290 research outputs found
Radio emission models of Colliding-Wind Binary Systems
We present calculations of the spatial and spectral distribution of the radio
emission from a wide WR+OB colliding-wind binary system based on
high-resolution hydrodynamical simulations and solutions to the radiative
transfer equation. We account for both thermal and synchrotron radio emission,
free-free absorption in both the unshocked stellar wind envelopes and the
shocked gas, synchrotron self-absorption, and the Razin effect. The
applicability of these calculations to modelling radio images and spectra of
colliding-wind systems is demonstrated with models of the radio emission from
the wide WR+OB binary WR147. Its synchrotron spectrum follows a power-law
between 5 and 15 GHz but turns down to below this at lower and higher
frequencies. We find that while free-free opacity from the circum-binary
stellar winds can potentially account for the low-frequency turnover, models
that also include a combination of synchrotron self-absorption and Razin effect
are favoured. We argue that the high-frequency turn down is a consequence of
inverse-Compton cooling. We present our resulting spectra and intensity
distributions, along with simulated MERLIN observations of these intensity
distributions. From these we argue that the inclination of the WR147 system to
the plane of the sky is low. We summarise by considering extensions of the
current model that are important for models of the emission from closer
colliding wind binaries, in particular the dramatically varying radio emission
of WR140.Comment: 18 pages, 18 figures; Accepted by Astronomy and Astrophysics, July 8,
200
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
XMM-Newton X-ray Observations of the Wolf-Rayet Binary System WR 147
We present results of a 20 ksec X-ray observation of the Wolf-Rayet (WR)
binary system WR 147 obtained with XMM-Newton. Previous studies have shown that
this system consists of a nitrogen-type WN8 star plus an OB companion whose
winds are interacting to produce a colliding wind shock. X-ray spectra from the
pn and MOS detectors confirm the high extinction reported from IR studies and
reveal hot plasma including the first detection of the Fe K-alpha line complex
at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock
model give a shock temperature kT(shock) = 2.7 keV [T(shock) ~ 31 MK], close to
but slightly hotter than the maximum temperature predicted for a colliding wind
shock. Optically thin plasma models suggest even higher temperatures, which are
not yet ruled out. The X-ray spectra are harder than can be accounted for using
2D numerical colliding wind shock models based on nominal mass-loss parameters.
Possible explanations include: (i) underestimates of the terminal wind speeds
or wind abundances, (ii) overly simplistic colliding wind models, or (iii) the
presence of other X-ray emission mechanisms besides colliding wind shocks.
Further improvement of the numerical models to include potentially important
physics such as non-equilibrium ionization will be needed to rigorously test
the colliding wind interpretation.Comment: 8 pages, 7 figure
Future sea level change from Antarctica's Lambert-Amery glacial system
Future global mean sea level (GMSL) change is dependent on the complex response of the Antarctic ice sheet to ongoing changes and feedbacks in the climate system. The Lambert-Amery glacial system has been observed to be stable over the recent period yet is potentially at risk of rapid grounding line retreat and ice discharge given that a significant volume of its ice is grounded below sea level, making its future contribution to GMSL uncertain. Using a regional ice sheet model of the Lambert-Amery system, we find that under a range of future warming and extreme scenarios, the simulated grounding line remains stable and does not trigger rapid mass loss from grounding line retreat. This allows for increased future accumulation to exceed the mass loss from ice dynamical changes. We suggest that the Lambert-Amery glacial system will remain stable or gain ice mass and mitigate a portion of potential future sea level rise over the next 500 years, with a range of +3.6 to −117.5 mm GMSL equivalent
High-Resolution X-ray Imaging of the Colliding Wind Shock in WR147
We analyze new high-resolution Chandra X-ray images of the Wolf-Rayet binary
system WR147. This system contains a WN8 star with an early-type companion
located 0.6'' to its north, and is the only known early-type binary with a
separation on the sky large enough for the wind-wind collision between the
stars to currently be resolved at X-ray energies. The 5 ksec Chandra HRC-I
image provides the first direct evidence for spatially extended X-ray emission
in an early-type binary system. The X-ray emission peaks close to the position
of the radio bow shock and north of the WN8 star. A deeper X-ray image is
needed to accurately determine the degree of spatial extension, to exactly
align the X-ray and optical/radio frames, and to determine whether part of the
detected X-ray emission arises in the individual stellar winds. Simulated X-ray
images of the wind-wind collision have a FWHM consistent with the data, and
maximum likelihood fits suggest that a deeper observation may also constrain
the inclination and wind momentum ratio of this system. However, as the WR wind
dominates the colliding wind X-ray emission it appears unlikely that the
mass-loss rate and the terminal velocity of the companion wind can be
separately determined from X-ray observations. We also note an inconsistency
between numerical and analytical estimates of the X-ray luminosity ratio of the
stronger and weaker wind components, and conclude that the analytical results
are in error.Comment: 13 pages, 6 figures, accepted by A&
VLT/FLAMES-ARGUS observations of stellar wind--ISM cloud interactions in NGC 6357
We present optical/near-IR IFU observations of a gas pillar in the Galactic
HII region NGC 6357 containing the young open star cluster Pismis 24. These
observations have allowed us to examined in detail the gas conditions of the
strong wind-clump interactions taking place on its surface. We identify the
presence of a narrow (~20 km/s) and broad (50-150 km/s) component to the
H_alpha emission line, where the broadest broad component widths are found in a
region that follows the shape of the eastern pillar edge. These connections
have allowed us to firmly associate the broad component with emission from
ionized gas within turbulent mixing layers on the pillar's surface set up by
the shear flows of the O-star winds from the cluster. We discuss the
implications of our findings in terms of the broad emission line component that
is increasingly found in extragalactic starburst environments. Although the
broad line widths found here are narrower, we conclude that the mechanisms
producing both must be the same. The difference in line widths may result from
the lower total mechanical wind energy produced by the O stars in Pismis 24
compared to that from a typical young massive star cluster found in a starburst
galaxy. The pillar's edge is also clearly defined by dense (<5000 cm^-3), hot
(>20000 K), and excited (via [NII]/H_a and [SII]/H_a ratios) gas conditions,
implying the presence of a D-type ionization front propagating into the pillar
surface. Although there must be both photoevaporation outflows produced by the
ionization front, and mass-loss through mechanical ablation, we see no evidence
for any significant bulk gas motions on or around the pillar. We postulate that
the evaporated/ablated gas must be rapidly heated before being entrained.Comment: 9 pages, 5 figures (3 colour). Accepted for publication in MNRA
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD\168112 (O5.5III(f^+))
We report the results of a multiwavelength study of the non-thermal radio
emitter HD168112 (O5.5III(f^+)). The detailed analysis of two
quasi-simultaneous XMM-Newton and VLA observations reveals strong variability
of this star both in the X-ray and radio ranges. The X-ray observations
separated by five months reveal a decrease of the X-ray flux of ~30%. The radio
emission on the other hand increases by a factor 5-7 between the two
observations obtained roughly simultaneously with the XMM-Newton pointings. The
X-ray data reveal a hard emission that is most likely produced by a thermal
plasma at kT ~2-3 keV while the VLA data confirm the non-thermal status of this
star in the radio waveband. Comparison with archive X-ray and radio data
confirms the variability of this source in both wavelength ranges over a yet
ill defined time scale. The properties of HD168112 in the X-ray and radio
domain point towards a binary system with a significant eccentricity and an
orbital period of a few years. However, our optical spectra reveal no
significant changes of the star's radial velocity suggesting that if HD168112
is indeed a binary, it must be seen under a fairly low inclination.Comment: 17 pages, 11 figures (10 postscript + 1 gif
V444 Cygni X-ray and polarimetric variability: radiative and coriolis forces shape the wind collision region
We present results from a study of the eclipsing, colliding-wind binary V444 Cyg that uses a combination of X-ray and optical spectropolarimetric methods to describe the 3D nature of the shock and wind structure within the system. We have created the most complete X-ray light curve of V444 Cyg to date using 40 ks of new data from Swift, and 200 ks of new and archived XMM-Newton observations. In addition, we have characterized the intrinsic, polarimetric phase-dependent behavior of the strongest optical emission lines using data obtained with the University of Wisconsin's Half-Wave Spectropolarimeter. We have detected evidence of the Coriolis distortion of the wind-wind collision in the X-ray regime, which manifests itself through asymmetric behavior around the eclipses in the system's X-ray light curves. The large opening angle of the X-ray emitting region, as well as its location (i.e. the WN wind does not collide with the O star, but rather its wind) are evidence of radiative braking/inhibition occurring within the system. Additionally, the polarimetric results show evidence of the cavity the wind-wind collision region carves out of the Wolf-Rayet star's wind
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