arXiv subjects

Gloria Koenigsberger

Publications and source records attributed to Gloria Koenigsberger.

28 records · Page 2Linked to original sources

arXiv2013

Spectral modelling of massive binary systems

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 and we used the TIDES code for surface computation 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 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 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.

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arXiv2012

Tidal effects on the radial velocity curve of HD77581 (Vela X-1)

The mass of the neutron star in Vela X-1 has been found to be more massive than the canonical 1.5 Mo. This result relies on the assumption that the amplitude of the optical component's measured radial velocity curve is not seriously affected by the interactions in the system. In this paper we explore the effect on the radial velocity curve caused by surface motions excited by tidal interactions. We use a calculation from first principles that involves solving the equations of motion of a Lagrangian grid of surface elements. The velocities on the visible surface of the star are projected along the line-of-sight to the observer to obtain the absorption-line profile in the observer's reference frame. The centroid of the line-profiles for different orbital phases is then measured and a simulated RV curve constructed. Models are run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiant rotation velocities. We find that the surface velocity field is complex and includes fast, small-spatial scale structures. It leads to strong variability in the photospheric line profiles which, in turn, causes significant deviations from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are in all cases larger than the amplitude of the orbital motion. Keplerian fits to RV curves obtained with the "standard" rotation velocity imply a neutron star >1.7 Mo. However, a similar analysis of the "slow" rotational velocity models allows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role in determining the characteristics of the perturbed RV curve. Given the observational uncertainty in GP Vel's projected rotation velocity and the strong perturbations seen in the published and the model RV curves, we are unable to rule out a small (~1.5 Mo) mass for the neutron star companion.

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arXiv2011

Wind structure and luminosity variations in the WR/LBV HD 5980

Over the past 40 years, the massive LBV/WR system HD 5980 in the Small Magellanic Cloud has undergone a long-term S Doradus type variability cycle and two brief and violent eruptions in 1993 and 1994. In this paper we analyze a collection of UV and optical spectra obtained between 1979 and 2009 and perform CMFGEN model fits to spectra of 1994, 2000, 2002 and 2009. The results are as follows: a) The long term S Dor-type variability is associated with changes of the hydrostatic radius; b) The 1994 eruption involved changes in its bolometric luminosity and wind structure; c) the emission-line strength, the wind velocity and the continuum luminosity underwent correlated variations in the sense that a decreasing V$_\infty$ is associated with increasing emission line and continuum levels; and d) The spectrum of the third star in the system ({\it Star C}) is well-fit by a T$_{eff}$=32 K model atmosphere with SMC chemical abundances. For all epochs, the wind of the erupting star is optically thick at the sonic point and is thus driven mainly by the continuum opacity. We speculate that the wind switches between two stable regimes driven by the "hot" (during the eruption) and the "cool" (post-eruption) iron opacity bumps as defined by Lamers & Nugis (2002) and Gräfener and Hamann (2008), and thus the wind may undergo a bi-stability jump of a different nature from that which occurs in OB-stars.

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arXiv2009

Tidal Flows from Asynchronous Rotation in Binaries

Asynchronous rotation in binary stars produces non-radial oscillations that are known to cause observable variability on orbital timescales. The horizontal perturbations of the surface velocity fields are referred to as "tidal flows". In this paper we illustrate the manner in which tidal flows perturb the surface velocity field from that of uniform rotation, using a one-layer stellar model for the calculations. We justify the validity of this simplified model by the striking similarity between the photospheric absorption line-profiles it predicts and observational data of the binary system alpha Virginis. The velocity perturbations are used to compute the mechanical energy dissipation rates, dot-E, due to the shearing flows for the case of a massive (50+28 Mo) binary system having a moderately eccentric (e=0.3) orbit. The largest value of dot-E around periastron phases is found on the hemisphere facing the companion. However, at other orbital phases the maximum dot-E may migrate towards the poles. Assuming that dot-E plays a role in the mass-loss characteristics of massive binary systems, this suggests that peculiar binaries such as HD 5980 and eta Carinae may have a highly non-spherically symmetric mass-loss distribution which, in addition, is time-variable.

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arXiv2009

Tidal effects and periastron events in binary stars

Binary stars in eccentric orbits are frequently reported to present increasing levels of activity around periastron passage. In this paper we present results of a calculation from first principles of the velocity field on the surface of a star that is perturbed by a binary companion. This allows us to follow the orbital phase-dependence of the amount of kinetic energy that may be dissipated through the viscous shear, dot-E, driven by tidal interactions. For stars with relatively small stellar radii compared with the orbital separation at periastron (R/r_per< 0.14), a clear maximum occurs before periastron passage for sub-synchronous rotation and after periastron for super-synchronous rotation. For larger stellar radii however, dot-E oscillates over the orbital cycle and periastron passage does not cause a particularly greater enhancement in energy dissipation rates than some of the other orbital phases. Finally, we perform exploratory calculations for a WR/LBV binary system that in 1993-1994 underwent an LBV-like eruption, HD 5980. Our dot-E calculations reproduce the oscillations that appear around periastron passage in HD5980's recent visual light curve. We suggest that the energy dissipation from tidal flows in asynchronously rotating binary stars may provide a mechanism by which evolving stars may be driven into an active state. Given the nature of the tidal perturbations, the resulting mass-loss distribution is expected to be non-uniform over the stellar surface and highly time-dependent.

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arXiv2007

The extraordinary LBV/WR system HD 5980

The LBV/WR system HD 5980 contains a short-period, eccentric binary system with interacting stellar winds. In this paper we summarize results from model calculations of the tidal flows on the LBV component showing that energy dissipation rates, E-dot, associated with turbulent viscosity are orbital-phase dependent as well as variable over the stellar surface. We speculate that if E-dot contributes towards driving mass-loss, the strongest wind-wind interaction effects may occur after periastron passage. In addition, the model suggests the presence of stronger outflows localized at polar angles theta~30 --50 degrees during part of the orbital cycle. Thus, the analysis of wind-wind interactions in this system requires that models be revised to incorporate non-stationary and asymmetric wind structures.

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arXiv2006

Tides in asynchronous binary systems

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. In this paper we 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. In addition, calibrating our model results to fit Zahn's relationship between synchronization timescales and orbital separation, we are able to constrain the value of the kinematical viscosity parameter. For the 4Mo+5Mo binary modeled in this paper, the kinematical viscosity is in the range 0.0015 -- 0.0043 Ro^2/d for orbital periods in the range 2.5 -- 25 d. Furthermore, the energy dissipation rate due to the shear flow is found to decrease by ~2 orders of magnitude as synchronization is approached, implying that binary systems may approach synchronization relatively quickly but that it takes a much longer timescale to actually attain this condition. Our model can be used to calculate the energy dissipation rates throughout the orbital cycle for arbitrary values of eccentricity and stellar rotational velocity. We suggest that the asymmetric distribution of energy dissipation over the stellar surface may lead to the appearance of localized regions of enhanced surface activity.

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arXiv2006

FUSE observations of HD 5980: The wind structure of the eruptor

HD 5980 is a unique system containing one massive star (star A) that is apparently entering the luminous blue variable phase, and an eclipsing companion (star B) that may have already evolved beyond this phase to become a Wolf-Rayet star. In this paper we present the results from FUSE observations obtained in 1999, 2000, and 2002 and one far-UV observation obtained by ORFEUS/BEFS in 1993 shortly before the first eruption of HD 5980. The eight phase-resolved spectra obtained by FUSE in 2002 are analyzed in the context of a wind-eclipse model. This analysis shows that the wind of the eruptor obeyed a very fast velocity law in 2002, which is consistent with the line-driving mechanism. Large amplitude line-profile variations on the orbital period are shown to be due to the eclipse of star B by the wind of star A, although the eclipse due to gas flowing in the direction of star B is absent. This can only be explained if the wind of star A is not spherically symmetric, or if the eclipsed line radiation is "filled-in" by emission originating from somewhere else in the system, e.g., in the wind-wind collision region. Except for a slightly lower wind speed, the ORFEUS/BEFS spectrum is very similar to the spectrum obtained by FUSE at the same orbital phase: there is no indication of the impending eruption. However, the trend for decreasing wind velocity suggests the occurrence of the "bi-stability" mechanism, which in turn implies that the restructuring of the circumbinary environment caused by the transition from "fast, rarefied wind" to "slow, dense wind" was observed as the eruptive event. The underlying mechanism responsible for the long-term decrease in wind velocity that precipitated this change remains an open issue.

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arXiv2002

Optical Line profile variability of the B1+neutron star binary system LS I+65 010 = 2S 0114+650

We present high resolution spectroscopic observations of the massive X-ray binary system LS I+65 010=2S 0114+650 in the optical wavelength region. A correlation between equivalent width and radial velocity of photospheric absorption lines is found. The systemic velocity, inferred from the weaker lines is v_helio = -31 +/- 5 km s^-1, which, if attributed solely to the Galactic rotation curve, implies that LS I+65 010 lies within 3 kpc from the Sun. The ISM Na I D lines display 2 resolved high velocity components at v_helio = -70, -48 km s^-1, possibly associated with gas surrounding the binary system, in addition to the -24, -8 km s^-1 ISM features due to the Orion and the Perseus arm regions. Strong photospheric line profile variability is present on a night to night timescale, with He I 5875 A displaying an additional blue-shifted absorption in some of the spectra, similar to what is observed in the optical counterpart of Vela X-1. A connection between the extended blue wing and X-ray maximum is suggested. Short timescale variations in line profiles are detected on only two nights, but the evidence that these variations occur on the 2.78 hour X-ray flaring period is marginal.

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arXiv2002

A Supernova Remnant Collision with a Stellar Wind

Numerical simulations of the interaction between supernova ejecta and a stellar wind are presented. We follow the temporal evolution of the shock fronts that are formed through such an interaction and determine the velocities, temperatures and densities. We model the X-ray emission from the SNR-stellar wind collision region and we compare it with recent results from X-ray observations carried out with the Chandra satellite of the SMC supernova remnant SNR 0057-7226 which could be interacting with the wind of the Wolf-Rayet system HD 5980. The simulations predict the presence of shell-like regions of enhanced X-ray emission which are consistent with the presence of X-ray emitting arcs in the Chandra image. Also the observed X-ray luminosity is comparable to the X-ray luminosities we obtain from the simulations for a supernova with an initial energy in the (1-5)E50 erg range.

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