46 research outputs found
Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumably single
O stars, in terms of synchrotron emission from relativistic electrons
accelerated in wind-embedded shocks. These shocks are associated with an
unstable, chaotic wind. The main improvement with respect to earlier models is
the inclusion of the radial dependence of the shock velocity jump and
compression ratio, based on 1D hydrodynamical simulations. The decrease of the
velocity jump and the compression ratio as a function of radius produces a
rapidly decreasing synchrotron emissivity. This effectively prohibits the
models from reproducing the spectral shape of the observed non-thermal radio
emission. We investigate a number of "escape routes" by which the
hydrodynamical predictions might be reconciled with the radio observations.
Although these escape routes reproduce the observed spectral shape, none of
these escape routes are physically plausible. In particular, re-acceleration by
feeding an electron distribution through a number of shocks, is in
contradiction with current hydrodynamical simulations. These hydrodynamical
simulations have their limitations, most notably the use of 1D. At present, it
is not feasible to perform 2D simulations of the wind out to the distances
required for synchrotron-emission models. Based on the current hydrodynamic
models, we suspect that the observed non-thermal radio emission from O stars
cannot be explained by wind-embedded shocks associated with the instability of
the line-driving mechanism. The most likely alternative mechanism is
synchrotron emission from colliding winds. That would imply that all O stars
with non-thermal radio emission should be members of binary or multiple
systems.Comment: 10 pages, 8 figures, accepted for publication by A&
An XMM-Newton observation of the multiple system HD 167971 (O5-8V+O5-8V+(O81)) and the young open cluster NGC6604
We discuss the results of two XMM-Newton observations of the open cluster NGC 6604 obtained in April and September 2002. We concentrate mainly on the multiple system HD 167971 (O5- 8V + O5- 8V + (O8I)). The soft part of the EPIC spectrum of this system is thermal with typical temperatures of about 2 x 106 to 9 x 106 K. The nature ( thermal vs. non-thermal) of the hard part of the spectrum is not unambiguously revealed by our data. If the emission is thermal, the high temperature of the plasma (similar to 2.3 x 10(7) to 4.6 x 10(7) K) would be typical of what should be expected from a wind-wind interaction zone within a long period binary system. This emission could arise from an interaction between the combined winds of the O5- 8V + O5- 8V close binary system and that of the more distant O8I companion. Assuming instead that the hard part of the spectrum is non-thermal, the photon index would be rather steep (similar to 3). Moreover, a marginal variability between our two XMM-Newton pointings could be attributed to an eclipse of the O5 - 8V + O5 - 8V system. The overall X-ray luminosity points to a significant X-ray luminosity excess of about a factor 4 possibly due to colliding winds. Considering HD 167971 along with several recent X-ray and radio observations, we propose that the simultaneous observation of non-thermal radiation in the X-ray (below 10.0 keV) and radio domains appears rather unlikely. Our investigation of our XMM-Newton data of NGC 6604 reveals a rather sparse distribution of X-ray emitters. Including the two bright non-thermal radio emitters HD 168112 and HD 167971, we present a list of 31 X-ray sources along with the results of the cross-correlation with optical and infrared catalogues. A more complete spectral analysis is presented for the brightest X-ray sources. Some of the members of NGC 6604 present some characteristics suggesting they may be pre-main sequence star candidates
A pseudo-planar, periodic-box formalism for modelling the outer evolution of structure in spherically expanding stellar winds
We present an efficient technique to study the 1D evolution of
instability-generated structure in winds of hot stars out to very large
distances (more than 1000 stellar radii). This technique makes use of our
previous finding that external forces play little role in the outer evolution
of structure. Rather than evolving the entire wind, as is traditionally done,
the technique focuses on a representative portion of the structure and follows
it as it moves out with the flow. This requires the problem to be formulated in
a moving reference frame. The lack of Galilean invariance of the spherical
equations of hydrodynamics is circumvented by recasting them in a pseudo-planar
form. By applying the technique to a number of problems we show that it is fast
and accurate, and has considerable conceptual advantages. It is particularly
useful to test the dependence of solutions on the Galilean frame in which they
were obtained. As an illustration, we show that, in a one-dimensional
approximation, the wind can remain structured out to distances of more than
1300 stellar radii from the central star.Comment: submitted to Astronomy and Astrophysics, 12 pages, 13 figure
Non-thermal radio emission from O-type stars III. Is Cyg OB2 No. 9 a wind-colliding binary?
The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recent
theoretical work suggests that all such O-stars should be in a binary or a
multiple system. However, there is no spectroscopic evidence of a binary
component. Re-analysis of radio observations from the VLA of this system over
25 years has revealed that the non-thermal emission varies with a period of
2.35+-0.02 yr. This is interpreted as a strong suggestion of a binary system,
with the non-thermal emission arising in a wind-collision region. We derived
some preliminary orbital parameters for this putative binary and revised the
mass-loss rate of the primary star downward from previous estimates.Comment: 13 pages, 5 figures, includes online data, accepted by A&
A layered model for non-thermal radio emission from single O stars
We present a model for the non-thermal radio emission from bright O stars, in
terms of synchrotron emission from wind-embedded shocks. The model is an
extension of an earlier one, with an improved treatment of the cooling of
relativistic electrons. This improvement limits the synchrotron-emitting volume
to a series of fairly narrow layers behind the shocks. We show that the width
of these layers increases with increasing wavelength, which has important
consequences for the shape of the spectrum. We also show that the strongest
shocks produce the bulk of the emission, so that the emergent radio flux can be
adequately described as coming from a small number of shocks, or even from a
single shock.
A single shock model is completely determined by four parameters: the
position of the shock, the compression ratio and velocity jump of the shock,
and the surface magnetic field. Applying a single shock model to the O5 If star
Cyg OB2 No. 9 allows a good determination of the compression ratio and shock
position and, to a lesser extent, the magnetic field and velocity jump.
Our main conclusion is that strong shocks need to survive out to distances of
a few hundred stellar radii. Even with multiple shocks, the shocks needed to
explain the observed emission are stronger than predictions from time-dependent
hydrodynamical simulations.Comment: 10 pages, 5 figures, accepted by Astronomy and Astrophysic
Radio and submillimetre observations of wind structure in zeta Pup
We present radio and submillimetre observations of the O4I(n)f star zeta Pup,
and discuss structure in the outer region of its wind (~ 10-100 R*). The
properties of bremsstrahlung, the dominant emission process at these
wavelengths, make it sensitive to structure and allow us to study how the
amount of structure changes in the wind by comparing the fluxes at different
wavelengths. To look for variability, we acquired 3.6 and 6 cm observations
with the Australia Telescope Compact Array (ATCA). We supplemented these with
archive observations from the NRAO Very Large Array (VLA). We did not find
variability at more than the +- 20 % level. The long integration time does
allow an accurate determination of the fluxes at 3.6 and 6 cm. Converting these
fluxes into a mass loss rate, we find Mdot = 3.5 x 10^{-6} Msun/yr. This value
confirms the significant discrepancy with the mass loss rate derived from the
Halpha profile, making zeta Pup an exception to the usually good agreement
between Halpha and radio mass loss rates. We also observed zeta Pup at 850 mum
with the James Clerk Maxwell Telescope (JCMT) and at 20 cm with the VLA. A
smooth wind model shows that the millimetre fluxes are too high compared to the
radio fluxes. While recombination of helium in the outer wind cannot be
discounted as an explanation, the wealth of evidence for structure strongly
suggests this as the explanation for the discrepancy. Model calculations show
that the structure needs to be present in the inner ~ 70 R* of the wind, but
that it decays significantly, or maybe even disappears, beyond that radius.Comment: 13 pages, 8 figures, to be published in Astronomy and Astrophysic
Tomographic Separation of Composite Spectra. VIII. The Physical Properties of the Massive Compact Binary in the Triple Star System HD 36486 (delta Orionis A)
Double-lined spectroscopic orbital elements have recently been found for the
central binary in the massive triple, delta Orionis A based on radial
velocities from cross-correlation techniques applied to IUE high dispersion
spectra and He I 6678 spectra obtained at Kitt Peak. The primary and secondary
velocity amplitudes were found to be 94.9 +/- 0.6 km/s and 186 +/- 9 km/s
respectively. Tomographic reconstructions of the primary and secondary stars'
spectra confirm the O9.5 II classification of the primary and indicate a B0.5
III type for the secondary. The widths of the UV cross-correlation functions
are used to estimate the projected rotational velocities, Vsin i = 157 +/- 6
km/s and 138 +/- 16 km/s for the primary and secondary, respectively implying
that both stars rotate faster than their orbital motion. We used the
spectroscopic results to make a constrained fit of the Hipparcos light curve of
this eclipsing binary, and the model fits limit the inclination to the range
between 67 and 77 degrees. The i = 67 degrees solution, which corresponds to a
near Roche-filling configuration, results in a primary mass of 11.2 solar
masses and a secondary mass of 5.6 solar masses, both of which are
substantially below the expected masses for stars of their luminosity. This
binary may have experienced a mass ratio reversal caused by Case A Roche lobe
overflow, or the system may have suffered extensive mass loss through a binary
interaction, perhaps during a common envelope phase, in which most of the
primary's mass was lost from the system rather than transferred to the
secondary.Comment: 27 pages, 15 figures in press, the Astrophysical Journal, February 1,
200
XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient
IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of
supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching
luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been
ascribed to the combined effect of the intense magnetic field and rotation of
the compact object hosted in them and/or the presence of dense structures
("clumps") in the wind of their supergiant companion. IGR J18410-0535 was
observed for 45 ks by XMM-Newton as part of a program designed to study the
quiescent emission of supergiant fast X-ray transients and clarify the origin
of their peculiar X-ray variability. We carried out an in-depth spectral and
timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright
X-ray flare that started about 5 ks after the beginning of the observation and
lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board
XMM-Newton, the whole event could be followed in great detail. The results of
our analysis provide strong convincing evidence that the flare was produced by
the accretion of matter from a massive clump onto the compact object hosted in
this system. By assuming that the clump is spherical and moves at the same
velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl
\simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in
qualitative agreement with values expected from theoretical calculations. We
found no evidence of pulsations at \sim4.7 s after investigating coherent
modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and
Swift data of IGR J18410-0535, for which these pulsations were previously
detected, revealed that they were likely to be due to a statistical fluctuation
and an instrumental effect, respectively.Comment: Accepted for publication on A&A. V2: Inserted correct version of
Fig.1
The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X- ray luminosity of L-X less than or similar to 2.5 x 10(30) erg s(-1) and to the X-ray to bolometric luminosity ratio of L- X/L(bo)l less than or similar to 4 x 10(-9). This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and Chandra observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven stellar winds of this particular class are insignificant X-ray sources. We attribute this to photoelectronic absorption by the stellar wind. The high opacity of the metal-rich and dense winds from WC stars puts the radius of optical depth unity at hundreds or thousands of stellar radii for much of the X-ray band. We believe that the essential absence of hot plasma so far out in the wind exacerbated by the large distances and correspondingly high ISM column densities makes the WC stars too faint to be detectable with current technology. The result also applies to many WC stars in binary systems, of which only about 20% are identified X-ray sources, presumably due to colliding winds