302 research outputs found
MHD numerical simulations of colliding winds in massive binary systems - I. Thermal vs non-thermal radio emission
In the past few decades detailed observations of radio and X-rays emission
from massive binary systems revealed a whole new physics present in such
systems. Both thermal and non-thermal components of this emission indicate that
most of the radiation at these bands originates in shocks. OB and WR stars
present supersonic and massive winds that, when colliding, emit largely due to
the free-free radiation. The non-thermal radio and X-ray emissions are due to
synchrotron and inverse compton processes, respectively. In this case, magnetic
fields are expected to play an important role on the emission distribution. In
the past few years the modeling of the free-free and synchrotron emissions from
massive binary systems have been based on purely hydrodynamical simulations,
and ad hoc assumptions regarding the distribution of magnetic energy and the
field geometry. In this work we provide the first full MHD numerical
simulations of wind-wind collision in massive binary systems. We study the
free-free emission characterizing its dependence on the stellar and orbital
parameters. We also study self-consistently the evolution of the magnetic field
at the shock region, obtaining also the synchrotron energy distribution
integrated along different lines of sight. We show that the magnetic field in
the shocks is larger than that obtained when the proportionality between
and the plasma density is assumed. Also, we show that the role of the
synchrotron emission relative to the total radio emission has been
underestimated.Comment: MNRAS accepte
Non-thermal radio emission from O-type stars. IV. Cyg OB2 No. 8A
We study the non-thermal radio emission of the binary Cyg OB2 No. 8A, to see
if it is variable and if that variability is locked to the orbital phase. We
investigate if the synchrotron emission generated in the colliding-wind region
of this binary can explain the observations and we verify that our proposed
model is compatible with the X-ray data. We use both new and archive radio data
from the Very Large Array (VLA) to construct a light curve as a function of
orbital phase. We also present new X-ray data that allow us to improve the
X-ray light curve. We develop a numerical model for the colliding-wind region
and the synchrotron emission it generates. The model also includes free-free
absorption and emission due to the stellar winds of both stars. In this way we
construct artificial radio light curves and compare them with the observed one.
The observed radio fluxes show phase-locked variability. Our model can explain
this variability because the synchrotron emitting region is not completely
hidden by the free-free absorption. In order to obtain a better agreement for
the phases of minimum and maximum flux we need to use stellar wind parameters
for the binary components which are somewhat different from typical values for
single stars. We verify that the change in stellar parameters does not
influence the interpretation of the X-ray light curve. Our model has trouble
explaining the observed radio spectral index. This could indicate the presence
of clumping or porosity in the stellar wind, which - through its influence on
both the Razin effect and the free-free absorption - can considerably influence
the spectral index. Non-thermal radio emitters could therefore open a valuable
pathway to investigate the difficult issue of clumping in stellar winds.Comment: 19 pages, 10 figures, accepted by A&
Precession and Nutation in the eta Carinae binary system: Evidences from the X-ray light curve
It is believed that eta Carinae is actually a massive binary system, with the
wind-wind interaction responsible for the strong X-ray emission. Although the
overall shape of the X-ray light curve can be explained by the high
eccentricity of the binary orbit, other features like the asymmetry near
periastron passage and the short quasi-periodic oscillations seen at those
epochs, have not yet been accounted for. In this paper we explain these
features assuming that the rotation axis of eta Carinae is not perpendicular to
the orbital plane of the binary system. As a consequence, the companion star
will face eta Carinae on the orbital plane at different latitudes for different
orbital phases and, since both the mass loss rate and the wind velocity are
latitude dependent, they would produce the observed asymmetries in the X-ray
flux. We were able to reproduce the main features of the X-ray light curve
assuming that the rotation axis of eta Carinae forms an angle of 29 degrees
with the axis of the binary orbit. We also explained the short quasi-periodic
oscillations by assuming nutation of the rotation axis, with amplitude of about
5 degrees and period of about 22 days. The nutation parameters, as well as the
precession of the apsis, with a period of about 274 years, are consistent with
what is expected from the torques induced by the companion star.Comment: 9 pages, 8 figures, MNRAS accepte
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&
Mass-Loss Rate Determination for the Massive Binary V444 Cyg using 3-D Monte-Carlo Simulations of Line and Polarization Variability
A newly developed 3-D Monte Carlo model is used, in conjunction with a
multi-line non-LTE radiative transfer model, to determine the mass-loss rate of
the Wolf-Rayet (W-R) star in the massive binary \object{V444 Cyg} (WN5+O6).
This independent estimate of mass-loss rate is attained by fitting the observed
\HeI (5876) \AA and \HeII (5412) \AA line profiles, and the continuum light
curves of three Stokes parameters ((I, Q, U)) in the (V) band simultaneously.
The high accuracy of our determination arises from the use of many
observational constraints, and the sensitivity of the continuum polarization to
the mass-loss rate. Our best fit model suggests that the mass-loss rate of the
system is (\dot{M}_{\WR}=0.6(\pm 0.2) \times 10^{-5} M_{\sun} \mathrm{yr}^{-1}
), and is independent of the assumed distance to \object{V444 Cyg}. The fits
did not allow a unique value for the radius of the W-R star to be derived. The
range of the volume filling factor for the W-R star atmosphere is estimated to
be in the range of 0.050 (for R_{\WR}=5.0 R_{\sun}) to 0.075 (for
R_{\WR}=2.5 R_{\sun}). We also found that the blue-side of \HeI (5876 ) \AA
and \HeII (5412) \AA lines at phase 0.8 is relatively unaffected by the
emission from the wind-wind interaction zone and the absorption by the O-star
atmosphere; hence, the profiles at this phase are suitable for spectral line
fittings using a spherical radiative transfer model.Comment: 18 pages, 17 figures: Accepeted for publication in A&
Cold gas in the Intra Cluster Medium: implications for flow dynamics and powering optical nebulae
We show that the mechanical energy injection rate generated as the
intra-cluster medium (ICM) flows around cold clouds may be sufficient to power
the optical and near infra-red emission of nebulae observed in the central
regions of a sample of seven galaxy clusters. The energy injection rate is
extremely sensitive to the velocity difference between the ICM and cold clouds,
which may help to explain why optical and infra-red luminosity is often larger
than expected in systems containing AGNs. We also find that mass recycling is
likely to be important for the dynamics of the ICM. This effect will be
strongest in the central regions of clusters where there is more than enough
cold gas for its evaporation to contribute significantly to the density of the
hot phase.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Variable millimetre radiation from the colliding-wind binary Cygnus OB2 #8A
Context. Massive binaries have stellar winds that collide. In the colliding-wind region, various physically interesting processes occur, leading to enhanced X-ray emission, non-thermal radio emission, as well as non-thermal X-rays and gamma-rays. Non-thermal radio emission (due to synchrotron radiation) has so far been observed at centimetre wavelengths. At millimetre wavelengths, the stellar winds and the colliding-wind region emit more thermal free-free radiation, and it is expected that any non-thermal contribution will be difficult or impossible to detect. Aims. We aim to determine if the material in the colliding-wind region contributes substantially to the observed millimetre fluxes of a colliding-wind binary. We also try to distinguish the synchrotron emission from the free-free emission. Methods. We monitored the massive binary Cyg OB2 #8A at 3 mm with the NOrthern Extended Millimeter Array (NOEMA) interferometer of the Institut de Radioastronomie Millimétrique (IRAM). The data were collected in 14 separate observing runs (in 2014 and 2016), and provide good coverage of the orbital period. Results. The observed millimetre fluxes range between 1.1 and 2.3 mJy, and show phase-locked variability, clearly indicating that a large part of the emission is due to the colliding-wind region. A simple synchrotron model gives fluxes with the correct order of magnitude, but with a maximum that is phase-shifted with respect to the observations. Qualitatively this phase shift can be explained by our neglect of orbital motion on the shape of the colliding-wind region. A model using only free-free emission results in only a slightly worse explanation of the observations. Additionally, on the map of our observations we also detect the O6.5 III star Cyg OB2 #8B, for which we determine a 3 mm flux of 0.21 ± 0.033 mJy. Conclusions. The question of whether synchrotron radiation or free-free emission dominates the millimetre fluxes of Cyg OB2 #8A remains open. More detailed modelling of this system, based on solving the hydrodynamical equations, is required to give a definite answer
Chandra X-ray Grating Spectrometry of Eta Carinae near X-ray Minimum: I. Variability of the Sulfur and Silicon Emission Lines
We report on variations in important X-ray emission lines in a series of Chandra grating spectra of the supermassive colliding wind binary star eta Car, including key phases around the X-ray minimum/periastron passage in 2003.5. The X-rays arise from the collision of the slow, dense wind of eta Car with the fast, low-density wind of an otherwise hidden companion star. The X-ray emission lines provide the only direct measure of the flow dynamics of the companion's wind along the wind-wind collision zone. We concentrate here on the silicon and sulfur lines, which are the strongest and best resolved lines in the X-ray spectra. Most of the line profiles can be adequately fit with symmetric Gaussians with little significant skewness. Both the silicon and sulfur lines show significant velocity shifts and correlated increases in line widths through the observations. The R = forbidden-to-intercombination ratio from the Si XIII and S XV triplets is near or above the low-density limit in all observations, suggesting that the line-forming region is > 1.6 stellar radii from the companion star, and that the emitting plasma may be in a non-equilibrium state. We show that simple geometrical models cannot simultaneously fit both the observed centroid variations and changes in line width as a function of phase. We show that the observed profiles can be fitted with synthetic profiles with a reasonable model of the emissivity along the wind-wind collision boundary. We use this analysis to help constrain the line formation region as a function of orbital phase, and the orbital geometry. Subject headings: X-rays: stars -stars: early-type-stars: individual (q Car
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