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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 512; no. 3; pp. 4298 - 4307
• Main Authors: Guseva, N G, Thuan, T X, Izotov, Y I
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 06-04-2022; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Astronomy & Astrophysics; ASTRONOMY AND ASTROPHYSICS; galaxies: abundances; galaxies: dwarf; galaxies: ISM; galaxies: starburst; Sciences of the Universe; galaxies: dwarf; galaxies: starburst; galaxies: abundances; galaxies: ISM; Astrophysics - Astrophysics of Galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stac820

ABSTRACT
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.