As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity.

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Mass loss of stars on the asymptotic giant branch: Mechanisms, models and measurements

Here I review the current state of the field of optical stellar interferometry, concentrating on ground-based work although a brief report of space interferometry missions is included. We pause both to reflect on decades of immense progress in the field as well as to prepare for a new generation of large interferometers just now being commissioned (most notably, the CHARA, Keck and VLT Interferometers). First, this review summarizes the basic principles behind stellar interferometry needed by the lay-physicist and general astronomer to understand the scientific potential as well as technical challenges of interferometry. Next, the basic design principles of practical interferometers are discussed, using the experience of past and existing facilities to illustrate important points. Here there is significant discussion of current trends in the field, including the new facilities under construction and advanced technologies being debuted. This decade has seen the influence of stellar interferometry extend beyond classical regimes of stellar diameters and binary orbits to new areas such as mapping the accretion discs around young stars, novel calibration of the cepheid period-luminosity relation, and imaging of stellar surfaces. The third section is devoted to the major scientific results from interferometry, grouped into natural categories reflecting these current developments. Lastly, I consider the future of interferometry, highlighting the kinds of new science promised by the interferometers coming on-line in the next few years. I also discuss the longer-term future of optical interferometry, including the prospects for space interferometry and the possibilities of large-scale ground-based projects. Critical technological developments are still needed to make these projects attractive and affordable.

/pdf/optical-interferometry-in-astronomy-290ps14qag.pdf

Optical interferometry in astronomy

The purpose of this thesis is to investigate mixing in red-giant stars. Thanks particularly to the observations of globular clusters, it is clear that that standard stellar evolution fails to reproduce measured stellar abundances as a function of (red giant branch) luminosity. The empirical data has demonstrated that theorists are missing vital physics in their codes during this phase of evolution. Solutions to this problem have been postulated for many decades now, with mechanisms attributed (but not limited) to rotation, magnetic fields or internal gravity waves. A recently suggested instability, namely thermohaline mixing, serves as the focus of this investigation. It is caused by the competition between heat and chemical diffusion and is only possible due to the unique conditions following dredge-up events. Using observations of lithium and carbon we determine whether this mechanism and/or its current parameterisation can influence the surface composition of red giant branch stars. The quality of the data collected from the Very Large Telescope in Chile, some of which is gathered from our own observing programs, allows us to further constrain the mixing process(es)involved and improve the stellar models.

On the role of doubly-diffusive mixing in low-mass red giant branch stars

Here I review the current state of the field of optical stellar interferometry, concentrating on ground-based work although a brief report of space interferometry missions is included. We pause both to reflect on decades of immense progress in the field as well as to prepare for a new generation of large interferometers just now being commissioned (most notably, the CHARA, Keck and VLT Interferometers). First, this review summarizes the basic principles behind stellar interferometry needed by the lay-physicist and general astronomer to understand the scientific potential as well as technical challenges of interferometry. Next, the basic design principles of practical interferometers are discussed, using the experience of past and existing facilities to illustrate important points. Here there is significant discussion of current trends in the field, including the new facilities under construction and advanced technologies being debuted. This decade has seen the influence of stellar interferometry extend beyond classical regimes of stellar diameters and binary orbits to new areas such as mapping the accretion disks around young stars, novel calibration of the Cepheid Period-Luminosity relation, and imaging of stellar surfaces. The third section is devoted to the major scientific results from interferometry, grouped into natural categories reflecting these current developments. Lastly, I consider the future of interferometry, highlighting the kinds of new science promised by the interferometers coming on-line in the next few years. I also discuss the longer-term future of optical interferometry, including the prospects for space interferometry and the possibilities of large-scale ground-based projects. Critical technological developments are still needed to make these projects attractive and affordable.

/pdf/optical-interferometry-in-astronomy-3p7nq9avpn.pdf

Optical Interferometry in Astronomy

Various lines of evidence suggest that the cores of a large portion of early-type galaxies (ETGs) are virtually evacuated of warm ionised gas. This implies that the Lyman-continuum (LyC) radiation produced by an assumed active galactic nucleus (AGN) can escape from the nuclei of these systems without being locally reprocessed into nebular emission, which would prevent their reliable spectroscopic classification as Seyfert galaxies with standard diagnostic emission-line ratios. The spectral energy distribution (SED) of these ETGs would then lack nebular emission and be essentially composed of an old stellar component and the featureless power-law (PL) continuum from the AGN. A question that arises in this context is whether the AGN component can be detected with current spectral population synthesis in the optical, specifically, whether these techniques effectively place an AGN detection threshold in LyC-leaking galaxies. To quantitatively address this question, we took a combined approach that involves spectral fitting with Starlight of synthetic SEDs composed of stellar emission that characterises a 10 Gyr old ETG and an AGN power-law component that contributes a fraction 0 ≤ xAGN < 1 of the monochromatic luminosity at λ0 = 4020 Å. In addition to a set of fits for PL distributions Fν ∝ ν−α with the canonical α = 1.5, we used a base of multiple PLs with 0.5 ≤ α ≤ 2 for a grid of synthetic SEDs with a signal-to-noise ratio of 5–103. Our analysis indicates an effective AGN detection threshold at xAGN ' 0.26, which suggests that a considerable fraction of ETGs hosting significant accretion-powered nuclear activity may be missing in the AGN demographics.

Impact of AGN and nebular emission on the estimation of stellar properties of galaxies

Phase and cycle dependence of the photospheric structure and observable properties of mira variables

Calcul des couleurs en bandes larges et etroites d'une gamme de modeles d'atmosphere stellaire de type geante M. Ces couleurs synthetiques sont analysees en fonction de la gravite, la temperature, l'extension de l'enveloppe et la metallicite. Les resultats sont compares aux observations

Colors of extended static model photospheres of M giants

Erratum - Colors of Extended Static Model Photospheres of M Giants

Colors and stratifications of a grid of spherically extended M giant and supergiant model photospheres, described in Bessell M S et al (1989, Astron Astrophys Sup Ser, 77), have been supplemented and interpolated in metallicity, gravity and photospheric extension so that they correspond to the parameter values of theoretical evolutionary (super-)giant branches for stars with masses of 1, 15, 225, 5, 10 and 15 M ○ and metallicities log Z/Z ○ of +05, 00, −03, −06 and −10 These colors may be used, for example, for comparison with observed (super-)giant branches or for synthesizing integrated spectra of clusters and galaxies

M. Scholz

Papers

Phase and cycle dependence of the photospheric structure and observable properties of mira variables

Colors of extended static model photospheres of M giants

Erratum - Colors of Extended Static Model Photospheres of M Giants

Colors and stratifications of extended static model photospheres of M stars located on the FGB, AGB and supergiants branch.

The effects of photospheric extension upon the spectra of M-type Miravariables.