The Discordance of Mass-Loss Estimates for Galactic O-Type Stars

We have determined accurate values of the product of the mass-loss rate and the ion fraction of P^{4+}, Mdot q(P^{4+}), for a sample of 40 Galactic O-type stars by fitting stellar-wind profiles to observations of the P V resonance doublet obtained with FUSE, ORFEUS/BEFS, and Copernicus. When P^{4+} is the dominant ion in the wind, Mdot q(P^{4+}) approximates the mass-loss rate to within a factor of 2. Theory predicts that P^{4+} is the dominant ion in the winds of O7-O9.7 stars, though an empirical estimator suggests that the range from O4-O7 may be more appropriate. However, we find that the mass-loss rates obtained from P V wind profiles are systematically smaller than those obtained from fits to Halpha emission profiles or radio free-free emission by median factors of about 130 (if P^{4+} is dominant between O7 and O9.7) or about 20 (if P^{4+} is dominant between O4 and O7). These discordant measurements can be reconciled if the winds of O stars in the relevant temperature range are strongly clumped on small spatial scales. We use a simplified two-component model to investigate the volume filling factors of the denser regions. This clumping implies that mass-loss rates determined from "density squared" diagnostics have been systematically over-estimated by factors of 10 or more, at least for a subset of O stars. Reductions in the mass-loss rates of this size have important implications for the evolution of massive stars and quantitative estimates of the feedback that hot-star winds provide to their interstellar environments.Comment: 26 pages, 4 figures; accepted for publication in Ap

2D Simulations of the Line-Driven Instability in Hot-Star Winds: II. Approximations for the 2D Radiation Force

We present initial attempts to include the multi-dimensional nature of radiation transport in hydrodynamical simulations of the small-scale structure that arises from the line-driven instability in hot-star winds. Compared to previous 1D or 2D models that assume a purely radial radiation force, we seek additionally to treat the lateral momentum and transport of diffuse line-radiation, initially here within a 2D context. A key incentive is to study the damping effect of the associated diffuse line-drag on the dynamical properties of the flow, focusing particularly on whether this might prevent lateral break-up of shell structures at scales near the lateral Sobolev angle of ca. $1^{\rm o}$. We first explore nonlinear simulations that cast the lateral diffuse force in the simple, local form of a parallel viscosity. Second, to account for the lateral mixing of radiation associated with the radial driving, we next explore models in which the radial force is azimuthally smoothed over a chosen scale. Third, to account for both the lateral line-drag and the lateral mixing in a more self-consistent way, we explore further a method first proposed by Owocki (1999), which uses a restricted 3-ray approach that combines a radial ray with two oblique rays set to have an impact parameter $p < R_{\ast}$ within the stellar core. From numerical simulations, we find that, compared to equivalent 1-ray simulations, the high-resolution 3-ray models show systematically a much higher lateral coherence.... (Full abstract in paper)Comment: Accepted by A&A, 12 pages, 7 figures, 3 only shown in version available at http://www.mpa-garching.mpg.de/~luc/2778.ps.g

Review on quality assurance along the CFRP value chain - Non-destructive testing of fabrics, preforms and CFRP by HF radio wave techniques

Eddy current testing is well established for non-destructive testing of electrical conductive materials [1]. The development of radio frequency (RF) eddy current technology with frequency ranges up to 100 MHz made it possible to extend the classical fields of application even towards less conductive materials like CFRP [2][3](Table 2). It turns out that RF eddy current technology on CFRP generates a growing number of valuable information for comprehensive material diagnostic. Both permittivity and conductivity of CFRP influence the complex impedance measured with RF eddy current devices. The electrical conductivity contains information about fiber texture like orientations, gaps or undulations in a multilayered material. The permittivity characterization influenced by dielectric properties allows the determination of local curing defects on CFRP e.g. hot spots, thermal impacts or polymer degradation. An explanation for that effect is seen in the measurement frequency range and the capacitive structure of the carbon rovings. Using radio wave frequencies for testing, the effect of displacement currents cannot be neglected anymore. The capacitive structures formed by the carbon rovings is supposed to further strengthen the dielectric influences on eddy current measurement signal [3]. This report gives an overview of several realized applications and should be understood as a general introduction of CFRP testing by HF Radio Wave techniques

A Nozzle Analysis of Slow-Acceleration Solutions in One-Dimensional Models of Rotating Hot-Star Winds

We analyze the steady 1D flow equations for a rotating stellar wind based on a ``nozzle'' analogy for terms that constrain the local mass flux. For low rotation, we find the nozzle minimum occurs near the stellar surface, allowing a transition to a standard, CAK-type steep-acceleration solution; but for rotations > 75% of the critical rate, this inner nozzle minimum exceeds the global minimum, implying near-surface supercritical solutions would have an overloaded mass loss rate. In steady, analytic models in which the acceleration is assumed to be monotonically positive, this leads the solution to switch to a slow acceleration mode. However, time-dependent simulations using a numerical hydrodynamics code show that, for rotation rates 75 - 85% of critical, the flow can develop abrupt "kink" transitions from a steep acceleration to a decelerating solution. For rotations above 85% of critical, the hydrodynamic simulations confirm the slow acceleration, with the lower flow speed implying densities 5 - 30 times higher than the polar (or a nonrotating) wind. Still, when gravity darkening and 2D flow effects are accounted for, it seems unlikely that rotationally modified equatorial wind outflows could account for the very large densities inferred for the equatorial regions around B[e] supergiants.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 9 figure

Radio and submillimetre observations of wind structure in zeta Pup

We present radio and submillimetre observations of the O4I(n)f star zeta Pup, and discuss structure in the outer region of its wind (~ 10-100 R*). The properties of bremsstrahlung, the dominant emission process at these wavelengths, make it sensitive to structure and allow us to study how the amount of structure changes in the wind by comparing the fluxes at different wavelengths. To look for variability, we acquired 3.6 and 6 cm observations with the Australia Telescope Compact Array (ATCA). We supplemented these with archive observations from the NRAO Very Large Array (VLA). We did not find variability at more than the +- 20 % level. The long integration time does allow an accurate determination of the fluxes at 3.6 and 6 cm. Converting these fluxes into a mass loss rate, we find Mdot = 3.5 x 10^{-6} Msun/yr. This value confirms the significant discrepancy with the mass loss rate derived from the Halpha profile, making zeta Pup an exception to the usually good agreement between Halpha and radio mass loss rates. We also observed zeta Pup at 850 mum with the James Clerk Maxwell Telescope (JCMT) and at 20 cm with the VLA. A smooth wind model shows that the millimetre fluxes are too high compared to the radio fluxes. While recombination of helium in the outer wind cannot be discounted as an explanation, the wealth of evidence for structure strongly suggests this as the explanation for the discrepancy. Model calculations show that the structure needs to be present in the inner ~ 70 R* of the wind, but that it decays significantly, or maybe even disappears, beyond that radius.Comment: 13 pages, 8 figures, to be published in Astronomy and Astrophysic

Wind modelling of very massive stars up to 300 solar masses

Some studies have claimed a universal stellar upper-mass limit of 150 Msun. A factor that is often overlooked is that there might be a difference between the current and initial masses of the most massive stars, as a result of mass loss. We present Monte Carlo mass-loss predictions for very massive stars in the range 40-300 Msun, with large luminosities and Eddington factors Gamma. Using our new dynamical approach, we find an upturn in the mass-loss vs. Gamma dependence, at the point where the winds become optically thick. This coincides with the location where wind efficiency numbers surpass the single-scattering limit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests a transition from common O-type winds to Wolf-Rayet characteristics at the point where the winds become optically thick. This transitional behaviour is also revealed with respect to the wind acceleration parameter beta, which starts at values below 1 for the optically thin O-stars, and naturally reaches values as high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding concerns the transition in spectral morphology of the Of and WN characteristic He II line at 4686 Angstrom. When we express our mass-loss predictions as a function of the electron scattering Gamma_e (=L/M) only, we obtain a mass-loss Gamma dependence that is consistent with a previously reported power-law Mdot propto Gamma^5 (Vink 2006) that was based on our semi-empirical modelling approach. When we express Mdot in terms of both Gamma and stellar mass, we find Mdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm that the Gamma-effect on the mass-loss predictions is much stronger than that of an increased helium abundance, calling for a fundamental revision in the way mass loss is incorporated in evolutionary models of the most massive stars.Comment: minor language changes (Astronomy & Astrophysics in press - 11 pages, 10 figures

Large-scale wind structures in OB supergiants: a search for rotationally modulated H\alpha variability

We present the results of a long-term monitoring campaign of the H\alpha line in a sample of bright OB-supergiants (O7.5-B9) that aims at detecting rotationally modulated changes potentially related to the existence of large-scale wind structures. A total of 22 objects were monitor ed during 36 nights spread over 6 months in 2001-2002. Coordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in H\alpha is found for the stars displaying a feature contaminated by wind emission. Most changes take place on a daily time-scale, although hourly variations are also occasionally detected. Convincing evidence for a cyclical pattern of variability in H\alpha has been found in 2 stars: HD 14134 and HD 42087 (periodic signals are also detected in other stars, but independent confirmation is required). Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 days) and estimated periods of stellar rotation. We call attention to the atypical case of HD 14134 which exhibits a clear 12.8-d periodicity both in the photometric and in the spectroscopic data sets. This places this object among a handful of early-type stars where one may observe a clear link between extended wind structures and photospheric disturbances. Further modelling may test the hypothesis that azimuthally-extended wind streams are responsible for the patterns of spectral variability in our target stars.Comment: 18 pages, accepted for publication in MNRA

Mass and angular momentum loss via decretion disks

We examine the nature and role of mass loss via an equatorial decretion disk in massive stars with near-critical rotation induced by evolution of the stellar interior. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star's moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss depends crucially on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical mass loss the spherical, wind-like mass loss commonly assumed in evolutionary calculations. We discuss the physical processes that affect the outer disk radius, including thermal disk outflow, and ablation of the disk material via a line-driven wind induced by the star's radiation. We present parameterized scaling laws for taking account of decretion-disk mass loss in stellar evolution codes, including how these are affected by metallicity, or by presence within a close binary and/or a dense cluster. Effects similar to those discussed here should also be present in accretion disks during star formation, and may play an important role in shaping the distribution of rotation speeds on the ZAMS.Comment: 10 pages, accepted for publication in A&

Wind structure of late B supergiants I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)

We provide a quantitative analysis of time-variable phenomena in the photospheric, near-star, and outflow regions of the late-B supergiant (SG) HD 199478. The analysis is based primarily on optical spectroscopic datasets secured between 1999 and 2000 from the Bulgarian NAO, Tartu, and Ritter Observatories. The temporal behaviour of HD 199478 is characterised by three key empirical properties: (i) systematic central velocity shifts in the photospheric absorption lines, including C II and He I, over a characteristic time-scale of abou 20 days; (ii) extremely strong, variable H alpha emission with no clear modulation signal, and (iii) the occurrence in 2000 of a (rare) high-velocity absorption (HVA) event in H alpha, which evolved over about 60 days, showing the clear signature of mass infall and outflows. In these properties HD 199478 resembles few other late-B SGs with peculiar emission and HVAs in H alpha (HD 91619, HD 34085, HD 96919). Non-LTE line synthesis modelling is conducted using FASTWIND for these late-B SGs to constrain and compare their fundamental parameters within the context of extreme behaviour in the H alpha lines. Our analysis indicate that at the cooler temperature edge of B SGs, there are objects whose wind properties, as traced by H alpha, are inconsistent with the predictions of the smooth, spherically symmetric wind approximation. This discordance is still not fully understood and may highlight the role of a non-spherical, disk-like, geometry, which may result from magnetically-driven equatorial compression of the gas. Ordered dipole magnetic fields may also lead to confined plasma held above the stellar surface, which ultimately gives rise to transient HVA events.Comment: 12 pages. To be published in Astronomy and AStrophysic

Quantitative Spectroscopy of O Stars at Low Metallicity. O Dwarfs in NGC 346

We present the results of a detailed UV and optical spectral analysis of the properties of 6 dwarf O-type stars in the SMC H II region NGC 346. Stellar parameters, chemical abundances, and wind parameters have been determined using NLTE line blanketed models calculated with the photospheric code, Tlusty, and with the wind code, CMFGEN. The results, in particular iron abundances, obtained with the two NLTE codes compare very favorably, demonstrating that basic photospheric parameters of O dwarfs can be reliably determined using NLTE static model atmospheres. The two NLTE codes require a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the microturbulent velocity from early O stars to late O stars. Similarly to several recent studies of galactic, LMC and SMC stars, we derive effective temperatures lower than predicted from the widely-used relation between spectral type and Teff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the HR diagram, we find an age of 3 10^6 years for NGC 346. A majority of the stars in our sample reveal CNO-cycle processed material at their surface during the MS stage, indicating thus fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity, Z = 0.2 Zsun, in good agreement with other recent analyses of SMC stars. The derived mass loss rate of the three most luminous stars agrees with recent theoretical predictions. However, the three other stars of our sample reveal very weak wind signatures. We obtain mass loss rates that are significantly lower than 10^{-8} Msun/yr, which is below the predictions of radiative line-driven wind theory by an order of magnitude or more. (abridged version)Comment: 61 pages, 17 figures; to appear in ApJ, 595 (Oct 1, 2003); minor revisions and addition