Candidate LBV stars in galaxy NGC 7793 found via HST photometry + MUSE
  spectroscopy
  Only about 19 Galactic and 25 extra-galactic bona-fide Luminous Blue
Variables (LBVs) are known to date. This incomplete census prevents our
understanding of this crucial phase of massive star evolution which leads to
the formation of heavy binary black holes via the classical channel. With large
samples of LBVs one could better determine the duration and maximum stellar
luminosity which characterize this phase. We search for candidate LBVs (cLBVs)
in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution
images from two Hubble Space Telescope (HST) programs with optical spectroscopy
from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting
photometry measured on F547M, F657N, and F814W images, with restrictions on
point-like appearance (at HST resolution) and H{\alpha} luminosity, we find 100
potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36
sources are promising cLBVs which have M(V) less than or equal to -7 and a
combination of: H{\alpha} with a P-Cygni profile; no [O I] 6300 emission; weak
or no [O III] 5007 emission; large [N II]/H{\alpha} relative to H II regions;
and [S II] 6716 / [S II] 6731 similar to 1. It is not clear if these five cLBVs
are isolated from O-type stars, which would favor the binary formation scenario
of LBVs. Our study, which approximately covers one fourth of the optical disc
of NGC 7793, demonstrates how by combining the above HST surveys with
multi-object spectroscopy from 8-m class telescopes, one can efficiently find
large samples of cLBVs in nearby galaxies.
