**10. The Population of LBVs**

As mention above the first reports on Var 2 in M 33 was already in the 1920ties marks the first identification of an LBV-at that time without the knowledge that it is and what LBV are.

Since the Studies by Hubble & Sandage 1953 [1] and Sandage & Tamman 1974[4] we know that both M 31 and M 33, as well as NGC 2403 host several LBV and LBV candidates. S Dor added the LMC to the LBV host galaxies. The LMC has a remarkable population of LBVs [6,131]. Bernhard Wolf and his group in Heidelberg studied various LBVs and LBV candidates in several galaxies and with this first larger sample was able to identify the above mentioned amplitude luminosity relation. Beside that they found several LMC LBV candidates and confirmed many LBVs by observing their S Dor Cycles like R 127 [132], R 110 [133]. They also noticed an inverse P Cygni profile in the spectrum of S Dor [134]. and added HD 160529 to the galactic LBVs. Last but not least the group also identified with R 40 the very first LBV in the SMC [135,136] . Other LBV host galaxies now known are locally IC 10 and further out are the M 1 group members M 1, NGC 2366. LBV and LBV candidates are reported also in M 101, NGC 300, NGC 247, NGC 6822, NGC 4414, and IC 1613, just to name the most important galaxies.

In Table 2a list of known LBV and LBV candidates is given. True LBVs are those stars where the membership is clear since a complete S Dor cycle has been observed, this is not the case for the LBV candidates. For LBVs that had a giant eruption, those are classified separately and named giant eruption LBVs (or *η* Car Variables) to distinct them from LBVs with S Dor variability only.

Several more stars have for the one or other reason be classified as LBVs by one or more authors, but show no clear hints like S Dor cycle or giant eruption. For the Milky Way the objects HD 80077 and Schulte 12 the new GAIA parallax moves both to a closer distance and to a lower luminosity. Still, the GAIA parallaxes are at this time (GAIA DR2) prone to several systematics [137,138]. Therefore the distance of at least the Schulte 12 is still not settled ye<sup>t</sup> [139]. According to Humphreys et al. [125] in the LMC R 66, R 74, R 123 are B[e], R 149 is an Of star, HD 269604 an A supergiant and HD 34664 as well as HD 38489 are B[e]sg. Neither are R 81, R 84, R 99, R 126 LBVs. The SMC object R 50 is a B[e]sg while R 4 is a spectroscopic binary system with one B[e]sg. Finally the activity HD 5980 is more like a giant eruption but this most likely due to a binary interaction, see chapter on Multiplicity of LBVs below. Therefore its seen as a giant eruption LBV candidate with the above caveat. Just recently Humphreys [140] report that the following objects are not LBVs: I 8 in M 81 is an F supergiant, furthermore V 52 in NGC 2403 and I 3 in M 81 are foreground objects and not even part of those galaxies! HD 168625, He 3-519, Pistol star, and Sher 25 are LBV candidates due to the fact that they posses a circumstellar nebula. They however might indeed be LBVs, as members of what van Genderen classified as a group ex/dormant LBVs, just currently not showing any variability. Besides the variability searches, there is also a consistent search of luminous emission line stars using two or more broad band colors, H*α* as detection and [OIII] as veto filter, done as part of the NOAO Local Group Survey [141,142] and independently by our group [143]. Both searches covered M 31, M 33, NGC 6822, IC 10, Wolf-Lundmark-Melotte, Sextans A, and Sextans B and finding very few candidates in the dwarf galaxies.

We searched also NGC 3109, a low metallicity galaxy forming a subgroup with Sextans A and Sextans B at the fringes of the Local Group. We found one candidate [144], similar to the low candidate numbers for the low metallicity dwarfs in the Local Group. An earlier attempt with the same idea to detect very luminous stars, which are strong H*α* line emitters (either from strong mass loss, or from a circumstellar nebula), which are faint or absent in [OIII] (no stellar emission line, and faint for circumstellar nebulae of CNO processed material) was done by the Heidelberg group [145] for M 33, M 81, NGC 2403, and M 101, but was not published. We used e.g., these data to complement our list of good candidates for spectroscopy in M 33 [146,147]. It is interesting to note here, that coordinated searches for variable stars (in particular not only analyzing the Cepheids) is done only for small number of massive local galaxies since the photographic plate area. A new effort is ongoing with the LBT and yielded already interesting results [140]. Our group is currently working on a search for LBV and related objects in several nearby galaxies.


**Table 2.** LBVs and LBV candidates in alphabetic order. Giant eruption LBVs are italic. Objects marked bold have (optical) emission LBV nebula. Except for the MilkyWay and LMC which have a to large number of objects, references are given.

Another different approach was used by Khan et al. [165,166] to search for analogs of *η* Carinae. They applied SED fitting to HST and Spitzer IRAC data of resolved stars in nearby massive galaxies, and found 5 promising candidates (one each in M 51, M 101, NGC 6946, and two in M 83). Again, these are at best only candidate LBV, mainly due to the missing variability information.

### **11. LBVs in Low Metallicity Systems**

The situation is even worse for LBVs in lower mass galaxies. Detections are rare as metal-poor also implies low mass and even in actively starforming dwarf galaxies the numbers of massive stars are more limited as in large, massive spirals. The SMC, for example, on has only one confirmed LBV, see Table 2. An interesting LBV candidate is V 39 in the low metallicity Local Group dwarf galaxy IC 1613. Detailed analysis of its spectrum shows some patterns similar to other LBVs, but is also consistent with that of a sgB[e] star [161].

Besides the aforementioned Local Group galaxies, there are only chance detections up to now, including the exceptional case of NGC 2366 V1. NGC 2366 V1 [152,167,168] is located in a dwarf galaxy with a metallicity below 1/10 solar. Its "outburst", with a change of only ∼3 mag [167] was probably not really a giant eruption). Neither did it follow the classical S Dor pattern since it turned bluer (not redder) with increasing brightness.

The transient in UGC 5340 (DDO 68) was again a chance detection [162]. The brightening is 1 mag, and here again a blueing during the bright state is visible [163,164]. The galaxy is a morphologically peculiar, low mass system, which has with ∼1/30 solar (log (O/H) = 7.12) one of the lowest gas-phase metalicities in the nearby universe (distance 12.6 Mpc [169]. The transient in PHL 293B (= SDSS J223036.79-000636.9) [170] is difficult to study mainly due to its distance of ∼25 Mpc. The host galaxy is a dwarf galaxy and more metal-poor than the SMC (log O/H = 7.72). The transient discovery spectrum shows clear P Cygni profiles, but no details on the temporal variability were known, only 2 spectra (one without and one with P Cygni profiles). An additional spectrum brought the time baseline to 8 years and proved temporal variations of the broad stellar lines [171]. While being an interesting object, which may acquire LBV candidate status with a longer term photometric and spectroscopic monitoring, but the currently limited data makes the label LBV for this object a bit premature. As similar problem is the transient in the galaxy SDSS J094332.35+332657.6 [172], an apparent stellar transient in an very low mass and extremely low metallicity (log (O/H) = 7.03) galaxy at a distance of ∼ 8 Mpc. Only a very limited historical record is available, and therefore the LBV nature of the transient is quite unclear. It may be interesting to note here that the LBV GR 290 (= Romano's star) in M 33 also shows spectra variability, but not consistent with an S Dor pattern, see Maryeva et al., this volume. The star is located in the outer regions of the disk of M 33 (r= 4.3 kpc from the center of M 33. The observed metallicity gradient [173] therefore implies a low metallicity of log (O/H) = 8.2 (roughly between LMC and SMC [174]) for the star. Note in that context that metallicity gradients are a common feature in spiral galaxies, e.g., [175,176], so large spiral galaxies do not have one fixed metallicity.

Another intriguing object was detected by as a point source with high velocity dispersion in H*α* Fabry-Perot observations of the local (D ∼2.6 Mpc), low metallicity dwarf galaxy UGC 8508 [177]. An intermediate dispersion spectrum of the source shows a bright H*α* line with broad wings, a relatively strong Fe II *λ*4924 line, but also a strong He II *λ*4686 line. The classification of the authors as a massive star with strong mass loss is convincing, but if it is indeed a good LBV candidate is more uncertain, given the high temperature (and/or hard radiation field) implied by the presence of the strong, narrow He II line.

We detected another unusual point source [178] in NGC 1705, a starburst dwarf galaxy at D ∼ 5 Mpc with a metallicity similar to the LMC. The spectrum shows several very strong (and split) forbidden emission lines, all showing an expansion velocity of 50 km s<sup>−</sup>1, and an underlying spectrum of the source is that of an A supergiant. Again this is a massive star with an expanding circumstellar bubble, but its exact nature is not determined yet.

The starter for the question, how many galaxies do we know in the local universe (e.g., the Local Volume = D < 11 Mpc) based on the classic compilation of [179]. There is an obvious distance limit when using photometry from the ground, especially historic photographic plate material for long term light curves to identify LBV candidates. This limit is depending on seeing, size of the telescope used, and the detector. The limits for photographic plate work is about 7 Mpc (the distance of M 101) [24], and is for most telescopes more like ∼4 Mpc (the M 81 and IC 342 groups in the north, and Sculptor and Fornax groups in the south). Obviously with CCDs and good seeing this can be extended (and/or the quality of the photometry improved), but access of older CCD data is tricky, if the observatory does not run a well maintained archive. Clearly, HST and in the near future EUCLID and JWST, can go much farther out, but it gets hard beyond 20 Mpc (especially due to the crowding of stars).

Low metallicity LBVs are especially interesting, since the metallicity can influence opacity in the interior of the stars and in the wind. Metallicity also affects the path of the evolutionary tracks (at which mass stars still go RSG, return to the blue, or go through a LBV phase with S Dor-like variability and instability that caused these, etc...) Furthermore rotation rate, binary fraction, and potentially IMF as well as magnetic fields are important.

Several of this markers of LBV candidates are directly, or indirectly influenced by metallicity. Mass loss e.g., [180–182], emission lines of heavy elements (e.g., photospheric or wind emission lines of FeII, FeIII, [FeII], then HeI, and [NII] in a circumstellar nebula) [146,147], and variability due to the metallicity dependence of the instabilities involved (see above). It could be that at low enough metallicity, massive stars behave differently, e.g., not showing an typical S Dor variability pattern anymore. The cases of V1 in NGC 2366 [167] and the transient in UGC 5340 (DDO 68) [163,164] hints towards and seem to support such a scenario. No coordinated search for luminous variable sources in a sample of low metallicity dwarf galaxies outside the Local Group was done yet. A pilot search on a few selected very low metallicity galaxies was reported by [183] using HST archival data. While there are several interesting candidates of luminous stars with signs of variability and in some cases H*α* emission, the data yield not enough proofs to claim LBV candidates. Figure 7 demonstrates one of the problem, very low metallicity galaxies are rare and spatial resolution poses severe problems for ground based studies beyond ∼5 Mpc, requiring HST time. This aspect may improve with the upcoming EUCLID mission and more in the future by WFIRST, and is alleviated somewhat by the improving image quality of the large survey instruments, link e.g., SUBARU SuprimeCAM, DECam, and hopefully LSST. Another problem is the metallicity-luminosity relation, which implies that low metallicity is in the local universe the exclusive regime of dwarf galaxies. Therefore, even in a burst of starformation the absolute number of massive stars produced is, during a short time frame only, still comparable to the production rate of a massive spiral galaxy. With the current data situation it is to early to speculated on trends of LBV numbers and LBV nature at low metalicities, but as noted above, it is intriguing to see so many LBVs and LBV candidates in the LMC. With at the same time nearly non in the SMC.

**Figure 7.** Plot of the gas-phase metallicity of nearby galaxies versus their distance and spatial resolution. Only a selection of the galaxies in the Local volume are plotted, but the sample is complete for the significantly starforming galaxies in the Local Group. Metalicities of the inner disk are chosen for the spiral galaxies with metallicity gradients, the metalicities of stars in the outer disk of these galaxies can be a a few faction of tens solar lower. Galaxies with LBVs and/or LBV candidates are plotted as red dots, the other galaxies are plotted as blue dots. Plot was adapted and updated from [163].
