*4.4. Galactic Objects*

Finally, we return to our own Galaxy. Searching for B[e]SG stars in the Milky Way is a difficult task. Though many B[e] stars are known, the assignment of a supergiant status is significantly hampered due to highly uncertain distances, hence luminosities. The situation will hopefully change with the final data release from the GAIA mission, from which one hopes for accurate parallax measurements. However, for now, the luminosities of the objects are subject to large uncertainties, so that only objects with a reported luminosity of at least log *L*/*L* ≥ 5 are considered as serious B[e]SG candidates.

The lower luminosity boundary of log *L*/*L* ∼ 4.0 for an evolved star to be assigned a supergiant status is a further hindrance in the classification of objects as B[e]SGs, because this luminosity domain is shared with the massive pre-main sequence (HAeBe) stars. The latter have emission-line spectra with numerous forbidden emission lines from [Fe II] and [O I] similar to the B[e]SGs, and the stars are surrounded by significant amounts of circumstellar dust within their massive accretion disks causing considerable IR excess emission, just as the B[e]SGs. Hence, it is not surprising that confusion exists about the proper classification for a number of objects within this luminosity domain of 4.0 < log *L*/*L* < 4.5, and that Galactic B[e]SG candidates also appear as candidates in catalogs of HAeBe stars (see, e.g., [160]). In the absence of clear indications for infall of material, which is a typical characteristic of pre-main sequence stars, alternative discriminators for the classification of such objects are needed.

A reasonable approach to this is to search for 13CO emission from the circumstellar environments of the uncertain candidates. Many HAeBe stars have been reported to display CO band emission from their massive accretion disks (e.g., [161–165]). As these disks form from material provided by the interstellar medium in which the 12C/13C isotope abundance ratio has typically a value of about 90 [88], these pre-main sequence disks can clearly be distinguished from the disks around evolved massive stars, which should be enriched in 13C and hence give rise to clearly measurable emission in 13CO. K-band spectra of these objects, covering the first-overtone bands of both 12CO and 13CO (see Section 2.3 and Figure 3), are thus key for a proper discrimination between a young (pre-main sequence) and an evolved status.

From the currently proposed 15 Galactic B[e]SGs listed in Table 2, nine have been reported to display CO band emission. However, thus far, only four of them have been observed in the region around the 13CO bands. The spectra of all four stars have been found to display clear signatures of 13CO emission, and model results revealed that the environments of all four objects are clearly enriched in 13C. Two of these objects were already known to be supergiants based on their confirmed high luminosities: the stars GG Car [41,42] and Hen 2-398 [41]. For the other two, which so far have also been considered as HAeBe candidates (see Table 9), the detection of chemically processed material can hence be regarded as the ultimate proof of their evolved, supergiant nature. These are the objects MWC 137 [59] and MWC 349 [62]. These results are very promising and encouraging, and they demonstrate that the 13CO molecular emission provides a solid tool to unambiguously classify a star as either a pre-main sequence or an evolved object. Clearly, more observational effort needs to be undertaken to search also for the signatures of 13CO in the spectra of the remaining objects.

When collecting the IR magnitudes of the Galactic sample, it turned out that only the near-IR measurements are reliable, whereas the WISE measurements for all objects have been flagged as being contaminated by neighboring objects. The latter are hence useless for classification purposes, and one can currently only rely on the JHK-band magnitudes. The list of objects, their observed colors, and literature values of their color excess are listed in Table 9. The relatively high values of the color excess requires correction for extinction before placing the objects to the near-IR diagram. Corrections have been performed with the galactic extinction curve using an *R*V value of 3.1 [117]. The extinction corrected colors are included in Table 9, and the positions of the objects are shown in Figure 8 separately for the confirmed (Figure 8, top) and candidate objects (Figure 8, bottom).


**Table 9.** Confirmed and candidate B[e]SGs in the Milky Way.

Note: IR photometry is taken from the 2MASS point source catalog (J, H, and K [104]). a Star appears also in HAeBe catalogs (see, e.g., [160]). b Confirmed or suspected binary.

The separation of confirmed from candidate B[e]SGs is based on two characteristics: (i) stars with detected enrichment in 13CO of their circumstellar environments are considered as confirmed; and (ii) stars with reported (by more than one research team) luminosity values of log *L*/*L* ≥ 5.0. Objects with lower luminosities 4.0 < log *L*/*L* < 5.0 are assigned a candidate status. Based on these criteria, the Galactic sample splits into nine confirmed B[e]SGs and six candidates (see Table 9).

### MWC 137, MWC 349, GG Car, Hen 3-298

These four objects are considered as confirmed B[e]SGs based on the detected enrichment of their circumstellar disk material with 13CO (see Table 2). They all fall into the region of the confirmed B[e]SGs in the near-IR diagram, regardless of the large error bar for MWC 349.

### CPD-52 9243, HD 327083, MWC 300

All three stars fulfill the high luminosity criterion. The near-IR colors of CPD-52 9243 and HD 327083 place these two objects within the B[e]SG domain. MWC 300 appears regularly in studies of HAeBe stars. Its near-IR colors locate this star close to the B[e]SGs but far away from the HAeBe region, making a pre-main sequence nature of this object rather unlikely. Its relatively high color values might be influenced by a possible companion [51,172].
