520 Astronomie und zugeordnete Wissenschaften
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Gasausströmungen, oft in der Form hoch kollimierter Jets, sind ein allgegenwärtiges Phänomen bei der Geburt neuer Sterne. Emission von stossangeregtem molekularem Wasserstoff bei Wellenlängen im nahen Infrarotbereich ist ein Merkmal ihrer Existenz und auch in eingebetteten, im Optischen obskurierten Ausströmungen generell gut zu beobachten. In dieser Arbeit werden die Resultate einer von Auswahleffekten freien, empfindlichen, grossflächigen Suche nach solchen Ausströmungen von Protosternen in der v=1-0 S(1) Linie molekularen Wasserstoffs bei einer Wellenlänge von 2.12 µm vorgestellt. Die Durchmusterung umfasst eine Fläche von etwa einem Quadratgrad in der Orion A Riesenmolekülwolke. Weitere Daten aus einem grossen Wellenlängenbereich werden benutzt, um die Quellen der Ausströmungen zu identifizieren. Das Ziel dieser Arbeit ist es, eine Stichprobe von Ausströmungen zu bekommen, die so weit wie möglich frei von Auswahleffekten ist, um die typischen Eigenschaften protostellarer Ausströmungen und deren Entwicklung festzustellen, sowie um die Rückwirkung der Ausströmungen auf die umgebende Wolke zu untersuchen. Das erste Ergebnis ist, dass Ausströmungen in Sternentstehungsgebieten tatsächlich sehr häufig sind: mehr als 70 Jet-Kandidaten werden identifiziert. Die meisten zeigen eine sehr irreguläre Morphologie anstelle regulärer oder symmetrischer Strukturen. Dies ist auf das turbulente, klumpige Medium zurückzuführen, in das sich die Jets hineinbewegen. Die Ausrichtung der Jets ist zufällig verteilt. Insbesondere gibt es keine bevorzugte Ausrichtung der Jets parallel zum grossräumigen Magnetfeld in der Wolke. Das legt nahe, dass die Rotations- und Symmetrieachse in einem protostellaren System durch zufällige, turbulente Bewegung in der Wolke bestimmt wird. Mögliche Ausströmungsquellen werden für 49 Jets identifiziert; für diese wird der Entwicklungsstand und die bolometrische Leuchtkraft abgeschätzt. Die Jetlänge und die H2 Leuchtkraft entwickeln sich gemeinsam mit der Ausströmungsquelle. Von null startend, dehnen sich die Jets schnell bis auf eine Länge von einigen Parsec aus und werden dann langsam wieder kürzer. Sie sind zuerst sehr leuchtkräftig, die H2 Helligkeit nimmt aber im Lauf der protostellaren Entwicklung ab. Die Längen- und H2 Leuchtkraftentwicklung lässt sich im Wesentlichen durch eine zuerst sehr hohe, dann niedriger werdende Massenausflussrate erklären, die auf eine zuerst sehr hohe, dann niedriger werdende Gasakkretionsrate auf den Protostern schliessen lässt (Akkretion und Ejektion sind eng verknüpft!). Die Längenabnahme der Jets erfordert eine ständig wirkende Abbremsung der Jets. Ein einfaches Modell einer simultanen Entwicklung eines Protosterns, seiner zirkumstellaren Umgebung und seiner Ausströmung (Smith 2000) kann die gemessenen H2- und bolometrischen Leuchtkräfte der Jets und ihrer Quellen reproduzieren, unter der Annahme, dass die starke Akkretionsaktivität zu Beginn der protostellaren Entwicklung mit einer überproportional hohen Massenausflussrate verbunden ist. Im Durchmusterungsgebiet sind 125 dichte Molekülwolkenkerne bekannt (Tatematsu et al. 1993). Jets (bzw. Sterne) entstehen in ruhigen Wolkenkernen, d.h. solchen mit einem niedrigen Verhältnis von interner kinetischer Energie zu gravitativer potentieller Energie; dies sind die Wolkenkerne höherer Masse. Die Wolkenkerne mit Jets haben im Mittel grössere Linienbreiten als die ohne Jets. Dies ist darauf zurückzuführen, dass sie bevorzugt in den massereicheren Wolkenkernen zu finden sind, welche generell eine grössere Linienbreite haben. Es gibt keinen Hinweis auf stärkere interne Bewegungen in Wolkenkernen mit Jets, die durch eine Wechselwirkung der Jets mit den Wolkenkernen erzeugt sein könnte. Es gibt, wie von der Theorie vorausgesagt, eine Beziehung zwischen der Linienbreite der Wolkenkerne und der H2 Leuchtkraft der Jets, wenn Jets von Klasse 0 und Klasse I Protosternen separat betrachtet werden; dabei sind Klasse 0 Jets leuchtkräftiger als Klasse I Jets, was ebenfalls auf eine zeitabhängige Akkretionsrate mit einer frühzeitigen Spitze und einem darauffolgenden Abklingen hinweist. Schliesslich wird die Rückwirkung der Jetpopulation auf eine Molekülwolke unter der Annahme strikter Vorwärtsimpulserhaltung betrachtet. Die Jets können auf der Skala einer ganzen Riesenmolekülwolke und auf den Skalen von Molekülwolkenkernen nicht genügend Impuls liefern, um die abklingende Turbulenz wieder anzuregen. Auf der mittleren Skala von molekularen Klumpen, mit einer Grösse von einigen parsec und Massen von einigen hundert Sonnenmassen liefern die Jets jedoch genügend Impuls in hinreichend kurzer Zeit, um die Turbulenz “am Leben zu erhalten” und können damit helfen, einen Klumpen gegen seinen Kollaps zu stabilisieren.
Strong damped Lyman alpha absorption (DLA) lines seen spectra of distant quasar are believed to arise when the sight line to the quasar goes trough the disc of a galaxy or a proto galaxy. Most of the neutral matter in the universe is contained in these clouds of neutral hydrogen that cause the absorption lines. Hence these DLAs are reservoirs for the formation of stars and galaxies throughout the universe. Despite intensive efforts over more than two decades only few galaxies responsible for the DLAs have been found. The problem is that the galaxies that harbour the neutral clouds are not necessarily bright, and selecting galaxies based on absorption lines could well select different types of galaxies than found in large surveys. If we are to understand how galaxies form out of neutral gas clouds it is essential to locate the galaxies in which DLAs reside. This thesis explores the use of integral field spectroscopy (IFS) to observe quasars known to have strong absorption lines in their spectra. IFS allows us to obtain a spectrum at many spatial points close to the quasar, thus providing images and spectroscopy simultaneously. From the imaging part, we can directly identify objects, and from the spectroscopy we can calculate the distances to the objects. When the distance of the object found in emission matches the distance to the object that cause the DLA line, we have identified the absorbing galaxy. Using this technique, we have showed that we can successfully recover a few DLA galaxies known previously from the literature. In a survey aimed to increase the number of DLA galaxies we have identified eight new candidate DLA galaxies. The projected distances from the candidates to the quasar sight lines indicate that the DLA galaxies have sizes similar to local disc galaxies. Hence our results suggest that large discs may be present when the universe was just 2 billion years old. We furthermore find no differences between the sizes of the very distant DLA galaxies and those that are not so distant. The large sizes imply that their neutral hydrogen masses are also similar to those in local galaxies, but we argue that the DLA galaxies are not necessarily as luminous as the present day disc galaxies. Taking advantage of the three-dimensional view provided by the IFS data, the second part of this thesis investigates extended emission line regions arising in the quasar neighborhood. We find that extended emission line nebulae are common around quasars, and explore the effects that may be the cause. Some quasars are known to be powerful radio emitters while others are not detected at radio wavelengths. We find that significantly larger and brighter emission line nebulae are found around the quasars which have the brightest radio emission, and in particular those that have large radio jets. The existence of the nebulae can be interpreted as an interaction of the radio jet with the surrounding medium, but we can not rule out a scenario where there are density or temperature differences in the surrounding environment. Only for the brightest object, where additional velocity information can be derived from the IFS data, can we argue for an interaction. In conclusion the use of IFS to search for faint emission lines, both from point sources and extended nebulae provides exciting new results within the scientific areas studied here.
I perform and analyse the first ever calculations of rotating stellar iron core collapse in {3+1} general relativity that start out with presupernova models from stellar evolutionary calculations and include a microphysical finite-temperature nuclear equation of state, an approximate scheme for electron capture during collapse and neutrino pressure effects. Based on the results of these calculations, I obtain the to-date most realistic estimates for the gravitational wave signal from collapse, bounce and the early postbounce phase of core collapse supernovae. I supplement my {3+1} GR hydrodynamic simulations with 2D Newtonian neutrino radiation-hydrodynamic supernova calculations focussing on (1) the late postbounce gravitational wave emission owing to convective overturn, anisotropic neutrino emission and protoneutron star pulsations, and (2) on the gravitational wave signature of accretion-induced collapse of white dwarfs to neutron stars.
The H.E.S.S. collaboration recently reported the discovery of VHE γ-ray emission coincident with the young stellar cluster Westerlund 2. This system is known to host a population of hot, massive stars, and, most particularly, the WR binary WR 20a. Particle acceleration to TeV energies in Westerlund 2 can be accomplished in several alternative scenarios, therefore we only discuss energetic constraints based on the total available kinetic energy in the system, the actual mass loss rates of respective cluster members, and implied gamma-ray production from processes such as inverse Compton scattering or neutral pion decay. From the inferred gammaray luminosity of the order of 1035erg/s, implications for the efficiency of converting available kinetic energy into non-thermal radiation associated with stellar winds in the Westerlund 2 cluster are discussed under consideration of either the presence or absence of wind clumping.
Mass loss is a very important aspect of the life of massive stars. After briefly reviewing its importance, we discuss the impact of the recently proposed downward revision of mass loss rates due to clumping (difficulty to form Wolf-Rayet stars and production of critically rotating stars). Although a small reduction might be allowed, large reduction factors around ten are disfavoured. We then discuss the possibility of significant mass loss at very low metallicity due to stars reaching break-up velocities and especially due to the metal enrichment of the surface of the star via rotational and convective mixing. This significant mass loss may help the first very massive stars avoid the fate of pair-creation supernova, the chemical signature of which is not observed in extremely metal poor stars. The chemical composition of the very low metallicity winds is very similar to that of the most metal poor star known to date, HE1327-2326 and offer an interesting explanation for the origin of the metals in this star. We also discuss the importance of mass loss in the context of long and soft gamma-ray bursts and pair-creation supernovae. Finally, we would like to stress that mass loss in cooler parts of the HR-diagram (luminous blue variable and yellow and red supergiant stages) are much more uncertain than in the hot part. More work needs to be done in these areas to better constrain the evolution of the most massive stars.
The P v λλ1118, 1128 resonance doublet is an extraordinarily useful diagnostic of O-star winds, because it bypasses the traditional problems associated with determining mass-loss rates from UV resonance lines. We discuss critically the assumptions and uncertainties involved with using P v to diagnose mass-loss rates, and conclude that the large discrepancies between massloss rates determined from P v and the rates determined from “density squared” emission processes pose a significant challenge to the “standard model” of hot-star winds. The disparate measurements can be reconciled if the winds of O-type stars are strongly clumped on small spatial scales, which in turn implies that mass-loss rates based on Hα or radio emission are too large by up to an order of magnitude.
We have analyzed the spectra of seven Galactic O4 supergiants, with the NLTE wind code CMFGEN. For all stars, we have found that clumped wind models match well lines from different species spanning a wavelength range from FUV to optical, and remain consistent with Hα data. We have achieved an excellent match of the P V λλ1118, 1128 resonance doublet and N IV λ1718, as well as He II λ4686 suggesting that our physical description of clumping is adequate. We find very small volume filling factors and that clumping starts deep in the wind, near the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more compared to those obtained from smooth-wind models.
The spatially-resolved winds of the massive binary, Eta Carinae, extend an arcsecond on the sky, well beyond the 10 to 20 milliarcsecond binary orbital dimension. Stellar wind line profiles, observed at very different angular resolutions of VLTI/AMBER, HST/STIS and VLT/UVES, provide spatial information on the extended wind interaction structure as it changes with orbital phase. These same wind lines, observable in the starlight scattered off the foreground lobe of the dusty Homunculus, provide time-variant line profiles viewed from significantly different angles. Comparisons of direct and scattered wind profiles observed in the same epoch and at different orbital phases provide insight on the extended wind structure and promise the potential for three-dimensional imaging of the outer wind structures. Massive, long-lasting clumps, including the nebularWeigelt blobs, originated during the two historical ejection events. Wind interactions with these clumps are quite noticeable in spatially-resolved spectroscopy. As the 2009.0 minimum approaches, analysis of existing spectra and 3-D modeling are providing bases for key observations to gain further understanding of this complex massive binary.
Observational evidence exists that winds of massive stars are clumped. Many massive star systems are known as non-thermal particle production sites, as indicated by their synchrotron emission in the radio band. As a consequence they are also considered as candidate sites for non-thermal high-energy photon production up to gamma-ray energies. The present work considers the effects of wind clumpiness expected on the emitting relativistic particle spectrum in colliding wind systems, built up from the pool of thermal wind particles through diffusive particle acceleration, and taking into account inverse Compton and synchrotron losses. In comparison to a homogeneous wind, a clumpy wind causes flux variations of the emitting particle spectrum when the clump enters the wind collision region. It is found that the spectral features associated with this variability moves temporally from low to high energy bands with the time shift between any two spectral bands being dependent on clump size, filling factor, and the energy-dependence of particle energy gains and losses.
We present preliminary results of a tailored atmosphere analysis of six Galactic WC stars using UV, optical, and mid-infrared Spitzer IRS data. With these data, we are able to sample regions from 10 to 10³ stellar radii, thus to determine wind clumping in different parts of the wind. Ultimately, derived wind parameters will be used to accuratelymeasure neon abundances, and to so test predicted nuclear-reaction rates.