19,860 research outputs found
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
INTEGRAL-ISGRI observations of the CygOB2 region: earching for hard X-ray point sources in a region containing several non-thermal emitting massive stars
Aims: We analyze INTEGRAL-ISGRI data in order to probe the hard X-ray
emission (above 20 keV) from point sources in the Cyg OB2 region and to
investigate the putative non-thermal high-energy emission from early-type stars
(Wolf-Rayet and O-type stars). Among the targets located in the field of view,
we focus on the still unidentified EGRET source 3EG 2033+4118 that may be
related to massive stars known to produce non-thermal emission in the radio
domain, and on the wide colliding-wind binary WR 140. Methods: Using a large
set of data obtained with the IBIS-ISGRI imager onboard INTEGRAL, we run the
OSA software package in order to find point sources in the fully coded field of
view of the instrument. Results: Our data do not allow the detection of a
lower-energy counterpart of 3EG J2033+4118 nor of any other new point sources
in the field of view, and we derive upper limits on the high-energy flux for a
few targets: 3EG J2033+4118, TeV J2032+4130, WR140, WR146 and WR147. The
results are discussed in the context of the multiwavelength investigation of
these objects. Conclusions: The upper limits derived are valuable constraints
for models aimed at understanding the acceleration of particles in non-thermal
emitting massive stars, and of the still unidentified very-high gamma-ray
source TeV J2032+4130.Comment: 6 page, 2 figures including one figure in GIF format, accepted for
publication by A&
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
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&
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
Resonant enhancements of high-order harmonic generation
Solving the one-dimensional time-dependent Schr\"odinger equation for simple
model potentials, we investigate resonance-enhanced high-order harmonic
generation, with emphasis on the physical mechanism of the enhancement. By
truncating a long-range potential, we investigate the significance of the
long-range tail, the Rydberg series, and the existence of highly excited states
for the enhancements in question. We conclude that the channel closings typical
of a short-range or zero-range potential are capable of generating essentially
the same effects.Comment: 7 pages revtex, 4 figures (ps files
The investigation of particle acceleration in colliding-wind massive binaries with SIMBOL-X
An increasing number of early-type (O and Wolf-Rayet) colliding wind binaries
(CWBs) is known to accelerate particles up to relativistic energies. In this
context, non-thermal emission processes such as inverse Compton (IC) scattering
are expected to produce a high energy spectrum, in addition to the strong
thermal emission from the shock-heated plasma. SIMBOL-X will be the ideal
observatory to investigate the hard X-ray spectrum (above 10 keV) of these
systems, i.e. where it is no longer dominated by the thermal emission. Such
observations are strongly needed to constrain the models aimed at understanding
the physics of particle acceleration in CWB. Such systems are important
laboratories for investigating the underlying physics of particle acceleration
at high Mach number shocks, and probe a different region of parameter space
than studies of supernova remnants.Comment: 2 pages, 2 figures, to appear in the proceedings of the workshop
"Simbol-X: the hard X-ray universe in focus", held in Bologna, Italy (14-16
May 2007
Evidence for a physically bound third component in HD 150136
Context. HD150136 is one of the nearest systems harbouring an O3 star.
Although this system was for a long time considered as binary, more recent
investigations have suggested the possible existence of a third component.
Aims. We present a detailed analysis of HD 150136 to confirm the triple nature
of this system. In addition, we investigate the physical properties of the
individual components of this system. Methods. We analysed high-resolution,
high signal-to-noise data collected through multi-epoch runs spread over ten
years. We applied a disentangling program to refine the radial velocities and
to obtain the individual spectra of each star. With the radial velocities, we
computed the orbital solution of the inner system, and we describe the main
properties of the orbit of the outer star such as the preliminary mass ratio,
the eccentricity, and the orbital-period range. With the individual spectra, we
determined the stellar parameters of each star by means of the CMFGEN
atmosphere code. Results. We offer clear evidence that HD 150136 is a triple
system composed of an O3V((f\ast))-3.5V((f+)), an O5.5-6V((f)), and an
O6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical
masses of about 64, 40, and 35 Msun for the primary, the secondary and the
third components by assuming an inclination of 49{\deg}. It currently
corresponds to one of the most massive systems in our galaxy. The third star
moves with a period in the range of 2950 to 5500 d on an outer orbit with an
eccentricity of at least 0.3. This discovery makes HD 150136 the first
confirmed triple system with an O3 primary star. However, because of the long
orbital period, our dataset is not sufficient to constrain the orbital solution
of the tertiary component with high accuracy.Comment: 13 pages, 11 figures, accepted at A&
Quantum Gravity Corrections for Schwarzschild Black Holes
We consider the Matrix theory proposal describing eleven-dimensional
Schwarzschild black holes. We argue that the Newtonian potential between two
black holes receives a genuine long range quantum gravity correction, which is
finite and can be computed from the supergravity point of view. The result
agrees with Matrix theory up to a numerical factor which we have not computed.Comment: 14 pages, Tex, no figure
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