OB Companion Research Articles

Is WR 104 a face-on, colliding-wind binary?

ABSTRACT Two decades of extensive spectroscopic monitoring of WR 104 is used to confront what we think we understand about this iconic prototype of the dust-producing pinwheel stars. Convincing SB1 orbital solutions are obtained for both the WC9d star and its OB companion. The period ($241.54 \pm 0.14$ d) and circular orbit agree perfectly with results from modelling images of the rotating dust spiral. Contrary to those results though, the orbit is found to be significantly inclined instead of face-on. The two SB1 solutions each indicate $i \sim 45^{\circ }$ but could be as low as $\sim 34^{\circ }$. This result naturally provides an explanation for the long puzzling photometric and emission line strength variations but is difficult to reconcile with the imaging. Confirmation that colliding winds are present is found by the detection of variable excess emission in two of the lines where it would be most expected. The changing shapes of this excess emission are not perfectly phase-locked though, which will complicate future modelling.

Hubble Space Telescope Imaging Reveals That SN 2015bh Is Much Fainter than Its Progenitor

We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four years after the peak of the explosion. In 2017–2018, the transient fades steadily in optical filters before declining more slowly to F814W = −7.1 mag in 2019, ≈4 mag below the level of its eruptive luminous blue variable (LBV) progenitor observed with HST in 2008–2009. The source fades at a constant color of F555W − F814W = 0.4 mag until 2018, similar to SN 2009ip and consistent with a spectrum dominated by interaction of the ejecta with circumstellar material (CSM). A deep optical spectrum obtained in 2021 lacks signatures of ongoing interaction (L Hα ≲ 1038 erg s−1 for broadened emission ≲2000 km s−1), but indicates the presence of a nearby H ii region (≲300 pc). The color evolution of the fading source makes it unlikely that emission from a scattered-light echo or binary OB companion of the progenitor contributes significantly to the flattening of the late-time light curve. The remaining emission in 2019 may plausibly be attributed an evolved/inflated companion or an unresolved (≲3 pc), young stellar cluster. Importantly, the color evolution of SN 2015bh rules out scenarios in which the surviving progenitor is obscured by nascent dust and does not clearly indicate a transition to a hotter, optically faint state. The simplest explanation is that the massive progenitor did not survive. SN 2015bh likely represents a remarkable example of the terminal explosion of a massive star preceded by decades of end-stage eruptive variability.

Monitoring clumpy wind accretion in supergiant fast-X-ray transients with XMM-Newton

Supergiant fast-X-ray transients (SFXTs) are a sub-class of supergiant high-mass X-ray binaries hosting a neutron star accreting from the stellar wind of a massive OB companion. Compared to the classical systems, SFXTs display a pronounced variability in X-rays that has long been (at least partly) ascribed to the presence of clumps in the stellar wind. Here, we report on the first set of results of an ongoing XMM-Newton observational program searching for spectroscopic variability during the X-ray flares and outbursts of the SFXTs. The goal of the paper is to present the observational program and show that the obtained results are in agreement with expectations, with a number of flares (between one and four) generally observed per source and per observation (20 ks-long, on average). We base our work on a systematic and uniform analysis method optimized to consistently search for spectral signatures of a variable absorption column density, as well as other parameters of the spectral continuum. Our preliminary results show that the program is successful and the outcomes of the analysis support previous findings that most of the X-ray flares seem associated to the presence of a massive structure approaching and being accreted by the compact object. However, we cannot rule out that other mechanisms are at work together with clumps to enhance the X-ray variability of SFXTs. This is expected according to current theoretical models. The success of these observations shows that our observational program can be a powerful instrument to deepen our understanding of the X-ray variability in SFXTs. Further observations will help us to obtain a statistically robust sample. This will be required to conduct a systematic analysis of the whole SFXT class with the ultimate goal being to disentangle the roles of the different mechanisms giving rise to these events.

Determination of the mass of the neutron star in SMC X-1, LMC X-4, and Cen X-3 with VLT/UVES

We present the results of a spectroscopic monitoring campaign of the OB-star companions to the eclipsing X-ray pulsars SMC X-1, LMC X-4 and Cen X-3. High-resolution optical spectra obtained with UVES on the ESO Very Large Telescope are used to determine the radial-velocity orbit of the OB (super)giants with high precision. The excellent quality of the spectra provides the opportunity to measure the radial-velocity curve based on individual lines, and to study the effect of possible distortions of the line profiles due to e.g. X-ray heating on the derived radial-velocity amplitude. Several spectral lines show intrinsic variations with orbital phase. The magnitude of these variations depends on line strength, and thus provides a criterion to select lines that do not suffer from distortions. The undistorted lines show a larger radial-velocity amplitude than the distorted lines, consistent with model predictions. Application of our line-selection criteria results in a mean radial-velocity amplitude K(Opt) of 20.2 +/- 1.1, 35.1 +/- 1.5, and 27.5 +/- 2.3 km/s (1 sigma errors), for the OB companion to SMC X-1, LMC X-4 and Cen X-3, respectively. Adding information on the projected rotational velocity of the OB companion (derived from our spectra), the duration of X-ray eclipse and orbital parameters of the X-ray pulsar (obtained from literature), we arrive at a neutron star mass of 1.06^{+0.11}_{-0.10}, 1.25^{+0.11}_{-0.10} and 1.34^{+0.16}_{-0.14} M{sun} for SMC X-1, LMC X-4 and Cen X-3, respectively. The mass of SMC X-1 is near the minimum mass (~1 M{sun}) expected for a neutron star produced in a supernova. We discuss the implications of the measured mass distribution on the neutron-star formation mechanism, in relation to the evolutionary history of the massive binaries.

XMM–Newton X-ray observations of the Wolf–Rayet binary system WR 147

We present results of an ≈20-ks X-ray observation of the Wolf–Rayet (WR) binary system WR 147 obtained with XMM–Newton. Previous studies have shown that this system consists of a nitrogen-type WN8 star plus an OB companion whose winds are interacting to produce a colliding wind shock. X-ray spectra from the pn and MOS detectors confirm the high extinction reported from infrared studies and reveal hot plasma including the first detection of the Fe Kα line complex at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock model give a shock temperature kTshock= 2.7 keV (Tshock≈ 31 MK), close to but slightly hotter than the maximum temperature predicted for a colliding wind shock. Optically thin plasma models suggest even higher temperatures, which are not yet ruled out. The X-ray spectra are harder than can be accounted for using 2D numerical colliding wind shock models based on nominal mass-loss parameters. Possible explanations include: (i) underestimates of the terminal wind speeds or wind abundances, (ii) overly simplistic colliding wind models or (iii) the presence of other X-ray emission mechanisms besides colliding wind shocks. Further improvement of the numerical models to include potentially important physics such as non-equilibrium ionization will be needed to rigorously test the colliding wind interpretation.

Dust Formation around Wolf-Rayet Stars

We discuss the observational evidence in favour of dust formation around massive, evolved carbon-rich Wolf-Rayet stars. Large dust shells containing (amorphous carbon) grains are observed in persistent dust makers, such as the pinwheel system WR104 (WC9+OB) and periodic/episodic dust makers, such asWR140 (WC7+O) close to periastron. Problems with the nucleation and growth of dust in such harsh environments are discussed. It is probable that mixing of carbon-rich material from the WC star plus hydrogen from an OB companion, together with efficient shielding via the shocked region of their colliding winds, provides the necessary ingredients. Many problems with the nucleation and growth remain to be addressed.

Colliding winds in the WR+OB binary θ Muscae (WR 48, WC5+O6-7V)

Spectra, providing full phase coverage, of the 19 d WC6+OB binary θ Mus (WR 48, HD 113904), have been obtained and show dramatic variations of the C iii λ5696 emission line profile. We have modeled these line profile variations assuming the winds from the WR star and its close OB companion are colliding and forming a shock region from which extra emission originates.

The orbital modulation in the radio emission of Cygnus X-1

We present model light curves which have been created in order to explain the orbital modulation observed in the radio emission of Cyg X-1. We invoke variable absorption by the stellar wind as the black hole jet orbits around the OB companion star and find that a very simple model is able to reproduce the amplitudes and frequency dependence of the observed light curves.

Modelling the colliding-winds spectra of the 19-d WR + OB binary in the massive triple system Muscae

High signal-to-noise ratio, moderate-resolution spectra, providing full phase coverage of the 19-d WC6 + OB binary θ Mus (WR 48, HD 113904), have been obtained and show dramatic variations of the C III λ5696 emission-line profile. We have modelled these line profile variations using a purely geometrical model which assumes that the emission arises from two regions, an optically thin spherical shell around the WR star and a cone-shaped region that partially wraps around the OB star. The cone-shaped region represents the shock front arising from the collision between the winds of the two stars. This work builds upon our earlier study of WR 42 and WR 79, and uses a completely new code for the modelling, which includes the effects of turbulence. We now find much better agreement between the orbital inclination angles found for these stars with those determined using other methods. The fitting parameters found via modelling the C III λ5696 profile variations of θ Mus are used to infer that the OB companion most likely has a spectral type of 06V or 07V. The modelling presented here continues to show the exciting promise of a better understanding of WR star fundamental parameters.

WR 146 -- observing the OB-type companion

We present new radio and optical observations of the colliding-wind system WR 146 aimed at understanding the nature of the companion to the Wolf–Rayet (WR) star and the collision of their winds. The radio observations reveal emission from three components: the WR stellar wind, the non-thermal wind–wind interaction region and, for the first time, the stellar wind of the OB companion. This provides the unique possibility of determining the mass-loss rate and terminal wind velocity ratios of the two winds, independent of distance. Respectively, these ratios are 0.20±0.06 and 0.56±0.17 for the OB-companion star relative to the WR star. A new optical spectrum indicates that the system is more luminous than had been believed previously. We deduce that the ‘companion’ cannot be a single, low-luminosity O8 star as suggested previously, but is either a high-luminosity O8 star, or possibly an O8+WC binary system.

Further Evidence that 1RXS J170849.0−400910 Is an Anomalous X-Ray Pulsar

We report the results of two ROSAT HRI observations of the recently discovered ~11 s X-ray pulsar 1RXS J170849.0-400910. A refined position with a smaller error radius (10'' uncertainty) and a new spin-period measurement were obtained. These results allowed us to derive a period derivative of ~7 × 10-4 s yr-1 and to perform a photometric and spectroscopic study of the possible optical counterparts of the source. The limits derived from the optical-to-X-ray flux ratio exclude the presence of a massive OB companion. These findings, together with the nearly constant X-ray flux across observations spanning 3 years, strongly support the inclusion of this ~11 s pulsar in the class of anomalous X-ray pulsars.

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object.

Rapid variability of P Cygni lines in massive X-ray binaries

view Abstract Citations (10) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rapid Variability of P Cygni Lines in Massive X-Ray Binaries Kallman, Timothy R. ; McCray, Richard ; Voit, G. Mark Abstract In early-type X-ray binaries, the recombination time scales for ions such as C IV, Si IV, and N V in the stellar winds of their massive OB companion stars can be very short compared to typical X-ray pulsation or fluctuation time scales. Thus, the authors predict that the 'bleached zones', in which these ions are removed from the stellar wind by X-ray photoionization, will track the X-ray beam as it rotates and that the P Cygni line profiles due to these ions will pulsate. With the IUE satellite the authors obtained low-resolution UV spectra of the binary X-ray system HD 77581/4U 0900-40 resolved into six phase bins of the 283 s pulse period; the observed upper limit to the pulse amplitude of the equivalent width of C IV 1550 A was less than the predicted value. However, the pulsations should be easy to observe with the Hubble Space Telescope. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165322 Bibcode: 1987ApJ...317..746K Keywords: B Stars; Early Stars; O Stars; Spectral Line Width; Stellar Spectra; X Ray Binaries; Iue; Stellar Winds; Ultraviolet Spectra; Variability; Astrophysics; X-RAYS: BINARIES; STARS: WINDS; ULTRAVIOLET: SPECTRA; LINE PROFILES full text sources ADS | data products SIMBAD (1) MAST (1) INES (1)

Asymmetric Supernova Explosions: The Missing Link between Wolf-Rayet Binaries, Run-Away OB Stars and Pulsars

The WR binary systems, consisting of a WR star and an O or B star companion, are supposed to be the progenitors of the massive X-ray binaries. The missing link is generally accepted to be the SN explosion of the WR star which leaves a pulsar remnant. As most pulsars originate from single stars, observations of their proper motions indicate that they receive at their birth a “kick” velocity of about 100 km s−1. We assume this velocity to be due to the asymmetry of the SN explosion. This asymmetry, together with the loss of the SN shell and its impact on the OB star, may cause to disrupt the remaining system.For the ten best known WR binaries we evaluated the survival probability P after an instantaneous SN explosion, leaving a 1.5 M⊙ collapsar with a random orientated kick velocity of 75 kms−1 (case a) and 150 kms−1 (case b) respectively. The influence of the impact is found to be marginal. The run-away velocity of the remaining system and of the disrupted OB star are comparable and of the same order of magnitude, but smaller than the initial orbital velocity of the OB companion; which decreases for increasing values of the initial orbital period. They are found to be independent of the kick velocity.

Spectroscopic studies of Wolf-Rayet stars with absorption lines. I - The probable single WN5 star HD 193077

view Abstract Citations (63) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopic studies of Wolf-Rayet stars with absorption lines. I. The probable single WN5 star HD 193077. Massey, P. Abstract The WN5 Wolf-Rayet star HD 193077 has been called a binary because of the presence of absorption lines in its spectrum. I have studied this star at high dispersion and can set an upper limit on the orbital semiamplitude of 30 km s-1 for the emission lines and 80 km s-1 for the absorption lines, for periods less than 6 months. There is no evidence of velocity variations greater than this amount during the past 15 years. The absorption lines are rotationally broadened to 500 km s-1 (greater than that of any known early-type star), but have strengths normal for an 0 star. If these lines originate in an OB companion, then this high rotation velocity, combined with the limits on the velocity variations, sets very stringent limits on the allowed orbital inclinations and periods. Furthermore, there is evidence of hydrogen in the emission-producing envelope, with N(H)IN(He) ∼0.8, as large as that known for any early WN star. Taken together, these observations suggest that the absorption lines are intrinsic to the W-R star itself, and that HD 193077 is a newly arrived, single Wolf-Rayet star, although the possibility that it is a binary system with a highly eccentric orbit or extremely long period cannot be completely excluded. Publication: The Astrophysical Journal Pub Date: March 1980 DOI: 10.1086/157770 Bibcode: 1980ApJ...236..526M Keywords: Absorption Spectra; Astronomical Spectroscopy; Binary Stars; Line Spectra; O Stars; Stellar Spectra; Wolf-Rayet Stars; Emission Spectra; Radial Velocity; Stellar Evolution; Astronomy full text sources ADS | data products SIMBAD (15) Related Materials (7) Part 2: 1980ApJ...242..638M Part 3: 1981ApJ...244..157M Part 4: 1981ApJ...244..169M Part 5: 1981ApJ...244..173M Part 6: 1981ApJ...245..195M Part 7: 1981ApJ...246..145M Part 8: 1984ApJ...282..693C