**2. B[e] Supergiants**

The early-type supergiants include a class of emission-line objects, whose optical spectra display a peculiar character with strong Balmer emission along with narrow emission lines from permitted and forbidden transitions (e.g., [7–10]). The latter are indicative of a cool and slowly expanding medium. With the advent of ultraviolet (UV) observations taken with the International Ultraviolet Explorer (IUE), these stars were found to display very broad blueshifted resonance lines of highly ionized elements in this spectral range. These resonance lines originate from a hot and fast stellar line-driven wind which is very typical for supergiants in this temperature and luminosity range.

Another peculiar property of these stars was discovered in the near-infrared, in which these objects possess a pronounced excess emission pointing to hot circumstellar dust [11–16]. This dust was proposed to be most likely produced within the slow and cool component and to possibly populate a ring or disk-like region at far distances from the luminous central objects [15].

In the HR diagram, these objects are all found beyond the main-sequence and with luminosities spreading from about log *L*/*L* ∼ 4 to about log *L*/*L* ∼ 6, implying that they are all evolved, massive stars. This luminosity range was determined from the sample residing in the Magellanic Clouds (MCs), for which the luminosity determination is unquestionable, due to the low extinction towards the MCs and their well constraint distances. The classification of Galactic objects as supergiants bears much higher uncertainties due to their often poorly constrained distances, hence luminosities. We come back to this issue in Section 4.4.

The position of the MC sample in the HR diagram is shown in Figure 1 for the values of luminosity and effective temperature listed in Table 1. The stellar parameters (effective temperature *T*eff, visual magnitude V, and color excess E(B-V)) of the sample have been taken from the references listed in the last column of Table 1. For the calculations of the luminosities, distance moduli of 18.5 and 18.9 mag, respectively, for the Large and Small Magellanic Clouds have been utilized (see the review paper by Humphreys, this volume) along with bolometric corrections from [17].

**Figure 1.** HR diagram showing the positions of the classical MC B[e]SG sample [18]. The stellar evolutionary tracks at SMC metallicity for stars rotating initially with 40% of their critical velocity are also included (from [19]). The dotted square contains objects that display CO band emission (except for S 89, see Section 2.3). For brevity and readability, the identifiers LHA 120 and LHA 115 for objects within the LMC and SMC, respectively, have been omitted.



Note: Former designations of some of the objects as Hen S # (see, e.g., [25]) were omitted here, as these are not SIMBAD identifiers. However, LHA 120-S and LHA 115-S, respectively, and the former Hen S numbers refer to the same objects. a The star is also listed as RMC 105 in SIMBAD, but this designation should be used for a neighboring, normal B-type star in this dense cluster (see [26]). b Confirmed or suspected binary. c X-ray source [27–29].

The presence of dust around an early-type (typically of spectral type B) supergiant, along with the often pure emission-line spectra with numerous forbidden lines predominantly of [Fe II] and [O I] finally resulted in the designation of these objects as B[e] supergiants (B[e]SGs)1.

### *2.1. General Aspects of B[e]SG Stars' Disks*

There is compelling evidence that B[e]SGs are surrounded by gaseous and dusty disks. The simultaneous presence of a hot and fast polar wind traced in the UV, and the cool and slow equatorial wind traced at optical wavelengths led to the assignment of a so-called hybrid or two-component wind model [15]. For this two-component wind, a density contrast between the equatorial and polar components of 100–1000 was proposed, meaning that the equatorial wind might be assigned the character of an outflowing disk [23].

The degree of non-sphericity of the envelopes and the latitude dependence of the wind density, respectively, are pursued by the measured net intrinsic polarization [30,31] and from spectropolarimetric observations [32,33]. The often high degree of intrinsic polarization support the idea of a combination of Thomson scattering by free electrons and Mie scattering by dust in a circumstellar disk [34].

If the disks of B[e]SGs are supposed to form from a high-density equatorial stellar outflow, there should be a transition zone between the atomic gas and the location of the dust, in which molecules can form in substantial amounts, because the high gas density can shield the material from the direct irradiation with dissociating UV photons coming from the hot luminous star. In fact, molecular emission, in particular of the first-overtone bands of carbon monoxide (CO), has been detected in the K-band spectra of a number of B[e]SGs in the Galaxy and the MCs (e.g., [21,35–44], see Table 2). To produce the characteristic observed emission spectra with several individual band heads, temperatures of the CO gas higher than ∼2000 K are required. These temperatures are in excess of the dust sublimation temperature, which is on the order of ∼1500 K, placing the CO emitting region closer to the star than the dust.

Additional hot molecular emission from silicon oxide (SiO) has been identified in four Galactic B[e]SGs [45], and a feature arising in the optical spectrum, which has been tentatively identified as emission from titanium oxide (TiO), was reported from six MC B[e]SGs [23,43,44,46]. However, to date, no systematic surveys for molecular emission has been performed, so that these numbers are not representative for the existence or absence of molecules in the environments of B[e]SGs. For instance, SiO emission has not been searched for ye<sup>t</sup> in any of the MC B[e]SGs, and only those Galactic B[e]SGs with the most intense CO band emission have been observed in the wavelength range of the first-overtone band of SiO arising in the L-band. Hence, one might expect to find molecular emission from SiO in many more objects, but also emission from other ye<sup>t</sup> undiscovered molecules that might form in the environments of B[e]SGs. What is interesting though is the fact that all MC stars displaying TiO emission also have CO emission, whereas the opposite does not hold. No detection of TiO from Galactic B[e]SGs has been reported so far.

Finally, the power of optical interferometry operating at near- and mid-infrared wavelengths should be mentioned when talking about the disks of B[e]SGs. Based on this technique, the disks of the closest and infrared brightest Galactic objects could be spatially resolved, providing precise measurements of the disk inclinations, disk sizes, and the distances of the emitting material (dust, CO gas, and ionized gas traced by the Br *γ* emission) from the central star (see [38,47–52]).

<sup>1</sup> Note that these objects have previously been abbreviated sgB[e] [25], to separate them from other stars showing the B[e] phenomenon. We prefer the designation B[e]SG, to be in line with the naming and abbreviation of other types of supergiants such as blue supergiant (BSG) and red supergiant (RSG).


**Table 2.** Presence of disk tracers in the optical and near-IR spectra of the Galactic and Magellanic Cloud B[e]SG samples.

Note: TW, This work; abs, in absorption; ?, uncertain detection/no value available; . . . , no information. a Refers to the presence of [O I]*λ*5577. All sample stars show emission of [O I]*λλ*6300,6364. b Based on (unpublished) high-resolution optical spectra taken between 2005 and 2017 with FEROS at the MPG 2.2 m telescope. c A[e]SG due to early-A spectral type assignment [68,69]. d No indication of CO band features seen in a K-band spectrum (unpublished) taken on 20 October 2013 with OSIRIS at the Southern Astrophysical Research (SOAR) Telescope. e No CO emission was detected during the observations taken between 1987 and 1989 [35,36].
