Gloria Koenigsberger

AN X-RAY INVESTIGATION OF THE NGC 346 FIELD IN THE SMALL MAGELLANIC CLOUD I. THE LUMINOUS BLUE VARIABLE HD 5980 AND THE NGC 346 CLUSTER

We present results from a Chandra observation of the NGC346 cluster. This cluster contains numerous massive stars and is responsible for the ionization of N66, the most luminous HII region and the largest star formation region in the SMC. In this first paper, we will focus on the characteristics of the main objects of the field. The NGC346 cluster itself shows only relatively faint X-ray emission (with L_X^{unabs} ~ 1.5 times 10^{34} erg s^{-1}), tightly correlated with the core of the cluster. In the field also lies HD5980, a LBV star in a binary (or possibly a triple system) that is detected for the first time at X-ray energies. The star is X-ray bright, with an unabsorbed luminosity of L_X^{unabs} ~ 1.7 times 10^{34} erg s^{-1}, but needs to be monitored further to investigate its X-ray variability over a complete 19d orbital cycle. The high X-ray luminosity may be associated either with colliding winds in the binary system or with the 1994 eruption. HD5980 is surrounded by a region of diffuse X-ray emission, which is a supernova remnant. While it may be only a chance alignment with HD5980, such a spatial coincidence may indicate that the remnant is indeed related to this peculiar massive star.We present results from a Chandra observation of the NGC 346 cluster. This cluster contains numerous massive stars and is responsible for the ionization of N66, the most luminous H ii region and the largest star formation region in the SMC. In this first paper we will focus on the characteristics of the main objects of the field. The NGC 346 cluster itself shows only relatively faint X-ray emission (with L unabs � 1:5 � 10 34 ergs s � 1 ), tightly correlated with the core of the cluster. In the field also lies HD 5980, a luminous blue variable star in a binary (or possibly a triple) system that is detected for the first time at X-ray energies. The star is X-ray bright, with an unabsorbed luminosity of L unabs � 1:7 � 10 34 ergs s � 1 , but needs to be monitored further to investigate its X-ray variability over a complete 19 day orbital cycle. The high X-ray luminosity may be associated either with colliding winds in the binary system or with the 1994 eruption. HD 5980 is surrounded by a region of diffuse X-ray emission, which is a supernova remnant. While it may be only a chance alignment with HD 5980, such a spatial coincidence may indicate that the remnant is indeed related to this peculiar massive star. Subject headings: galaxies: star clusters — Magellanic Clouds — X-rays: individual (NGC 346, HD 5980) — X-rays: stars

A 2.3 Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?

The apparently single WN 6 type star WR 134 (HD 191765) is distinguished among the Wolf-Rayet star population by its strong, presumably cyclical (≈2.3 day) spectral variations. A true periodicity—which is still very much debated—would render WR 134 a prime candidate for harboring either a collapsed companion or a rotating, large-scale, inhomogeneous outflow. We have carried out an intensive campaign of spectroscopic and photometric monitoring of WR 134 from 1989 to 1997 in an attempt to reveal the true nature of this object. This unprecedentedly large data set allows us to confirm unambiguously the existence of a coherent 2.25±0.05 day periodicity in the line-profile changes of He II λ4686, although the global pattern of variability is different from one epoch to another. This period is only marginally detected in the photometric data set. Assuming the 2.25 day periodic variability to be induced by orbital motion of a collapsed companion, we develop a simple model that aims to investigate (1) the effect of this strongly ionizing, accreting companion on the Wolf-Rayet wind structure, and (2) the expected emergent X-ray luminosity. We argue that the predicted and observed X-ray fluxes can only be matched if the accretion on the collapsed star is significantly inhibited. Additionally, we performed simulations of line-profile variations caused by the orbital revolution of a localized, strongly ionized wind cavity surrounding the X-ray source. A reasonable fit is achieved between the observed and modeled phase-dependent line profiles of He II λ4686. However, the derived size of the photoionized zone substantially exceeds our expectations, given the observed low-level X-ray flux. Alternatively, we explore rotational modulation of a persistent, largely anisotropic outflow as the origin of the observed cyclical variability. Although qualitative, this hypothesis leads to greater consistency with the observations.

Wind Velocity Variations in the Luminous Blue Variable-Type Erupting Star of the Wolf-Rayet Binary HD 5980

We present the wind velocity and UV luminosity variations in the Wolf-Rayet system HD 5980 obtained over a time span during which one of the stars of the system was transformed into a luminous blue variable and underwent an eruption. We are able to separate the velocity components of the two stars in the system: a stable velocity component at -1700 km s-1 is associated with the nonerupting star, while the variable wind with velocities ranging from -500 to -3000 km s-1 corresponds to the eruptor. The development of a fast wind following the slow wind eruptive phase is observed. Under the assumption of radiatively driven winds, these changing velocities indicate that the radius of the photosphere gradually increased during at least 12 years prior to the 1994 eruption, decreasing rapidly thereafter. An estimate of the stellar parameters indicates that the erupting star is massive (M > 40 M☉) and very luminous (L > 106 L☉), and that during the eruption its radius extended beyond the binary orbit (R* > 100 R☉).

Hubble Space Telescope Observations of the Luminous Blue Variable/W-R Eclipsing Binary System HD 5980*

We report the results of Hubble Space Telescope STIS observations of the intriguing luminous blue variable/Wolf-Rayet binary system HD 5980 in the Small Magellanic Cloud. Although its spectral char- acteristics (WN6) are currently very similar to those observed in 1995 in the FUV, some of the line —uxes continue with the increasing trend observed since the 1980s. The erupting star still dominates the emission-line spectrum, and a radial velocity curve from UV lines is derived, supporting previous esti- mates of its mass (50 A rough estimate of 2 ) 10~4 yr~1 for the current mass-loss rate of this M _ ). M _ star is derived. The only spectral lines attributable to the close companion are the very extended P Cygni absorption components of the strong UV lines, present only during the eclipse of the eruptor, implying that the wind-wind collision shock cone winds tightly around this companion. A third stellar component contributing to the spectrum is detected through the presence of stationary photospheric absorption lines in the UV spectrum. This object contributes D30% to the UV continuum luminosity. A new set of ISM absorption components at (680 km s~1 has appeared as a result of the shock interface between the slow wind (D400 km s~1) that emerged during the 1994 eruption and the subsequently emerging fast wind (D2000 km s~1) phases. Subject headings: binaries: generalstars: evolutionstars: individual (HD 5980) ¨ stars: variables: otherstars: winds, out—owsstars: Wolf-Rayet

Mass-Loss Rate and He/H Abundance of the Erupting Component in the Small Magellanic Cloud System HD 5980

The binary Wolf-Rayet system HD 5980 in the Small Magellanic Cloud underwent a major luminous blue variable-type eruptive event in 1994. It is the first such recorded event in which the apparent precursor transits from WNE to WNL spectral types prior to the eruption. In this paper we analyze the spectrum of the system obtained when the outburst was declining (1994 December), but the dominant spectrum was that of the eruptor. From non-LTE analysis we obtain =10−3 M☉ yr-1, N[He]/N[H] = 0.43 (by number; Y = 0.63 by mass), v∞ = 600 km s-1, T* = 35,500 K, and Leruptor = 3 × 106L☉. A comparison of the He/H abundance derived here and an estimate obtained from published data of 20 yr ago leads us to conclude that the member of the binary system that underwent the eruption is the star formerly classified as an O7 supergiant. The considerable amount of He in this star indicates that it is in transition to becoming an H-poor W-R. By comparing the stellar parameters with single-star evolutionary tracks, we derive that the progenitor was more massive than 120 M☉, and that its current mass is close to 80 M☉.

Tides in asynchronous binary systems

Context. Stellar oscillations are excited in non-synchronously rotating stars in binary systems due to the tidal forces. Tangential components of the tides can drive a shear flow which behaves as a differentially forced rotating structure in a stratified outer medium. Aims. The aims of this paper are to show that our single-layer approximation for the calculation of the forced oscillations yields results that are consistent with the predictions for the synchronization timescales in circular orbits, τsync ∼ a 6 , thus providing a simplified means of computing the energy dissipation rates, u E. Furthermore, by calibrating our model results to fit the relationship between

Coupled Line-Profile and Continuum Variations in EZ Canis Majoris: Implications for the Driving Mechanism of Global Wind Structures in Wolf-Rayet Winds

EZ CMa is an apparently unusual Wolf-Rayet star of the nitrogen sequence that exhibits strong variations on a period of 3.77 days with coherency lasting typically about 10 cycles. We have used an extensive set of optical spectroscopic observations to investigate a possible link between its line-profile and photometric continuum variability. Despite the strong epoch dependency of the variations, a persistent correlation is found between changes in the wind line profiles (N V λλ4604, 4620 in particular) and in continuum flux emanating near the stellar core. We suggest that these observations give further support to the idea that the physical conditions prevailing in the vicinity of the stars photosphere have a significant impact on the wind structure and that a spatial dependence of these conditions at the base of the outflow induces the formation of azimuthal wind structures in EZ CMa. The epoch-dependent nature of the variability could be related to long-term behavior of corotating magnetic structures, although pulsational instabilities constitute a viable alternative.

Remarkable long-term changes in the small Magellanic Cloud Wolf-Rayet system HD 5980

In this paper we report the remarkable changes which occured in the Small Magellanic Cloud W-R system HD5980 = AB5 between 1978 and 1991. Within this timescale, there has been a systematic enhancement (by factors of 2-10 depending on the line) in the equivalent widths of all emission lines, and a change in the relative strengths of N III, N IV, and N V lines. Currently, the W-R spectrum is more typical of a WN6 star than a WN3 or WN4, as it was originally classified. The terminal speed of the wind has diminished by approximately 600 km/s, while the system has brightened in the visual by 0.45 mag. The UV (1850 A) continuum changed by less than 0.13 mag. The change from WN3 or WN4 to WN6 is unprecedented. The system appears to be composed at least three stars: two WNs in mutual 19.266 day orbit and an O-type supergiant. We propose that the changes observed in HD 5980 are related to an increase in wind density of one (or both?) of the W-R components, where the brighter WN6 component will dominate the W-R spectrum after the change, and we speculate that this modification of the wind structure is driven by tidal interaction induced by a possible current periastron passage of the third component in the system.

Eccentric binaries - Tidal flows and periastron events

Context. A number of binary systems present evidence of enhanced activity around periastron passage, suggesting a connection between tidal interactions and these periastron effects. Aims. The aim of this investigation is to study the time-dependent response of a star’s surface as it is perturbed by a binary companion. Here we focus on the tidal shear energy dissipation. Methods. We derive a mathematical expression for computing the rate of dissipation, u E, of the kinetic energy by the viscous flows that are driven by tidal interactions on the surface layer of a binary star. The method is tested by comparing the results from a grid of model calculations with the analytical predictions of Hut (1981, AA 2008, EAS Pub. Ser., 29, 67). Results. Our results for the dependence of the average (over orbital cycle) energy dissipation, u Eave, on orbital separation are consistent with those of Hut (1981) for model binaries with an orbital separation at periastron rper/R1 > 8, where R1 is the stellar radius. The model also reproduces the predicted pseudo-synchronization angular velocity for moderate eccentricities (e ≤ 0.3). In addition, for circular orbits our approach yields the same scaling of synchronization timescales with orbital separation as given by Zahn (1977, 2008) for convective envelopes. The computations give the distribution of u E over the stellar surface, and show that it is generally concentrated at the equatorial latitude, with maxima generally located around four clearly defined longitudes, corresponding to the fastest azimuthal velocity perturbations. Maximum amplitudes occur around periastron passage or slightly thereafter for supersynchronously rotating stars. In very eccentric binaries, the distribution of u E over the surface changes significantly as a function of orbital phase, with small spatial structures appearing after periastron. An exploratory calculation for a highly eccentric binary system with parameters similar to those of δ Sco (e = 0.94, P = 3944.7 d) indicates that u Eave changes by ∼5 orders of magnitude over the 82 days before periastron, suggesting that the sudden and large amplitude variations in surface properties around periastron may, indeed, contribute toward the activity observed around this orbital phase.

Spectral modelling of massive binary systems

Context. The spectra of massive binaries may be affected by interactions between the stars in the system. These are believed to produce observational phenomena such as the Struve-Sahade effect. Aims. We simulate the spectra of massive binaries at different phases of the orbital cycle, accounting for the gravitational influence of the companion star on the shape and physical properties of the stellar surface. Methods. We used the Roche potential modified to account for radiation pressure to compute the stellar surface of close circular systems. We furthermore used the tidal interactions with dissipation of energy through shear code for surface computations of eccentric systems. In both cases, we accounted for gravity darkening and mutual heating generated by irradiation to compute the surface temperature. We then interpolated non-local thermodynamic equilibrium (NLTE) plane-parallel atmosphere model spectra in a grid to obtain the local spectrum at each surface point. We finally summed all contributions, accounting for the Doppler shift, limb-darkening, and visibility to obtain the total synthetic spectrum. We computed different orbital phases and different sets of physical and orbital parameters. Results. Our models predict line strength variations through the orbital cycle, but fail to completely reproduce the Struve-Sahade effect. Including radiation pressure allows us to reproduce a surface temperature distribution that is consistent with observations of semi-detached binary systems. Conclusions. Radiation pressure effects on the stellar surface are weak in (over)contact binaries and well-detached systems but can become very significant in semi-detached systems. The classical von Zeipel theorem is sufficient for the spectral computation. Broad-band light curves derived from the spectral computation are different from those computed with a model in which the stellar surfaces are equipotentials of the Roche potential scaled by the instantaneous orbital separation. In many cases, the fit of two Gaussian/Lorentzian profiles fails to properly measure the equivalent width of the lines and leads to apparent variations that could explain some of the effects reported in the literature.