Colliding-wind Binary Systems Research Articles

9 Sagittarii: uncovering an O-type spectroscopic binary with an 8.6 year period

The O-type object 9 Sgr is a well-known synchrotron radio emitter. This feature is usually attributed to colliding-wind binary systems, but 9 Sgr was long considered a single star. We have conducted a long-term spectroscopic monitoring of this star to investigate its multiplicity and search for evidence for wind-wind interactions. Radial velocities are determined and analysed using various period search methods. Spectral disentangling is applied to separate the spectra of the components of the binary system. We derive the first ever orbital solution of 9 Sgr. The system is found to consist of an O3.5 V((f+)) primary and an O5- 5.5 V((f)) secondary moving around each other on a highly eccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emission lines that could be formed in the wind interaction zone in agreement with the expected properties of the interaction in such a wide system. Our results provide further support to the paradigm of synchrotron radio emission from early-type stars being a manifestation of interacting winds in a binary system.

IDENTIFICATION OF A POPULATION OF X-RAY-EMITTING MASSIVE STARS IN THE GALACTIC PLANE

We present X-ray, infrared, optical and radio observations of four previously unidentified Galactic plane X-ray sources, AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. Detection of each source with the Chandra X-ray Observatory has provided sub-arcsecond localizations, which we use to identify bright infrared counterparts to all four objects. Infrared and optical spectroscopy of these counterparts demonstrate that all four X-ray sources are extremely massive stars, with spectral classifications Ofpe/WN9 (AX J163252-4746), WN7 (AX J184738-0156 = WR121a), WN7-8h (AX J144701-5919) and OIf+ (AX J144547-5931). AX J163252-4746 and AX J184738-0156 are both luminous, hard, X-ray emitters with strong Fe XXV emission lines in their X-ray spectra at ~6.7 keV. The multi-wavelength properties of AX J163252-4746 and AX J184738-0156 are not consistent with isolated massive stars or accretion onto a compact companion; we conclude that their X-ray emission is most likely generated in a colliding-wind binary system. For both AX J144701-5919 and AX J144547-5931, the X-ray emission is an order of magnitude less luminous and with a softer spectrum. These properties are consistent with a colliding-wind binary interpretation for these two sources also, but other mechanisms for the generation of X-rays cannot be excluded. There are many other as yet unidentified X-ray sources in the Galactic plane, with X-ray properties similar to those seen for AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. This may indicate a substantial population of X-ray-emitting massive stars and colliding-wind binaries in the Milky Way.

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit

Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high spatial resolution near-IR photometry. We derive high T* for the WN stars in NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~ 2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars make a disproportionate contribution to the global budget of their host clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3 systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar masses are supported by dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted L_X of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars in such a dense environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa IMF, both clusters are consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster is either too old, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - M_init for the most luminous stars in the Arches cluster approach 200 Msun according to contemporary stellar & photometric results. The potential for stars greatly exceeding 150 Msun within metal-poor galaxies suggests that such pair-instability SNe could occur within the local universe, as has been claimed for SN 2007bi (abridged).

Overview of the first AGILE catalog of high-confidence gamma-ray sources

AGILE is an Italian Space Agency mission launched in April 2007, devoted to γ ‐ray observations in the 30 MeV–50 GeV energy range, with simultaneous X-ray imaging capability in the 18–60 keV band. During its first year in orbit AGILE surveyed the γ ‐ray sky and detected many galactic and extragalactic sources. The first catalog of high-confidence γ ‐ray sources detected by AGILE during observations performed from July 9, 2007 to June 30, 2008, for energies greater than 100 MeV was recently published [1], and preliminary versions of the catalog were published on the AGILE Data Center webpage to allow AGILE Cycle-2 guest observers to benefit of the Catalog in the preparation of their proposals. We present here the main results of the final catalog, that includes 47 high-confidence sources, 21 of which are associated with confirmed or candidate pulsars, 13 with Blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 with HMXRBs, 2 with SNRs, 1 with a colliding-wind binary system, 8 with unidentified sources.

THE FERMI /LAT SKY AS SEEN BY INTEGRAL/IBIS

In this letter we present the result of the cross correlation between the 4th INTEGRAL/IBIS soft gamma-ray catalog, in the range 20-100 keV, and the Fermi LAT bright source list of objects emitting in the 100 MeV - 100 GeV range. The main result is that only a minuscule part of the more than 720 sources detected by INTEGRAL and the population of 205 Fermi LAT sources are detected in both spectral regimes. This is in spite of the mCrab INTEGRAL sensitivity for both galactic and extragalactic sources and the breakthrough, in terms of sensitivity, achieved by Fermi at MeV-GeV energies. The majority of the 14 Fermi LAT sources clearly detected in the 4th INTEGRAL/IBIS catalog are optically identified AGNs (10) complemented by 2 isolated pulsars (Crab and Vela) and 2 High Mass X-Ray Binaries (HMXB, LS I +61 303 and LS 5039). Two more possible associations have been found: one is 0FGL J1045.6-5937, possibly the counterpart at high energy of the massive colliding wind binary system Eta Carinae, discovered to be a soft gamma ray emitter by recent INTEGRAL observations and 0FGL J1746.0-2900 coincident with IGR J17459-2902, but still not identified with any known object at lower energy. For the remaining 189 Fermi LAT sources no INTEGRAL counterpart was found and we report the 2 sigma upper limit in the energy band 20-40 keV.

First AGILE catalog of high-confidence gamma-ray sources

We present the first catalog of high-confidence gamma-ray sources detected by the AGILE satellite during observations performed from July 9, 2007 to June 30, 2008. Catalogued sources are detected by merging all the available data over the entire time period. AGILE, launched in April 2007, is an ASI mission devoted to gamma-ray observations in the 30 MeV - 50 GeV energy range, with simultaneous X-ray imaging capability in the 18-60 keV band. This catalog is based on Gamma-Ray Imaging Detector (GRID) data for energies greater than 100 MeV. For the first AGILE catalog we adopted a conservative analysis, with a high-quality event filter optimized to select gamma-ray events within the central zone of the instrument Field of View (radius of 40 degrees). This is a significance-limited (4 sigma) catalog, and it is not a complete flux-limited sample due to the non-uniform first year AGILE sky coverage. The catalog includes 47 sources, 21 of which are associated with confirmed or candidate pulsars, 13 with Blazars (7 FSRQ, 4 BL Lacs, 2 unknown type), 2 with HMXRBs, 2 with SNRs, 1 with a colliding-wind binary system, 8 with unidentified sources.

Dust formation by the colliding wind WC5+O9 binary WR 19 at periastron passage

We present infrared photometry of the episodic dust-making Wolf-Rayet system WR19 (LS3), tracking its fading from a third observed dust-formation episode in 2007 and strengthening the view that these episodes are periodic (P = 10.1+/-0.1 y). Radial velocities of the O9 component observed between 2001 and 2008 show RV variations consistent with WC19 being a spectroscopic binary of high eccentricity (e=0.8), having periastron passage in 2007.14, shortly before the phase of dust formation. In this respect, WR19 resembles the archetypical episodic dust-making colliding-wind binary system WR140.

Modelling the RXTE light curve of η Carinae from a 3D SPH simulation of its binary wind collision

Abstract The very massive star system η Carinae exhibits regular 5.54 yr (2024 d) period disruptive events in wavebands ranging from the radio to X-ray. There is a growing consensus that these events likely stem from periastron passage of an (as yet) unseen companion in a highly eccentric (e∼ 0.9) orbit. This Letter presents 3D smoothed particle hydrodynamics (SPH) simulations of the orbital variation of the binary wind–wind collision, and applies these to modelling the X-ray light curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global 3D model of the phase variation of the density of the interacting winds, the simulations allow computation of the associated variation in X-ray absorption, presumed here to originate from near the apex of the wind–wind interaction cone. We find that the observed RXTE light curve can be readily fitted if the observer's line of sight is within this cone along the general direction of apastron. Specifically, the data are well fitted by an assumed inclination i= 45° for the orbit's polar axis, which is thus consistent with orbital angular momentum being along the inferred polar axis of the Homunculus nebula. The fits also constrain the position angle φ that an orbital-plane projection makes with the apastron side of the semimajor axis, strongly excluding positions φ < 9° along or to the retrograde side of the axis, with the best-fitting position given by φ= 27°. Overall the results demonstrate the utility of a fully 3D dynamical model for constraining the geometric and physical properties of this complex colliding wind binary system.

Effect of the Coriolis Force on the Hydrodynamics of Colliding‐Wind Binaries

Using fully three-dimensional hydrodynamic simulations, we investigate the effect of the Coriolis force on the hydrodynamic and observable properties of colliding wind binary systems. To make the calculations tractable, we assume adiabatic, constant velocity winds. The neglect of radiative driving, gravitational deceleration, and cooling limit the application of our models to real systems. However, these assumptions allow us to isolate the effect of the Coriolis force, and by simplifying the calculations, allow us to use a higher resolution (up to 640^3) and to conduct a larger survey of parameter space. We study the dynamics of collidng winds with equal mass loss rates and velocities emanating from equal mass stars on circular orbits, with a range of values for the ratio of the wind to orbital velocity. We also study the dynamics of winds from stars on elliptical orbits and with unequal strength winds. Orbital motion of the stars sweeps the shocked wind gas into an Archimedean spiral, with asymmetric shock strengths and therefore unequal postshock temperatures and densities in the leading and trailing edges of the spiral. We observe the Kelvin-Helmholtz instability at the contact surface between the shocked winds in systems with orbital motion even when the winds are identical. The change in shock strengths caused by orbital motion increases the volume of X-ray emitting post-shock gas with T > 0.59 keV by 63% for a typical system as the ratio of wind velocity to orbital velocity decreases to V_w/V_o = 2.5. This causes increased free-free emission from systems with shorter orbital periods and an altered time-dependence of the wind attenuation. We comment on the importance of the effects of orbital motion on the observable properties of colliding wind binaries.

The first orbital solution for the massive colliding-wind binary HD 93162 (≡WR 25)

Context.Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD 93162 (≡WR 25), this object has been predicted to be a colliding-wind binary system. However, radial-velocity variations that would prove the suspected binary nature have yet to be found.

Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR 140

We use hydrodynamical models of the wind-collision region in the archetype colliding-wind system WR 140 to determine the spatial and spectral distributions of the radio, X-ray, and γ-ray emission from shock-accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission at this phase in conjunction with the radio emission to constrain the mass-loss rates, we find that the O star mass-loss rate is consistent with the reduced estimates for O4-5 supergiants by Fullerton, Massa & Prinja, and the wind-momentum ratio, η= 0.02. This is independent of the opening angle deduced from radio very long baseline interferometry observations of the WCR that we demonstrate fail to constrain the opening angle. We show that the turnover at ∼3 GHz in the radio emission is due to free–free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal electrons. We find that the spectral index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p 2 does not exclude the possibility of shock modification, but stronger evidence than that which currently exists is necessary for its support. Tighter constraints on p and the nature of the shocks in WR 140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than those in previous works. Since the high stellar photon fluxes prevent the acceleration of electrons beyond γ≳ 105–106, TeV emission from colliding-wind binary systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre.

A Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1

Recent optical and infrared studies have revealed that the heavily reddened starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (W-R) stars, constituting the richest W-R population of any Galactic cluster. We present results of a sensitive Chandra X-ray observation of Wd 1 that detected 12 of the 22 known W-R stars and the mysterious emission-line star W9. The fraction of detected WN stars is nearly identical to that of WC stars. The WN stars WR-A and WR-B, as well as W9, are exceptionally luminous in X-rays and have similar hard, heavily absorbed X-ray spectra with strong Si XIII and S XV emission lines. The luminous high-temperature X-ray emission of these three stars is characteristic of colliding-wind binary systems, but their binary status remains to be determined. Spectral fits of the X-ray-bright sources WR-A and W9 with isothermal plane-parallel shock models require high absorption column densities, log NH = 22.56 (cm-2), and yield characteristic shock temperatures kTs ≈ 3 keV (Ts ≈ 35 MK).

Radio emission models of colliding-wind binary systems. Inclusion of IC cooling

Radio emission models of colliding wind binaries (CWBs) have been discussed by Dougherty et al. (2003). We extend these models by considering the temporal and spatial evolution of the energy distribution of relativistic electrons as they advect downstream from their shock acceleration site. The energy spectrum evolves significantly due to the strength of inverse-Compton (IC) cooling in these systems, and a full numerical evaluation of the synchrotron emission and absorption coefficients is made. We have demonstrated that the geometry of the WCR and the streamlines of the flow within it lead to a spatially dependent break frequency in the synchrotron emission. We therefore do not observe a single, sharp break in the synchrotron spectrum integrated over the WCR, but rather a steepening of the synchrotron spectrum towards higher frequencies. We also observe that emission from the wind-collision region (WCR) may appear brightest near the shocks, since the impact of IC cooling on the non-thermal electron distribution is greatest near the contact discontinuity (CD), and demonstrate that the impact of IC cooling on the observed radio emission increases significantly with decreasing binary separation. We study how the synchrotron emission changes in response to departures from equipartition, and investigate how the thermal flux from the WCR varies with binary separation. Since the emission from the WCR is optically thin, we see a substantial fraction of this emission at certain viewing angles, and we show that the thermal emission from a CWB can mimic a thermal plus non-thermal composite spectrum if the thermal emission from the WCR becomes comparable to that from the unshocked winds. We demonstrate that the observed synchrotron emission depends upon the viewing angle and the wind-momentum ratio, and find that the observed synchrotron emission decreases as the viewing angle moves through the WCR from the WR shock to the O shock. We obtain a number of insights relevant to models of closer systems such as WR 140. Finally, we apply our new models to the very wide system WR 147. The acceleration of non-thermal electrons appears to be very efficient in our models of WR 147, and we suggest that the shock structure may be modified by feedback from the accelerated particles.

High‐Resolution Radio Observations of the Colliding‐Wind Binary WR 140

Milliarcsecond resolution Very Long Baseline Array (VLBA) observations of the archetype W-R+O star colliding-wind binary (CWB) system WR 140 are presented for 23 epochs between orbital phases 0.74 and 0.97. At 8.4 GHz, the emission in the wind-collision region (WCR) is clearly resolved as a bow-shaped arc that rotates as the orbit progresses. We interpret this rotation as due to the O star moving from southeast to approximately east of the W-R star, which leads to solutions for the orbital inclination of 122° ± 5°, longitude of the ascending node of 353° ± 3°, and an orbit semimajor axis of 9.0 ± 0.5 mas. The distance to WR 140 is determined to be 1.85 ± 0.16 kpc, which requires the O star to be a supergiant. The inclination implies that the mass of the W-R and O star is 20 ± 4 and 54 ± 10 M☉, respectively. We determine a wind momentum ratio of 0.22, with an expected half-opening angle for the WCR of 63°, consistent with 65° ± 10° derived from the VLBA observations. Total flux measurements from Very Large Array (VLA) observations show that the radio emission from WR 140 is very closely the same from one orbit to the next, pointing strongly toward emission, absorption, and cooling mechanism(s) that are controlled largely by the orbital motion. The synchrotron spectra evolve dramatically through the orbital phases observed, exhibiting both optically thin and optically thick emission. We discuss a number of absorption and cooling mechanisms that may determine the evolution of the synchrotron spectrum with orbital phase.

An X‐Ray Investigation of the NGC 346 Field in the Small Magellanic Cloud. III.XMM‐NewtonData

We present new XMM-Newton results on the field around the NGC 346 star cluster in the SMC. This continues and extends previously published work on Chandra observations of the same field. The two XMM-Newton observations were obtained, respectively, 6 months before and 6 months after the previously published Chandra data. Of the 51 X-ray sources detected with XMM-Newton, 29 were already detected with Chandra. Comparing the properties of these X-ray sources in each of our three data sets has enabled us to investigate their variability on timescales of a year. Changes in the flux levels and/or spectral properties were observed for 21 of these sources. In addition, we discovered long-term variations in the X-ray properties of the peculiar system HD 5980, a luminous blue variable star that is likely to be a colliding wind binary system, which displayed the largest luminosity during the first XMM-Newton observation.

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 eect. To calculate the synchrotron emission several simplifying assumptions are adopted: the relativistic particle energy density is a simple fraction of the thermal particle energy density, in equipartition with the magnetic energy density, and a power-law in energy. We also assume that the magnetic field is tangled such that the resulting emission is isotropic. 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 WR 147. 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 eect 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 WR 147 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 WR 140.

In hot pursuit of the hidden companion of η Carinae: an X-ray determination of the wind parameters

We perform X-ray spectral fits to a recently obtained Chandra grating spectrum of η Carinae, one of the most massive and powerful stars in the Galaxy and which is strongly suspected to be a colliding wind binary system. The good fit that we obtain gives us further confidence in the binary hypothesis, and we find M ≈ 2.5 × 10–4 M⊙ yr–1 for the mass loss rate of η Car.

Radio emission models of colliding-wind binary systems

We present preliminary calculations of the spatial distribution of the radio emission from a WR+OB colliding wind system, based on high-resolution hydrodynamical simulations and solutions to the radiative transfer equation. We account for both thermal and non-thermal radio emission, under the assumption of equipartition between magnetic and relativistic particle energy densities, and that the latter is a simple fraction of the thermal particle energy density. These calculations provide the foundation for modeling high resolution radio images and light curves of colliding-wind systems like WR 140, WR 146 and WR 147.

X-rays from stars.

More than two years of observation with Chandra and XMM-Newton has provided a rich harvest of new results on the physics of stellar coronae and winds. High-resolution X-ray spectroscopy in particular has opened new windows to the structure, the dynamics and the composition of stellar atmospheres. The present paper presents selected results from the areas of hot and cool stars and star formation, summarizing new views of the thermal structure and energy release in stellar coronae, observations of magnetically active brown dwarfs, the structure of winds in hot stars, the physics in colliding-wind binary systems, and X-rays from protostars and stellar jets.

In hot pursuit of the hidden companion ofη Carinae: An X-ray determination of the wind parameters

We present X-ray spectral ts to a recently obtained Chandra grating spectrum of Carinae, one of the most massive and powerful stars in the Galaxy and which is strongly suspected to be a colliding wind binary system. Hydrodynamic models of colliding winds are used to generate synthetic X-ray spectra for a range of mass-loss rates and wind velocities. They are then tted against newly acquired Chandra grating data. We nd that due to the low velocity of the primary wind (500 km s 1 ), most of the observed X-ray emission appears to arise from the shocked wind of the companion star. We use the duration of the lightcurve minimum to x the wind momentum ratio at =0 :2. We are then able to obtain a good t to the data by varying the mass-loss rate of the companion and the terminal velocity of its wind. We nd that _ M 2 10 5 M yr 1 and v12 3000 km s 1 . With observationally determined values of500{700 km s 1 for the velocity of the primary wind, our t implies a primary mass-loss rate of _ M 1 2:5 10 4 M yr 1 . This value is smaller than commonly inferred, although we note that a lower mass-loss rate can reduce some of the problems noted by Hillier et al. (2001) when a value as high as 10 3 M yr 1 is used. The wind parameters of the companion are indicative of a massive star which may or may not be evolved. The line strengths appear to show slightly sub-solar abundances, although this needs further conrmation. Based on the over-estimation of the X-ray line strengths in our model, and re-interpretation of the HST/FOS results, it appears that the Homunculus nebula was produced by the primary star.