722 research outputs found
Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation
We have used VLT FLAMES data to constrain the uncertain physics of rotational
mixing in stellar evolution models. We have simulated a population of single
stars and find two groups of observed stars that cannot be explained: (1) a
group of fast rotating stars which do not show evidence for rotational mixing
and (2) a group of slow rotators with strong N enrichment. Binary effects and
fossil magnetic fields may be considered to explain those two groups. We
suggest that the element boron could be used to distinguish between rotational
mixing and the binary scenario. Our single star population simulations quantify
the expected amount of boron in fast and slow rotators and allow a comparison
with measured nitrogen and boron abundances in B-stars.Comment: to appear in Comm. in Astroseismology - Contribution to the
Proceedings of the 38th LIAC, 200
The nature of B supergiants: clues from a steep drop in rotation rates at 22000 K. The possibility of Bi-stability braking
The location of B supergiants in the Hertzsprung-Russell diagram (HRD)
represents a long-standing problem in massive star evolution. Here we propose
their nature may be revealed utilising their rotational properties, and we
highlight a steep drop in massive star rotation rates at an effective
temperature of 22000 K. We discuss two potential explanations for it. On the
one hand, the feature might be due to the end of the main sequence, which could
potentially constrain the core overshooting parameter. On the other hand, the
feature might be the result of enhanced mass loss at the predicted location of
the bi-stability jump. We term this effect "bi-stability breaking" and discuss
its potential consequences for the evolution of massive stars.Comment: Accepted by A&A Letters (4 pages, 5 figures); typos correcte
The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds
Aims. We compare the predictions of evolutionary models for early-type stars
with atmospheric parameters, projected rotational velocities and nitrogen
abundances estimated for a sample of Be-type stars. Our targets are located in
4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11
region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.
Methods. Atmospheric parameters and photospheric abundances have been
determined using the non-LTE atmosphere code tlusty. Effective temperature
estimates were deduced using three different methodologies depending on the
spectral features observed; in general they were found to yield consistent
estimates. Gravities were deduced from Balmer line profiles and
microturbulences from the Si iii spectrum. Additionally the contributions of
continuum emission from circumstellar discs were estimated. Given its
importance in constraining stellar evolutionary models, nitrogen abundances (or
upper limits) were deduced for all the stars analysed. Results. Our nitrogen
abundances are inconsistent with those predicted for targets spending most of
their main sequence life rotating near to the critical velocity. This is
consistent with the results we obtain from modelling the inferred rotational
velocity distribution of our sample and of other investigators. We consider a
number of possibilities to explain the nitrogen abundances and rotational
velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&
A brief description of geological and geophysical exploration of the Marysville geothermal area
Extensive geological and geophysical surveys were carried out at the Marysville geothermal area during 1973 and 1974. The area has high heat flow (up to microcalories per square centimeter-second, a negative gravity anomaly, high electrical resistivity, low seismic ground noise, and nearby microseismic activity. Significant magnetic and infrared anomalies are not associated with the geothermal area. The geothermal anomaly occupies the axial portion of a dome in Precambrian sedimentary rocks intruded by Cretaceous and Cenozoic granitic rocks. The results from a 2.4-km-deep test well indicate that the cause of the geothermal anomaly is hydrothermal convection in a Cenozoic intrusive. A maximum temperature of 95 C was measured at a depth of 500 m in the test well
The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
Rotation has become an important element in evolutionary models of massive
stars, specifically via the prediction of rotational mixing. Here, we study a
sample of stars, including rapid rotators, to constrain such models and use
nitrogen enrichments as a probe of the mixing process. Chemical compositions
(C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the
Large Magellanic Cloud with projected rotational velocities up to ~300km/s
using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including
rotational mixing, have been generated attempting to reproduce these
observations by adjusting the overshooting and rotational mixing parameters and
produce reasonable agreement with 60% of our core hydrogen burning sample. We
find (excluding known binaries) a significant population of highly nitrogen
enriched intrinsic slow rotators vsini less than 50km/s incompatible with our
models ~20% of the sample). Furthermore, while we find fast rotators with
enrichments in agreement with the models, the observation of evolved (log g
less than 3.7dex) fast rotators that are relatively unenriched (a further ~20%
of the sample) challenges the concept of rotational mixing. We also find that
70% of our blue supergiant sample cannot have evolved directly from the
hydrogen burning main-sequence. We are left with a picture where invoking
binarity and perhaps fossil magnetic fields are required to understand the
surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio
A hydrodynamic study of the circumstellar envelope of alpha Scorpii
Context: Both the absolute mass-loss rates and the mechanisms that drive the
mass loss of late-type supergiants are still not well known. Binaries such as
alpha Sco provide the most detailed empirical information about the winds of
these stars.
Aims: The goal was to improve the binary technique for the determination of
the mass-loss rate of alpha Sco A by including a realistic density distribution
and velocity field from hydrodynamic and plasma simulations.
Methods: We performed 3D hydrodynamic simulations of the circumstellar
envelope of alpha Sco in combination with plasma simulations accounting for the
heating, ionization, and excitation of the wind by the radiation of alpha Sco
B. These simulations served as the basis for an examination of circumstellar
absorption lines in the spectrum of alpha Sco B as well as of emission lines
from the Antares nebula.
Results: The present model of the extended envelope of alpha Sco reproduces
some of the structures that were observed in the circumstellar absorption lines
in the spectrum of alpha Sco B. Our theoretical density and velocity
distributions of the outflow deviate considerably from a spherically expanding
model, which was used in previous studies. This results in a higher mass-loss
rate of (2 +/- 0.5) x 10^-6 M_sun/yr. The hot H II region around the secondary
star induces an additional acceleration of the wind at large distances from the
primary, which is seen in absorption lines of Ti II and Cr II at -30 km/s.Comment: 12 pages, 14 figures, accepted for publication in A&
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters
We introduce a HST/STIS stellar census of R136a, the central ionizing star
cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA
spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together
provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide
spectral types of 90% of the 57 sources brighter than m_F555W = 16.0 mag within
a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including
R136b (O4\,If/WN8). We measure wind velocities for 52 early-type stars from CIV
1548-51, including 16 O2-3 stars. For the first time we spectroscopically
classify all Weigelt & Baier members of R136a, which comprise three WN5 stars
(a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A
complete Hertzsprung-Russell diagram for the most massive O stars in R136 is
provided, from which we obtain a cluster age of 1.5+0.3_-0.7 Myr. In addition,
we discuss the integrated ultraviolet spectrum of R136, and highlight the
central role played by the most luminous stars in producing the prominent HeII
1640 emission line. This emission is totally dominated by very massive stars
with initial masses above ~100 Msun. The presence of strong HeII 1640 emission
in the integrated light of very young star clusters (e.g A1 in NGC 3125)
favours an initial mass function extending well beyond a conventional upper
limit of 100 Msun. We include montages of ultraviolet spectroscopy for LMC O
stars in the Appendix. Future studies in this series will focus on optical
STIS/CCD medium resolution observations.Comment: 20 pages plus four Appendices providing LMC UV O spectral templates,
UV spectral atlas in R136, wind velocities of LMC O stars and photometry of
additional R136 source
A quantitative study of O stars in NGC2244 and the Mon OB2 association
Our goal is to determine the stellar and wind properties of seven O stars in
the cluster NGC2244 and three O stars in the OB association MonOB2. These
properties give us insight into the mass loss rates of O stars, allow us to
check the validity of rotational mixing in massive stars, and to better
understand the effects of the ionizing flux and wind mechanical energy release
on the surrounding interstellar medium and its influence on triggered star
formation. We collect optical and UV spectra of the target stars which are
analyzed by means of atmosphere models computed with the code CMFGEN. The
spectra of binary stars are disentangled and the components are studied
separately. All stars have an evolutionary age less than 5 million years, with
the most massive stars being among the youngest. Nitrogen surface abundances
show no clear relation with projected rotational velocities. Binaries and
single stars show the same range of enrichment. This is attributed to the youth
and/or wide separation of the binary systems in which the components have not
(yet) experienced strong interaction. A clear trend of larger enrichment in
higher luminosity objects is observed, consistent with what evolutionary models
with rotation predict for a population of O stars at a given age. We confirm
the weakness of winds in late O dwarfs. In general, mass loss rates derived
from UV lines are lower than mass loss rates obtained from Ha. The UV mass loss
rates are even lower than the single line driving limit in the latest type
dwarfs. These issues are discussed in the context of the structure of massive
stars winds. The evolutionary and spectroscopic masses are in agreement above
25 Msun but the uncertainties are large. Below this threshold, the few
late-type O stars studied here indicate that the mass discrepancy still seems
to hold.Comment: 20 pages, 12 figures. A&A accepte
The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
VFTS 682 is located in an active star-forming region, at a projected distance
of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the
Large Magellanic Cloud. It was previously reported as a candidate young stellar
object, and more recently spectroscopically revealed as a hydrogen-rich
Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as
its intrinsic luminosity, and to investigate the origin of VFTS 682. To this
purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the
non-LTE stellar atmosphere code CMFGEN, as well as the spectral energy
distribution from complementary optical and infrared photometry. We find the
extinction properties to be highly peculiar (RV ~4.7), and obtain a
surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a
present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 2.5kK
might be explained by chemically homogeneous evolution - suggested to be the
key process in the path towards long gamma-ray bursts. Lightcurves of the
object show variability at the 10% level on a timescale of years. Such changes
are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of
Luminous Blue Variables. Finally, we discuss two possibilities for the origin
of VFTS 682: (i) the star either formed in situ, which would have profound
implications for the formation mechanism of massive stars, or (ii) VFTS 682 is
a slow runaway star that originated from the dense cluster R136, which would
make it the most massive runaway known to date.Comment: 5 pages, 5 figures, accepted by A&A Letter
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