Free Access
Issue
A&A
Volume 639, July 2020
Article Number L6
Number of page(s) 7
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202038275
Published online 10 July 2020

© ESO 2020

1. Introduction

The extraordinary binary system LS V +22 25 (LB-1 hereafter) was reported by Liu et al. (2019) to comprise a B-type star (the primary) in a 79-day orbit with a ≈70 M black hole (BH). This BH mass significantly exceeded mass measurements of massive stellar BHs, including the record-breaking measurements from gravitational-wave events (Abbott et al. 2019). The formation of such a massive BH, especially in solar metallicity environments, challenges our current understanding of stellar evolution, a problem that has been addressed by a series of recent studies (Groh et al. 2019; Eldridge et al. 2020; Safarzadeh et al. 2019; Belczynski et al. 2020).

The mass measurement of the BH by Liu et al. (2019) relies on the derived orbit and spectral properties of the primary and on radial velocity (RV) measurements of the wings of the Hα emission line. As Liu et al. (2019) notes, the Hα line appears to exhibit a low-amplitude, anti-phase motion relative to the primary star, leading the authors to speculate that it originates in an accretion disk around a ≈70 M BH secondary. However, Abdul-Masih et al. (2020) and El-Badry & Quataert (2020) have since independently shown that this anti-phase signature can be explained as a spurious result that emerges from the motion of the broad absorption wings of the B-type primary on top of a static Hα line. These results cast doubt on the measurement of the large mass of the secondary from Liu et al. (2019). The derived binary mass function of 1.2 M sets a definite lower limit on the mass of the unseen secondary. However, its nature remains unknown.

Simón-Díaz et al. (2020) performed a spectroscopic analysis of LB-1 while relaxing the assumption of local thermodynamic equilibrium (non-LTE). They classified the primary star as B6 IV and derived significantly lower values for the effective temperature (Teff = 14 kK) and surface gravity (logg = 3.5 [cgs]) compared with Liu et al. (2019). Similar values were reported from an LTE analysis performed by Abdul-Masih et al. (2020).

Even lower values of Teff = 12.7 kK and logg = 3.0 [cgs] were recently reported by Irrgang et al. (2020), who utilised hybrid LTE and non-LTE models. They suggested that the unseen secondary component is a neutron star or a faint main-sequence star and that the primary star is a stripped helium star that lost its H-rich envelope due to a past binary mass-transfer event. This claim was based on anomalous abundances derived for the star, consistent with processed material from the CNO cycle (enriched N, depleted C and O), also confirmed by Simón-Díaz et al. (2020). A notable peculiarity that was consistently reported in the aforementioned studies is the sub-solar abundance derived for several heavy elements such as Mg, Si, and Fe, which are not expected to change throughout the evolution of the star. This led Abdul-Masih et al. (2020) to speculate that the secondary may be a rapidly rotating star that dilutes some of the primary spectral lines, leading to an apparent sub-solar metal content.

In this Letter, we present firm evidence that the unseen secondary is not a compact object, but a rapidly-rotating Be star, and that the properties of the primary star in LB-1 are indeed consistent with it being a stripped star. We show this through the spectral disentangling and analysis of 26 recently obtained optical spectra that cover the full 79 d orbit.

2. Observations and data reduction

From December 2019 to March 2020, we obtained 24 epochs of observations with the HERMES spectrograph mounted on the Mercator telescope (Raskin et al. 2011) that uniformly sample the orbit of LB-1. HERMES spectra cover the wavelength range from 3770 to 9000 Å with a spectral resolving power of ∼85 000. Additionally, we obtained two epochs with the FEROS spectrograph (Kaufer et al. 1999) mounted on the MPG/ESO 2.2 m telescope at the La Silla observatory. The wavelength coverage of FEROS ranges from 3500 to 9200 Å with a spectral resolving power of 48 000. The journal of the observations, including typical signal-to-noise ratios (S/N), is given in Table A.1.

Standard calibrations, including bias and flat-field corrections and wavelength calibrations, were performed for both data sets using their respective pipelines. Barycentric correction was applied. On some of the nights, multiple observations were taken consecutively (see Table A.1). Concomitant spectra obtained on the same night were combined using a S/N-weighted average. The spectra were individually normalized by fitting a spline through anchor points in the continuum of each spectrum.

3. Orbital analysis

3.1. The orbit of the primary

We measured the RVs of the primary in the 26 available spectra using the method described in Sana et al. (2013), which relies on Gaussian fitting to the spectral lines simultaneously at all observing epochs. Here, we used three high-S/N He I lines at λλ 4713, 5015, and 6678 Å.

We subsequently fitted an orbit to the RV data (Fig. A.1) using the PYTHON package SPINOS1. Given that the derived eccentricity of the orbit (e = 0.0036 ± 0.0021) is not significant (Lucy & Sweeney 1971), we adopted a circular orbit. As our orbital solution (root-mean-square; rms = 0.86 km s−1) is in good agreement with the orbital solution presented in Liu et al. (2019), we joined the two data sets, which, when combined, gave us a time base of 4 yr, yielding an unweighted rms of 2.7 km s−1. The best-fit parameters using the combined data set are included in Table 1.

Table 1.

Orbital parameters derived based on our orbital analysis and grid disentanglement, along with their 1σ errors (upper) and estimated physical parameters of the components of LB-1 based on a spectral analysis with GSSP and PoWR (lower).

3.2. Evidence for Balmer emission tracing the primary

The morphologies of the Balmer emission lines in LB-1 are complex and variable. Liu et al. (2019) suggested that the Hα line comprises two contributions: a broad Hα emission that moves with the putative BH secondary in anti-phase to Hα absorption from the primary. Here, we show that the primary appears to exhibit Hα emission, and not absorption.

To illustrate this, Fig. A.2 shows dynamic spectra of the Hα and He Iλ6678 lines, demonstrating a complex emission pattern that traces the orbit of the primary. Figure A.2 also shows two HERMES spectra taken close to quadrature, in which the narrow Hα emission is shown to follow the orbit of the primary. This is in contrast to the behaviour of the Hα-line wings, which appear to exhibit a slight anti-phase motion. However, it is not readily clear if this anti-phase motion contains a reflex motion of the secondary (Abdul-Masih et al. 2020; El-Badry & Quataert 2020).

4. Spectral disentangling: unveiling the companion

The fact that in previous studies there was no companion to the primary star to be readily seen in the spectra was interpreted as evidence for the presence of a dark companion (i.e. neutron star or BH). However, within the S/N levels, it is possible that a combination of rapid rotation and low light contribution could push a non-degenerate stellar companion below the detection threshold (Abdul-Masih et al. 2020).

To extract the spectra of the individual components in LB-1 from the observations, we utilise spectral disentangling using two different methods: the shift-and-add technique in wavelength space and disentangling in Fourier space. These methods are described below and the comparison of their results offers a consistency check of the robustness of the disentangling process. Both methods assume prior knowledge of the RVs of the two binary components. Since the orbit of the secondary object is not constrained, we perform a disentangling along the K2-axis. We infer K2 by minimising the reduced χ2(K2). The scaling of the final disentangled spectra depends on the light contribution of each component (see Appendix B). However, the χ2 statistic is independent of the light ratio.

4.1. Disentangling using shift-and-add

Shift-and-add is a widely used disentangling technique (e.g. Marchenko et al. 1998; González & Levato 2006; Mahy et al. 2012; Shenar et al. 2018). It is an iterative procedure that uses the disentangled spectra obtained in the jth iteration, Aj and Bj, to calculate the disentangled spectra for the j + 1th iteration.

Fixing the orbital parameters (Table 1), we evaluate the reduced χ2(K2) for a series of K2 values in the range of 0 to 100 km s−1 in steps of ΔK2 = 0.5 km s−1. We consider different wavelength ranges: the full spectrum, He I lines, and Balmer lines. The Hα line implies a relatively well-defined minimum at K2  ≈  10 km s−1. The Hβ, Hγ, and Hδ lines imply similar values, but due to their much lower S/N, their minima are poorly localised (Fig. A.3). A parabola fit to the minimum region in the combined reduced χ2 of all Balmer lines yields K2 = 11.3 ± 1.4 km s−1, where the 1σ error is calculated from the corresponding χ2(K2) contour. The disentangled spectra are shown in Fig. 1.

thumbnail Fig. 1.

Disentangled spectra obtained with the shift-and-add technique. The spectrum of the Be secondary is binned at Δλ = 0.1 Å for clarity. The disentangled spectra obtained with FDBinary (Fourier disentangling) are identical within the S/N. The spectra are scaled assuming the light ratios 55% and 45% for the primary and secondary, respectively, which is justified in Appendix B.1.

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4.2. Fourier disentangling with FDBinary

Unlike the shift-and-add technique, which works in wavelength space, the FDBinary disentangling tool operates in Fourier space. The method was developed by Hadrava (1995) and implemented in FDBinary by Ilijic et al. (2004), where a detailed description can be found.

Using the orbital parameters listed in Table 1, we perform a χ2 minimisation by varying K2. Errors are estimated by running 3000 Monte Carlo samples of the minimisation process, while altering the spectra and the orbit by adding white noise. Our grid ranges from 0.5 to 25 km s−1 in steps of 0.5 km s−1 for K2. Instead of disentangling the entire wavelength range, we focus specifically on Balmer and He I lines. We find that the minimal χ2 distance is achieved for K2 = 11.2  ±  1.0 km s−1 (1σ confidence interval).

4.3. Disentangled spectra of the two components

The results obtained by the two different techniques in Sects. 4.1 and 4.2 agree well. The derived semi-amplitudes K2 are consistent within the 1σ errors and the spectra for the two components derived from both methods are consistent within the S/N. The derived K2 value of 11 km s−1 also agrees with the recent independent measurement provided by Liu et al. (2020).

As is evident from Fig. 1, our algorithm successfully separates the observed spectra into two stellar components. The first exhibits a multitude of narrow lines and is clearly associated with the primary star seen in the individual observations. We note the Hα and Hβ emission features observed for the primary (see Sect. 3.2). In contrast, the secondary component exhibits significantly broadened lines belonging primarily to the Balmer series and to He I. The Balmer lines portray double-peak emission profiles that are characteristic of classical Be stars: B-type stars possessing a decretion disk (e.g. Rivinius et al. 2013).

We provide a spectral classification and analysis of both components in Appendix B. The derived parameters are given in Table 1. The properties of the system suggest that LB-1 is a Be binary that formed through a past mass-transfer event (Pols et al. 1991; de Mink et al. 2014), currently comprising a stripped star (the mass donor) and a critically rotating Be star (the mass accretor). In contrast to Liu et al. (2019), we find no evidence for the presence of a compact object in LB-1, nor do we find evidence that the Be component is a static tertiary, as has recently been proposed by Rivinius et al. (2020).

5. Conclusions

We performed an orbital analysis and spectral disentangling of the intriguing LB-1 binary system using newly-acquired spectroscopic observations that cover its 79-day orbit. We show that the binary does not contain a compact object, but rather, it consists of two non-degenerate components: a stripped primary and a rapidly rotating B3 Ve secondary.

The orbital inclination and the mass of the primary can be estimated by calibrating the mass of the B3 Ve secondary to a typical value for its spectral type and estimated parameters. Adopting M2  =  7 ± 2 M (Cox 2000) for the B3 Ve secondary, we obtain a narrow constraint on the inclination of 39 ± 4°. This, in turn, implies that the equatorial rotational velocity of the Be star is veq  ≈  470 km s−1, close to critical (cf. Townsend et al. 2004). The orbital mass of the stripped star is then M1  =  1.5 ± 0.4 M, making it a potential core-collapse supernova progenitor (e.g. Zapartas et al. 2019).

LB-1 thus represents a Be binary system in which the Be secondary has formed through a previous mass transfer event, having gained mass from the originally more massive primary star. This is a clear example of how binary interactions act as an important agent in producing rapid stellar rotators and Be stars (e.g. Pols et al. 1991; de Mink et al. 2013).

The low temperature of the stripped primary implies that it is thermally unstable, most likely contracting towards the He main sequence. LB-1 is therefore a rare progenitor to Be binaries that host subdwarf stars, such as ϕ Per (Gies et al. 1998; Schootemeijer et al. 2018) and o Pup (Koubský et al. 2012), potentially the most massive of its kind detected to date. These results pave the way to tailored binary evolution modeling of the LB-1 system, which will shed new light onto the evolutionary status of this extraordinary object and contribute to our understanding of binary-interaction processes.


Acknowledgments

Based on observations obtained with the HERMES spectrograph, which is supported by the Research Foundation – Flanders (FWO), Belgium, the Research Council of KU Leuven, Belgium, the Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Genève, Switzerland and the Thüringer Landessternwarte Tautenburg, Germany. The authors gratefully acknowledge MPIA, Heidelberg for access to the FEROS spectrograph for use in this work. The authors acknowledge support from the European Research Council (ERC) innovation programme of the Horizon 2020 (programmes DLV-772225-MULTIPLES and ERC-AdG-670519-MAMSIE), and the FWO Odysseus program under project G0F8H6N. TS acknowledges helpful discussions with Helge Todt and Avishai Gilkis.

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Appendix A: Supplementary material

thumbnail Fig. A.1.

Best-fit orbital solution for a circular orbit overlaid with the RV measurements from HERMES and FEROS and the data from Liu et al. (2019). For our data, the errors are smaller than the symbol size (see Table A.1).

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thumbnail Fig. A.2.

Top: dynamical spectra of the Hα (left) and He Iλ6678 (right) lines in Doppler space phased using the ephemeris given in Table 1, compared to the derived orbit (red line). The Hα line is mean-subtracted to enhance the contrast. Bottom: two HERMES spectra taken close to quadrature (see legend) of the Hα and He Iλ6678 lines, as in the top panel. The Hα line exhibits a narrow emission peak on top that traces the orbit of the primary star.

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thumbnail Fig. A.3.

Reduced χ2(K2) obtained through shift-and-add grid disentangling for the Hα, Hβ, and Hγ lines (see legend), normalised to unity to allow for an easy comparison between the different curves.

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Table A.1.

Journal of the observations of LB-1.

thumbnail Fig. A.4.

Absolute values of the reduced χ2(K2) for Hα and Hβ.

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Appendix B: Spectral analysis

B.1. The stripped primary and the light ratio

Without eclipses or further constraints on photometry, the spectra can only be disentangled up to a scaling factor that reflects the light ratio of the two components. Before determining the parameters of the two stars, it is necessary to estimate the contribution of each star to the total flux. We find that for the primary, the depths of the narrow lines belonging to heavy elements (e.g. S, Si, Fe) are consistent with solar abundance when scaling the primary spectrum to a contribution of about 55%, implying a 45% contribution for the Be secondary.

The disentangled, scaled spectrum of the primary is analysed using the Grid Search in Stellar Parameters (GSSP) tool (Tkachenko 2015), constructed with the LTE radiative transfer code SYNTHV (Tsymbal et al. 1996) from a grid of LLMODEL atmospheres (Shulyak et al. 2004). Given the degeneracy in the parameters, we perform the analysis based on the following procedure: we fix the metallicity to solar and set the microturbulence to vmicro  =  2 km s−1 while we fit for the macroturbulent velocity vmacro and the projected rotational velocity v sini. In a second step, we use diagnostic line ratios such as He I to Mg II and Si II to Si III to constrain Teff, and we rely on the wings of the Balmer and He I lines to estimate logg. Finally, we vary the He abundance to obtain a best-fit model that reproduces the strength of the He lines. In Fig. B.1, we compare the observed spectrum to the best-fit model as well as a model with the same parameters but a standard He abundance. Our best-fit estimates are given in Table 1. Typical GSSP errors are 2 kK on Teff and 0.2 dex on logg.

We confirm the comparably low Teff and logg values that were reported by Irrgang et al. (2020). Furthermore, we confirm the more widely accepted low rotational velocity reported previously. Our consistent fit implies that the sub-solar metallicity derived by Irrgang et al. (2020) for heavy elements was a spurious result caused by the dilution of the secondary star. Unlike Irrgang et al. (2020), we can consistently fit the wings and cores of the He I lines, having removed the contribution of the secondary component.

Despite the overall solar metallicity, we find that the number density ratio of He to H is about three times the solar value. Moreover, we confirm strong evidence of CNO-processed material (N enhanced, C and O depleted) as reported by Irrgang et al. (2020). This is evident from Fig. B.1, which shows a similar trend in the comparison of the observed CNO lines with the ones predicted by the best-fit model. The Balmer-line emission (see Fig. 1) is not considered in this analysis. It may originate in a wind or a disk around the stripped star, but it could also be indicative of a perturbation in the Be-star disk that mimics the primary’s orbit (e.g. due to the irradiation of the Be disk by the primary Liu et al. 2020).

thumbnail Fig. B.1.

Comparison between the disentangled spectrum of the primary with the best-fit GSSP model and a model of identical parameters but a solar He abundance (see legend). The three bottom panels focus on C, N, and O lines. The main spectral lines are indicated.

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thumbnail Fig. B.2.

Comparison between the disentangled spectrum of the Be secondary and other Be-type stars (see legend and text). The spectra are binned at Δλ = 0.2 Å for clarity.

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thumbnail Fig. B.3.

Comparison between the disentangled spectrum of the Be secondary (blue) with a PoWR model calculated with the parameters given in Table 1 (black dashed line). The region shown is least affected by disk emission.

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B.2. The Be-type secondary

To classify the Be-type secondary, we searched the HERMES archive for representative Be-type stars. Figure B.2 shows a comparison to HERMES observations of three Be stars: o Cas (B5 IVe, Slettebak 1982), HD 224559 (B4 Vne, Herman et al. 1959), and HD 37657 (B3 Ve, Guetter 1968). Since the spectral lines of o Cas and HD 37657 are somewhat narrower than observed for the secondary, we convolve their absorption-line profiles with rotational profiles that match the observed width of the Be secondary’s He I lines. The equivalent widths (EW) of the He I lines peak around B2-B3 V, as is also evident from Fig. B.2. Since the light ratio was constrained in Appendix B.1, we can use the absolute EW of the He I lines to obtain a rough estimate of the spectral type. Figure B.2 indicates that a reasonable match with the He I and Mg II lines is reached for a B3 Ve star. Hence, we classify the secondary as B3 Ve.

The Balmer emission depends primarily on the properties of the disk (e.g. density, inclination). As Fig. B.2 shows, the morphology of the Balmer lines is best matched with the B4 Ve star HD 224559. The Hγ line is stronger than typically observed, while Hβ shows little absorption compared to the other stars. However, these slight discrepancies are probably a result of the disentangling and normalisation procedure.

We refrain from utilising an LTE analysis for the hotter Be secondary, where non-LTE effects may become more important. Instead, we estimate its physical parameters by comparing its absorption spectrum to synthetic spectra calculated with the non-LTE Potsdam Wolf-Rayet (PoWR) atmosphere code (Hamann & Gräfener 2003; Sander et al. 2015), relying on pre-calculated grids (Hainich et al. 2019) extended for our purpose. We find a good match for Teff = 18 ± 2 kK, logg = 4.0 ± 0.3 [cgs], and vsini = 300 ± 50 km s−1 (Fig. B.3). The reddening and luminosities of the two components are estimated by fitting the observed spectral energy distribution of LB-1 to the sum of two PoWR models calculated with the parameters given in Table 1, assuming the Gaia distance of 2.13 kpc (Bailer-Jones et al. 2018). Despite being fainter in the visual, the Be secondary is the more luminous component due to its higher temperature. We note that the comparable light contribution of the two components provides a natural explanation for the lack of evidence of binary motion in the Gaia astrometry (see Simón-Díaz et al. 2020).

All Tables

Table 1.

Orbital parameters derived based on our orbital analysis and grid disentanglement, along with their 1σ errors (upper) and estimated physical parameters of the components of LB-1 based on a spectral analysis with GSSP and PoWR (lower).

Table A.1.

Journal of the observations of LB-1.

All Figures

thumbnail Fig. 1.

Disentangled spectra obtained with the shift-and-add technique. The spectrum of the Be secondary is binned at Δλ = 0.1 Å for clarity. The disentangled spectra obtained with FDBinary (Fourier disentangling) are identical within the S/N. The spectra are scaled assuming the light ratios 55% and 45% for the primary and secondary, respectively, which is justified in Appendix B.1.

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In the text
thumbnail Fig. A.1.

Best-fit orbital solution for a circular orbit overlaid with the RV measurements from HERMES and FEROS and the data from Liu et al. (2019). For our data, the errors are smaller than the symbol size (see Table A.1).

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In the text
thumbnail Fig. A.2.

Top: dynamical spectra of the Hα (left) and He Iλ6678 (right) lines in Doppler space phased using the ephemeris given in Table 1, compared to the derived orbit (red line). The Hα line is mean-subtracted to enhance the contrast. Bottom: two HERMES spectra taken close to quadrature (see legend) of the Hα and He Iλ6678 lines, as in the top panel. The Hα line exhibits a narrow emission peak on top that traces the orbit of the primary star.

Open with DEXTER
In the text
thumbnail Fig. A.3.

Reduced χ2(K2) obtained through shift-and-add grid disentangling for the Hα, Hβ, and Hγ lines (see legend), normalised to unity to allow for an easy comparison between the different curves.

Open with DEXTER
In the text
thumbnail Fig. A.4.

Absolute values of the reduced χ2(K2) for Hα and Hβ.

Open with DEXTER
In the text
thumbnail Fig. B.1.

Comparison between the disentangled spectrum of the primary with the best-fit GSSP model and a model of identical parameters but a solar He abundance (see legend). The three bottom panels focus on C, N, and O lines. The main spectral lines are indicated.

Open with DEXTER
In the text
thumbnail Fig. B.2.

Comparison between the disentangled spectrum of the Be secondary and other Be-type stars (see legend and text). The spectra are binned at Δλ = 0.2 Å for clarity.

Open with DEXTER
In the text
thumbnail Fig. B.3.

Comparison between the disentangled spectrum of the Be secondary (blue) with a PoWR model calculated with the parameters given in Table 1 (black dashed line). The region shown is least affected by disk emission.

Open with DEXTER
In the text

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