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&

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 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

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

Cultural considerations at end of life in a geriatric inpatient rehabilitiation setting

Aim: To explore the impact of cultural factors on the provision of end-of-life care in a geriatric inpatient rehabilitation setting. Background: Australia’s ageing population is now also one of the most culturally diverse. Individuals from culturally and linguistically diverse backgrounds may have specific care needs at the end of life according to various aspects of their culture. Design: A mixed method approach using a retrospective audit of existing hospital databases, deceased patients’ medical records, and in-depth interviews with clinicians. Findings: Patients’ and families’ cultural needs were not always recognised or facilitated in end-of-life care, resulting in missed opportunities to tailor care to the individual’s needs. Clinicians identified a lack of awareness of cultural factors, and how these may influence end-of-life care needs. Clinicians expressed a desire for education opportunities to improve their understanding of how to provide patient-specific, culturally sensitive end-of-life care. Conclusion: The findings highlight that dying in geriatric inpatient rehabilitation settings remains problematic, particularly when issues of cultural diversity further compound end-of-life care provision. There is a need for recognition and acceptance of the potential sensitivities associated with cultural diversity and how it may influence patients’ and families’ needs at the end of life. Health service organisations should prioritise and make explicit the importance of early referral and utilisation of existing support services such as professional interpreters, specialist palliative care and pastoral care personnel in the provision of end-of-life care. Furthermore, health service organisations should consider reviewing end-of-life care policy documents, guidelines and care pathways to ensure there is an emphasis on respecting and honouring cultural diversity at end of life. If use of a dying care pathway for all dying patients was promoted, or possibly mandated, these issues would likely be addressed. © 2018 Published by Elsevier Ltd on behalf of Australian College of Nursing Ltd

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

INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302

In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds.Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1-200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity.Comment: 9 pages, 7 figures, accepted for publication in A&

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

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