A&A 633, L5 (2020) Astronomy https://doi.org/10.1051/0004-6361/201937343 & c ESO 2020 Astrophysics LETTER TO THE EDITOR A stripped helium star in the potential black hole binary LB-1 A. Irrgang1, S. Geier2, S. Kreuzer1, I. Pelisoli2, and U. Heber1 1 Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg (FAU), Sternwartstr. 7, 96049 Bamberg, Germany e-mail: [email protected] 2 Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany Received 18 December 2019 / Accepted 1 January 2020 ABSTRACT +11 Context. The recently claimed discovery of a massive (MBH = 68−13 M ) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution. Aims. A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LS V+22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass. Methods. To this end, we study the spectral energy distribution of LS V+22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe. Results. Our analysis clearly shows that LS V+22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LS V+22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of 1:1 ± 0:5 M . Based on the binary system’s mass function, this yields a minimum mass of 2–3 M for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star. Conclusions. The star LS V+22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object. Key words. stars: abundances – stars: chemically peculiar – stars: early-type – pulsars: individual: LS V+22 25 1. Introduction the radial-velocity curve yielded a semi-amplitude K1 = 52:8 ± 0:7 km s−1 and a very small eccentricity e = 0:03 ± 0:01. Adopt- Stellar black holes are hard to find if they are not in interacting ing the above-mentioned mass for the visible B-star, the mass of +0:4 binaries, that is, in X-ray binaries. Recently, Giesers et al.(2018) the companion would have to be higher than 6:3−1:0 M . Since found a single-lined binary in the globular cluster NGC 3201 no spectral features of another star are detectable in the spectra, consisting of a turn-off star orbited by a detached compact object Liu et al.(2019) concluded that the companion cannot be a star, of minimum mass 4:36 ± 0:41 M in a highly eccentric orbit but instead has to be a black hole. with a period of 166:8 d. Because this minimum mass exceeds Based on the complex shape and substantial width the Oppenheimer-Volkoff limit, the discovery represents the first (∼240 km s−1) of the Hα emission line, the authors concluded direct mass estimate of a detached black hole in a globular that it can only be a Keplerian disk. Since its radial velocity cluster. does not follow the motion of the star, but shows a very small −1 Most recently, Liu et al.(2019) reported the discovery of variation (Kα = 6:4 ± 0:8 km s ) in anti-phase, the authors the long-period (single-lined) spectroscopic binary system LB-1 propose that the disk is connected to the black hole compan- (LS V+22 25). The visible component of this system is a bright ion. If so, the orbital inclination of the binary would be very ◦ B-type star with a prominent and broad Hα emission line very small (io = 15–18 ) and the black hole extremely massive +11 similar to a Be star. A fit of TLUSTY B-star model spectra (MBH = 68−13 M ). This result strongly challenges stellar evo- lead to an effective temperature Teff = 18 104 ± 825 K, a sur- lution models, which do not predict black holes of such high face gravity of log(g) = 3:43 ± 0:15, and a metallicity consis- masses to be formed in metal-rich environments (Liu et al. 2019; tent with the solar value. Comparing these atmospheric parame- Eldridge et al. 2019; Groh et al. 2019; Belczynski et al. 2019). ters with stellar evolution models of B-type main sequence (MS) The conclusion of Liu et al.(2019) rests on two main pillars: stars, the authors concluded that it is a young, massive star with first, the B-star is a massive MS star; and second, the emission +0:9 M = 8:2−1:2 M and R = 9 ± 2 R located in the direction of the line is connected to the compact companion. However the lat- Galactic anti-center at a distance of d = 4:23 ± 0:24 kpc. ter was severely questioned by Abdul-Masih et al.(2019) and The absorption lines of the star show radial-velocity varia- El-Badry & Quataert(2019) who argue that the radial velocity tions with a period P = 78:9 ± 0:3 d. Fitting a binary orbit to variation of the emission line is actually caused by the variability Article published by EDP Sciences L5, page 1 of9 A&A 633, L5 (2020) of the underlying absorption line of the visible star and is there- fore not related to the compact companion. Because the Hα emission appears to be stationary, it might result from a cir- 0.5 cumbinary disk. Consequently, the measured mass function 0 M sin3(i ) K3P f M 2 o −e2 3=2 1 : ± : M ; ( ) B 2 = (1 ) = 1 20 0 05 (1) (1 + M1=M2) 2πG )) (number fraction) -0.5 x ( n seems to remain as the only constraint on the mass M2 of the compact companion, which is then a function of two unknowns, log( -1 ∆ namely the mass of the visible star M1 and the orbital incli- nation io. The spectral analysis by Abdul-Masih et al.(2019) 0.5 revised the atmospheric parameters to Teff = 13 500 ± 700 K, log(g) = 3:3±0:3, and a projected rotational velocity of v sin(i) = 7:5 ± 4:0 km s−1. Those parameter values might still be con- 0 sistent with a massive B-type subgiant. However, the very low projected rotational velocity – although it can be explained by )) (mass fraction) -0.5 x seeing the object relatively pole-on – is quite unusual for such an ( n object and could also be indicative of a different nature. Indeed, log( -1 some evolved low-mass stars are known that can mimic nor- ∆ mal B-type stars spectroscopically (see, e.g., Hambly et al. 1997; Ramspeck et al. 2001); these can be distinguished from massive He C N O CNO Ne Mg Al Si S Ar Fe B stars by their slow rotation and chemical anomalies. There- Chemical species x fore, in order to unravel the nature of the visible star and hence Fig. 1. Differential abundance pattern of LS V+22 25 with respect to its mass, we embarked on a quantitative spectroscopic analysis the cosmic abundance standard by Nieva & Przybilla(2012) in terms of LS V+22 25. This analysis shows that this object is very likely of fractional particle numbers (upper panel) and mass fractions (lower a rare stripped helium star in an evolutionary stage, at which panel). Solar abundances by Asplund et al.(2009) are used for Al, S, it simply mimics a massive MS star. This has important conse- and Ar because cosmic abundances are not available for these ele- quences for the mass and potentially also for the nature of its ments. The error bars are the square roots of the quadratic sums of compact companion. all given individual 1σ uncertainties. The gray-shaded area marks ele- ments whose abundances show signatures of hydrogen burning via the CNO bi-cycle. To get rid of this effect, the invariant sum of the bi-cycle- 2. Analysis catalysts C, N, and O (“CNO”) is also plotted. The abundance pattern of metals heavier than oxygen is nonstandard as well. 2.1. Quantitative spectroscopic analysis Our spectral investigation is based on the publicly available high- from those of Liu et al.(2019). Furthermore, the low values resolution Keck/HIRES spectra described by Liu et al.(2019). for the projected rotational velocity and for the microturbu- The analysis strategy and the applied models are explained lence, which were adopted or measured by Liu et al.(2019) in detail in Irrgang et al.(2014). Briefly, model computations and Abdul-Masih et al.(2019), are more or less confirmed here follow a so-called hybrid approach in which the structure of (v sin(i) = 8:7 ± 0:2 km s−1, ξ ≤ 0:1 km s−1).