| 1. |
- Barklem, Paul S.
(författare)
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Accurate abundance analysis of late-type stars : advances in atomic physics
- 2016
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Ingår i: The Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 24
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Forskningsöversikt (refereegranskat)abstract
- The measurement of stellar properties such as chemical compositions, masses and ages, through stellar spectra, is a fundamental problem in astrophysics. Progress in the understanding, calculation and measurement of atomic properties and processes relevant to the high-accuracy analysis of F-, G-, and K-type stellar spectra is reviewed, with particular emphasis on abundance analysis. This includes fundamental atomic data such as energy levels, wavelengths, and transition probabilities, as well as processes of photoionisation, collisional broadening and inelastic collisions. A recurring theme throughout the review is the interplay between theoretical atomic physics, laboratory measurements, and astrophysical modelling, all of which contribute to our understanding of atoms and atomic processes, as well as to modelling stellar spectra.
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| 2. |
- Höfner, Susanne, et al.
(författare)
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Mass loss of stars on the asymptotic giant branch: Mechanisms, models and measurements
- 2018
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Ingår i: Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 26:1
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Forskningsöversikt (refereegranskat)abstract
- 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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| 3. |
- Kochukhov, Oleg
(författare)
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Magnetic fields of M dwarfs
- 2021
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Ingår i: The Astronomy and Astrophysics Review. - : Springer. - 0935-4956 .- 1432-0754. ; 29:1
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Forskningsöversikt (refereegranskat)abstract
- Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields.
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| 4. |
- Lammer, H., et al.
(författare)
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What makes a planet habitable?
- 2009
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Ingår i: The Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 17:2, s. 181-249
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Forskningsöversikt (refereegranskat)abstract
- This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane-ethane surface lakes on Saturn's large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.
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| 5. |
- Nissen, Poul Erik, et al.
(författare)
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High-precision stellar abundances of the elements : methods and applications
- 2018
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Ingår i: The Astronomy and Astrophysics Review. - : SPRINGER. - 0935-4956 .- 1432-0754. ; 26
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Tidskriftsartikel (refereegranskat)abstract
- Efficient spectrographs at large telescopes have made it possible to obtain high-resolution spectra of stars with high signal-to-noise ratio and advances in model atmosphere analyses have enabled estimates of high-precision differential abundances of the elements from these spectra, i.e. with errors in the range 0.01-0.03dex for F, G, and K stars. Methods to determine such high-precision abundances together with precise values of effective temperatures and surface gravities from equivalent widths of spectral lines or by spectrum synthesis techniques are outlined, and effects on abundance determinations from using a 3D non-LTE analysis instead of a classical 1D LTE analysis are considered. The determination of high-precision stellar abundances of the elements has led to the discovery of unexpected phenomena and relations with important bearings on the astrophysics of galaxies, stars, and planets, i.e. (i) Existence of discrete stellar populations within each of the main Galactic components (disk, halo, and bulge) providing new constraints on models for the formation of the Milky Way. (ii) Differences in the relation between abundances and elemental condensation temperature for the Sun and solar twins suggesting dust-cleansing effects in proto-planetary disks and/or engulfment of planets by stars; (iii) Differences in chemical composition between binary star components and between members of open or globular clusters showing that star- and cluster-formation processes are more complicated than previously thought; (iv) Tight relations between some abundance ratios and age for solar-like stars providing new constraints on nucleosynthesis and Galactic chemical evolution models as well as the composition of terrestrial exoplanets. We conclude that if stellar abundances with precisions of 0.01-0.03dex can be achieved in studies of more distant stars and stars on the giant and supergiant branches, many more interesting future applications, of great relevance to stellar and galaxy evolution, are probable. Hence, in planning abundance surveys, it is important to carefully balance the need for large samples of stars against the spectral resolution and signal-to-noise ratio needed to obtain high-precision abundances. Furthermore, it is an advantage to work differentially on stars with similar atmospheric parameters, because then a simple 1D LTE analysis of stellar spectra may be sufficient. However, when determining high-precision absolute abundances or differential abundance between stars having more widely different parameters, e.g. metal-poor stars compared to the Sun or giants to dwarfs, then 3D non-LTE effects must be taken into account.
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| 6. |
- Pérez-Torres, M.A., et al.
(författare)
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Star formation and nuclear activity in luminous infrared galaxies: an infrared through radio review
- 2021
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Ingår i: Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 29:1
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Forskningsöversikt (refereegranskat)abstract
- Nearby galaxies offer unique laboratories allowing multi-wavelength spatially resolved studies of the interstellar medium, star formation and nuclear activity across a broad range of physical conditions. In particular, detailed studies of individual local luminous infrared galaxies (LIRGs) are crucial for gaining a better understanding of these processes and for developing and testing models that are used to explain statistical studies of large populations of such galaxies at high redshift for which it is currently impossible to reach a sufficient physical resolution. Here, we provide an overview of the impact of spatially resolved infrared, sub-millimetre and radio observations in the study of the interstellar medium, star formation and active galactic nuclei as well as their interplay in local LIRGs. We also present an overview of the modelling of their spectral energy distributions using state-of-the-art radiative transfer codes. These contribute necessary and powerful ‘workhorse’ tools for the study of LIRGs (and their more luminous counterparts) at higher redshifts which are unresolved in observations. We describe how spatially-resolved time-domain observations have recently opened a new window to study the nuclear activity in LIRGs. We describe in detail the observational characteristics of Arp 299 which is one of the best studied local LIRGs and exemplifies the power of the combination of time-domain and high-resolution observations at infrared to radio wavelengths together with radiative transfer modelling used to explain the spectral energy distributions of its different components. We summarise the previous achievements obtained using high-spatial resolution observations and provide an outlook into what we can expect to achieve with future facilities.
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| 7. |
- Serenelli, Aldo, et al.
(författare)
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Weighing stars from birth to death : mass determination methods across the HRD
- 2021
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Ingår i: Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 29:1
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Forskningsöversikt (refereegranskat)abstract
- The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exist a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between [0.3 , 2] % for the covered mass range of M∈[0.1,16]M⊙, 75 % of which are stars burning hydrogen in their core and the other 25 % covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a “mass-ladder” for stars.
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| 8. |
- Veilleux, Sylvain, et al.
(författare)
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Cool outflows in galaxies and their implications
- 2020
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Ingår i: Astronomy and Astrophysics Review. - : Springer Science and Business Media LLC. - 0935-4956 .- 1432-0754. ; 28:1
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Forskningsöversikt (refereegranskat)abstract
- Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.
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| 9. |
- Quillaguaman, Jorge, et al.
(författare)
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Optimizing conditions for poly(beta-hydroxybutyrate) production by Halomonas boliviensis LC1 in batch culture with sucrose as carbon source
- 2007
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Science and Business Media LLC. - 1432-0614 .- 0175-7598. ; 74:5, s. 981-986
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Tidskriftsartikel (refereegranskat)abstract
- Halomonas boliviensis LC1 is able to accumulate poly(beta-hydroxybutyrate) (PHB) under conditions of excess carbon source and depletion of essential nutrients. This study was aimed at an efficient production of PHB by growing H. boliviensis to high cell concentrations in batch cultures. The effect of ammonium, phosphate, and yeast extract concentrations on cell concentration [cell dry weight (CDW)] and PHB content of H. boliviensis cultured in shake flasks was assayed using a factorial design. High concentrations of these nutrients led to increments in cell growth but reduced the PHB content to some extent. Cultivations of H. boliviensis under controlled conditions in a fermentor using 1.5% (w/v) yeast extract as N source, and intermittent addition of sucrose to provide excess C source, resulted in a polymer accumulation of 44 wt.% and 12 g l(-1) CDW after 24 h of cultivation. Batch cultures in a fermentor with initial concentrations of 2.5% (w/v) sucrose and 1.5% (w/v) yeast extract, and with induced oxygen limitation, resulted in an optimum PHB accumulation, PHB concentration and CDW of 54 wt.%, 7.7 g l(-1) and 14 g l(-1), respectively, after 19 h of cultivation. The addition of casaminoacids in the medium increased the CDW to 14.4 g l(-1) in 17 h but reduced the PHB content in the cells to 52 wt.%.
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| 10. |
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