K. A. van der Hucht

Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries

Since close WR+O binaries are the result of a strong interaction of both stars in massive close binary systems, they can be used to constrain the highly uncertain mass and angular momentum budget during the major mass transfer phase. We explore the progenitor evolution of the three best suited WR+O binaries HD 90657, HD 186943 and HD 211853, which are characterized by a WR/O mass ratio of ~0.5 and periods of 6...10 days. We are doing so at three different levels of approximation: predicting the massive binary evolution through simple mass loss and angular momentum loss estimates, through full binary evolution models with parametrized mass transfer efficiency, and through binary evolution models including rotation of both components and a physical model which allows to compute mass and angular momentum loss from the binary system as function of time during the mass transfer process. All three methods give consistently the same answers. Our results show that, if these systems formed through stable mass transfer, their initial periods were smaller than their current ones, which implies that mass transfer has started during the core hydrogen burning phase of the initially more massive star. Furthermore, the mass transfer in all three cases must have been highly non-conservative, with on average only ~10% of the transferred mass being retained by the mass receiving star. This result gives support to our system mass and angular momentum loss model, which predicts that, in the considered systems, about 90% of the overflowing matter is expelled by the rapid rotation of the mass receiver close to the Omega-limit, which is reached through the accretion of the remaining 10%.

WR 20a: A massive cornerstone binary system comprising two extreme early-type stars

We analyse spectroscopic observations of WR 20a revealing that this star is a massive early-type binary system with a most probable orbital period of ∼3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If ∗ /WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 ± 4.0 and 68.8± 3.8 Mfor the two stars. These properties make WR 20a a cornerstone system for the study of massive star evolution.

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

We analyse the optical spectrum of the very massive binary system WR 20a (WN6ha + WN6ha). The most prominent emission lines, Hα and He  λ 4686, 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 WR 20a indicates a reddening of AV � 6.0 mag and a distance of ∼7.9 kpc, suggesting that the star actually belongs to the open cluster Westerlund 2. The location of the system at ∼1.1 pc from the cluster core could indicate that WR 20a was gently ejected from the core via dynamical interactions. Using a non-LTE model atmosphere code, we derive the fundamental parameters of each component: Teff = 43 000 ± 2000 K, log Lbol/L� � 6.0, u M = 8.5 × 10 −6 Myr −1 (assuming a clumped wind with a volume filling factor f = 0.1). Nitrogen is enhanced in the atmospheres of the components of WR 20a, 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 as ingle star of same age.

New Galactic Wolf-Rayet stars, and candidates (Research Note) An annex to The VIIth Catalogue of Galactic Wolf-Rayet Stars

This paper gathers, from the literature and private communication, 72 new Galactic Population I Wolf-Rayet stars and 17 candidate WCLd stars, recognized and/or discovered after the publication of The VIIth Catalogue of Galactic Wolf-Rayet Stars. This brings the total number of known Galactic Wolf-Rayet stars to 298, of which 24 (8%) are in open cluster Westerlund 1, and 60 (20%) are in open clusters near the Galactic Center.

New Galactic Wolf-Rayet stars, and candidates (Research note)

This paper gathers, from the literature and private communication, 72 new Galactic Population I Wolf-Rayet stars and 17 candidate WCLd stars, recognized and/or discovered after the publication of The VIIth Catalogue of Galactic Wolf-Rayet Stars. This brings the total number of known Galactic Wolf-Rayet stars to 298, of which 24 (8%) are in open cluster Westerlund 1, and 60 (20%) are in open clusters near the Galactic Center.

XMM-Newton high-resolution X-ray spectroscopy of the Wolf-Rayet object WR 25 in the Carina OB1 association ?

We report the analysis of the first high-resolution X-ray spectra of the Wolf-Rayet (WR) object WR 25 (HD 93162, WN6ha+O4f) obtained with the reflection grating spectrometers (rgs )a nd theeuropean photon imaging cameras (epic-mos and pn) ccd spectrometers on board the XMM-Newtonsatellite. The spectrum exhibits bright emission lines of the H- and He- like ions of Ne, Mg, Si and S, as well as Fexviii to Fexx and Fexxv lines. Line fluxes have been measured. The rgs and epic spectra have been simultaneously fitted to obtain self-consistent temperatures, emission measures, and elemental abundances. Strong absorption by the dense WR stellar wind and the interstellar medium (ISM) is observed equivalent to NH= 710 21 cm 2 . Multi-temperature (dem) fitting yields two dominant components around temperatures of 7.0 and 32 MK, respectively. The XMM intrinsic (i.e. unabsorbed, corrected for the stellar wind absorption and the absorption of ISM) X-ray luminosity of WR 25 is Lx(0.5-10 keV)= 1:3 10 34 erg s 1 ,a ndLx(0.5-10 keV)= 0:85 10 34 erg s 1 , (when correcting for the ISM only) assuming d= 3:24 kpc. The obtained chemical abundances are subsolar, except for S. This may be real, but could equally well be due to a weak coupling to the continuum, which is strongly influenced by the absorption column density and the subtracted background. The expected high N-abundance, as observed in the optical wavelength region, could not be confirmed due to the strong wind absorption, blocking out its spectral signature. The presence of the Fe xxv emission-line complex at 6.7 keV is argued as being indicative for colliding winds inside a WR+O binary system.

Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary gamma ² Velorum observations at high and low state. XMM-Newton observations at high and low state

We present XMM-Newton observations of γ 2 Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Rayet binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate . M = 8 × 10 −6 Myr −1 for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12 A) show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5 MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of C  (25.3 A) and C  (31.6 A) radiative recombination continua at both phases indicates the presence of a cool (∼40 000 K) recombination region located far out in the binary system.We present XMM-Newton observations of 2 Velorum (WR11, WC8+O7.5III, P =78.53 d), a nearby Wolf-Ray binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate . M =8×10 −6 M⊙yr −1 for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12 u show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of C v (25.3 u and C vi (31.6 u radiative recombination continua at both phases indicates the presence of a cool (�40,000 K) recombination region located far out in the binary system.

Episodic dust formation by HD 192641 (WR 137) – II

Observations of the dust and gas around embedded stellar clusters reveal some of the processes involved in their formation and evolution. Large scale mass infall with rates dM/dt=4e-4 solar masses/year is found to be disrupted on small scales by protostellar outflows. Observations of the size and velocity dispersion of clusters suggest that protostellar migration from their birthplace begins at very early times and is a potentially useful evolutionary indicator.

Multi-frequency variations of the Wolf-Rayet system HD 193793 (WC7pd + O4-5). III. IUE observations.

The colliding-wind binary system WR140 (HD193793, WC7pd+O4-5, P=7.94 yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35 short-wavelength high-resolution spectra. An absorption-line radial-velocity solution is obtained from the photospheric lines of the O component, by comparison with a single O star. The resulting orbital parameters, e=0.87 +/- 0.05, w=31 degrees +/- 9 degrees and Ko star=25 +/- 15 km s(-1), confirm the large eccentricity of the orbit, within the uncertainties of previous optical studies. This brings the weighted mean UV-optical eccentricity to e=0.85 +/- 0.04. Occultation of the O-star light by the WC wind and the WC+O colliding-wind region results into orbital modulation of the P-Cygni profiles of the C II, CIV and Si IV resonance lines. Near periastron passage, the absorption troughs of those resonance-line profiles increase abruptly in strength and width, followed by a gradual decrease. In particular, near periastron the blue black-edges of the P-Cygni absorption troughs shift to larger out ow velocities. We discuss that the apparently larger wind velocity and velocity dispersion observed at periastron could be explained by four phenomena: (i) geometrical resonance-line eclipse effects being the main cause of the observed UV spectral variability, enhanced by sightline crossing of the turbulent wind-wind collision zone; (ii) the possibility of an orbital-plane enhanced WC7 stellar wind; (iii) possible common-envelope acceleration by the combined WC and O stellar radiation fields; and (iv) possible enhanced radiatively driven mass loss due to tidal stresses, focused along the orbiting line of centers.

WSRT 1.4 and 5-GHz light curves for WR 147 (AS 431, WN8(h)+OB)

The results of more than 8-yr monitoring (1988-1997) of the Wolf-Rayet binary WR 147 (WN8(h)-OB) with the Westerbork Synthesis Radio Telescope (WSRT) are presented. When the strong winds of the Wolf-Rayet (WR) and OB binary components collide. they produce non-thermal excess radiation in the region where the two winds interact. The binary system, monitored at 1.4 and 5 GHz (21 and 6cm), is not resolved by the WSRT, thus we observed the total flux density of the system. The time-averaged 5 and 1.4-GHz flux densities are 35.4 +/-0.4 mJy and 26.4 +/- 0.3 mJy, respectively These give a time-averaged spectral index of alpha (5-1.4 GHz) approximate to 0.23 +/- 0.04, where S-nu proportional to nu (alpha). The departure from the value expected for thermal radiation from a spherically symmetric stellar wind, alpha = 0.6, call be attributed to non-thermal emission from a bow-shaped source to the north of the thermal source associated with the WN8 star. With a possible detection at 350 WHz of 16 +/- mJy, in our separate study of the Cygnus region, the spectral energy distribution, after the contribution of the southern thermal source is subtracted, can be fitted by a synchrotron emission model which includes free-free absorption. The nonthermal emission originates in the region where the winds of the binary components collide. This region, therefore, contains a mixture of relativistic particles accelerated by shocks and thermal particles, responsible for the free-free absorption. We show, in a simplified model of the system, that additional free-free absorption may occur when the line of sight to the collision region passes through the radiophotosphere of the WR wind. The 1.4-GHz flux density of WR 147 varied between similar to 20 mJy and similar to 30 mJy. We attribute the irregular, stochastic variations with a typical timescale of about 60 days to inhomogeneities in the wind, with different mechanisms involved in the flux-density increase than in the flux-density decrease. A flux-density increase results when the inhomogeneities in the wind/clumps enter the wind collision region, fuelling the synchrotron emission. The typical timescale of the flux-density decrease is shorter than the timescale of synchrotron loss (similar to 10(3) yr) or the Inverse-Compton lifetime (approximate to4.5 yr), but of the order of the Row time in the colliding-wind region (similar to 80 d). Therefore, we suggest that the flux-density decrease is due to plasma outflow from the system. Furthermore. variable free-free absorption due to large clumps passing the line of sight may also cause variations in the flux density. We observe a possible long-term flux-density variation oil top of the stochastic variation. This variation is fitted with a sinusoid with a similar to7.9-yr period, with a reduced chi (2) of 1.9. However. as the period of the sinusoid is too close to the monitoring time span, further monitoring is needed to confirm this lon-term variation.