25 research outputs found
Research Note: Rotation and the wind momentum-luminosity relation for extragalactic distances
The effects of axial stellar rotation on the wind-momentum relation (WLR) for
determining the extragalactic distances are investigated. Despite the fact that
the mass loss rates grow quite a lot with rotation, remarkably the effects on
the WLR are found to be very small on the average. As an example, for an
average orientation angle between the rotation axis and the line of sight, the
luminosity would be overestimated by 5.9 % for a star rotating at 90% of its
break-up rotational velocity. Different orientation angles between the rotation
axis and the line of sight produce some limited scatter.Comment: 4 pages, 1 figure, in press in A&
A detailed X-ray investigation of zeta Puppis I. The dataset and some preliminary results
Aims: zeta Puppis, one of the closest and brightest massive stars, was the
first early-type object observed by the current generation of X-ray
observatories. These data provided some surprising results, confirming partly
the theoretical predictions while simultaneously unveiling some problematic
mismatches with expectations. In this series of papers, we perform a thorough
study of zeta Puppis in X-rays, using a decade of XMM observations. Methods:
zeta Puppis was observed 18 times by XMM, totaling 1Ms in exposure. This
provides the highest-quality high-resolution X-ray spectrum of a massive star
to date, as well as a perfect dataset for studying X-ray variability in an
"archetype" object. Results: This first paper reports on the data reduction of
this unique dataset and provides a few preliminary results. On the one hand,
the analysis of EPIC low-resolution spectra shows the star to have a remarkably
stable X-ray emission from one observation to the next. On the other hand, the
fitting by a wind model of individual line profiles recorded by RGS confirms
the wavelength dependence of the line morphology.Comment: 9 pages, 3 figures, accepted for publication by A&
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
The chemical evolution of the solar neighbourhood
Recent models of galactic chemical evolution account for updated evolutionary
models of massive stars (with special emphasis on stellar winds) and for the
effects of intermediate mass and massive binaries. The results are summarised.
We also present a critical discussion on possible effects of stellar rotation
on overall galactic chemical evolutionary simulations.Comment: 12 pages, 3 figures, Pacific Rim Conference, Xi'an, China, 11-17 July
200
The Diversity of Gamma-Ray Bursts and the Surroundings of Massive Stars
The finding of a Type Ic supernova connected with GRB 030329 showed a massive
star origin for this burst, supporting evidence for this association in
previous bursts with lightcurve bumps at the appropriate time for a supernova.
Here, we explore the possibility that all long bursts have massive star
progenitors, interacting with either the freely expanding wind of the
progenitor or the shocked wind. We present models for the afterglows of GRB
020405 and GRB 021211, which are a challenge to wind interaction models.
Considering sources for which wind interaction models are acceptable, a range
of wind densities is required, from values typical of Galactic Wolf-Rayet stars
to values ~100 times smaller. The reason for the low densities is unclear, but
may involve low progenitor masses and/or low metallicities. If mass is a
factor, a low density event should be associated with a low mass supernova. The
interpretation of bursts apparently interacting with constant density media as
interaction with a shocked wind requires both a range of mass loss densities
and a range of external pressures. The highest pressures, p/k > 10^8 cm^{-3} K,
may be due to an extreme starburst environment, which would imply that the
burst is superposed on an active star forming region. Although the range of
observed events can be accomodated by the shocked wind theory, special
circumstances are necessary to bring this about. Finally, we consider the high
velocity, high ionization absorption features observed in some afterglow
spectra. If the features are circumstellar, the presence of the burst in a
starburst region may be important for the formation of clumps near the burst.Comment: 31 pages, 1 figure, ApJ, submitte
Multi-periodic photospheric pulsations and connected wind structures in HD64760
We report on the results of an extended optical spectroscopic monitoring
campaign on the early-type B supergiant HD64760 (B0.5Ib) designed to probe the
deep-seated origin of spatial wind structure. This new study is based on
high-resolution echelle spectra obtained with the FEROS instrument at ESO La
Silla. 279 spectra were collected over 10 consecutive nights in 2003. From the
period analysis of the line-profile variability of the photospheric lines we
identify three closely spaced periods around 4.810 hrs and a splitting of
+/-3%. The velocity - phase diagrams of the line-profile variations for the
distinct periods reveal characteristic prograde non-radial pulsation patterns
of high order corresponding to pulsation modes with l and m in the range 6-10.
The three pulsation modes have periods clearly shorter than the characteristic
pulsation time scale and show small horizontal velocity fields and hence are
identified as p-modes. The beating of the three pulsation modes leads to a
retrograde beat pattern with two regions of constructive interference
diametrically opposite on the stellar surface and a beat period of 162.8hrs
(6.8days). This beat pattern is directly observed in the spectroscopic time
series of the photospheric lines. The wind-sensitive lines display features of
enhanced emission, which appear to follow the maxima of the photospheric beat
pattern.Comment: 18 pages, 21 figures (reduced resolution
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
Models of Forbidden Line Emission Profiles from Axisymmetric Stellar Winds
A number of strong infrared forbidden lines have been observed in several
evolved Wolf-Rayet star winds, and these are important for deriving metal
abundances and testing stellar evolution models. In addition, because these
optically thin lines form at large radius in the wind, their resolved profiles
carry an imprint of the asymptotic structure of the wind flow. This work
presents model forbidden line profile shapes formed in axisymmetric winds. It
is well-known that an optically thin emission line formed in a spherical wind
expanding at constant velocity yields a flat-topped emission profile shape.
Simulated forbidden lines are produced for a model stellar wind with an
axisymmetric density distribution that treats the latitudinal ionization
self-consistently and examines the influence of the ion stage on the profile
shape. The resulting line profiles are symmetric about line centre. Within a
given atomic species, profile shapes can vary between centrally peaked, doubly
peaked, and approximately flat-topped in appearance depending on the ion stage
(relative to the dominant ion) and viewing inclination. Although application to
Wolf-Rayet star winds is emphasized, the concepts are also relevant to other
classes of hot stars such as luminous blue variables and Be/B[e] stars.Comment: To appear in MNRA
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