Decade-long time-monitoring of candidate luminous blue variable stars in the two very metal-deficient star-forming galaxies DDO 68 and PHL 293B
Résumé
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 × 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 × 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.
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