546 research outputs found
An exceptional X-ray view of the young open cluster NGC 6231: what XMM-Newton has taught us
Considered as the core of the Sco OB1 association, the young open cluster NGC
6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite
targeted the cluster for a nominal duration of about 180 ks. Thanks to the
detector sensitivity, the EPIC cameras provided an unprecedented X-ray view of
NGC 6231, revealing about 600 point-like sources. In this contribution, we
review the main results that have been obtained thanks to this unprecedented
data set. Concerning the O-type stars, we present the latest developments
related to the so-called 'canonical' Lx-Lbol relation. The dispersion around
this relation might actually be much smaller than previously thought. In our
data set, the sole mechanism that yields a significant deviation from this
scheme is wind interaction. It is also the sole mechanism that induces a
significant variation of the early-type star X-ray flux. In a second part of
this contribution, we probe the properties of the optically faint X-ray
sources. Most of them are believed to be low mass pre-main sequence stars.
Their analysis provides direct insight into the star formation history of the
cluster.Comment: 6 pages, 5 figures, to appear in "The X-Ray Universe 2005", ESA
Symposium held at El Escorial, Madrid (Spain), 26-30 Sep 200
Are WC9 Wolf-Rayet stars in colliding-wind binaries?
We present results from a spectroscopic search for massive companions to
dust-making Galactic WC9 stars as a step to testing the paradigm that dust
formation in these systems requires colliding winds to produce over densities.
We find evidence for OB companions to the WC9 stars WR 59 and WR 65, but not WR
121 or WR 117. We identify lines of N III-V and possibly N II in the spectrum
of WR 88, one of the few Galactic WC9 stars which do not make circumstellar
dust, and suggest that WR 88 is a transitional WN-WC9 object and less evolved
than the other WC9 stars. On the other hand, the possible identification of a
strong emission line at 4176A in the spectrum of WR 117 with Ne I suggests that
this star is more evolved than other WC9 stars studied.Comment: 4 pages, 3 figures, contribution to "Massive Stars and High-Energy
Emission in OB Associations"; JENAM 2005, held in Liege (Belgium
Compact star clusters of the LMC HII region N11C
Based on imaging and spectroscopy obtained at the ESO NTT telescope and using
an efficient image analysis algorithm, we study the core of the LMC OB
association LH13, particularly the two compact stellar clusters Sk-6641 and HNT
in the HII, region N11C. We resolve Sk-6641 into 15 components and for the
first time the HNT cluster into 70 stars, and derive photometry for the
members. Moreover, from medium resolution spectroscopy we determine the
spectral types for sixteen stars in N11C. We compare the color-magnitude
diagrams of the clusters with that of the field stars and discuss the cluster
ages. With an age of ~100 Myr, the HNT cluster appears significantly older than
the very young (< 5 Myr) Sk-6641 starburst. We suggest that most of the `field'
O-stars in the core of N11C have actually been ejected from Sk-6641 through
dynamical interactions in the compact cluster. The properties of the Sk-6641
and HNT clusters suggest that we are viewing different star formation regions
lying at different distances along the same line of sight.Comment: 12 pages, 8 figure
Search for magnetic fields in particle-accelerating colliding-wind binaries
Some colliding-wind massive binaries, called particle-accelerating
colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is
assumed to be generated by a stellar magnetic field. However, no measurement of
magnetic fields in these stars has ever been performed. We aim at quantifying
the possible stellar magnetic fields present in PACWB to provide constraints
for models. We gathered 21 high-resolution spectropolarimetric observations of
9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed
these observations with the Least Squares Deconvolution method. We separated
the binary spectral components when possible. No magnetic signature is detected
in any of the 9 PACWB stars and all longitudinal field measurements are
compatible with 0 G. We derived the upper field strength of a possible field
that could have remained hidden in the noise of the data. While the data are
not very constraining for some stars, for several stars we could derive an
upper limit of the polar field strength of the order of 200 G. We can therefore
exclude the presence of strong or moderate stellar magnetic fields in PACWB,
typical of the ones present in magnetic massive stars. Weak magnetic fields
could however be present in these objects. These observational results provide
the first quantitative constraints for future models of PACWB.Comment: Accepted in A&
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
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
A binary signature in the non-thermal radio-emitter Cyg OB2 #9
Aims: Non-thermal radio emission associated with massive stars is believed to
arise from a wind-wind collision in a binary system. However, the evidence of
binarity is still lacking in some cases, notably Cyg OB2 #9 Methods: For
several years, we have been monitoring this heavily-reddened star from various
observatories. This campaign allowed us to probe variations both on short and
long timescales and constitutes the first in-depth study of the visible
spectrum of this object. Results: Our observations provide the very first
direct evidence of a companion in Cyg OB2 #9, confirming the theoretical
wind-wind collision scenario. These data suggest a highly eccentric orbit with
a period of a few years, compatible with the 2yr-timescale measured in the
radio range. In addition, the signature of the wind-wind collision is very
likely reflected in the behaviour of some emission lines.Comment: accepted by A&A, 4 p, 3figure
Spectroscopic study of the O-type runaway supergiant HD 195592
The scope of this paper is to perform a detailed spectroscopic study of the
northern O-type supergiant HD 195592. We use a large sample of high quality
spectra in order to investigate its multiplicity, and to probe the line profile
variability. Our analysis reveals a clear spectroscopic binary signature in the
profile of the He {\sc i} 6678 line, pointing to a probable O + B
system. We report on low amplitude radial velocity variations in every strong
absorption line in the blue spectrum of HD 195592. These variations are ruled
by two time-scales respectively of 5.063 and about 20 days. The former is
firmly established, whilst the latter is poorly constrained. We report also on
a very significant line profile variability of the H line, with time
scales strongly related to those of the radial velocities. Our results provide
significant evidence that HD 195592 is a binary system, with a period that
might be the variability time-scale of about 5 days. The second time scale may
be the signature of an additional star moving along a wider orbit provided its
mass is low enough, even though direct evidence for the presence of a third
star is still lacking. Alternatively, the second time-scale may be the
signature of a variability intrinsic to the stellar wind of the primary,
potentially related to the stellar rotation.Comment: 9 pages, 5 postscript figures, accepted for publication in New
Astronom
A spectroscopic investigation of the O-type star population in four Cygnus OB associations. II. Determination of the fundamental parameters
Aims. Having established the binary status of nineteen O-type stars located
in four Cygnus OB associations, we now determine their fundamental parameters
to constrain their properties and their evolutionary status. We also
investigate their surface nitrogen abundances, which we compare with other
results from the literature obtained for galactic O-type stars. Methods. Using
optical spectra collected for each object in our sample and some UV data from
the archives, we apply the CMFGEN atmosphere code to determine their main
properties. For the binary systems, we have disentangled the components to
obtain their individual spectra and investigate them as if they were single
stars. Results. We find that the distances of several presumably single O-type
stars seem poorly constrained because their luminosities are not in agreement
with the "standard" luminosities of stars with similar spectral types. The ages
of these O-type stars are all less than 7 Myrs. Therefore, the ages of these
stars agree with those, quoted in the literature, of the four associations,
except for CygOB8 for which the stars seem older than the association itself.
However, we point out that the distance of certain stars is debatable relative
to values found in the literature. The N content of these stars put in
perspective with N contents of several other galactic O-type stars seems to
draw the same five groups as found in the "Hunter" diagram for the O and B-type
stars in the LMC even though their locations are obviously different. We
determine mass-loss rates for several objects from the Halpha line and UV
spectra. Finally, we confirm the "mass discrepancy" especially for O stars with
masses smaller than 30 Msun. .Comment: 11 pages, and 26 pages of Appendix. A&A in pres
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