arXiv subjects

A. A. C. Sander

Publications and source records attributed to A. A. C. Sander.

At least 73 records · Page 4Linked to original sources

arXiv2022

The Link between Hot and Cool Outflows

The link between hot and cool stellar outflows is shown to be critical for correctly predicting the masses of the most massive black holes (BHs) below the so-called pair-instability supernova (PISN) mass gap. Gravitational Wave (GW) event 190521 allegedly hosted an "impossibly" heavy BH of 85 Solar Masses. Here we show how our increased knowledge of both metallicity Z and temperature dependent mass loss is critical for our evolutionary scenario of a low-Z blue supergiant (BSG) progenitor of an initially approx 100 Solar Masses star to work. We show using MESA stellar evolution modelling experiments that as long as we can keep such stars above 8000 K such low-Z BSGs can avoid strong winds, and keep a very large envelope mass intact before core collapse. This naturally leads to the Cosmic Time dependent maximum BH function below the PISN gap.

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arXiv2022

The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV 476: A comprehensive analysis reveals surprisingly low stellar masses

Massive stars at low metallicity are among the main feedback agents in the early Universe and in present-day star forming galaxies. When in binaries, these stars are potential progenitors of gravitational-wave events. Knowledge of stellar masses is a prerequisite to understanding evolution and feedback of low-metallicity massive stars. Using abundant spectroscopic and photometric measurements of an outstandingly bright eclipsing binary, we compare its dynamic, spectroscopic, and evolutionary mass estimates and develop a binary evolution scenario. We comprehensively studied the eclipsing binary system, AzV 476, in the Small Magellanic Cloud. The light curve and radial velocities were analyzed to obtain the orbital parameters. The photometric and spectroscopic data in the UV and optical were analyzed using the Potsdam Wolf-Rayet model atmospheres. The obtained results are interpreted using binary-evolution tracks. AzV 476 consists of an O4IV-III((f))p primary and an O9.5:Vn secondary. Both components have similar current masses (~20 M$_{\odot}$) obtained from both the orbital and spectroscopic analysis. The wind mass-loss rate of log($\dot{M}$/(M$_{\odot}$/yr))=-6.2 of the primary is a factor of ten higher than a recent empirical prescription for single O stars in the SMC. Only close-binary evolution with mass transfer can reproduce the current stellar and orbital parameters. The binary evolutionary model reveals that the primary has lost about half of its initial mass and is already core helium burning. Our comprehensive analysis of AzV 476 yields a consistent set of parameters and suggests previous case B mass transfer. The derived stellar masses agree within their uncertainties. The moderate masses of AzV 476 underline the scarcity of bright massive stars in the SMC. The core helium burning nature of the primary indicates that stripped stars might be hidden among OB-type populations.

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arXiv2021

A meeting at z~3: Young massive galaxies and an AGN within 30kpc of the luminous QSO LBQS0302-0019

Contrary to expectations from scenarios of black hole growth driven by galaxy interactions and mergers, dual active galactic nuclei (AGN) with kiloparsec separations are rarely observed and are very difficult to identify, in particular at high redshifts (i.e. z>2). Focussing on the recently discovered dual AGN system LBQS 0302-0019 at z=3.29, we seek to identify further group members in its environment and to understand their formation history through deep high-angular-resolution imaging. We present deep Hubble Space Telescope (HST) Wide-field Camera 3 near-infrared imaging of LBQS 0302-0019. In combination with ground-based VLT/HAWK-I imaging, we infer accurate sizes, colours, ages, and stellar masses of companion galaxies. We clearly detect four companion objects close to LBQS 0302-0019 that also have faint signatures in the ground-based images. We constrain light-weighted ages and masses for the two most prominent companions, Jil1 and Jil2, to $t_\star=252_{-109}^{+222}$Myr with $\log(M_\star/[\mathrm{M}_\odot])= 11.2_{-0.1}^{+0.3}$ and $t_{\star}=19_{-14}^{+74}$Myr with $\log(M_\star/[\mathrm{M}_\odot])= 9.4_{-0.4}^{+0.9}$, respectively. The HST data also show that the obscured AGN, previously identified by strong nebular HeII emission, is associated with the young massive companion Jil2. Because very massive stars of the starburst cannot be solely responsible for the HeII emission, we strengthen our initial conclusion that Jil2 has been hosting an AGN. If the young starburst of Jil2 had been accompanied by sustained black hole growth, Jil2 may have contributed HeII-ionising flux to create the large HeII Ly$α$ proximity zone around LBQS 0302-0019. Hence, the duration of the current luminous AGN episode of LBQS 0302-0019 may have been overestimated.

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arXiv2020

Properties of OB star-black hole systems derived from detailed binary evolution models

The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify the contribution of binary evolution to the formation of double black holes. A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples such as the population in 30 Doradus in the Large Magellanic Cloud (LMC). Methods. To this purpose, we analyse a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40 Msun, and identify which model systems potentially evolve into a binary consisting of a black hole and a massive main sequence star. We then derive the observable properties of such systems, as well as peculiarities of the OB star component. We find that about 3% of the LMC late O and early B stars in binaries are expected to possess a black hole companion, when assuming stars with a final helium core mass above 6.6 M to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these may be identified in spectroscopic binaries, either by large amplitude radial velocity variations ( > 50 km s ) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity ( > 10 km/s ) and simultaneously rapid rotation of the OB star. The predicted mass ratios are such that main sequence companions could be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be/X-ray binaries, and known massive BH binaries supports our conclusion. We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general, and for the formation of double-black hole mergers in particular.

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arXiv2020

Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity

Classical Wolf-Rayet (WR) stars are massive, hydrogen depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity Z, stars exceeding a certain initial mass M_single(Z) can reach the WR phase through intrinsic mass-loss (single-star channel). Stars of lower masses can reach the WR phase via binary mass transfer (binary channel). It is commonly assumed that the binary channel dominates the formation of WR stars in environments with low Z such as the SMC and LMC. However, their reported WR binary fractions of 30-40% are comparable to that of the Galaxy. Here, we explain this apparent contradiction by considering the minimum initial mass M_spec(Z) needed for the stripped product to appear as a WR star. We calibrate M_spec(Z) using the lowest-luminosity WR stars in the Clouds and the Galaxy. A range of M_single(Z) values are explored using various evolution codes. We estimate the additional contribution of the binary channel by considering the interval [M_spec(Z), M_single(Z)], which characterises the initial-mass range in which binaries can form additional WR stars. Results: The WR-phenomenon ceases below luminosities of logL = 4.9, 5.25, and 5.6 [Lsun] in the Galaxy, LMC, and SMC, which translates to He-star masses of 7.5, 11, 17 Msun and initial masses of M_spec = 18, 23, 37 Msun. Stripped stars with lower initial masses in the respective galaxies would tend to not appear as WR stars. M_single lies in the range 20-30, 30-60, and > 40 Msun for the Galaxy, LMC, and SMC. We find that that the additional contribution of the binary channel is a non-trivial function of Z that cannot be conclusively claimed to be monotonically increasing with decreasing Z. Hence, one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low Z, or that the WR binary fraction grows with decreasing Z.

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arXiv2020

The stellar and wind parameters of six prototypical HMXBs and their evolutionary status

High-mass X-ray binaries (HMXBs) are exceptional astrophysical laboratories that offer a rare glimpse into the physical processes that govern accretion on compact objects, massive-star winds, and stellar evolution. In a subset of the HMXBs, the compact objects accrete matter solely from winds of massive donor stars. These so-called wind-fed HMXBs are divided in persistent HMXBs and supergiant fast X-ray transients (SFXTs) according to their X-ray properties. While it has been suggested that this dichotomy depends on the characteristics of stellar winds, they have been poorly studied. With this investigation, we aim to remedy this situation by systematically analyzing donor stars of wind-fed HMXBs that are observable in the UV, concentrating on those with neutron star (NS) companions. We obtained Swift X-ray data, HST UV spectra, and additional optical spectra for all our targets. Our multi-wavelength approach allows us to provide stellar and wind parameters for six donor stars (four wind-fed systems and two OBe X-ray binaries). The wind properties are in line with the predictions of the line-driven wind theory. Three of the donor stars are in an advanced evolutionary stage, while for some of the stars, the abundance pattern indicates that processed material might have been accreted. When passing by the NS in its tight orbit, the donor star wind has not yet reached its terminal velocity but it is still significantly slower; its speed is comparable with the orbital velocity of the NS companion. There are no systematic differences between the two types of wind-fed HMXBs (persistent versus transients) with respect to the donor stars. For the SFXTs in our sample, the orbital eccentricity is decisive for their transient X-ray nature. Based on the orbital parameters and the further evolution of the donor stars, the investigated HMXBs will presumably form Thorne-Żytkow objects in the future.

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arXiv2019

Vela X-1 as a laboratory for accretion in High-Mass X-ray Binaries

Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a prototype of wind-accreting HMXB and it has been used as a baseline in different theoretical or modelling studies. We discuss the observational properties of the system and the use of the observational data as laboratory to test recent developments in modelling the accretion process in High-Mass X-ray Binaries (e.g., Sander et al. 2018; El Mellah et al. 2018), which range from detailed descriptions of the wind acceleration to modelling of the structure of the flow of matter close to the neutron star and its variations.

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arXiv2019

Testing massive star evolution, star-formation history and feedback at low metallicity : Spectroscopic analysis of OB stars in the SMC Wing

Stars which start their lives with spectral types O and early-B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 OB stars in the Small Magellanic Cloud. The data, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models. We find that stellar winds of our sample stars are much weaker than theoretically expected. The stellar rotation rates show a bi-modal distribution. The well-populated upper Hertzsprung-Russell diagram including our sample OB stars from SMC Wing as well as additional evolved stars all over SMC from the literature shows a strict luminosity limit. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with Minit<30M$_{\odot}$ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with Minit>30M$_{\odot}$ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We report extended star-formation episodes in a quiescent low-density region of the Wing, which is progressing stochastically. We measure the key parameters of stellar feedback and establish the links between the rates of star formation and supernovae. Our study reveals that in metal-poor environments the stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars.

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arXiv2019

The Galactic WN stars revisited. Impact of Gaia distances on fundamental stellar parameters

Comprehensive spectral analyses of the Galactic Wolf-Rayet stars of the nitrogen sequence (i.e.\ the WN subclass) have been performed in a previous paper. However, the distances of these objects were poorly known. Distances have a direct impact on the "absolute" parameters, such as luminosities and mass-loss rates. The recent Gaia Data Release (DR2) of trigonometric parallaxes includes nearly all WN stars of our Galactic sample. In the present paper, we apply the new distances to the previously analyzed Galactic WN stars and rescale the results accordingly. On this basis, we present a revised catalog of 55 Galactic WN stars with their stellar and wind parameters. The correlations between mass-loss rate and luminosity show a large scatter, for the hydrogen-free WN stars as well as for those with detectable hydrogen. The slopes of the $\log L - \log \dot{M}$ correlations are shallower than found previously. The empirical Hertzsprung-Russell diagram (HRD) still shows the previously established dichotomy between the hydrogen-free early WN subtypes that are located on the hot side of the zero-age main sequence (ZAMS), and the late WN subtypes, which show hydrogen and reside mostly at cooler temperatures than the ZAMS (with few exceptions). However, with the new distances, the distribution of stellar luminosities became more continuous than obtained previously. The hydrogen-showing stars of late WN subtype are still found to be typically more luminous than the hydrogen-free early subtypes, but there is a range of luminosities where both subclasses overlap. The empirical HRD of the Galactic single WN stars is compared with recent evolutionary tracks. Neither these single-star evolutionary models nor binary scenarios can provide a fully satisfactory explanation for the parameters of these objects and their location in the HRD.

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arXiv2019

The extreme O-type spectroscopic binary HD 93129A

HD 93129A was classified as the earliest O-type star in the Galaxy (O2~If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum. We analyzed UV and optical observations acquired with the Hubble Space Telescope and ESO's Very Large Telescope. A multiwavelength analysis of the system was performed using the latest version of the Potsdam Wolf-Rayet model atmosphere code. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive $\log (L/L_\odot) = 6.15$, $T_{\textrm{eff}}=52$ kK, and $\log \dot{M}=-4.7 [M_\odot\text{yr}^{-1}]$ for the primary Aa, and $\log (L/L_\odot)=5.58$, $T_{\textrm{eff}}=45$ kK, and $\log\dot{M}=-5.8 [M_\odot\text{yr}^{-1}]$ for the secondary Ab. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2~If* and O3~III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system.

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arXiv2019

Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars

Context. Metal-poor massive stars are supposed to be progenitors of certain supernovae, gamma-ray bursts and compact object mergers, potentially contributing to the early epochs of the Universe with their strong ionizing radiation. However, they remain mainly theoretical as individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the SMC. Aims. This work aims at exploring what our state-of-the-art theories of stellar evolution combined with those of stellar atmospheres predict about a certain type of metal-poor (0.02 Z$_{\odot}$) hot massive stars, the chemically homogeneously evolving ones, called TWUIN stars. Methods. Synthetic spectra corresponding to a broad range in masses (20-130 M$_{\odot}$) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage were computed. Results. We find that TWUIN stars show almost no emission lines during most of their {core hydrogen-burning} lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission and even some metal lines (oxygen and carbon, but still almost no nitrogen) show up. Mass loss and clumping play a significant role in line-formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early O type star observed in a low-metallicity galaxy could be the outcome of chemically homogeneous evolution $-$ and therefore the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe due to their being hot without showing prominent emission lines during the majority of their lifetimes.

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arXiv2018

On the apparent absence of WR+NS systems: the curious case of WR124

Among different types of massive stars in advanced evolutionary stages is the enigmatic WN8h type. There are only a few Wolf-Rayet (WR) stars with this spectral type in our Galaxy. It has long been suggested that WN8h-type stars are the products of binary evolution that may harbor neutron stars (NS). One of the most intriguing WN8h stars is the runaway WR124 surrounded by its magnificent nebula M1-67. We test the presence of an accreting NS companion in WR124 using 100 ks long observations by the Chandra X-ray observatory. The hard X-ray emission from WR124 with a luminosity of $L_\mathrm{X}$= 10$^{31}$ erg s$^{-1}$ is marginally detected. We use the non-LTE stellar atmosphere code PoWR to estimate the WR wind opacity to the X-rays. The wind of a WN8-type star is effectively opaque for X-rays, hence the low X-ray luminosity of WR124 does not rule out the presence of an embedded compact object. We suggest that, in general, high opacity WR winds could prevent X-ray detections of embedded NS, and be an explanation for the apparent lack of WR+NS systems.

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arXiv2018

PoWR grids of non-LTE model atmospheres for OB-type stars of various metallicities

The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website.

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arXiv2018

Formation of wind-captured discs in Supergiant X-ray binaries : consequences for Vela X-1 and Cygnus X-1

In Supergiant X-ray binaries (SgXB), a compact object captures a fraction of the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the efficiency of mass and angular momentum accretion but they depend so dramatically on the wind speed that given the current uncertainties, they only set loose constrains. Furthermore, they often bypass the impact of orbital and shock effects on the flow structure. We study the wind dynamics and the angular momentum gained as the flow is accreted. We identify the conditions for the formation of a disc-like structure around the accretor and the observational consequences for SgXB. We use recent results on the wind launching mechanism to compute 3D streamlines, accounting for the gravitational and X-ray ionizing influence of the compact companion on the wind. Once the flow enters the Roche lobe of the accretor, we solve the hydrodynamics equations with cooling. A shocked region forms around the accretor as the flow is beamed. For wind speeds of the order of the orbital speed, the shock is highly asymmetric compared to the axisymmetric bow shock obtained for a purely planar homogeneous flow. With net radiative cooling, the flow always circularizes for wind speeds low enough. Although the donor star does not fill its Roche lobe, the wind can be significantly beamed and bent by the orbital effects. The net angular momentum of the accreted flow is then sufficient to form a persistent disc-like structure. This mechanism could explain the proposed limited outer extension of the accretion disc in Cygnus X-1 and suggests the presence of a disc at the outer rim of the neutron star magnetosphere in Vela X-1, with dramatic consequences on the spinning up of the accretor.

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arXiv2018

The Shortest-period Wolf-Rayet binary in the Small Magellanic Cloud: Part of a high-order multiple system

SMC AB 6 is the shortest-period (6.5d) Wolf-Rayet (WR) binary in the Small Magellanic Cloud, and is therefore crucial for the study of binary interaction and formation of WR stars at low metallicity. The WR component in AB 6 was previously found to be very luminous (logL=6.3[Lsun]) compared to its reported orbital mass (8Msun), placing it significantly above the Eddington limit. Through spectroscopy and orbital analysis of newly acquired optical data taken with UVES, we aim to understand the peculiar results reported for this system and explore its evolutionary history. Results: We find that AB 6 contains at least four stars. The 6.5d period WR binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating early O-type companion (O5.5 V, star B). Static N and He lines suggest the presence of an emission line star (O5.5 I(f), star C). Finally, narrow absorption lines portraying a long-term radial velocity variation show the existence of a fourth star (O7.5 V, star D). Star D appears to form a second 140d period binary together with a fifth stellar member, which is a B-type dwarf or a black hole. It is not clear that these additional components are bound to the WR binary. The WR star is found to be less luminous than previously thought (logL = 5.9[Lsun]) and, adopting 41Msun for star B, more massive (18Msun). Correspondingly, the WR star does not exceed the Eddington limit. We derive the initial masses of 60 and 40Msun for stars A and B and an age of 3.9 Myr for the system. The WR binary likely experienced nonconservative mass transfer in the past supported by the relatively rapid rotation of star B. Conclusion: Our study shows that AB 6 is a multiple -- probably quintuple -- system. This finding resolves the previously reported puzzle of the WR primary exceeding the Eddington limit and suggests that the WR star exchanged mass with its companion in the past.

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arXiv2018

Stellar population of the superbubble N206 in the LMC II. Parameters of the OB and WR stars, and the total massive star feedback

Clusters or associations of early-type stars are often associated with a 'superbubble' of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud exhibits a superbubble and a rich massive star population. We observed these massive stars using the FLAMES multi-object spectrograph at ESO-VLT. Available UV spectra from HST, IUE, and FUSE are also used. The spectral analysis is performed with Potsdam Wolf-Rayet (PoWR) model atmospheres. We present the stellar and wind parameters of the OB stars and the two WR binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The HRD of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is ~$2.3\times10^{52}$ erg, which exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.

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arXiv2017

The clumpy absorber in the high-mass X-ray binary Vela X-1

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase ~0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium and neon whose strengths and presence depend on the overall level of absorption. They imply a co-existence of cool and hot gas phases in the system that we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.

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arXiv2017

Observational properties of massive black hole binary progenitors

The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (PoWR), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. We provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.

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