arXiv subjects

Wolf-Rainer Hamann

Publications and source records attributed to Wolf-Rainer Hamann.

31 records · Page 2Linked to original sources

arXiv2014

Expansion of hydrogen-poor knots in the born-again planetary nebulae A30 and A78

We analyze the expansion of hydrogen-poor knots and filaments in the born-again planetary nebulae A30 and A78 based on Hubble Space Telescope (HST) images obtained almost 20 yr apart. The proper motion of these features generally increases with distance to the central star, but the fractional expansion decreases, i.e., the expansion is not homologous. As a result, there is not a unique expansion age, which is estimated to be 610-950 yr for A30 and 600-1140 yr for A78. The knots and filaments have experienced complex dynamical processes: the current fast stellar wind is mass loaded by the material ablated from the inner knots; the ablated material is then swept up until it shocks the inner edges of the outer, hydrogen-rich nebula. The angular expansion of the outer filaments shows a clear dependence on position angle, indicating that the interaction of the stellar wind with the innermost knots channels the wind along preferred directions. The apparent angular expansion of the innermost knots seems to be dominated by the rocket effect of evaporating gas and by the propagation of the ionization front inside them. Radiation-hydrodynamical simulations show that a single ejection of material followed by a rapid onset of the stellar wind and ionizing flux can reproduce the variety of clumps and filaments at different distances from the central star found in A30 and A78.

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arXiv2014

A new type of X-ray pulsar

X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. Here we report the discovery of pulsating X-rays from the massive B-type star Xi1 Canis Majoris. This star is a variable of beta Cephei type and has a strong magnetic field. Our observations with the XMM-Newton telescope reveal X-ray pulsations with the same period as the fundamental stellar pulsation. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission, and their magnetism.

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arXiv2014

The Wolf-Rayet stars in M31: I. Analysis of the late-type WN stars

Context: Comprehensive studies of Wolf-Rayet stars were performed in the past for the Galactic and the LMC population. The results revealed significant differences, but also unexpected similarities between the WR populations of these different galaxies. Analyzing the WR stars in M31 will extend our understanding of these objects in different galactic environments. Aims: The present study aims at the late-type WN stars in M31. The stellar and wind parameters will tell about the formation of WR stars in other galaxies with different metallicity and star formation histories. The obtained parameters will provide constraints to the evolution of massive stars in the environment of M31. Methods: We used the latest version of the Potsdam Wolf-Rayet model atmosphere code to analyze the stars via fitting optical spectra and photometric data. To account for the relatively low temperatures of the late WN10 and WN11 subtypes, our WN models have been extended into this temperature regime. Results: Stellar and atmospheric parameters are derived for all known late-type WN stars in M31 with available spectra. All of these stars still have hydrogen in their outer envelopes, some of them up to 50% by mass. The stars are located on the cool side of the zero age main sequence in the Hertzsprung-Russell diagram, while their luminosities range from $10^5$ to $10^6$ Lsun. It is remarkable that no star exceeds $10^6$ Lsun. Conclusions: If formed via single-star evolution, the late-type WN stars in M31 stem from an initial mass range between 20 and 60 Msun. From the very late-type WN9-11 stars, only one star is located in the S Doradus instability strip. We do not find any late-type WN stars with the high luminosities known in the Milky Way.

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arXiv2014

The impact of rotation on the line profiles of Wolf-Rayet stars

Massive Wolf-Rayet stars are recognized today to be in a very common, but short, evolutionary phase of massive stars. While our understanding of Wolf-Rayet stars has increased dramatically over the past decades, it remains unclear whether rapid rotators are among them. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. However, recently observed spectra of several Wolf-Rayet stars reveal peculiarly broad and round emission lines. Could these spectra imply rapid rotation? In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may be explained with the help of stellar rotation, infer appropriate rotation parameters, and discuss the implications of our results. We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code. Since the observed spectra of Wolf-Rayet stars are mainly formed in their expanding atmospheres, rotation must be accounted for with a 3D integration scheme of the formal integral. For this purpose, we assume a rotational velocity field consisting of an inner co-rotating domain and an outer domain, where the angular momentum is conserved. We find that rotation can reproduce the unique spectra analyzed here. However, the inferred rotational velocities at the stellar surface are large (~200 km/s), and the inferred co-rotation radii (~10 stellar radii) suggest the existence of very strong photospheric magnetic fields (~20 kG).

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arXiv2012

WR 120bb and WR 120bc: a pair of WN9h stars with possibly interacting circumstellar shells

Two optically obscured Wolf-Rayet (WR) stars have been recently discovered by means of their infrared (IR) circumstellar shells, which show signatures of interaction with each other. Following the systematics of the WR star catalogues, these stars obtain the names WR\,120bb and WR\,120bc. In this paper, we present and analyse new near-IR, $J$, $H$, and $K$-band, spectra using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. For that purpose, the atomic data base of the code has been extended in order to include all significant lines in the near-IR bands. The spectra of both stars are classified as WN9h. As their spectra are very similar the parameters that we obtained by the spectral analyses hardly differ. Despite their late spectral subtype, we found relatively high stellar temperatures of 63 kK. The wind composition is dominated by helium, while hydrogen is depleted to 25 per cent by mass. Because of their location in the Scutum-Centaurus arm, WR\,120bb and WR\,120bc appear highly reddened, $A_{K_{\rm s}} \approx 2$ mag. We adopt a common distance of 5.8\,kpc to both stars, which complies with the typical absolute $K$-band magnitude for the WN9h subtype of -6.5 mag, is consistent with their observed extinction based on comparison with other massive stars in the region, and allows for the possibility that their shells are interacting with each other. This leads to luminosities of $\log(L/L_\odot) = 5.66$ and 5.54 for WR\,120bb and WR\,120bc, with large uncertainties due to the adopted distance. The values of the luminosities of WR\,120bb and WR\,120bc imply that the immediate precursors of both stars were red supergiants (RSG). This implies in turn that the circumstellar shells associated with WR\,120bb and WR\,120bc were formed by interaction between the WR wind and the dense material shed during the preceding RSG phase.

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arXiv2012

The Galactic WC stars: Stellar parameters from spectral analyses indicate a new evolutionary sequence

[Abridged] [...] AIMS: We aim to establish the stellar parameters and mass-loss rates of the Galactic WC stars. These data provide the empirical basis of studies of (i) the role of WC stars in the evolution of massive stars, (ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as input to models of the chemical and dynamical evolution of galaxies. Methods: We analyze the nearly complete sample of un-obscured Galactic WC stars, using optical spectra as well as ultraviolet spectra when available. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. A large grid of line-blanked models has been established for the range of WC subtypes WC4 - WC8, and smaller grids for the WC9 parameter domain. Both WO stars and WN/WC transit types are also analyzed using special models. Results: Stellar and atmospheric parameters are derived for more than 50 GalacticWC and two WO stars, covering almost the whole GalacticWC population as far as the stars are single, and un-obscured in the visual. In the Hertzsprung-Russell diagram, theWC stars reside between the hydrogen and the helium zero-age main sequences, having luminosities L from 10^4.9 to 10^5.6 Lsun. The mass-loss rates scale very tightly with L^0.8. The two WO stars in our sample turn out to be outstandingly hot (\approx200 kK) and do not fit into the WC scheme. Conclusions: By comparing the empirical WC positions in the Hertzsprung-Russell diagram with evolutionary models, and from recent supernova statistics, we conclude that WC stars have evolved from initial masses between 20 solar masses and 45 Msun. In contrast to previous assumptions, it seems that WC stars in general do not descend from the most massive stars. Only the WO stars might stem from progenitors that have been initially more massive than 45 Msun.

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arXiv2012

3-D radiative transfer in clumped hot star winds I. Influence of clumping on the resonance line formation

The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping turned out to be very important. In order to incorporate clumping into radiative transfer calculation, 3-D models are required. Various properties of the clumps may have strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. We incorporate the 3-D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. We develop a full 3-D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, realistic 3-D models that describe the dense as well as the tenuous wind components are used to model the formation of resonance lines in a clumped stellar wind. At the same time, non-monotonic velocity fields are accounted for. The 3-D density and velocity wind inhomogeneities show very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3-D models show that the line opacity is reduced for larger clump separation and for more shallow velocity gradients within the clumps. Our new model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3-D nature must be taken into account.

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arXiv2012

3-D Monte Carlo radiative transfer calculation of resonance line formation in the inhomogeneous expanding stellar wind

We study the effects of optically thick clumps, non-void inter-clump medium, variation of the onset of clumping, and velocity dispersion inside clumps on the formation of resonance lines. For this purpose we developed a full 3-D Monte Carlo Radiative Transfer (MCRT) code that is able to handle 3-D shapes of clumps and arbitrary 3-D velocity fields. The method we developed allows us to take into account contributions from density and velocity wind inhomogeneities to the total opacity very precisely. The first comparison with observation shows that 3-D density and velocity wind inhomogeneities have a very strong influence on the resonance line formation, and that they have to be accounted for in order to obtain reliable mass-loss rate determinations.

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arXiv2010

X-rays, clumping and stellar wind structures

X-ray emission is ubiquitous among massive stars. In the last decade, X-ray observations revolutionized our perception of stellar winds but opened a Pandora's box of urgent problems. X-rays penetrating stellar winds suffer mainly continuum absorption, which greatly simplifies the radiative transfer treatment. The small and large scale structures in stellar winds must be accounted for to understand the X-ray emission from massive stars. The analysis of X-ray spectral lines can help to infer the parameters of wind clumping, which is prerequisite for obtaining empirically correct stellar mass-loss rates. The imprint of large scale structures, such as CIRs and equatorial disks, on the X-ray emission is predicted, and new observations are testing theoretical expectations. The X-ray emission from magnetic stars proves to be more diverse than anticipated from the direct application of the magnetically-confined wind model. Many outstanding questions about X-rays from massive stars will be answered when the models and the observations advance.

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arXiv2010

The most luminous stars in the Galaxy and the Magellanic Clouds

Some of the Wolf-Rayet stars are found to have very high bolometric luminosities (more than 1000000 solar). We employ the Potsdam Wolf-Rayet (PoWR) model atmospheres for their spectral analysis, which yields the bolometric corrections. Distance and interstellar reddening also enter the luminosity estimates. Among the Galactic stars, there is a group of very luminous WNL stars (i.e. WR stars of late subtype from nitrogen sequence with hydrogen being depleted in their atmospheres, but not absent). Their distances are often the major source of uncertainty. From K-band spectroscopy we found a very luminous star ($\log L/L_\odot = 6.5$) in the Galactic center region, which we termed the Peony Star because of the form of its surrounding dusty nebula. A similar group of very luminous WNL stars is found in the Large Magellanic Cloud (LMC). In the Small Magellanic Cloud (SMC) the majority of WR stars resides in binary systems. The single WNL stars in the SMC are not very luminous. We conclude that a significant number of very luminous WNL stars exist in the Galaxy and the LMC. With initial masses above 60$M_\odot$, they apparently evolved directly to the WNL stage without a prior excursion to the red side of the HRD. At the low metallicity of the SMC, the binary channel may be dominant for the formation of WR stars.

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arXiv2008

A new outburst in LMC-N66

This is the first report on the new outburst presented by the central star of the LMC-N66 nebula. This object was classified as a planetary nebula, however, its true nature is under debate. In the period 1955-1990 the central star was almost undetectable and only nebular emission lines were observed. In 1990, the beginning of an outburst was detected and in few months it became much brighter and developed wide He and N lines, typical of a Wolf Rayet star of the N-sequence. The maximum occurred in 1994 and afterwards the star slowly faded. Analysis of its evolution showed that it has a variable mass-loss rate which occasionally increases enormously, creating a false photosphere at a much larger radius, making it appear a few magnitudes brighter. The present outburst has occurred 13 years after the episode from 1994 to 2000. So far this new event has similar characteristics although there are some significant differences in the spectral features. We present optical and FUSE spectra showing the main properties of this latter event.

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arXiv2003

X-ray line emission from a fragmented stellar wind

We discuss X-ray line formation in dense O star winds. A random distribution of wind shocks is assumed to emit X-rays that are partially absorbed by cooler wind gas. The cool gas resides in highly compressed fragments oriented perpendicular to the radial flow direction. For fully opaque fragments, we find that the blueshifted part of X-ray line profiles remains flat-topped even after severe wind attenuation, whereas the red part shows a steep decline. These box-type, blueshifted profiles resemble recent Chandra observations of the O3 star zeta Pup. For partially transparent fragments, the emission lines become similar to those from a homogeneous wind.

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arXiv2001

Runaway Acceleration of Line-Driven Winds: the Role of the Outer Boundary

Observations and theory suggest that line driven winds from hot stars and luminous accretion disks adopt a unique, critical solution which corresponds to maximum mass loss rate. We analyze the numerical stability of the infinite family of shallow wind solutions, which resemble solar wind breezes, and their transition to the critical wind. Shallow solutions are sub-critical with respect to radiative (or Abbott) waves. These waves can propagate upstream through shallow winds at high speeds. If the waves are not accounted for in the Courant time step, numerical runaway results. The outer boundary condition is equally important for wind stability. Assuming pure outflow conditions, as is done in the literature, triggers runaway of shallow winds to the critical solution or to accretion flow.

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