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X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. Generally, to correctly model the emerging Xray spectra, a detailed knowledge of the cool wind opacities based on stellar atmosphere models is prerequisite. A nearly grey stellar wind opacity for the X-rays is deduced from the analyses of high-resolution X-ray spectra. This indicates that the stellar winds are strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission line profiles can be explained if the wind clumps are radially compressed. In massive binaries the orbital variations of X-ray emission allow to probe the opacity of the stellar wind; results support the picture of strong wind clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the extend of the stellar-wind part photoionized by X-rays provide further strong evidence that stellar winds consist of dense clumps.
We summarize Chandra observations of the emission line profiles from 17 OB stars. The lines tend to be broad and unshifted. The forbidden/intercombination line ratios arising from Helium-like ions provide radial distance information for the X-ray emission sources, while the H-like to He-like line ratios provide X-ray temperatures, and thus also source temperature versus radius distributions. OB stars usually show power law differential emission measure distributions versus temperature. In models of bow shocks, we find a power law differential emission measure, a wide range of ion stages, and the bow shock flow around the clumps provides transverse velocities comparable to HWHM values. We find that the bow shock results for the line profile properties, consistent with the observations of X-ray line emission for a broad range of OB star properties.
Giacconi et al. (1962) discovered a diffuse cosmic X-ray background with rocket experiments when they searched for lunar X-ray emission. Later satellite missions found a spectral peak in the cosmic X-ray background at ~30 keV. Imaging X-ray satellites such as ROSAT (1990-1999) were able to resolve up to 80% of the background below 2 keV into single point sources, mainly active galaxies. The cosmic X-ray background is the integration of all accreting super-massive (several million solar masses) black holes in the centre of active galaxies over cosmic time. Synthesis models need further populations of X-ray absorbed active galaxy nuclei (AGN) in order to explain the cosmic X-ray background peak at ~30 keV. Current X-ray missions such as XMM-Newton and Chandra offer the possibility of studying these additional populations. This Ph.D. thesis studies the populations that dominate the X-ray sky. For this purpose the 120 ksec XMM-Newton Marano field survey, named for an earlier optical quasar survey in the southern hemisphere, is analysed. Based on the optical follow-up observations the X-ray sources are spectroscopically classified. Optical and X-ray properties of the different X-ray source populations are studied and differences are derived. The amount of absorption in the X-ray spectra of type II AGN, which are considered as a main contributor to the X-ray background at ~30 keV, is determined. In order to extend the sample size of the rare type II AGN, this study also includes objects from another survey, the XMM-Newton Serendipitous Medium Sample. In addition, the dependence of the absorption in type II AGN with redshift and X-ray luminosity is analysed. We detected 328 X-ray sources in the Marano field. 140 sources were spectroscopically classified. We found 89 type I AGN, 36 type II AGN, 6 galaxies, and 9 stars. AGN, galaxies, and stars are clearly distinguishable by their optical and X-ray properties. Type I and II AGN do not separate clearly. They have a significant overlap in all studied properties. In a few cases the X-ray properties are in contradiction to the observed optical properties for type I and type II AGN. For example we find type II AGN that show evidence for optical absorption but are not absorbed in X-rays. Based on the additional use of near infra-red imaging (K-band), we were able to identify several of the rare type II AGN. The X-ray spectra of type II AGN from the XMM-Newton Marano field survey and the XMM-Newton Serendipitous Medium Sample were analysed. Since most of the sources have only ~40 X-ray counts in the XMM-Newton PN-detector, I carefully studied the fit results of simulated X-ray spectra as a function of fit statistic and binning method. The objects revealed only moderate absorption. In particular, I do not find any Compton-thick sources (absorbed by column densities of NH > 1.5 x 10^24 cm^−2). This gives evidence that type II AGN are not the main contributor of the X-ray background around 30 keV. Although bias effects may occur, type II AGN show no noticeable trend of the amount of absorption with redshift or X-ray luminosity.
Writing an alternative Australia : women and national discourse in nineteenth-century literature
(2007)
In this thesis, I want to outline the emergence of the Australian national identity in colonial Australia. National identity is not a politically determined construct but culturally produced through discourse on literary works by female and male writers. The emergence of the dominant bushman myth exhibited enormous strength and influence on subsequent generations and infused the notion of “Australianness” with exclusively male characteristics. It provided a unique geographical space, the bush, on and against which the colonial subject could model his identity. Its dominance rendered non-male and non-bush experiences of Australia as “un-Australian.” I will present a variety of contemporary voices – postcolonial, Aboriginal, feminist, cultural critics – which see the Australian identity as a prominent topic, not only in the academia but also in everyday culture and politics. Although positioned in different disciplines and influenced by varying histories, these voices share a similar view on Australian society: Australia is a plural society, it is home to millions of different people – women, men, and children, Aboriginal Australians and immigrants, newly arrived and descendents of the first settlers – with millions of different identities which make up one nation. One version of national identity does not account for the multitude of experiences; one version, if applied strictly, renders some voices unheard and oppressed. After exemplifying how the literature of the 1890s and its subsequent criticism constructed the itinerant worker as “the” Australian, literary productions by women will be singled out to counteract the dominant version by presenting different opinions on the state of colonial Australia. The writers Louisa Lawson, Barbara Baynton, and Tasma are discussed with regard to their assessment of their mother country. These women did not only present a different picture, they were also gifted writers and lived the ideal of the “New Women:” they obtained divorces, remarried, were politically active, worked for their living and led independent lives. They paved the way for many Australian women to come. In their literary works they allowed for a dual approach to the bush and the Australian nation. Louisa Lawson credited the bushwoman with heroic traits and described the bush as both cruel and full of opportunities not known to women in England. She understood women’s position in Australian society as oppressed and tried to change politics and culture through the writings in her feminist magazine the Dawn and her courageous campaign for women suffrage. Barbara Baynton painted a gloomy picture of the Australian bush and its inhabitants and offered one of the fiercest critiques of bush society. Although the woman is presented as the able and resourceful bushperson, she does not manage to survive in an environment which functions on male rules and only values the economic potential of the individual. Finally, Tasma does not present as outright a critique as Barbara Baynton, however, she also attests the colonies a fascination with wealth which she renders questionable. She offers an informed judgement on colonial developments in the urban surrounds of the city of Melbourne through the comparison of colonial society with the mother country England. Tasma attests that the colonies had a fascination with wealth which she renders questionable. She offers an informed judgement on colonial developments in the urban surrounds of the city of Melbourne through the comparison of colonial society with the mother country England and demonstrates how uncertainties and irritations emerged in the course of Australia’s nation formation. These three women, as writers, commentators, and political activists, faced exclusion from the dominant literary discourses. Their assessment of colonial society remained unheard for a long time. Now, after much academic excavation, these voices speak to us from the past and remind us that people are diverse, thus nation is diverse. Dominant power structures, the institutions and individuals who decide who can contribute to the discourse on nation, have to be questioned and reassessed, for they mute voices which contribute to a wider, to the “full”, and maybe “real” picture of society.
Luminous Blue Variables show strong changes in their stellar wind on time scales of typically years to decades when they expand and contract radially at approximately constant luminosity. Micro-variability on shorter time scales and amplitudes can be observed superimposed to the larger scale radial changes. I will show long-term time series of high resolution spectra which we have collected in the past 20 years for many of the well known LBVs together with a few time series of weekly sampling (HR Car, R40, R71, R110, R127, S Dor) covering a time windows of up to a few months. Wind variability is seen on short and intermediate time scales with the line profiles changing from P Cygni to inverse P Cygni and double peeked profiles sometimes for the same star and spectral line. On longer time scales the ionisation levels for all chemical elements change drastically due to the strong change of the temperature on the stellar surface. While on the long term the characteristic radial changes may have impact on the over all mass loss rates, the variabilities and asymmetries on short and intermediate time scales may cause false estimates of the mass loss rates when confronting models with the observed line profiles
The most massive stars are those with the shortest but most active life. One group of massive stars, the Luminous Blue Variables (LBVs), of which only a few objects are known, are in particular of interest concerning the stability of stars. They have a high mass loss rate and are close to being instable. This is even more likely as rotation becomes an important factor in stellar evolution of these stars. Through massive stellar winds and sometimes giant eruptions, LBV nebulae are formed. Various aspects in the evolution in the LBV phase lead, beside the large scale morphological and kinematical differences, to a diversity of small structures like clumps, rims, and outflows in these nebulae.
We discuss the results of time-resolved spectroscopy of three presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud, where the ambient metallicity is $\sim 1/5 Z_\odot$. We were able to detect and follow numerous small-scale wind-embedded inhomogeneities in all observed stars. The general properties of the moving features, such as their velocity dispersions, emissivities and average accelerations, closely match the corresponding characteristics of small-scale inhomogeneities in the winds of Galactic Wolf-Rayet stars.
The influence of the wind to the total continuum of OB supergiants is discussed. For wind velocity distributions with β > 1.0, the wind can have strong influence to the total continuum emission, even at optical wavelengths. Comparing the continuum emission of clumped and unclumped winds, especially for stars with high β values, delivers flux differences of up to 30% with maximum in the near-IR. Continuum observations at these wavelengths are therefore an ideal tool to discriminate between clumped and unclumped winds of OB supergiants.