New Colliding-Wind Massive Binaries

in Massive Stars in Interactive Binaries (2007)

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile ... [more ▼]

We report on the recent discovery of two new massive binary systems: WR20a and CygOB2#8A. For both systems, we briefly present the orbital solutions and we discuss the strong phase-locked line profile variability of the Halpha and HeII lambda 4686 lines for WR20a, and the HeII &; 4686 line in the case of CygOB2#8A. In both cases, we show that the profile variability reveals the signature of a wind interaction. The prospects for future high-energy observations are also discussed. [less ▲]

On the nature of the hard X-ray source IGR J2018+4043

in Astrophysical Journal (2006), 649(1, Part 2), 21-24

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV ... [more ▼]

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018 + 4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) ergs cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018 + 4043 field revealed a hard pointlike source with an observed 0.5-10 keV flux of 3.4(-0.8)(+0.7) x 10(-12) ergs cm(-2) s(-1) (90% confidence level) at alpha = 20(h)18(m)38(s).55, delta = +40 degrees 41'00.4" (with a 4".2 uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index Gamma = 1.3 +/- 0.2 and N-H = 6.1(-2.2)(+3.3) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beamwidth distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018 + 4043 are consistent with those expected for an obscured accreting object, i.e., an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018 + 4043 to the bright gamma-ray source GEV J2020 + 4023 detected by COS B and CGRO EGRET. [less ▲]

The 4m international liquid mirror telescope (ILMT)

in Stepp, Larry (Ed.) Ground-based and Airborne Telescopes (2006, July 01)

The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct ... [more ▼]

The entire funding has recently been obtained in Belgium for the construction of a 4m Liquid Mirror Telescope. Its prime focus will be equipped with a semi-conventional glass corrector allowing to correct for the TDI effect and a thinned, high quantum efficiency, 4K × 4K pixel equivalent CCD camera. It will be capable of subarcsecond imaging in the i'(760 nm) and possibly r', g' band(s) over a field of ~ 30' in diameter. This facility will be entirely dedicated to a deep photometric and astrometric variability survey over a period of ~ 5 years. In this paper, the working principle of liquid mirror telescopes is first recalled, along with the advantages and disadvantages of the latter over classical telescopes. Several science cases are described. For a good access to one of the galactic poles, the best image quality sites for the ILMT are located either in Northern Chile (latitude near -29°30') or in North-East India (Nainital Hills, latitude near +29°30'). At those geographic latitudes, a deep (i' = 22.5 mag.) survey will approximately cover 90 square degrees at high galactic latitude, which is very useful for gravitational lensing studies as well as for the identification of various classes of interesting galactic and extragalactic objects (cf. microlensed stars, supernovae, clusters, etc.). A description of the telescope, its instrumentation and the handling of the data is also presented. [less ▲]

A mutliwavelength observational study of the non-thermal emission from O-type stars.

Doctoral thesis (2005)

The present work is devoted to an observational multiwavelength study of the non-thermal emission of O-type stars. The non-thermal radio emission is believed to be synchrotron radiation, therefore ... [more ▼]

The present work is devoted to an observational multiwavelength study of the non-thermal emission of O-type stars. The non-thermal radio emission is believed to be synchrotron radiation, therefore requiring the existence of a magnetic field as well as of a population of relativistic electrons. The physical circumstances responsible for the acceleration of electrons up to relativistic velocities are not yet completely elucidated, but such an acceleration most probably occurs through the first order Fermi mechanism in the presence of hydrodynamic shocks, also called the Diffusive Shock Acceleration (DSA) mechanism. In the context of massive stars, the shocks responsible for this acceleration process may be intrinsic to their stellar winds, or arise from the collision between stellar winds in binary systems. In addition, the existence of non-thermal emission processes such as inverse Compton scattering suggests the possibility to detect a high-energy counterpart to the non-thermal emission observed in the radio domain. The objectives of this work are to (1) study the impact of the multiplicity on the non-thermal emission, (2) investigate the issue of the non-thermal emission of massive stars in the high-energy domain, and (3) discuss the possibility to constrain physical quantities such as the magnetic field strength through the study of these non-thermal emission processes. We investigate in detail several non-thermal emitting O-type stars, namely HD168112, HD167971, Cyg OB2 #8A, Cyg OB2 #9, and HD15558. For most of these targets, high quality XMM-Newton data are used in order to study their X-ray properties. In most case, the question of the multiplicity is also addressed through optical studies performed on the basis of data obtained with ground-based telescopes. The most intensive study is carried out on the Cyg OB2 region, for which we also obtained and analysed INTEGRAL data, in order to investigate the possible relation between the massive stars in Cyg OB2 and the unidentified EGRET source 3EG J2033+4118. In addition, high quality XMM-Newton data of the close massive binary HD159176 are also studied. Even though this latter target does not belong to the category of non-thermal radio emitters, its study unexpectedly turns out to be crucial in the context of this campaign as it might be the first O + O system likely to be classified as an n! on-thermal X-ray emitter. Considering our observational results, along with results from radio observations mainly coordinated by colleagues from the Royal Observatory of Belgium, we propose a general qualitative schematic view of the non-thermal emission from massive stars. First, it seems that the multiplicity plays a crucial role in the non-thermal emission processes. Second, we have found that the simultaneous detection of non-thermal radiation in the radio and X-ray (below 10.0 keV) domains is unlikely. Non-thermal radio emitters are indeed most probably binaries with periods longer than a few weeks, whilst only close binaries with periods of at most a few days may be non-thermal emitters in the soft X-ray domain. However, such a simultaneous detection is possible if the very hard X-rays or soft gamma-rays are considered, i.e. in a domain where the thermal emission from the colliding winds does not overwhelm the non-thermal emission. Moreover, it appears that the existence of non-thermal X-ray emitters without a non-thermal radio counterpart should be envisaged. Finally, the simultaneous detection of radio and high-energy non-thermal emissions is likely to lead to an indirect method to estimate the local magnetic field strength in the colliding-wind region of massive binaries. [less ▲]

On the multiplicity of the non-thermal radio emitters 9 Sgr and HD 168112

in Massive Stars and High-Energy Emission in OB Associations (2005, November)

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as ... [more ▼]

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as single stars. Based on a large set of high-resolution spectra, we find that 9 Sgr is clearly an eccentric SB2 binary with an orbital period of several years. On the other hand, no evidence for radial velocity variations attributable to binary motion is found in our spectra of HD 168112. [less ▲]

Evidence for phase-locked X-ray variations from the colliding-wind massive binary Cyg OB2 #8A

in Rauw, G.; Nazé, Y.; Blomme, R. (Eds.) et al Massive Stars and High-Energy Emission in OB Associations (2005, November)

We report on preliminary results of a multi-observatory investigation of the X-ray emission from the massive colliding wind binary Cyg OB2 #8A (O6If + O5.5III(f)). On the basis of our new XMM-Newton-EPIC ... [more ▼]

We report on preliminary results of a multi-observatory investigation of the X-ray emission from the massive colliding wind binary Cyg OB2 #8A (O6If + O5.5III(f)). On the basis of our new XMM-Newton-EPIC observations, along with archive ASCA-SIS and ROSAT-PSPC data, we show strong evidence for a significant phase-locked variability of the X-ray emission from Cyg OB2 #8A with the period of 21.9 days determined by De Becker et al. (2004). These results lend further support to the colliding wind scenario that was already suggested by optical data (De Becker & Rauw 2005). We briefly discuss the behaviour of the X-ray emission from this binary system as a function of phase in the context of the colliding wind scenario. [less ▲]

Non-thermal radio emission from O-type stars. I. HD168112

in Astronomy and Astrophysics (2005), 436(3), 1033-1040

We present a radio lightcurve of the O5.5 III(f(+)) star HD 168112, based on archive data from the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). The fluxes show considerable ... [more ▼]

We present a radio lightcurve of the O5.5 III(f(+)) star HD 168112, based on archive data from the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). The fluxes show considerable variability and a negative spectral index, thereby confirming that HD 168112 is a non-thermal radio emitter. The non-thermal radio emission is believed to be due to synchrotron radiation from relativistic electrons that have been Fermi accelerated in shocks. For HD 168112, it is not known whether these shocks are due to a wind-wind collision in a binary system or to the intrinsic instability of the stellar wind driving mechanism. Assuming HD 168112 to be a single star, our synchrotron model shows that the velocity jump of the shocks should be very high, or there should be a very large number of shocks in the wind. Neither of these is compatible with time-dependent hydrodynamical calculations of O star winds. If, on the other hand, we assume that HD 168112 is a binary, the high velocity jump is easily explained by ascribing it to the wind-wind collision. By further assuming the star to be an eccentric binary, we can explain the observed radio variability by the colliding-wind region moving in and out of the region where free-free absorption is important. The radio data presented here show that the binary has a period of between one and two years. By combining the radio data with X-ray data, we find that the most likely period is similar to 1.4 yr. [less ▲]

The spectrum of the very massive binary system WR20a (WN6ha+WN6ha): Fundamental parameters and wind interactions

in Astronomy and Astrophysics (2005), 432(3), 985-998

We analyse the optical spectrum of the very massive binary system WR20a (WN6ha + WN6ha). The most prominent emission lines, Halpha and He II lambda4686, display strong phase-locked profile variability ... [more ▼]

We analyse the optical spectrum of the very massive binary system WR20a (WN6ha + WN6ha). The most prominent emission lines, Halpha and He II lambda4686, display strong phase-locked profile variability. From the variations of their equivalent widths and from a tomographic analysis, we find that part of the line emission probably arises in a wind interaction region between the stars. Our analysis of the optical spectrum of WR20a indicates a reddening of A(nu) similar or equal to 6.0 mag and a distance of similar to 7.9 kpc, suggesting that the star actually belongs to the open cluster Westerlund 2. The location of the system at similar to 1.1 pc from the cluster core could indicate that WR20a was gently ejected from the core via dynamical interactions. Using a non-LTE model atmosphere code, we derive the fundamental parameters of each component: T-eff = 43 000 +/- 2000 K, log L-bol/ L-. similar or equal to 6.0, (M) over dot = 8.5 x 10(-6) M-. yr(-1) (assuming a clumped wind with a volume filling factor f = 0.1). Nitrogen is enhanced in the atmospheres of the components of WR20a, while carbon is definitely depleted. Finally, the position of the binary components in the Hertzsprung-Russell diagram suggests that they are core hydrogen burning stars in a pre-LBV stage and their current atmospheric chemical composition probably results from rotational mixing that might be enhanced in a close binary compared to a single star of same age. [less ▲]

Line profile variability in the spectra of Oef stars - II. HD192281, HD14442 and HD14434

in Astronomy and Astrophysics (2004), 427(3), 995-1008

We present the very first analysis of the spectroscopic variability of the three rapidly rotating Oef stars HD 192281 (O5(ef)), HD14442 (O5.5ef) and HD 14434 (O6.5( ef)). Radial velocities of the He II ... [more ▼]

We present the very first analysis of the spectroscopic variability of the three rapidly rotating Oef stars HD 192281 (O5(ef)), HD14442 (O5.5ef) and HD 14434 (O6.5( ef)). Radial velocities of the He II lambda 4541 line reveal no evidence of binarity on time scales of a few days, or from one year to the next, for any of the targets. The He II lambda 4686 double-peaked emission and, to some extent, the H absorption line display significant profile variability in the spectra of all three stars. Data gathered during different observing runs spread over six years reveal a rather stable time scale for HD 192281 and HD14442, whereas the variability pattern changes significantly from one year to the other. The case of HD 14434 is less clear as no obvious time scale emerges from our analysis. In a tentative way to interpret this variability, stellar rotation remains a possible clock for HD 192281 and HD14442. However, currently available models addressing stellar rotation fail to explain some crucial aspects of the observed variability behaviour, which appear to be even more complex in the case of HD 14434. [less ▲]

Early-type stars in the young open cluster IC 1805 - I. The SB2 system BD+60 degrees 497 and the probably single stars BD+60 degrees 501 and BD+60 degrees 513

in Astronomy and Astrophysics (2004), 421(2), 693-702

We investigate the multiplicity of three O-type stars in the very young open cluster IC 1805. All our targets were previously considered as spectroscopic binaries, but no orbital solution was available ... [more ▼]

We investigate the multiplicity of three O-type stars in the very young open cluster IC 1805. All our targets were previously considered as spectroscopic binaries, but no orbital solution was available for any of them. Our results confirm the binarity of BD+60degrees 497 and we provide the very first orbital solution for this double-lined spectroscopic binary. This is only the second O-star binary in IC 1805, and the first SB2 system, for which an orbital solution is now available. BD+60degrees 497 has an orbital period of 3.96 days and consists of an evolved 06.5 V((f)) primary and an 08.5-9.5 V((f)) secondary with minimum masses of m, sin(3) i = 13.9 M-circle dot and m(2) sin 3 i = 10.9 M-circle dot. The observed primary/secondary mass ratio (1.28) appears lower than expected from a comparison with single star evolutionary models (1.60-1.74). For the other two stars, BD+60degrees 501 and BD+60degrees 513, we find no significant radial velocity variations, suggesting that they are most probably single. Although a fraction of binaries among the early-type stars of IC 1805 as high as 80% has been advocated in the literature, our results suggest that this number might be overestimated. [less ▲]