**5. Discussion and Conclusions**

B[e]SGs form a special class of evolved massive stars and are thought to represent a short-lived transition phase either in their red-ward, post-main sequence evolution, or in their blue-ward post-RSG evolution. The total number of currently known B[e]SGs is low, supporting the idea of a short transition phase. However, in which direction the stars evolve, and whether all B[e]SGs evolve in the same direction, is still an open issue. For the objects in M31 and M33, it has been argued that the B[e]SGs are more isolated than LBVs and hardly found in stellar associations, so that a post-RSG (or post-yellow supergiant) evolution was proposed to be more likely [142]. This assessment was based on the available numbers of putative B[e]SGs in these two galaxies at that time. However, after revision of the two samples (Tables 6 and 7), two of the four confirmed B[e]SGs in M31 and both confirmed B[e]SGs in M33 are associated with stellar groups, questioning the conclusion that B[e]SGs are isolated and thus post-RSGs.

The best age indicator for B[e]SGs we have to date is their surface abundance enrichment in 13C, as discussed in Section 2.3. If B[e]SGs were post-RSGs, their progenitors would all have started with a (very) low rotation speed. While such a scenario cannot be excluded, it may not be very likely considering that stars are born on average with a rotation rate of about 40% of their critical velocity [88].

Interaction within a (close) binary system, possibly even up to a binary merger, seems to be an alternative and popular scenario (e.g., [51,177]), but the number of currently confirmed binaries amongs<sup>t</sup> B[e]SGs is still rather low to give preference to the binary channel as the sole possible way for the formation of B[e]SGs. Likewise, some B[e]SGs have been suggested as suitable supernova candidates. For example, the Galactic object MWC 137 appears similar to Sher 25 [178] and SBW1 [179], which both look like the progenitor of SN1987A. In addition, the SMC object LHA 115-S 18 has been proposed to be a viable SN1987A progenitor [27]. In this respect, it is vital to resolve B[e]SG populations and to study their properties.

In this review, a census of the currently known B[e]SG population in the Milky Way and in nearby star-forming galaxies within and beyond the Local Group is presented. The proposed candidates have been undertaken a critical examination, sorted into confirmed B[e]SGs and B[e]SG candidates, and unsuitable objects have been flagged as "misclassified"10.

During these investigations, a fundamental difference has been recognized between the identification issues for objects in our Galaxy compared to those in other galaxies. Extragalactic B[e]SGs bare the risk of being confused with LBVs in quiescence, which share very similar optical spectroscopic characteristics. To separate these two classes of objects, one can make use of clearly defined classification criteria based on certain sets of emission features identified in their optical and near-IR spectra, as outlined in Section 3. In addition, inspection of the location of possible B[e]SG candidates in

<sup>10</sup> We would like to caution that, with insufficient knowledge of stellar properties, individual objects may easily be misclassified, as it happened in recently published catalogs [6,102,180,181], in which erroneously a number of (even confirmed) B[e]SGs are listed as LBV candidates.

the IR color–color diagrams is highly advisable, because B[e]SGs and LBVs populate clearly separate domains. This fact might also be used as starting point for future investigations of extragalactic samples. However, for such future studies, infrared photometry with higher spatial resolution than what is currently provided by 2MASS and WISE is desirable to prevent from contamination with neighboring sources in densely populated regions. Moreover, precise distances, as soon provided by GAIA, will help to separate foreground stars in the directions to other galaxies that might have been misclassified as luminous extragalactic stars.

In the Milky Way, an additional complication occurs due to the often uncertain luminosity estimates, and the overlap of low-luminosity B[e]SGs with the most luminous massive pre-main sequence objects (HAeBe). Here, special care needs to be taken, especially since both classes of objects occupy adjacent regions in the near-IR color–color diagram with a probable overlap. Without additional distinctive features for such low-luminosity, borderline B[e]SG candidates, their real nature remains elusive. One complementary classification criteria that was discussed, is provided by the enrichment of the circumstellar material of evolved objects with processed material that has been released from the stellar surface, as opposed to the non-processed material with interstellar abundance patterns found around HAeBes. The most ideal element to search for is 13C, which is bound in 13CO molecules in the circumstellar disks. Measuring the 13CO amount with respect to 12CO provides immediate insight into the nature of the object. The current HAeBe samples might hide such low-luminosity B[e]SG candidates, which can only be identified as such by careful and honest analysis.

The result from this census, after strict application of the classification criteria, is that we count nine confirmed and six candidate B[e]SGs in the Galaxy. Moving out to the MCs, the numbers amount to 13 (+2) in the LMC and 5 (+1) in the SMC. The situation in other members of the Local Group is not much better, where the numbers drop to four (+7) in M31 and two (+6) in M33. Even further away, only three candidates have been reported from M81. The total number of B[e]SGs found in the various galaxies are too small for statistical analyses with respect to a metallicity dependence of their number, but the comparable quantities within the Milky Way and the LMC sugges<sup>t</sup> only a mild dependence of the amount of B[e]SGs on (i.e., a possible drop with decreasing) metallicity.

Further, we report that the Galactic sample of 15 B[e]SGs and candidates contains currently seven confirmed or suspected binaries (see Table 9), which is less than half of the population. In the MCs, this number is even lower, because thus far only three stars have been reported to be possible binaries, of which one, the SMC star LHA 115-S 6 (= RMC 4) has been proposed to be the remnant of a binary merger within an initially triple system [182,183]. The other two objects, the LMC star LHA 120-S 134 and the SMC object LHA 115-S 18, have been identified as optical counterparts of X-ray sources [27,29]. Nothing is known about possible binarity in the B[e]SG samples from the other galaxies, although photometric variability was seen in at least two M31 objects: the confirmed B[e]SG star J004417.10+411928.0 [137,184] and the candidate J004444.52+412804.0 [185,186]. Whether this variability is a sign of binarity or just of semi-regular variability which might be interpreted with pulsation activity such as reported from the *α* Cygni variables, needs to be studied in more detail.

For the sake of completeness, a special class of objects, which have not been discussed yet, should be briefly mentioned as well. These are the high-mass X-ray binaries (HMXB) with possible B[e] supergiant (candidate) companion. Some objects with an assigned B[e]SG status have been found to be too luminous in X-rays for being considered single stars. These objects have been proposed to be binary systems, in which the high energy emission is caused by either accretion onto a compact object, or by shocks in a colliding wind binary with a second massive star. Members of this group are the Galactic objects Cl\*Westerlund 1W 9 (=Wd1-9), which is considered a colliding wind system [85]; CI Cam (=MWC 84), which might be interpreted as supernova imposter [187]; and the high-mass X-ray binary (HMXB) IGR J16318-4848, in which the compact object was proposed to be a neutron star [188]. Two ultra-luminous X-ray sources (ULXs), Holmberg II X-1 and NGC 300 ULX1, the latter being also named as supernova imposter SN2010da, have also been proposed to be HMXBs including a B[e]SG [189–191]. Whether these objects indeed host a B[e]SG clearly needs to be investigated in more detail. However, since the behavior of these sources is considerably different from the confirmed B[e]SGs, I hesitate to include them into the census.

The presented populations represent the current knowledge of B[e]SGs and B[e]SG candidates in the closest star-forming galaxies. With the ever growing sensitivities of instruments and telescope sizes, the future in B[e]SG star research is bright, because many more candidates will be identified in even more distant galaxies and with metallicities spreading over a large range. With statistically meaningful samples, it will finally be possible to unveil the nature and fate of these fascinating objects.

**Funding:** This project received funding from the Grant Agency of the Czech Republic (GACR, gran<sup>t</sup> number ˇ 17-02337S). The Astronomical Institute of the Czech Academy of Sciences, Ondˇrejov, is supported by the project RVO:67985815.

**Acknowledgments:** I wish to thank the editor, Roberta Humphreys, for the invitation to write this review. In addition, I am grateful to Michalis Kourniotis for inspiring discussions on searching for extragalactic B[e]SGs, and to Dieter Nickeler and Lydia Cidale for passionate discussions about B[e]SGs as well as for their proofreading of and suggestions on the draft versions. Moreover, I thank the anonymous referees for their careful reading and suggestions on the draft version. This research made use of the NASA Astrophysics Data System (ADS) and of the SIMBAD database, operated at CDS, Strasbourg, France. This publication makes use of data products from the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

**Conflicts of Interest:** The author declares no conflict of interest.
