4.1.2. Small Magellanic Cloud

In the SMC, a total of six new B[e]SGs has been reported. Five of them were discovered as by-products from the spectroscopic survey of the hot, luminous stars in the SMC (2dFS [122]), from the Runaways and Isolated O Star Spectroscopic Survey of the SMC (RIOTS4 [123]), and from the SMC photometric catalog [124]. One more object was serendipitously detected [125]. From a critical inspection of the properties of these six stars, only one object appears to fulfill all required criteria, one object appears as very promising candidate that lacks only some complementary information for unambiguous classification, whereas the remaining four stars are considered as misclassified. All objects are presented in the following, their IR colors are provided in Table 5, and their positions within the two color–color diagrams are included in Figure 6.

### [MA93] 1116 = NGC 346:KWBBe 200

The star is one of the few objects residing in a cluster, the SMC cluster NGC 346. Due to its H*α* emission, it was first classified as a compact H II region [126], then as a Be star based on its optical photometry [127], and in the following as B[e]SG based on its optical appearance and near-IR excess emission [125]. In the mid-IR, [MA93] 1116 displays silicate emission and strong PAH bands, features often seen in HAeBe stars (e.g., [128,129]). Additional cold dust is surrounding the object as is implied by its detection at 24 μm with Spitzer/MIPS. These characteristics led to the suggestion that [MA93] 1116 might be an evolved young stellar object [130] or a HAeBe star [131], despite the lack of clear evidence for infall, e.g., in form of inverse P Cygni profiles. If true, its luminosity of log *L*/*L* ∼ 4.4 [54,125] would place it at the upper limit for Herbig objects, while it would ge<sup>t</sup> in lane with the group of less-luminous B[e]SGs in the evolved scenario. The position of [MA93] 1116 within the WISE diagram leaves room for a possible classification as HAeBe star where it falls on the edge of the region populated by HAeBe stars, whereas it appears clearly off the HAeBe region in the near-IR color–color diagram. It is hence considered as B[e]SG, and the fifth object within a cluster (the first one in the SMC).


**Table 5.** Confirmed and candidate B[e]SGs in the SMC. Misclassified objects are listed in the bottom part of the table.

Note: IR photometry for all objects is taken from the 2MASS point source catalog (J, H, and K [104]) and from the WISE All-Sky Data Release (W1, W2, and W4 [105]).

### (LHA 115-S 38 = 2dFS 1804)

This object was found from the spectroscopic survey of the hot, luminous stars in the SMC [122]. The blue spectrum displays numerous emission lines, in particular of [Fe II], but no information about [O I] emission is available. The spectral energy distribution indicates a near-IR excess [124]. With a luminosity estimate of log *L*/*L* 4.1 the star was also considered as a post-AGB object [132], but its positions within the two color–color diagrams places the star clearly within the B[e]SG domain. As such, it is a strong candidate for another low-luminosity B[e]SG.

### (LHA 115-N82 = 2dFS 2837 = LIN 495)

This star displays all spectroscopic B[e]SG characteristics [122,133] and an IR excess emission [124]. The optical spectrum was reported to be composite [122], and the detected photospheric lines display radial velocity variations, whereas the emission from the circumstellar matter appears to be stable [54]. The IR colors of LHA 115-N82 locate it clearly within the B[e]SG domains. Curiously, the *V* and *I* band light curves display a long-term brightening, which resembles LBV outbursts and is not common in B[e]SGs. On the other hand, it displays both [O I] and [Ca II] emission, which are typically not seen in LBVs. Since the star has too low luminosity (log *L*/*L* 3.8) to be an LBV (or even a B[e]SG), it was recently assigned a classification as "LBV imposter" [54]. This object clearly requires further investigations to pin down its status.

### (LHA 115-S 29 = RMC 15), (LHA 115-S 46 = RMC 38), (LHA 115-S 62 = RMC 48)

The blue optical spectra of these three objects show emission-line features similar to B[e]SGs, and the luminosities derived for these objects, ranging from log *L*/*L* 4.4 to 4.8, assigns them a supergiant status [123]. However, it is currently not known whether these objects display [O I] line emission, one of the defining characters of the B[e] phenomenon. Due to the lack of a pronounced near-IR excess emission, these three stars have been proposed to be dust-poor B[e]SGs [123]. However, the presence of warm dust is another main classification criteria for a star to be considered as B[e]SG. It is not surprising that all three objects fall clearly outside the B[e]SG domains in the color–color diagrams. Instead, their positions coincide with the regions populated by LBVs, which implies that these three stars have dense winds and circumstellar ionized gas, which is exemplified by their emission-line spectra. These objects require further investigations to unveil their true nature.

### *4.2. Local Group Galaxies beyond the Magellanic Clouds*

Moving further away, beyond the Magellanic Clouds, we may expect to find B[e]SG stars and candidates in those galaxies, in which star-formation is ongoing. For these galaxies, surveys such as the LGGS project mentioned in Section 3, but also earlier surveys (e.g., [134,135]) provide indispensable information on the population of luminous, evolved massive stars, and provide the base for systematic investigations for unambiguous classification of these objects.

To date, systematic spectroscopic studies have been performed in the two large spiral galaxies of the Local Group, M31 and M33, in which a number of B[e]SGs were found amongs<sup>t</sup> the putative LBV candidates [97,99,136–138]. The suggested B[e]SG populations in each of these galaxies is presented in the following subsections. The samples are provided in Tables 6 and 7, in which all objects are listed under their Local Group Galaxy Survey (LGGS) identifiers (Column 1). The properties, based on which the decision to categorize an individual star as either a candidate or a misclassified object has been made, are briefly depicted. The locations of the confirmed, candidate, and possibly misclassified B[e]SGs in the two color–color diagrams are shown in Figure 7.

**Table 6.** Confirmed and candidate B[e]SGs in M31. Misclassified objects are listed at the bottom of the table.


Note: IR photometry is taken from the 2MASS point source catalog (J, H, K [104]) and from the WISE All-Sky Data Release (W1, W2, W4 [105]). Colors resulting from uncertain photometric values are written in italic. a Possible contamination in the photometric bands J, W1 and W2 due to crowding. b Possible contamination in the photometric bands J and H due to crowding. c Has CO band emission [136,137]. d Has also been classified as warm hypergiant [140]. e Possible contamination in the photometric bands W1 and W2 due to crowding.
