2,191 research outputs found
On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud
We re-analyze the age distribution (dN/dt) of star clusters in the Small
Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud
Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can
be approximated by a power-law distribution, dN/dt propto t^-beta, with
-beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive
the ages. Predictions for a cluster population without dissolution limited by a
V-band detection result in a power-law dN/dt distribution with an index of
~-0.7. This is because the limiting cluster mass increases with age, due to
evolutionary fading of clusters, reducing the number of observed clusters at
old ages. When a mass cut well above the limiting cluster mass is applied, the
dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is
of small importance in shaping the dN/dt distribution and incompleteness causes
dN/dt to decline. The reason that no (mass independent) infant mortality of
star clusters in the first ~10-20 Myr is found is explained by a detection bias
towards clusters without nebular emission, i.e. cluster that have survived the
infant mortality phase. The reason we find no evidence for tidal (mass
dependent) cluster dissolution in the first Gyr is explained by the weak tidal
field of the SMC. Our results are in sharp contrast to the interpretation of
Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid
cluster dissolution. This is due to their erroneous assumption that the sample
is limited by cluster mass, rather than luminosity.Comment: 8 pages, 4 figures, accepted for publication in Ap
Disk winds of B[e] supergiants
The class of B[e] supergiants is characterized by a two-component stellar
wind consisting of a normal hot star wind in the polar zone and a slow and
dense disk-like wind in the equatorial region. The properties of the disk wind
are discussed using satellite UV spectra of stars seen edge-on, i.e. through
the equatorial disk. These observations show that the disk winds are extremely
slow, 50-90 km/s, i.e. a factor of about 10 slower than expected from the
spectral types. Optical emission lines provide a further means to study the
disk wind. This is discussed for line profiles of forbidden lines formed in the
disk.Comment: 7 pages, LaTeX, 3 ps figures, uses lamuphys.sty from Springer-Verlag,
to be published in the proceedings of IAU Coll. 169 "Variable and
Non-spherical Stellar Winds in Luminous Hot Stars" held in Heidelberg 199
The enigmatic B[e]-star Henize 2-90: The non-spherical mass loss history from an analysis of forbidden lines
(abridged) We study the optical spectrum of the exciting B[e] star Hen 2-90
based on new high-resolution observations that cover the innermost 2". Our
investigation is splitted in two parts, a qualitative study of the presence of
the numerous emission lines and the classification of their line profiles which
indicate a circumstellar environment of high complexity, and a quantitative
analysis of numerous forbidden lines, e.g. [OI], [OII], [OIII], [SII], [SIII],
[ArIII], [ClII], [ClIII] and [NII]. We find a correlation between the different
ionization states of the elements and the velocities derived from the line
profiles: the highly ionized atoms have the highest outflow velocity while the
neutral lines have the lowest outflow velocity. The recent HST image of Hen
2-90 reveals a bipolar, highly ionized region, a neutral disk-like structure
and an intermediate region of moderate ionization. It seems that a
non-spherical stellar wind model is a good option to explain the ionization and
spatial distribution of the circumstellar material. We modelled the forbidden
lines under the assumption of a non-spherically symmetric wind based on the HST
image. We find that in order to fit the observed line luminosities, the mass
flux, surface temperature, and terminal wind velocities need to be latitude
dependent, which might be explained in terms of a rapidly rotating central
star. A rotation speed of 75-80 % of the critical velocity has been derived.
The total mass loss rate of the star was determined to be of order 3 10^{-5}
M_sun/yr. Such a wind scenario and the fact that compared to solar abundances
C, O, and N seem to be underabundant while S, Ar and Cl have solar abundances,
might be explained in terms of a rapidly rotating post-AGB star.Comment: 16 pages, 13 figures, accepted for publication in A&A. Table 4 is
included at the end of the paper. This table will only be available in the
online version of the paper and will not appear in the printed versio
The peculiar B-type supergiant HD327083
Coude spectroscopic data of a poorly-studied peculiar supergiant, HD327083,
are presented. Halpha and Hbeta line profiles have been fitted employing a
non-LTE code adequate for spherically expanding atmospheres. Line fits lead to
estimates of physical parameters. These parameters suggest that HD327083 may be
close to the Luminous Blue Variable phase but it is also possible that it could
be a B[e] Supergiant.Comment: 4 pages, 4 figures, Accepted for publication in A&A Lette
Eliciting Tacit Knowledge with a Grammar-targeted Interview Method
Tacit knowledge represents a challenge to knowledge elicitation due to the assumption that this type of knowledge cannot be articulated. We argue that Polanyi's (1966:4) widely cited notion that âwe know more than we can tellâ represents a weak model of language that does not acknowledge the grammatical patterns in spoken discourse that we, as speakers, apply tacitly. We investigate the hypothesis that individuals articulate what they know through grammatical patterns, referred to as under-representation, without direct awareness. This thesis develops and pilots a grammar-targeted interview method aimed at unpacking specific grammatical features that occur in spoken discourse. The model of language from which these features are derived is Systemic Functional Linguistics. We report findings from three empirical studies of tacit knowledge in corporate organisations where we used the grammar-targeted interview technique to elicit tacit knowledge in the areas of knowledge management, requirements analysis and performance reviews. We compare this interview method with a content-targeted approach. The results show that the grammar-targeted technique produces less under-represented discourse thus allowing tacit knowledge held by the interviewees to be made visible. Based on the linguistic analyses undertaken in these field studies we propose that Polanyiâs expression âwe know more than we tellâ be reformulated to âwe tell more than we realise we knowâ
Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI
B[e] stars are hot stars surrounded by circumstellar gas and dust responsible
for the presence of emission lines and IR-excess in their spectra. How dust can
be formed in this highly illuminated and diluted environment remains an open
issue. HD 62623 is one of the very few A-type supergiants showing the B[e]
phenomenon. We obtained nine calibrated visibility measurements using the
VLTI/MIDI instrument in SCI-PHOT mode and PRISM spectral dispersion mode with
projected baselines ranging from 13 to 71 m and with various position angles.
We used geometrical models and physical modeling with a radiative transfer code
to analyze these data. The dusty circumstellar environment of HD 62623 is
partially resolved by the VLTI/MIDI even with the shortest baselines. The
environment is flattened and can be separated into two components: a compact
one whose extension grows from 17 mas at 8 microns to 30 mas at 9.6 microns and
stays almost constant up to 13 microns, and a more extended one that is
over-resolved even with the shortest baselines. Using the radiative transfer
code MC3D, we managed to model HD 62623's circumstellar environment as a dusty
disk with an inner radius of 3.85+-0.6 AU, an inclination angle of 60+-10 deg,
and a mass of 2x10^-7Mo. It is the first time that the dusty disk inner rim of
a supergiant star exhibiting the B[e] phenomenon is significantly constrained.
The inner gaseous envelope likely contributes up to 20% to the total N band
flux and acts like a reprocessing disk. Finally, the hypothesis of a stellar
wind deceleration by the companion's gravitational effects remains the most
probable case since the bi-stability mechanism does not seem to be efficient
for this star.Comment: 13 pages, 11 figures. A&A accepted pape
On the hydrogen neutral outflowing disks of B[e] supergiants
(abridged) B[e] supergiants are known to possess geometrically thick dusty
disks. Disk-forming wind models have, however, been found to be insufficient in
reproducing the observed dust emission. This problem arises due to the severe
assumption that, as for classical Be stars, the near-infrared excess emission
originates in the disk. Modeling of the free-free and free-bound emission
therefore results in an upper limit for the disk mass loss rate, hampering dust
condensation in the disk. We propose a revised scenario for the non-spherical
winds of B[e] supergiants: a normal B-type line-driven polar wind and an
outflowing disk-forming wind that is neutral in hydrogen at, or very close to
the stellar surface. We concentrate on the pole-on seen LMC B[e] supergiant
R126 and calculate the line luminosities of the optical [OI] emission lines
with an outflowing disk scenario. In addition, we compute the free-free and
free-bound emission from a line-driven polar wind and model the spectral energy
distribution in the optical and near-infrared. Good fits to the [OI] line
luminosities are achieved for an outflowing disk that is neutral in hydrogen
right from the stellar surface. Neutral thereby means that hydrogen is ionized
by less than 0.1%. Consequently, the free-free and free-bound emission cannot
(dominantly) arise from the disk and cannot limit the disk mass loss rate. The
hydrogen neutral outflowing disk scenario therefore provides an ideal
environment for efficient dust formation. The spectral energy distribution in
the optical and near-infrared range can be well fitted with the stellar
continuum plus free-free and free-bound emission from the polar line-driven
wind. Our modeling further delivers minimum values for \dot{M}(disk) > 2.5d-5
M_sun/yr and for the density contrast between equatorial and polar wind of ~10.Comment: 9 pages, 8 figures, accepted for publication in A&
Crowded field 3D spectroscopy of LBV candidates in M33
We present integral field spectroscopy of the LBV candidate stars B416 and
v532 in the local group galaxy M33. B416 is surrounded by an elongated
ring-like nebula, which has a projected radius of 20x30 pc. From the datacube
we create ionization and radial velocity maps of the nebula. The excitation of
the gas decreases towards the outer part of the ring, while the inner part of
the nebula is filled with a more excited gas. In the EW direction the ring is
seen to expand with a maximum projected velocity amplitude of about 40 km/s.
The eastern part approaches the observer. We estimate the nebula dynamical
lifetime 8*10E5 years. It could be a residual MS bubble, which indicates a
main-sequence or pre-LBV status of the star. We classify B416 as an "iron star"
or B[e]-supergiant. In v532 an elongated nebula has been marginally detected.
The total projected size of the nebula along the main axis is 30 pc, and the
total radial velocity gradient is 44+/-11 km/s. v532 exhibits both strong
photometric and spectral variability. At the time of our observations it was in
an intermediate brightness state with a rich nitrogen spectrum. We classify
v532 as an LBV, showing LBV Ofpe/WN transitions. We stress the importance
of integral field spectroscopy as the optimal technique for studying nebulae
and the evolution of LBV-like stars in nearby galaxies.Comment: 12 pages, 10 Postscript figures, A&A accepte
Where's the Doughnut? LBV bubbles and Aspherical Fast Winds
In this paper we address the issue of the origin of LBV bipolar bubbles.
Previous studies have explained the shapes of LBV nebulae, such as Car,
by invoking the interaction of an isotropic fast wind with a previously
deposited, slow aspherical wind (a ``slow torus''). In this paper we focus on
the opposite scenario where an aspherical fast wind expands into a previously
deposited isotropic slow wind. Using high resolution hydrodynamic simulations,
which include the effects of radiative cooling, we have completed a series of
numerical experiments to test if and how aspherical fast winds effect wind
blown bubble morphologies. Our experiments explore a variety of models for the
latitudinal variations of fast wind flow parameters. The simulations
demonstrate that aspherical fast winds can produce strongly bipolar outflows.
In addition the properties of outflows recover some important aspects of LBV
bubbles which the previous "slow torus" models can not.Comment: 23 pages, 6 figures, to appear the Astrophysical Journa
XMM-Newton observations of a superbubble in N 158 in the LMC
Aims: We study the diffuse X-ray emission observed in the field of view of
the pulsar B 0540-69 in the Large Magellanic Cloud (LMC) by XMM-Newton. We want
to understand the nature of this soft diffuse emission, which coincides with
the superbubble in the HII region N 158, and improve our understanding of the
evolution of superbubbles. Methods: We analyse the XMM-Newton spectra of the
diffuse emission. Using the parameters obtained from the spectral fit, we
perform calculations of the evolution of the superbubble. The mass loss and
energy input rates are based on the initial mass function (IMF) of the observed
OB association inside the superbubble. Results: The analysis of the spectra
shows that the soft X-ray emission arises from hot shocked gas surrounded by a
thin shell of cooler, ionised gas. We show that the stellar winds alone cannot
account for the energy inside the superbubble, but the energy release of 2 - 3
supernova explosions in the past ~1 Myr provides a possible explanation.
Conclusions: The combination of high sensitivity X-ray data, allowing spectral
analysis, and analytical models for superbubbles bears the potential to reveal
the evolutionary state of interstellar bubbles, if the stellar content is
known.Comment: 7 pages. Accepted for publication in A&
- âŠ