ISSN 1783-3426
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A rare early-type star revealed in the
Wing of the Small Magellanic Cloud
9
Sgr: uncovering an O-type spectroscopic binary with an 8.6 year
period
Nitrogen line spectroscopy
in O-stars -- III. The earliest O-stars
Radiation-driven
winds of hot luminous stars
Numerical
Models for the Diffuse Ionized Gas in Galaxies. I. Synthetic spectra
of thermally excited gas with turbulent magnetic reconnection as
energy source
A peculiar Of star in
the Local Group galaxy IC,1613
The
Transition Mass-loss Rate: Calibrating the Role of Line-driven Winds
in Massive Star Evolution
Low-frequency
GMRT observations of the magnetic Bp star HR Lup (HD 133880)
Diagnosing small- and large-scale structure in the winds of hot, massive OB-stars
Positions on Massive Stars at the National Observatory of Athens
C. J. Evans (1), R. Hainich
(2), L. M. Oskinova (2), J. S. Gallagher (3), Y.-H. Chu (4), R. A.
Gruendl (4), W.-R. Hamann (2), V. Hénault-Brunet (5), H. Todt
(2)
(1) UK Astronomy Technology Centre, Edinburgh; (2)
University of Potsdam; (3) University of Wisconsin-Madison; (4)
University of Illinois; (5) University of Edinburgh
Sk 183 is
the visually-brightest star in the N90 nebula, a young star-forming
region in the Wing of the Small Magellanic Cloud (SMC). We present
new optical spectroscopy from the Very Large Telescope which reveals
Sk 183 to be one of the most massive O-type stars in the SMC.
Classified as an O3-type dwarf on the basis of its nitrogen spectrum,
the star also displays broadened He 1 absorption which suggests a
later type. We propose that Sk 183 has a composite spectrum and that
it is similar to another star in the SMC, MPG 324. This brings the
number of rare O2- and O3-type stars known in the whole of the SMC to
a mere three. We estimate physical parameters for Sk 183 from
analysis of its spectrum. For a single-star model, we estimate an
effective temperature of 46+/-2 kK, a low mass-loss rate of ~10^-7
Msun yr^-1, and a spectroscopic mass of 46^9_-8 Msun (for an adopted
distance modulus of 18.7 mag to the young population in the SMC
Wing). An illustrative binary model requires a slightly hotter
temperature (~47.5 kK) for the primary component. In either scenario,
Sk 183 is the earliest-type star known in N90 and will therefore be
the dominant source of hydrogen-ionising photons. This suggests Sk
183 is the primary influence on the star formation along the inner
edge of the nebula.
Reference: Accepted by ApJ
Status:
Manuscript has been accepted
Weblink:
http://arxiv.org/abs/1205.2387
Comments:
Email: chris.evans@stfc.ac.uk
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G. Rauw$^1$, H. Sana$^2$, M.
Spano$^3$, E. Gosset$^1$, L. Mahy$^1$, M. De Becker$^1$, and P.
Eenens$^4$
1 - Institut d’Astrophysique et de
Geophysique, Liege University, Belgium;
2 - Sterrenkundig
Instituut ‘Anton Pannekoek’, Amsterdam University, The
Netherlands;
3 - Observatoire de Geneve, Geneva University,
Switzerland;
4 - Departamento de Astronomia, Guanajuato
University, Mexico
Context. The O-type object 9 Sgr is a
well-known synchrotron radio emitter. This feature is usually
attributed to colliding-wind binary systems, but 9 Sgr was long
considered a single star.
Aims. We have conducted a long-term
spectroscopic monitoring of this star to investigate its multiplicity
and search for evidence for wind-wind interactions.
Methods.
Radial velocities are determined and analysed using various period
search methods. Spectral disentangling is applied to separate the
spectra of the components of the binary system.
Results. We
derive the first ever orbital solution of 9 Sgr. The system is found
to consist of an O3.5 V((f+)) primary and an O5-5.5 V((f)) secondary
moving around each other on a highly eccentric (e = 0.7), 8.6 year
orbit. The spectra reveal no variable emission lines that could be
formed in the wind interaction zone in agreement with the expected
properties of the interaction in such a wide system.
Conclusions.
Our results provide further support to the paradigm of synchrotron
radio emission from early-type stars being a manifestation of
interacting winds in a binary system.
Reference:
arXiv:1205.3314
Status: Manuscript has been accepted
Weblink:
http://arxiv.org/abs/1205.3314
Comments:
Accepted for publication in Astronomy & Astrophysics
Email:
rauw@astro.ulg.ac.be
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J.G. Rivero Gonzalez (1), J.
Puls (1), P. Massey (2), and F. Najarro (3)
(1)
Universitaetssternwarte Muenchen, Germany
(2) Lowell Observatory,
Flagstaff, USA
(3) Centro de Astrobiologia, (CSIC-INTA), Torrejon
de Ardoz, Spain
Context: The classification scheme proposed by
Walborn et al. (2002, AJ, 123, 2754), based primarily on the relative
strengths of the NIV4058 and NIII4640 emission lines, has been used
in a variety of studies to spectroscopically classify early O-type
stars. Owing to the lack of a solid theoretical basis, this scheme
has not yet been universally accepted though.
Aims: We
provide first theoretical predictions for the NIV4058/NIII4640
emission line ratio in dependence of various parameters, and confront
these predictions with results from the analysis of a sample of
early-type LMC/SMC O-stars.
Methods:Stellar and wind
parameters of our sample stars are determined by line profile fitting
of hydrogen, helium and nitrogen lines, exploiting the helium and
nitrogen ionization balance. Corresponding synthetic spectra are
calculated by means of the NLTE atmosphere/spectrum synthesis code
FASTWIND.
Results: Though there is a monotonic relationship
between the NIV/NIII emission line ratio and the effective
temperature, all other parameters being equal, theoretical
predictions indicate additional
dependencies on surface gravity,
mass-loss, metallicity, and, particularly, nitrogen abundance. For a
given line ratio (i.e., spectral type), more enriched objects should
be typically hotter. These basic predictions are confirmed by results
from the alternative model atmosphere code CMFGEN.
The
effective temperatures for the earliest O-stars, inferred from the
nitrogen ionization balance, are partly considerably hotter than
indicated by previous studies. Consistent with earlier results,
effective temperatures increase from supergiants to dwarfs for all
spectral types in the LMC. The relation between observed
NIV4058/NIII4640 emission line ratio and effective temperature, for a
given luminosity class, turned out to be quite monotonic for our
sample stars, and to be fairly consistent with our model predictions.
The scatter within a spectral sub-type is mainly produced by
abundance effects.
Conclusions: Our findings suggest that the
Walborn et al.(2002) classification scheme is able to provide a
meaningful relation between spectral type and effective temperature,
as long as it is possible to discriminate for the luminosity class.
In terms of spectral morphology, this might
be difficult to
achieve in low-Z environments such as the SMC, owing to rather low
wind-strengths. According to our predictions, the major bias of the
classification scheme is due to nitrogen content, and the overall
spectral type-Teff relation for low-metallicity (e.g., SMC) O-stars
might be non-monotonic around O3.5/O4.
Reference: Accepted
by Astronomy & Astrophysics
Status: Manuscript has been
accepted
Weblink: http://arxiv.org/abs/1205.4444
Comments:
Email: uh101aw@usm.uni-muenchen.de
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C. B. Kaschinski, A.W.A.
Pauldrach, T. L. Hoffmann
Institut fuer Astronomie und
Astrophysik der Universitaet Muenchen, Scheinerstrasse 1, 81679
Muenchen, Germany
Context: The commonly accepted
mass-luminosity relation of central stars of planetary nebulae
(CSPNs) might not be universally valid. While earlier optical
analyses could not derive masses and luminosities independently
(instead taking them from theoretical evolutionary models)
hydrodynamically consistent
modelling of the stellar winds allows
using fits to the UV spectra to consistently determine also stellar
radii, masses, and luminosities without assuming a mass-luminosity
relation. Recent application to a sample of CSPNs raised questions
regarding the validity of the theoretical mass-luminosity relation of
CSPNs.
Aims: The results of the earlier UV analysis are
reassessed by means of a simultaneous comparison of observed optical
and UV spectra with corresponding synthetic spectra.
Methods:
Using published stellar parameters (a) from a consistent UV analysis
and (b) from fits to optical H and He lines, we calculate
simultaneous optical and UV spectra with our model atmosphere code,
which has been improved by implementing Stark broadening for H and He
lines.
Results: Spectra computed with the parameter sets from the
UV analysis yield good agreement to the observations, but spectra
computed with the stellar parameters from the published optical
analysis and using corresponding consistent wind parameters show
large discrepancies to both the observed optical and UV spectra. The
published optical analyses give good fits to the observed spectrum
only because the wind parameters assumed in these analyses are
inconsistent with their stellar parameters. By enforcing consistency
between stellar and wind parameters, stellar parameters are obtained
which disagree with
the core-mass-luminosity relation for the
objects analyzed. This disagreement is also evident from a completely
different approach: an investigation of the dynamical wind
parameters.
Reference: Publication in A&A
Status:
Manuscript has been accepted
Weblink:
http://arxiv.org/abs/1204.1200
Comments:
Email: corni@usm.uni-muenchen.de
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T. L. Hoffmann, S. Lieb, A. W. A. Pauldrach, H. Lesch, P.
J. N. Hultzsch, G. T. Birk
Institut fuer Astronomie und
Astrophysik der Universitaet Muenchen, Scheinerstraße 1, 81679
Muenchen, Germany
Aims: The aim of this work is to verify
whether turbulent magnetic reconnection can provide the additional
energy input required to explain the up to now only poorly understood
ionization mechanism of the diffuse ionized gas (DIG) in galaxies and
its observed emission line spectra.
Methods: We use a detailed
non-LTE radiative transfer code that does not make use of the usual
restrictive gaseous nebula approximations to compute synthetic
spectra for gas at low densities. Excitation of the gas is via an
additional heating term in the energy balance as well as by
photoionization. Numerical values for this heating term are derived
from three-dimensional resistive magnetohydrodynamic two-fluid
plasma--neutral-gas simulations to compute energy dissipation rates
for the DIG under typical conditions.
Results: Our simulations
show that magnetic reconnection can liberate enough energy to by
itself fully or partially ionize the gas. However, synthetic spectra
from purely thermally excited gas are incompatible with the observed
spectra; a photoionization source must additionally be present to
establish the correct (observed) ionization balance in the
gas.
Reference: Accepted by Astronomy &
Astrophysics
Status: Manuscript has been accepted
Weblink:
arXiv:1206.0394v1
Comments:
Email: hoffmann@usm.lmu.de
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A. Herrero$^{1,2}$, M.
Garcia$^{1,2}$, J. Puls$^3$, K. Uytterhoeven$^{1,2}$, F. Najarro$^4$,
D.J. Lennon$^5$ and J.G. Rivero-Gonzalez$^3$
1. Instituto
de Astrof'{i}sica de Canarias, C/ Via Lactea s/n, E-38200 La Laguna,
Tenerife, Spain.
2. Departamento de Astrofisica, Universidad de
La Laguna, Avda. Astrofisico Francisco Sanchez s/n, E-38071 La
Laguna, Tenerife, Spain.
3. Universitatssternwarte Munchen,
Scheinerstr. 1, 81679 Munchen, Germany.
4. Centro de
Astrobiologia (CSIC-INTA), Ctra. de Torrejon a Ajalvir km-4, E-28850,
Torrej'on de Ardoz, Madrid, Spain.
5. ESA, Space Telescope
Science Institute, 3700 San Martin Drive, Baltimore, MD 21218,
USA.
Context. Results from the theory of radiatively driven
winds are nowadays incorporated in stellar evolutionary and
population synthesis models, and are used in our interpretation of
the observations of the deep Universe. Yet, the theory has been
confirmed only until Small Magellanic Cloud (SMC) metallicities.
Observations and analyses of O-stars at lower metallicities are
difficult, but much needed to prove the theory.
Aims. We have
observed GHV-62024, an O6.5 IIIf star in the low-metallicity galaxy
IC,1613 (Z~0.15 Zsun) to study its evolution and wind. According to a
previous preliminary analysis that was subject to significant
restrictions this star could challenge the radiatively driven wind
theory at low metallicities. Here we present a complete analysis of
this star.
Methods. Our observations were obtained with VIMOS
at VLT, at R~2000 and covered approximately between 400 and 700 nm.
The observations were analysed using the latest version of the model
atmosphere code FASTWIND, which includes the possibility of
calculating the N III spectrum.
Results. We obtain the
stellar parameters and conclude that the star follows the average
wind momentum--luminosity relationship (WLR) expected for its
metallicity, but with a high value for the exponent of the wind
velocity law, beta. Comparing this with values of other stars in the
literature, we suggest that this high value may be reached because
GHV-62024 could be a fast rotator seen at a low inclination angle. We
also suggest that this could favour the appearance of the spectral
"f"-characterictics. While the derived beta value does not
change by adopting a lower wind terminal velocity, we show that a
wrong V_infty has a clear impact on the position of the star in the
WLR diagram. The N and He abundances are very high, consistent with
strong CNO mixing that could have been caused by the fast rotation,
although we cannot discard a different origin with present data.
Stellar evolutionary model predictions are consistent with the star
being still a fast rotator. We find again the well-known
mass-discrepancy for this star.
Conclusions. We conclude that
the star follows the WLR expected for its metallicity. The results
are consistent with GHV-62024 being a fast rotator seen close to
pole-on, strongly contaminated at the surface with CNO products and
with a wind structure altered by the fast rotation but without
modifying the global WLR. We suggest that this could be a general
property of fast rotators.
Reference: Astronomy &
Astrophysics
Status: Manuscript has been accepted
Weblink:
http://arxiv.org/abs/1206.1238v1
Comments:
Email: ahd@iac.es
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Jorick S. Vink, G"otz
Gr"afener
Armagh Observatory
A debate has
arisen regarding the importance of stationary versus eruptive mass
loss for massive star evolution. The reason is that stellar winds
have been found to be clumped, which results in the reduction of
unclumped empirical mass-loss rates. Most stellar evolution models
employ theoretical mass-loss rates which are already reduced by a
moderate factor of ~2-3 compared to non-corrected empirical rates. A
key question is whether these reduced rates are of the correct order
of magnitude, or if they should be reduced even further, which would
mean that the alternative of eruptive mass loss becomes necessary.
Here we introduce the transition mass-loss rate (dM/dt)_trans between
O and Wolf-Rayet (WR) stars. Its novelty is that it is model
independent. All that is required is postulating the spectroscopic
transition point in a given data-set, and determining the stellar
luminosity, which is far less model dependent than the mass-loss
rate. The transition mass-loss rate is subsequently used to calibrate
stellar wind strength by its application to the Of/WNh stars in the
Arches cluster. Good agreement is found with two alternative
modelling/theoretical results, suggesting that the rates provided by
current theoretical models are of the right order of magnitude in the
~50Msun mass range. Our results do not confirm the specific need for
eruptive mass loss as Luminous Blue Variables, and current stellar
evolution modelling for Galactic massive stars seems sound. Mass loss
through alternative mechanisms might still become necessary at lower
masses, and/or metallicities, and the quantification of alternative
mass loss is desirable.
Reference: ApJL 751, 34 (2012)
Astro-ph/1205.0394
Status: Manuscript has been
accepted
Weblink: http://arxiv.org/abs/1205.0394
Comments:
Email: jsv@arm.ac.uk
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Samuel J. George$^{1,2}$, Ian R.
Stevens$^{2}$
1 - Astrophysics Group, The Cavendish
Laboratory, JJ Thomson Avenue, University of Cambridge, Cambridge,
CB3 0HE, UK;
2 - Astrophysics and Space Research Group, School of
Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT,
UK
We present radio observations of the magnetic chemically
peculiar Bp star HR Lup (HD 133880) at 647 and 277 MHz with the GMRT.
At both frequencies the source is not detected but we are able to
determine upper limits to the emission. The 647 MHz limits are
particularly useful, with a 5$sigma$ value of 0.45 mJy. Also, no
large enhancements of the emission were seen. The non-detections,
along with previously published higher frequency detections, provide
evidence that an optically thick gyrosynchrotron model is the correct
mechanism for the radio emission of HR Lup.
Reference:
accepted for publication in the Bulletin of the Astronomical
Society of India, to appear in the June issue;
http://arxiv.org/abs/1206.1155
Status: Manuscript has been
accepted
Weblink: http://arxiv.org/abs/1206.1155
Comments:
Email: sgeorge@mrao.cam.ac.uk
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J.O. Sundqvist(1), and S.P.
Owocki(1)
1. University of Delaware, USA
It is
observationally as well as theoretically well established that the
winds of hot, massive OB-stars are highly structured on a broad range
of spatial scales. This paper first discusses consequences of the
small-scale structures associated with the strong instability
inherent to the line-driving of these winds. We demonstrate the
importance of a proper treatment of such wind clumping to obtain
reliable estimates of mass-loss rates, and also show that instability
simulations that are perturbed at the lower boundary indeed display
significant clumping quite close to the wind base, in general
agreement with observations.
But a growing subset of massive
stars has also been found to possess strong surface magnetic fields,
which may channel the star's outflow and induce also large-scale wind
structures and cyclic behavior of spectral diagnostics. The paper
concludes by showing that multi-dimensional, magneto-hydrodynamical
wind simulations, together with detailed radiative-transfer modeling,
can reproduce remarkably well the periodic Balmer line emission
observed in slowly rotating magnetic O stars like
HD191612.
Reference: To appear in the proceedings of
"Circumstellar Dynamics at High Resolution", Foz do Iguacu,
Feb. 2012. Available at astro-ph.
Status: Conference
proceedings
Weblink:
http://arxiv.org/abs/1205.3190
Comments:
Email: jon@bartol.udel.edu
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A. Bonanos
Institute
of Astronomy, Astrophysics, Space Applications and Remote Sensing
National Observatory of Athens
Metaxa & Vas. Pavlou,
Palaia Penteli 15236
Greece
Applications are invited for
(i) a 2 to 3-year postdoctoral research position and (ii) a 3-year
PhD position, on characterizing massive stars in the Milky Way and in
nearby galaxies at the National Observatory of Athens, Greece.
Applicants with previous experience in optical and infrared
spectroscopy, photometry, and the field of massive stars are
particularly encouraged to apply. For more information visit:
http://www.astro.noa.gr/~bonanos/Homepage/Job_Openings.html
Attention/Comments:
Consideration of applications will begin on August 1st, 2012 and
will continue until the positions are filled. A starting date in the
Fall of 2012 is anticipated.
Weblink:
http://www.astro.noa.gr/~bonanos/Homepage/Job_Openings.html
Email:
bonanos@astro.noa.gr
Deadline: August 1, 2012
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