92 research outputs found
VLT observations of the highly ionized nebula around Brey2
We present the first high resolution HeII 4686 images of the high excitation
nebula around the WR star Brey 2 in the LMC. This nebula presents a striking
morphology: a small arc-like feature some 3.6pc in radius is particularly
prominent in the HeII 4686 line. We further discover a previously unknown faint
HeII emission that extends over an area of 22*17 pc^2. An even fainter HeII
emission is apparently associated with the interstellar bubble blown by the
progenitor of Brey2. The total HeII flux corresponds to an ionizing flux of
4*10^{47} photons/s. Halpha, [OIII], and HeI 5876 images and long-slit spectra
are also examined in this letter, enabling us to investigate the detailed
physical properties at various locations of the nebula.Comment: 4 pages, 3 figures (2 in jpg), accepted by A&A Letters, also
available from http://vela.astro.ulg.ac.be/Preprints/P80/index.htm
The atypical emission-line star Hen3-209
We analyse observations, spanning 15 years, dedicated to the extreme
emission-line object Hen3-209. Our photometric data indicate that the
luminosity of the star undergoes marked variations with a peak-to-peak
amplitude of 0.65mag. These variations are recurrent, with a period of
16.093+-0.005d. The spectrum of Hen3-209 is peculiar with many different lines
(HI, HeI, FeII,...) showing P Cygni profiles. The line profiles are apparently
changing in harmony with the photometry. The spectrum also contains [OIII]
lines that display a saddle profile topped by three peaks, with a maximum
separation of about 600km/s. Hen3-209 is most likely an evolved luminous object
suffering from mass ejection events and maybe belonging to a binary system.Comment: 6p, 5 fig, accepted for publication in MNRAS
(www.blackwell-synergy.com
Phase resolved X-ray spectroscopy of HDE228766: Probing the wind of an extreme Of+/WNLha star
HDE228766 is a very massive binary system hosting a secondary component,
which is probably in an intermediate evolutionary stage between an Of
supergiant and an WN star. The wind of this star collides with the wind of its
O8 II companion, leading to relatively strong X-ray emission. Measuring the
orbital variations of the line-of-sight absorption toward the X-ray emission
from the wind-wind interaction zone yields information on the wind densities of
both stars. X-ray spectra have been collected at three key orbital phases to
probe the winds of both stars. Optical photometry has been gathered to set
constraints on the orbital inclination of the system. The X-ray spectra reveal
prominent variations of the intervening column density toward the X-ray
emission zone, which are in line with the expectations for a wind-wind
collision. We use a toy model to set constraints on the stellar wind parameters
by attempting to reproduce the observed variations of the relative fluxes and
wind optical depths at 1 keV. The lack of strong optical eclipses sets an upper
limit of about 68 degrees on the orbital inclination. The analysis of the
variations of the X-ray spectra suggests an inclination in the range 54 - 61
degrees and indicates that the secondary wind momentum ratio exceeds that of
the primary by at least a factor 5. Our models further suggest that the bulk of
the X-ray emission arises from the innermost region of the wind interaction
zone, which is from a region whose outer radius, as measured from the secondary
star, lies between 0.5 and 1.5 times the orbital separation
A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
Previously, the X-ray emission of zeta Puppis was found to be variable with
light curves harbouring "trends" with a typical timescale longer than the
exposure length. The origin of these changes was proposed to be linked to
large-scale structures in the wind, but further characterization of the
variability at high energies was needed. Since then, a number of new X-ray
observations have become available. Furthermore, a cyclic behaviour with a
1.78d period was identified in long optical photometric runs, which is thought
to be associated with the launching mechanism of large-scale wind structures.
We analysed these new X-ray data, revisited the old data, and compared X-ray
with optical data, including when simultaneous. We found that the behaviour in
X-rays cannot be explained in terms of a perfect clock because the amplitude
and shape of its variations change with time. For example, zeta Puppis was much
more strongly variable between 2007 and 2011 than before and after this
interval. Comparing the X-ray spectra of the star at maximum and minimum
brightness yields no compelling difference beyond the overall flux change: the
temperatures, absorptions, and line shapes seem to remain constant, well within
errors. The only common feature between X-ray datasets is that the variation
amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no
clear and coherent correlation can be found between simultaneous X-ray and
optical data. Only a subgroup of observations may be combined coherently with
the optical period of 1.78d, although the simultaneous optical behaviour is
unknown. The currently available data do not reveal any obvious, permanent, and
direct correlation between X-ray and optical variations. The origin of the
X-ray variability therefore still needs to be ascertained, highlighting the
need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.Comment: accepted for publication by A&
How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems
Amongst O-type stars with detected magnetic fields, the fast rotator in the
close binary called Plaskett's star shows a variety of unusual properties.
Since strong binary interactions are believed to have occurred in this system,
one may wonder about their potential role in generating magnetic fields. Stokes
V spectra collected with the low-resolution FORS2 and high-resolution ESPaDOnS
and Narval spectropolarimeters were therefore used to search for magnetic
fields in 15 interacting or post-interaction massive binaries. No magnetic
field was detected in any of them, with 0G always being within 2sigma of the
derived values. For 17 out of 25 stars in the systems observed at
high-resolution, the 90% upper limit on the individual dipolar fields is below
the dipolar field strength of Plaskett's secondary; a similar result is found
for five out of six systems observed at low resolution. If our sample is
considered to form a group of stars sharing similar magnetic properties, a
global statistical analysis results in a stringent upper limit of ~200G on the
dipolar field strength. Moreover, the magnetic incidence rate in the full
sample of interacting or post-interaction systems (our targets + Plaskett's
star) is compatible with that measured from large surveys, showing that they
are not significantly different from the general O-star population. These
results suggest that binary interactions play no systematic role in the
magnetism of such massive systems.Comment: 11 pages, accepted for publication in MNRA
The 2.35 year itch of Cyg OB2 #9. II. Radio monitoring
Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star
binaries. The non-thermal radiation is due to synchrotron emission in the
colliding-wind region. Cyg OB2 #9 was only recently discovered to be a binary
system and a multi-wavelength campaign was organized to study its 2011
periastron passage. We report here on the results of the radio observations
obtained in this monitoring campaign. We used the Expanded Very Large Array
(EVLA) radio interferometer to obtain 6 and 20 cm continuum fluxes. The
observed radio light curve shows a steep drop in flux sometime before
periastron. The fluxes drop to a level that is comparable to the expected
free-free emission from the stellar winds, suggesting that the non-thermal
emitting region is completely hidden at that time. After periastron passage,
the fluxes slowly increase. We introduce a simple model to solve the radiative
transfer in the stellar winds and the colliding-wind region, and thus determine
the expected behaviour of the radio light curve. From the asymmetry of the
light curve, we show that the primary has the stronger wind. This is somewhat
unexpected if we use the astrophysical parameters based on theoretical
calibrations. But it becomes entirely feasible if we take into account that a
given spectral type - luminosity class combination covers a range of
astrophysical parameters. The colliding-wind region also contributes to the
free-free emission, which can help to explain the high values of the spectral
index seen after periastron passage. Combining our data with older Very Large
Array (VLA) data allows us to derive a period P = 860.0 +- 3.7 days for this
system. With this period, we update the orbital parameters that were derived in
the first paper of this series.Comment: 10 pages, 4 figures, accepted for publication in A&
First Detection of Phase-dependent Colliding Wind X-ray Emission outside the Milky Way
After having reported the detection of X-rays emitted by the peculiar system HD 5980, we assess here the origin of this high-energy emission from additional X-ray observations obtained with XMM-Newton. This research provides the first detection of apparently periodic X-ray emission from hot gas produced by the collision of winds in an evolved massive binary outside the Milky Way. It also provides the first X-ray monitoring of a Luminous Blue Variable only years after its eruption and shows that the source of the X-rays is not associated with the ejecta
The Herschel view of the nebula around the luminous blue variable star AG Carinae
Far-infrared Herschel PACS imaging and spectroscopic observations of the
nebula around the luminous blue variable (LBV) star AG Car have been obtained
along with optical imaging in the Halpha+[NII] filter. In the infrared light,
the nebula appears as a clumpy ring shell that extends up to 1.2 pc with an
inner radius of 0.4 pc. It coincides with the Halpha nebula, but extends
further out. Dust modeling of the nebula was performed and indicates the
presence of large grains. The dust mass is estimated to be ~ 0.2 Msun. The
infrared spectrum of the nebula consists of forbidden emission lines over a
dust continuum. Apart from ionized gas, these lines also indicate the existence
of neutral gas in a photodissociation region that surrounds the ionized region.
The abundance ratios point towards enrichment by processed material. The total
mass of the nebula ejected from the central star amounts to ~ 15 Msun, assuming
a dust-to-gas ratio typical of LBVs. The abundances and the mass-loss rate were
used to constrain the evolutionary path of the central star and the epoch at
which the nebula was ejected, with the help of available evolutionary models.
This suggests an ejection during a cool LBV phase for a star of ~ 55 Msun with
little rotation.Comment: accepted for publication in A&
First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds
Massive O-type stars play a dominant role in our Universe, but many of their
properties remain poorly constrained. In the last decade magnetic fields have
been detected in all Galactic members of the distinctive Of?p class, opening
the door to a better knowledge of all O-type stars. With the aim of extending
the study of magnetic massive stars to nearby galaxies, to better understand
the role of metallicity in the formation of their magnetic fields and
magnetospheres, and to broaden our knowledge of the role of magnetic fields in
massive star evolution, we have carried out spectropolarimetry of five
extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We
have been able to measure magnetic fields with typical error bars from 0.2 to
1.0 kG, depending on the apparent magnitude and on weather conditions. No
magnetic field has been firmly detected in any of our measurements, but we have
been able to estimate upper limits to the field values of our target stars. One
of our targets, 2dFS 936, exhibited an unexpected strengthening of emission
lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period
with two photometric peaks and one spectroscopic maximum. The observed
strengthening of the emission lines of 2dFS 936, and the lack of detection of a
strong magnetic field in a star with such strong emission lines is at odd with
expectations. Together with the unusual periodic behaviour of BI 57, it
represents a challenge for the current models of Of?p stars. The limited
precision that we obtained in our field measurements (in most cases as a
consequence of poor weather) has led to field-strength upper limits that are
substantially larger than those typically measured in Galactic magnetic O
stars. Further higher precision observations and monitoring are clearly
required.Comment: Accepted by A&
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