Two red supergiants (RSGs) of the Per OB1 association, RS Per and T Per, have been observed in the H band using the Michigan Infra-Red Combiner (MIRC) instrument at the CHARA array. The data show clear evidence of a departure from circular symmetry. We present here new techniques specially developed to analyze such cases, based on state-of-the-art statistical frameworks. The stellar surfaces are first modeled as limb-darkened disks based on SATLAS models that fit both MIRC interferometric data and publicly available spectrophotometric data. Bayesian model selection is then used to determine the most probable number of spots. The effective surface temperatures are also determined and give further support to the recently derived hotter temperature scales of RSGs. The stellar surfaces are reconstructed by our model-independent imaging code SQUEEZE, making use of its novel regularizer based on Compressed Sensing theory. We find excellent agreement between the model-selection results and the reconstructions. Our results provide evidence for the presence of near-infrared spots representing about 3%-5% of the stellar flux.

This MSc report discusses the attributes of W Ursae Majoris (W UMa) stars and an investigation into the Minimum Entropy (ME) method, a digital technique applied to the determination of their periods of variability. A Python code programme was written to apply the ME method to photometric data collected on W UMa stars by the All Sky Automated Survey (ASAS). Starting with the orbital period of the binaries estimated by ASAS, this programme systematically searches around this period for the period which corresponds to the lowest value of entropy. Low entropy here means low scatter (or spread) of data across the phase-magnitude plane. The ME method divides the light curve plot area into a number of elements of the investigators choosing. When a particular orbital period is applied to this photometric data, the resulting distribution of this data in the light curve plane corresponds to a speci c number of data points in each element into which this plane has been divided. This data spread is measured and calculated in terms of entropy and the lowest value of entropy corresponds to the lowest spread of data across the light curve plane. This should correspond to the best light curve shape available from the data and therefore the most accurate orbital period available. Subsequent to the testing of this Python code on perfect sine waves, it was applied, and its results compared, to the 62 ASAS eclipsing binary stars which were investigated by Deb and Singh (2011). The method was then applied to selected stars from the ASAS data base. / Environmental Sciences / M. Sc. (Astronomy)

From 5.5 months of dual-band optical photometric monitoring at the 1 mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star ζ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the 1.78-d signal previously detected by Coriolis/Solar Mass Ejection Imager, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inhomogeneities. By means of a constrained non-linear light-curve inversion algorithm, we mapped the locations of the bright surface spots and traced their evolution. Our simultaneous ground-based multisite spectroscopic monitoring of the star unveiled cyclical modulation of its He IIλ4686 wind emission line with the 1.78-d rotation period, showing signatures of corotating interaction regions that turn out to be driven by the bright photospheric spots observed by BRITE. Traces of wind clumps are also observed in the He II λ4686 line and are correlated with the amplitudes of the stochastic component of the light variations probed by BRITE at the photosphere, suggesting that the BRITE observations additionally unveiled the photospheric drivers of wind clumps in ζ Pup and that the clumping phenomenon starts at the very base of the wind. The origins of both the bright surface inhomogeneities and the stochastic light variations remain unknown, but a subsurface convective zone might play an important role in the generation of these two types of photospheric variability.

Within this past decade, advances in spectropolarimetric analyses allowed mapping surface characteristics of nearby non-resolved stars with unique characteristics. In this study, we attempt mapping of the magnetic field structure and surface distribution of elements for such a star, the magnetic Ap star phi Dra. Using high-precision photometric data from TESS satellite, we improve its rotation period. Spectrum synthesis calculations allow compilation of a list of lines present in its spectrum. The resulting synthetic spectrum and observed NARVAL spectra are used to re-estimate element abundances. Least-squares deconvolved (LSD) intensity and circular polarisation profiles are then computed from a sample of 1260 metal lines. To determine which element(s) are most suitable for in-depth analysis, variability of LSD profiles is studied qualitatively for Fe, Cr and Si. The longitudinal magnetic field of phi Dra is calculated from LSD circular polarisation profiles. Stellar magnetic field maps and distributions of Fe concentration are derived through Zeeman Doppler Imaging (ZDI). The resulting maps of this study show five areas of high Fe concentrations, in the Northern stellar hemisphere. The magnetic field topology of phi Dra resulting from our analysis is that of an offset dipole with small quadrupole contributions. Our abundance and magnetic maps suggest correlation between high concentrations of Fe and high magnetic field strength. The field is primarily radial in 4 out of 5 such regions, contrary to theoretical expectations.

Les hétérogénéités de surface des étoiles actives, telles que les taches et les facules, peuvent compliquer l'interprétation des spectres de transmission en introduisant des caractéristiques spectrales qui chevauchent celles d'atmosphère d'exoplanètes. Les courbes de lumière de transit d'HAT-P-18\(\,\)b et de WASP-52\(\,\)b, observées avec le mode SOSS de l'instrument NIRISS à bord du JWST, sont déformées par des occultations de taches. Avant le déploiement du JWST, ces régions actives étaient souvent simplement masquées, toutefois ceci peut mener à des mesures incorrectes des paramètres du transit. J'ai adapté et implémenté \(\texttt{spotrod}\), un modèle de transit avec occultation de taches, dans l'outil \(\texttt{Juliet}\) pour inférer conjointement les paramètres du transit et des taches occultées. J'ai ainsi ajusté les courbes de lumière de transit de ces deux Jupiters chaudes et récupéré la position de chaque tache, leur rayon et leur spectre de contraste, c'est-à-dire le rapport du flux de la tache sur le flux stellaire. J'ai contraint la température des taches et leur gravité de surface (pour prendre en compte les effets du champ magnétique local) en ajustant chaque spectre de contraste avec des spectres de modèles stellaires PHOENIX. Cependant, un certain degré de dégénérescence est présent, conduisant à une solution plus probable pour chaque tache, mais aussi à d'autres solutions qui ne peuvent être exclues. Le spectre de transmission d'HAT-P-18\(\,\)b nous a permis de détecter de l'H\(_2\)O (12,5\(\,\sigma\)) avec une abondance sub-solaire de \(\log\) H\(_2\)O \(\approx\) -4,4 \(\pm\) 0,3, des nuages (7,4\(\,\sigma\)) et du CO\(_2\) (7,3\(\,\sigma\)) dans l'atmosphère planétaire ainsi que des régions actives non occultées (5,8\(\,\sigma\)) qui imitent une pente de diffusion Rayleigh. / Surface heterogeneities on active stars, such as starspots and faculae, can complicate the interpretation of transmission spectra and introduce spectral features that overlap those of exoplanetary atmospheres. The transit light curves of HAT-P-18\(\,\)b and WASP-52\(\,\)b, observed in the SOSS mode of the NIRISS instrument aboard the JWST, are deformed by spot-crossings. These active regions were often simply masked before the launch of the JWST; however, this can prevent the correct measure of transit parameters. I adapted and implemented \(\texttt{spotrod}\), a model for transits of spotted stars, into the \(\texttt{Juliet}\) tool to simultaneously infer the transit and occulted starspots parameters. I fitted the transit light curves of these two hot Jupiters and retrieved for each spot its position, radius and spot-to-stellar flux contrast spectrum. I constrained the spots' temperature and surface gravity \(-\) attempting to capture the effects of the local magnetic pressure \(-\) by fitting each contrast spectrum with PHOENIX stellar model spectra. However, some degree of degeneracy is present, leading to a most likely solution for each starspot and other solutions that cannot be excluded. The transmission spectrum of HAT-P-18\(\,\)b enabled us to detect H\(_2\)O (12.5\(\,\sigma\)) with a sub-solar abundance of \(\log\) H\(_2\)O \(\approx\) -4.4 \(\pm\) 0.3, a cloud deck (7.4\(\,\sigma\)) and CO\(_2\) (7.3\(\,\sigma\)) in the planetary atmosphere as well as unocculted active regions (5.8\(\,\sigma\)) which mimic a Rayleigh scattering slope.

This MSc report discusses the attributes of W Ursae Majoris (W UMa) stars and an investigation into the Minimum Entropy (ME) method, a digital technique applied to the determination of their periods of variability. A Python code programme was written to apply the ME method to photometric data collected on W UMa stars by the All Sky Automated Survey (ASAS). Starting with the orbital period of the binaries estimated by ASAS, this programme systematically searches around this period for the period which corresponds to the lowest value of entropy. Low entropy here means low scatter (or spread) of data across the phase-magnitude plane. The ME method divides the light curve plot area into a number of elements of the investigators choosing. When a particular orbital period is applied to this photometric data, the resulting distribution of this data in the light curve plane corresponds to a speci c number of data points in each element into which this plane has been divided. This data spread is measured and calculated in terms of entropy and the lowest value of entropy corresponds to the lowest spread of data across the light curve plane. This should correspond to the best light curve shape available from the data and therefore the most accurate orbital period available. Subsequent to the testing of this Python code on perfect sine waves, it was applied, and its results compared, to the 62 ASAS eclipsing binary stars which were investigated by Deb and Singh (2011). The method was then applied to selected stars from the ASAS data base. / School of Environmental Sciences / M. Sc. (Astronomy)

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