Statistics – Computation
Scientific paper
Nov 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt........14n&link_type=abstract
PhD Thesis, University of Vienna, Austria
Statistics
Computation
4
Stars: Late-Type, Stars: Agb And Post-Agb, Stars: Atmospheres, Stars: Carbon, Infrared: Stars, Line: Profiles
Scientific paper
Light is the only source of information we have to study distant stars. Our knowledge about the state of the matter inside stars has been gathered by analysing star light (photometry, spectroscopy, interferometry, polarimetry, etc.). Of central importance in this context are stellar atmospheres, which are the transition regions from the optically thick stellar interiors where the electromagnetic radiation is generated to the optically thin outer layers from where the photons can leave the star. However, the atmosphere of a star is not only the region where most of the observable radiation is emitted or in other words the layers which are "visible from outside". The atmosphere also leaves an imprint on the stellar spectrum as the radiation passes through, most of the line spectrum is formed there. Thus, the light serves as a probe for the physical processes within stellar atmospheres, especially spectroscopy is one of the major tools in stellar astrophysics. Applying the underlying physical principles in numerical simulations (model atmospheres, synthetic spectra) is the second -- complementary and necessary -- step towards a deeper understanding of stellar atmospheres and for deriving stellar parameters (e.g. T_eff, L, log g, chemical composition) of observed objects.
This thesis is dedicated to the outer layers of Asymptotic Giant Branch (AGB) stars, which have rather remarkable properties compared to atmospheres of most other types of stars. AGB stars represent low- to intermediate mass stars at a late stage of their evolution. Forming a sub-group among all red giants, they exhibit large extensions, low effective temperatures and high luminosities. The evolutionary phase of the AGB -- complex but decisive for stellar evolution -- is characterised by several important phenomena as for example nucleo-synthesis in explosively burning shells (thermal pulses), convective processes (dredge up), large-amplitude pulsations with long periods or a pronounced mass loss.
Red giant stars generally have extremely extended atmospheres with extensions on the same order as the radii of the stars themselves (a few 100 R_sol). Within these cool and relatively dense environments, molecules can efficiently form. They have many internal degrees of freedom leading to a large number of possible transitions (electronic, vibrational, and rotational) and numerous absorption lines/bands. Thus, molecules significantly determine the spectral appearance of late-type stars which have characteristic line-rich spectra in the visual and infrared. At the upper part of the AGB, the stars become unstable to strong radial pulsations (e.g. Mira variables). Due to the large size variations of the stellar interior, the outer layers are levitated and the atmospheric structure is periodically modulated. Triggered by the pulsation, shock waves emerge and propagate outwards through the atmosphere. Efficient dust condensation can take place in the wake of the shock waves ( post-shock regions). Due to the large absorptivity of the formed dust grains, radiation pressure results in an outwards directed acceleration with the outflowing dust particles dragging along the surrounding gas. This leads to the development of a rather slow but dense stellar wind. The just mentioned dynamic effects -- pulsations of the stellar interior and dust-driven winds -- have substantial influence on the evolution of the outer layers of these red giants. As a consequence, the atmospheres of evolved AGB stars can eventually become even more extended. Being time-dependently changed on global and local scales, the resulting atmospheric structure strongly deviates from a hydrostatic configuration (e.g. shock fronts). Especially important in the context of this thesis are the complex, non-monotonic velocity fields with macroscopic motions on the order of 10 km/s, severly affecting the shapes of individual spectral lines (Doppler effect).
Observational studies have demonstrated that time series high-resolution spectroscopy in the near infrared (where AGB stars are bright and well observable) represents a valuable tool to study atmospheric kinematics of red giants. The spectra of these stars are densely populated by numerous absorption lines, making very high spectral resolutions (lambda/Dlambda of a few 10000) necessary. It turns out that spectral features of different vibration-rotation bands (or molecules) originate in separated regions of different atmospheric (geometrical) depth. The movements of the layers there can heavily influence the appearance of molecular line profiles in observed spectra (e.g. broadening, line doubling). Radial velocities (RV) derived from (Doppler-) shifts in wavelength of spectral lines provide clues on the gas velocities in the line-forming region of the respective feature. Monitoring line profiles of different individual molecular lines allows to probe atmospheric kinematics throughout the extended AGB atmospheres. Thus, we can trace the velocity field within all regions of the dynamic outer layers of AGB stars over time -- and thereby the mass loss process -- by repeated spectroscopic observations. Particularly useful in this context is the CO molecule, which is very stable against dissociation (due to its high bond energy) and therefore present at all depths. Three different vibration-rotation band systems originate in quite separated regions and are well observable in spectral windows of the earth's atmosphere. The corresponding NIR features nicely trace all layers from deep inside the atmosphere out to the cool wind region. Variations of CO line profiles can be used to systematically explore structure and dynamics of AGB atmospheres at different depths. For example, line splitting as a function of phase provides information about how shock waves progress up through the atmosphere. Spectral features of CN are prominent in visual and NIR spectra of carbon-rich AGB stars and can in addition to CO be used ! to infer velocity information.
Modelling the cool and very extended atmospheres of evolved AGB stars remains challenging due to the intricate interaction of different complex phenomena (convection, pulsation, radiation, molecular and dust formation/absorption, acceleration of winds). Standard hydrostatic models are not an adequate approach in the context of this thesis as they neglect the strong influence of dynamics on atmospheric structure and line profiles. Dynamic model atmospheres are constructed to simulate and understand the physical processes taking place in the outer layers of AGB stars, for example they are crucial for our understanding of the mass loss process. A combined and self-consistent solution of hydrodynamics, frequency-dependent radiative transfer and a detailed time-dependent treatment of dust formation/evolution is needed to reproduce the complex, temporally variable structures of AGB atmospheres properly. In this thesis, we utilised models for long period variables with well-pronounced and regular pulsations as well as moderate mass loss rates (Miras). These models represent the scenario of pulsation-enhanced dust-driven winds and provide a consistent and realistic description from the deep and dust-free photospheric layers (dominated by the pulsation of the stellar interior) out to the dust-forming layers and beyond to the stellar wind region. Taking into account the effects of dust in a consistent way is (so far) only possible for carbon-rich chemistries, therefore we concentrated on model atmospheres for C-type Miras in the work presented here.
The aim for this thesis was to see, if observed line profiles variations can be comprehended with state-of-the-art dynamic model atmospheres. The typical behaviour of different spectral features is an important observational aspect which should be reproduced by self-consistent numerical simulations. On the one hand, insights gained by such a line profile modelling may help to interpret the observed complex multi-component profiles. On the other hand, reproducing the temporal variability of various molecular line profiles is a crucial test for the atmosphe
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