2022MNRAS.512.4298G

We have studied the spectral time variations of candidate luminous blue variable (cLBV) stars in two low-metallicity star-forming galaxies, DDO 68 and PHL 293B. The LBV in DDO 68, located in H II region #3, shows an outburst, with an increase of more than 1000 times in H α luminosity during the period 2008-2010. The broad emission of the H I and He I lines display a P Cygni profile, with a relatively constant terminal velocity of ∼800 km s^–1, reaching a maximum luminosity L(H α) of ∼2 x 10³⁸ erg s^–1, with a full width at half-maximum (FWHM) of ∼1000-1200 km s^–1. On the other hand, since the discovery of a cLBV in 2001 in PHL 293B, the fluxes of the broad components and the broad-to-narrow flux ratios of the H I and He I emission lines in this galaxy have remained nearly constant over 16 yr, with small variations. The luminosity of the broad H α component varies between ∼2 x 10³⁸ erg s^–1 and ∼10³⁹ erg s^–1, with the FWHM varying in the range ∼500-1500 km s^–1. Unusually persistent P Cygni features are clearly visible until the end of 2020 despite a decrease of the broad-to-narrow flux ratio in the most recent years. A terminal velocity of ∼800 km s^–1 is measured from the P Cygni profile, similar to the one in DDO 68, although the latter is 3.7 more metal-deficient than PHL 293B. The relative constancy of the broad H α luminosity in PHL 293B suggests that it is due to a long-lived stellar transient of type LBV/SN IIn.