Institute For Physik Und Astronomie

Potsdam, Germany

Institute For Physik Und Astronomie

Potsdam, Germany
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Sundqvist J.O.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Feldmeier A.,Institute For Physik Und Astronomie
Astronomy and Astrophysics | Year: 2010

Context. The mass-loss rate is a key parameter of hot, massive stars. Small-scale inhomogeneities (clumping) in the winds of these stars are conventionally included in spectral analyses by assuming optically thin clumps, a void inter-clump medium, and a smooth velocity field. To reconcile investigations of different diagnostics (in particular, unsaturated UV resonance lines vs. Hα radio emission) within such models, a highly clumped wind with very low mass-loss rates needs to be invoked, where the resonance lines seem to indicate rates an order of magnitude (or even more) lower than previously accepted values. If found to be realistic, this would challenge the radiative line-driven wind theory and have dramatic consequences for the evolution of massive stars. Aims. We investigate basic properties of the formation of resonance lines in small-scale inhomogeneous hot star winds with non-monotonic velocity fields. Methods. We study inhomogeneous wind structures by means of 2D stochastic and pseudo-2D radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. A Monte-Carlo radiative transfer code, which treats the resonance line formation in an axially symmetric spherical wind (without resorting to the Sobolev approximation), is presented and used to produce synthetic line spectra. Results. The optically thin clumping limit is only valid for very weak lines. The detailed density structure, the inter-clump medium, and the non-monotonic velocity field are all important for the line formation. We confirm previous findings that radiation-hydrodynamic wind models reproduce observed characteristics of strong lines (e.g., the black troughs) without applying the highly supersonic "microturbulence" needed in smooth models. For intermediate strong lines, the velocity spans of the clumps are of central importance. Current radiation-hydrodynamic models predict spans that are too large to reproduce observed profiles unless a very low mass-loss rate is invoked. By simulating lower spans in 2D stochastic models, the profile strengths become drastically reduced, and are consistent with higher mass-loss rates. To simultaneously meet the constraints from strong lines, the inter-clump medium must be non-void. A first comparison to the observed Phosphorus V doublet in the O6 supergiant λ Cep confirms that line profiles calculated from a stochastic 2D model reproduce observations with a mass-loss rate approximately ten × higher than that derived from the same lines but assuming optically thin clumping. Tentatively this may resolve discrepancies between theoretical predictions, evolutionary constraints, and recent derived mass-loss rates, and suggests a re-investigation of the clump structure predicted by current radiation-hydrodynamic models. © ESO, 2010.

Sundqvist J.O.,Universitatssternwarte Munich | Puls J.,Universitatssternwarte Munich | Feldmeier A.,Institute For Physik Und Astronomie | Owocki S.P.,University of Delaware
Astronomy and Astrophysics | Year: 2011

Context. Mass loss is essential for massive star evolution, thus also for the variety of astrophysical applications relying on its predictions. However, mass-loss rates currently in use for hot, massive stars have recently been seriously questioned, mainly because of the effects of wind clumping. Aims. We investigate the impact of clumping on diagnostic ultraviolet resonance and optical recombination lines often used to derive empirical mass-loss rates of hot stars. Optically thick clumps, a non-void interclump medium, and a non-monotonic velocity field are all accounted for in a single model. The line formation is first theoretically studied, after which an exemplary multi-diagnostic study of an O-supergiant is performed. Methods. We used 2D and 3D stochastic and radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. To compute synthetic spectra, we developed and used detailed radiative transfer codes for both recombination lines (solving the "formal integral") and resonance lines (using a Monte-Carlo approach). In addition, we propose an analytic method to model these lines in clumpy winds, which does not rely on optically thin clumping. Results. The importance of the "vorosity" effect for line formation in clumpy winds is emphasized. Resonance lines are generally more affected by optically thick clumping than recombination lines. Synthetic spectra calculated directly from current radiation-hydrodynamic wind models of the line-driven instability are unable to in parallel reproduce strategic optical and ultraviolet lines for the Galactic O-supergiant λ Cep. Using our stochastic wind models, we obtain consistent fits essentially by increasing the clumping in the inner wind. A mass-loss rate is derived that is approximately two times lower than what is predicted by the line-driven wind theory, but much higher than the corresponding rate derived when assuming optically thin clumps. Our analytic formulation for line formation is used to demonstrate the potential importance of optically thick clumping in diagnostic lines in so-called weak-winded stars and to confirm recent results that resonance doublets may be used as tracers of wind structure and optically thick clumping. Conclusions. We confirm earlier results that a re-investigation of the structures in the inner wind predicted by line-driven instability simulations is needed. Our derived mass-loss rate for λ Cep suggests that only moderate reductions of current mass-loss predictions for OB-stars are necessary, but this nevertheless prompts investigations on feedback effects from optically thick clumping on the steady-state, NLTE wind models used for quantitative spectroscopy. © 2011 ESO.

Fox A.,Institute For Physik Und Astronomie | Richter P.,Institute For Physik Und Astronomie | Fechner C.,Institute For Physik Und Astronomie
Astronomy and Astrophysics | Year: 2014

We present a detailed analysis of chemical abundances in a sub-damped Lyman α absorber at z = 1:839 towards the quasar B1101..26, based on a very-high-resolution (R ∼ 75 000) and high-signal-to-noise (S=N > 100) spectrum observed with the UV Visual Echelle spectrograph (UVES) installed on the ESO Very Large Telescope (VLT). The absorption line profiles are resolved into a maximum of eleven velocity components spanning a rest-frame velocity range of ≈200 km s..1. Detected ions include Cii, C iv, Nii, Oi, Mgi, Mgii, Al ii, Al iii, Si ii, Si iii, Si iv, Fe ii, and possibly S ii. The total neutral hydrogen column density is log N(H i) = 19:48 ± 0:01. From measurements of column densities and Doppler parameters we estimate element abundances of the above-given elements. The overall metallicity, as traced by [O i/Hi], is ..1:56 ± 0:01. For the nitrogen-to-oxygen ratio we derive an upper limit of [N i/Oi] ≥ ..0:65, which suggests a chemically young absorption line system. This is supported by a supersolar α/Fe ratio of [Si ii/Fe ii] ≈ 0:5. The most striking feature in the observed abundance pattern is an unusually high sulphur-to-oxygen ratio of 0:69 ≥ [S ii/Oi] ≥ 1:26. We calculate detailed photoionisation models for two subcomponents with Cloudy, and can rule out that ionisation eαects alone are responsible for the high S/O ratio. We instead speculate that the high S/O ratio is caused by the combination of several eαects, such as specific ionisation conditions in multi-phase gas, unusual relative abundances of heavy elements, and/or dust depletion in a local gas environment that is not well mixed and/or that might be related to star-formation activity in the host galaxy. We discuss the implications of our findings for the interpretation of α-element abundances in metal absorbers at high redshift. © ESO 2014.

Fox A.,Institute For Physik Und Astronomie | Richter P.,Institute For Physik Und Astronomie
Astronomy and Astrophysics | Year: 2016

We present a detailed analysis of a very unusual sub-damped Lyman α (sub-DLA) system at redshift z = 2.304 towards the quasar Q 0453-423, based on high signal-to-noise (S/N), high-resolution spectral data obtained with VLT/UVES. With a neutral hydrogen column density of log N(H i) = 19.23 and a metallicity of -1.61 as indicated by [O i/H i] the sub-DLA mimics the properties of many other optically thick absorbers at this redshift. A very unusual feature of this system is, however, the lack of any C iv absorption at the redshift of the neutral hydrogen absorption, although the relevant spectral region is free of line blends and has very high S/N. Instead, we find high-ion absorption from C iv and O vi in another metal absorber at a velocity more than 220 km s-1 redwards of the neutral gas component. We explore the physical conditions in the two different absorption systems using Cloudy photoionisation models. We find that the weakly ionised absorber is dense and metal-poor while the highly ionised system is thin and more metal-rich. The absorber pair towards Q 0453-423 mimics the expected features of a galactic outflow with highly ionised material that moves away with high radial velocities from a (proto)galactic gas disk in which star-formation takes place. We discuss our findings in the context of C iv absorption line statistics at high redshift and compare our results to recent galactic-wind and outflow models. © ESO, 2016.

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