Measurements of secular period change probe real-time stellar evolution of classical Cepheids making these measurements powerful constraints for stellar evolution models, especially when coupled with interferometric measurements. In this work, we present stellar evolution models and measured rates of period change for two Galactic Cepheids: Polaris and l Carinae, both important Cepheids for anchoring the Cepheid Leavitt law (period-luminosity relation). The combination of previously-measured parallaxes, interferometric angular diameters and rates of period change allows for predictions of Cepheid mass loss and stellar mass. Using the stellar evolution models, We find that l Car has a mass of about 9 M S™ consistent with stellar pulsation models, but is not undergoing enhanced stellar mass loss. Conversely, the rate of period change for Polaris requires including enhanced mass-loss rates. We discuss what these different results imply for Cepheid evolution and the mass-loss mechanism on the Cepheid instability strip.

The stellar winds of hot stars have an important impact on both stellar and galactic evolution, yet their structure and internal processes are not fully understood in detail. One of the best nearby laboratories for studying such massive stellar winds is the O4I(n)fp star ζ Pup. After briefly discussing existing X-ray observations from Chandra and XMM, we present a simulation of X-ray emission line profile measurements for the upcoming 840 kilosecond Chandra HETGS observation. This simulation indicates that the increased S/N of this new observation will allow several major steps forward in the understanding of massive stellar winds. By measuring X-ray emission line strengths and profiles, we should be able to differentiate between various stellar wind models and map the entire wind structure in temperature and density. This legacy X-ray spectrum of ζ Pup will be a useful benchmark for future X-ray missions.

The structured winds of single massive stars can be classified into two broad groups: stochastic structure and organized structure. While the former is typically identified with clumping, the latter is typically associated with rotational modulations, particularly the paradigm of corotating interaction regions (CIRs). While CIRs have been explored extensively in the ultraviolet band, and moderately in the X-ray and optical, here we evaluate radio variability from CIR structures assuming free-free opacity in a dense wind. Our goal is to conduct a broad parameter study to assess the observational feasibility, and to this end, we adopt a phenomenological model for a CIR that threads an otherwise spherical wind. We find that under reasonable assumptions, it is possible to obtain radio variability at the 10 per cent level. The detailed structure of the folded light curve depends not only on the curvature of the CIR, the density contrast of the CIR relative to the wind, and viewing inclination, but also on wavelength. Comparing light curves at different wavelengths, we find that the amplitude can change, that there can be phase shifts in the waveform, and the entire waveform itself can change. These characterstics could be exploited to detect the presence of CIRs in dense, hot winds.

Theoretical models predict that the compressed interstellar medium around runaway O stars can produce high-energy non-thermal diffuse emission, in particular, non-thermal X-ray and γ-ray emission. So far, detection of non-thermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six well-determined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward ζ Oph and BD+43°3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.

The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.

Observations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.

Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013, ApJ, 762, L8).

Two equations in Ignace et al. (2019) have minor typos. The first correction is in Eq. (15). That expression has three lines. In the first line, there should be no multiplicative factor of R0 appearing. The correct equation is: (Formula Presented) The other two lines in Eq. (15) are correct. The second correction is in Eq. (19). As in Eq. (15), the appearance of R0 is in error. The correct expression is (Formula Presented) The mistakes appearing in Eqs. (15) and (19) are typos that have no bearing on the results appearing in the figures which were computed correctly, nor on the discussion or conclusions of the paper. (Equation Presented).

L'eau est un constituant essentiel de l'atmosphère de supergéantes rouges (RSG), mais dont l'influence reste mal comprise. Le spectre observé de l'eau de ces étoiles ne peut être reproduit que par l'ajout d'une coquille de gaz moléculaire, les MOLsphères. Cependant, l'hypothèse des MOLsphères reste fragile et sujette à caution. Dans le but de mieux interpréter les spectres observés, la synthèse de spectres hors équilibre thermodynamique local est une approche potentiellement importante. Les effets hors ETL étant potentiellement fort, ils pourraient être en mesure de lever les problèmes de l'interprétation des raies de l'eau sans ajout de MOLsphère et impliquer un rôle important dans la dynamique de l'atmosphère. Pour cela, nous avons développé une méthode originale en mesure de résoudre l'équation de transfert pour les nombreuses transitions radiatives de l'eau sans approximationETL. Cette méthode a été mise en oeuvre via le développement d'un code de transfert radiatif parallèle. Les premiers résultats montrent que les effets hors ETL dans l'atmosphère des RSG, et leur impact sur le spectre comme sur certaines observables utilisées pour sonder ces étoiles, sont importants / Water is an important constituent of the atmosphere of red supergiant stars (RSG), which influence remains however poorly understood. The water spectrum of these stars can apparently be only reproduced through the addition of a detached shell of cool molecular gas, the so-called MOLspheres. However, this hypothesis is still cautious. In order to better interpret observed spectra, non local thermodynamic equilibrium (NLTE) spectrum synthesis may be potentially important. NLTE effects being potentially important, they may alleviate the problems in the interpretation of water spectra, and affect the atmosphere dynamics. We thus developed an original method to solve the radiative transfer equation, adapted to the numerous water transitions and without the LTE approximation. This method has been implemented in an original parallel code. Preliminary results show that NLTE effects in RSG atmospheres and their impact on observables such as the emergent spectrum are very important.

The influence of macroclumps for free–free spectral energy distributions (SEDs) of ionized winds is considered. The goal is to emphasize distinctions between microclumping and macroclumping effects. Microclumping can alter SED slopes and flux levels if the volume filling factor of the clumps varies with radius; however, the modifications are independent of the clump geometry. To what extent does macroclumping alter SED slopes and flux levels? In addressing the question, two specific types of macroclump geometries are explored: shell fragments (pancake-shaped) and spherical clumps. Analytic and semi-analytic results are derived in the limiting case that clumps never obscure one another. Numerical calculations based on a porosity formalism is used when clumps do overlap. Under the assumptions of a constant expansion, isothermal, and fixed ionization wind, the fragment model leads to results that are essentially identical to the microclumping result. Mass-loss rate determinations are not affected by porosity effects for shell fragments. By contrast, spherical clumps can lead to a reduction in long-wavelength fluxes, but the reductions are only significant for extreme volume filling factors.