetric observations with the HARPS-Pol instrument mounted at the La Silla 3.6-m telescope.

The Hubble constant H0 gives us the Universe’s expansion rate at present-time and shows a persistent 4–6σ discrepancy between early- and late-time measurements. This discrepancy, known as the Hubble tension, may stem from systematics in the late-time measurement of the Cepheid-based distance ladder: differences in the properties of Type Ia supernovae (SNe Ia) and their host galaxies between the calibration sample (with Cepheids and SNe) and the Hubble Flow (HF) sample (more distant galaxies hosting only SNe). 
In this project, we test how closely matching the distributions of some properties such as the SNe colours (c), stretch (x₁), host stellar mass (M), and specific star formation rate (sSFR) between these samples can affect the H0​ estimation. We generate HF subsamples whose properties better match the calibration sample and use them to derive a more accurate correction for SNe Ia luminosities.
 We find that calibration hosts may not fully represent HF galaxies in mass and sSFR, and that while a HF subsample alone may not resolve the tension, certain SNe populations appear consistent with lower H0​ values.

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