Our research activities related to massive stars and high-energy astrophysics concern several aspects of the field that we will briefly describe below.
As in the previous years, a major effort was devoted to the study of massive binaries. Indeed, eclipsing spectroscopic binaries provide the only direct means to determine the masses of stars all across the Hertzsprung-Russell diagram and are particularly crucial for the most massive stars which have still poorly constrained properties (see e.g. Rauw 2004b). Here we highlight the results obtained for some specific systems. The results for some other systems (HD92024 and HD123335) are discussed in other sections of this report.
The analysis of O-star binary systems where both components are still on the main sequence allows determining stellar masses and radii that can be compared to the predictions of evolutionary models for single stars. In the 2003 IAP report, we described the results of our spectroscopic study of the O9 + B1 system CPD-41° 7742 in NGC6231. In 2004, we have analysed our optical and X-ray light curve of this system (Sana et al. 2005a, submitted). The photometric data indicate an orbital inclination of 77°, yielding masses of 18 and 10 MSun as well as radii of 7.5 and 5.4 RSun for the O9 and B1 component respectively. The X-ray light curve displays clear variability that we interpret as the signature of the O9 star wind crushing onto the surface of the B1 secondary (Sana et al. 2005a).
This colliding-wind O-type binary is the brightest X-ray source in NGC6231. Our XMM-Newton monitoring campaign of HD152248 reveals a clear phase-locked modulation of its X-ray flux and spectrum (Sana et al. 2004). We have performed 2D hydrodynamical simulations using different sets of parameters that closely reproduce the physical and orbital configuration of the binary system at the time of the six XMM-Newton observations, thus allowing a direct confrontation of the model predictions with the constraints deduced from the X-ray data. We have shown that the observed variation of the flux can be explained by a variation of the X-ray emission from the colliding-wind zone, diluted by the softer X-ray contribution of the two O-type stars of the system (Sana et al. 2004).
Eclipsing binaries in NGC6231
The VUB team has presented the first results of a long-term multi-colour photometric monitoring campaign of the very young open cluster NGC 6231. The preliminary results reveal a number of new variable stars, among which are several eclipsing and interacting binaries (Sterken & Bouzid 2004, Bouzid & Sterken 2004).
In 2004, we have shown that WR20a is a short period (3.675 days) binary system consisting of two WN6ha stars with orbital solutions indicating very large masses (Rauw et al. 2004a). The photometric eclipses that we had initially predicted were shortly after reported by the OGLE team. Combined with our orbital solutions these results yield very large masses of about 80 MSun for each component, thus making WR20a the most massive early-type star with well-established masses. In a follow-up analysis of the optical spectrum of this system (Rauw et al. 2005, in press), we have studied the wind interaction through a tomographic analysis of the prominent profile variations of the H-alpha and He II 4686 emission lines. Our analysis further indicates that WR20a is located at ~7.9 kpc and actually belongs to the Westerlund 2 open cluster. 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 the same age (Rauw et al. 2005).
As in the previous years, we also investigated various aspects of non-thermal phenomena in O-type stars. The various partners of the network contributed their expertise to the multi-wavelength observing campaigns (including radio, optical, X-ray and gamma-ray data) as well as to theoretical modelling of the phenomenon. It has been suggested that the non-thermal radio emission observed for a significant fraction of the O-stars could be related to binarity and colliding winds (see e.g. Rauw 2004a for a review). An important question is therefore whether all O-stars that produce non-thermal radio emission are indeed binaries. Another important issue is whether the relativistic electrons that produce the observed non-thermal (synchrotron) radio emission produce also an observable (inverse Compton) emission at X-ray and gamma-ray energies.
Cyg OB2 #8a
We have analysed the data of a spectroscopic monitoring campaign on the non-thermal radio emitter Cyg OB2 #8a (De Becker et al. 2004c). Our results show that this is an eccentric O6 + O5.5 binary system with an orbital period of 21.9 days. The spectra reveal also emission line profile variability that is probably related to the interaction between the winds of the stars; furthermore, Cyg OB2 #8a may also contribute to the gamma-ray emission of the yet unidentified EGRET source 3EG J2033+4118 (De Becker et al. 2005b & c). Further constraints on the wind interaction in this system are expected from the results of our XMM-Newton and INTEGRAL monitoring campaigns (the data have just been obtained) as well as from a VLA observing programme.
HD168112 and HD167971
Both stars are non-thermal radio emitters belonging to the NGC6604 cluster. The results of the coordinated VLA and XMM-Newton campaign on HD168112 (De Becker et al. 2004a) were already described in the 2003 IAP report and we shall not repeat them here. We have now finished the analysis and interpretation of the VLA and ATCA archive data on HD 168112. The fluxes show considerable variability and a negative spectral index, thereby confirming that HD 168112 is a non-thermal radio emitter. We explored both the single-star and binary hypothesis for the interpretation of the data. In the single-star hypothesis, we used a synchrotron model to fit the observations. Doing so, we found 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. In the binary hypothesis, we found that by folding the observed data with various periods, that the fluxes seem to repeat on a ~ 1.4 yr period. The variability is explained by assuming the star to be an eccentric binary: as the colliding-wind region moves in and out of the region where free-free absorption is important, the radio fluxes vary. We concluded that the data are much easier to explain by assuming that HD 168112 is a binary (Blomme et al., A&A, 2005 in press).
The X-ray spectra of the multiple system HD167971 (O5-8V + O5-8V + (O8I)) consist of a rather soft thermal component with typical temperatures of about 2 – 9 106K and a harder component that can be fitted equally well by a hot thermal model (23 – 46 106K) or a non-thermal (power-law) energy distribution (De Becker et al. 2005a). It seems likely that this harder emission is thermal and arises from an interaction between the combined winds of the O5-8V + O5-8V close binary system and that of the more distant O8I companion.
We continued our work on archive radio data on non-thermal emitters by finishing the reduction of the HD 167971 observations. The interpretation and modelling of the results still remains to be done. The reduction of the data on 9 Sgr = HD 164794 and the Cyg OB2 stars was also started.
Theoretical models of non-thermal radio emission
We have previously developed a theoretical model for the non-thermal radio emission from single hot stars, in terms of synchrotron radiation from relativistic electrons accelerated in wind-embedded shocks. This model has now been extended to include the effect of cooling on the relativistic particles. In the new model, the synchrotron radiation is limited to small layers behind a shock. The model has been applied to the non-thermal emitter Cyg OB2 No. 9, resulting in significant constraints on most of the model parameters (Van Loo et al., A&A, 2005, in press). The synchrotron model was also applied to all available observations of HD 168112 (Blomme et al., A&A, 2005 in press).
We also studied in detail the shock structure from time-dependent hydrodynamical simulations. This information was included in the synchrotron model to obtain a more realistic description of the stellar wind. Contrary to previous models, this most recent version of the synchrotron model can no longer explain the observations. This shows that the observed non-thermal radio emission cannot be ascribed to shocks in the wind, but is probably due to colliding stellar winds in a binary system, even for those stars where there is no spectroscopic indication for binarity. A publication detailing these results is nearly finished.
The ROB team has continued its work on thermal radio emitters. We reduced our VLA data and started the reduction of the VLA archive data of the thermal radio emitters HD37128, HD152236, HD152408, HD160529 and HD190603. A preliminary reduction of our SEST millimetre continuum observations of the O4I(n)f star ? Puppis shows that there might be some variability present. Considerable effort will be needed to judge the level of significance of this.
Hydrodynamical calculations have been performed with applications to both thermal and non-thermal radio emitters. In collaboration with S. P. Owocki (University of Delaware) a powerful method was developed to study the evolution of instability-generated structure in the outer winds of hot stars. We call this method the "pseudo-planar moving periodic box method". Its main feature is the formulation of the spherical equations of hydrodynamics in a moving reference frame, by virtue of a transformation of variables resulting in equations that still describe spherical symmetry but formally resemble the planar equations. This method allows us to follow wind structure out to very large distances (more than 1000 stellar radii from the central star). In addition, it does not suffer from certain numerical problems that cause artefacts in traditional models.
The periodic box method also makes it possible to measure the shock speeds with sufficient precision to translate the jump velocity of a shock to a compression ratio. We measured compression ratios and velocity jumps of typical strong shocks in the periodic box model and used them as a hydrodynamic input to models of non-thermal radio emission. A paper describing the periodic box model and key results, has been published (Runacres & Owocki 2005).
In 2004, we have also continued our analyses of the spectroscopic and photometric variability of early-type stars.
Our latest results on the mysterious Of?p star HD108 (Nazé et al. 2004a) were discussed in the previous IAP report. In 2004, we have contributed to the study of another object of this sparse category. HD191612 displays variations that are quite reminiscent of those of HD108 but occur on a much shorter time scale (18 months for HD191612 vs. six decades for HD108, see Nazé 2004 and Walborn et al. 2004). We have been involved in a spectroscopic monitoring campaign organised by N.R. Walborn (STScI Baltimore) that has finally allowed us to show that the spectroscopic variations of HD191612 are indeed periodic, and to successfully predict the most recent spectral type transition that occurred in September-October 2004 (Walborn et al. 2004). Although there is as yet no direct evidence for a companion, a model in which tidally induced oscillations drive an enhanced wind near periastron of an eccentric orbit appears promising. Taking advantage of the now well- determined (and rather short) period of HD191612, we have organised an observing campaign in the X-ray domain with XMM-Newton near the crucial phases of the cycle (P.I. Y. Nazé). These XMM observations will be obtained in 2005. The X-ray spectra of HD191612 will be confronted to those of HD108, thereby providing new insight into the origin of the Of?p phenomenon.
Another poorly studied category of O-type stars are the so-called Oef objects that display a double-peaked He II 4686 emission line in their spectrum. In 2004, we have published the results of the very first spectroscopic monitoring of three objects of this category: HD192281, HD14442 and HD14434 (De Becker & Rauw 2004d). The He II 4541 absorption lines indicate roughly constant radial velocities on time scales of a few days or a few years. The double-peaked He II 4686 emission and, to some extent, the H-beta 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 HD192281 and HD14442, whereas the variability pattern changes significantly from one year to the other. For HD14434 no obvious time scale emerges from our analysis. Whilst stellar rotation remains a possible clock for HD192281 and HD14442, current models fail to account for some crucial aspects of the observed variability behaviour of Oef stars.
Photometry of Luminous Blue Variables
The VUB team has analsed ground-based and HST photometry of the Homunculus nebula around eta Carinae obtained from 1998 to 2003 to search for a light-time effect in the Homunculus due to reflections. No measurable effect was found, except perhaps for the variable UV radiation originating in the equatorial plane, modulated by the 5.5yr binary period (van Genderen & Sterken 2004, 2005). A plausible reason for the homologous light behaviour is enhanced extinction decrease in the line-of-sight to the central region during a global brightening occurring after the 1998.0 spectroscopic event. From 1998 to 2004 there is indeed a brightness excess of ~0.3mag between the Homunculus and the extrapolated secular brightening (van Genderen & Sterken 2004).
We have also continued our investigation of the X-ray properties of early-type stars and their surroundings.
For instance, we have studied an XMM-Newton observation of the field of the WN8 Wolf-Rayet star WR40 (Gosset et al. 2005). Despite the high sensitivity of the satellite and despite the remarkable variability in the optical spectrum of WR40 (suggesting the existence of strong instabilities in its stellar wind), no X-ray emission from the star is detected. Using a simple model of the wind opacity of WR40, we have shown that any X-ray emission generated in the particular zone where the instabilities are supposed to be numerous will indeed have little chance to emerge from the dense wind. The same observation reveals no X-ray emission from the ejecta nebula RCW58 surrounding WR40.
Valuable information on the X-ray properties of the early-type stars and their surroundings can be obtained from XMM-Newton observations of very young open clusters. In the 2003 IAP report, we highlighted already the results obtained by Nazé et al. (2004b) for the diffuse X-ray emission of the giant H II region N11 in the Large Magellanic Cloud. In 2004, a major effort was devoted to the very young open cluster NGC6231 in our Galaxy. In addition to HD152248 and CPD-41°7742 (see above), the XMM-Newton data of this cluster reveal more than 600 X-ray sources. While most of them are associated with optically faint stars (most probably pre-main sequence objects in the cluster), 42 sources are associated with early-type stars. We have investigated their LX/Lbol relation (Sana et al. 2005b). We confirm the clear dichotomy between O- and B-type stars around Lbol ~ 1038 erg s-1 as previously proposed on the basis of ROSAT data. About 40% of the X-ray emitters in the field of view present consistent signs of variability in the EPIC data. Among the early-type stars, this fraction is about one third. More extensive investigations of the X-ray data of this cluster are currently under way and the results will be submitted for publication soon.
Finally, the GAPHE team has also been involved in an INTEGRAL study of the supernova remnant gamma-Cygni (Bykov et al. 2004). IBIS-ISGRI images suggest that the SNR produces a hard X-ray emission in the 25 - 120 keV energy band. The positions of these hard X-ray clumps correlate with bright patches of optical line emission. The observed morphology and spectra are consistent with model predictions of hard X-ray emission from non-thermal electrons accelerated by a radiative shock in the supernova interacting with an interstellar molecular cloud.