Abstract
(abridged) We present VLT/X-shooter spectroscopic observations in the wavelength range 3000-23000A of the extremely metal-deficient blue compact dwarf (BCD) galaxy PHL 293B containing a luminous blue variable (LBV) star. We determine abundances of N, O, Ne, S, Ar, and Fe and study the properties of the LBV from the fluxes and widths of broad emission lines. We derive an interstellar oxygen abundance of 12+log O/H = 7.71+/-0.02, which is in agreement with previous determinations. The observed fluxes of narrow Balmer, Paschen and Brackett hydrogen lines correspond to the theoretical recombination values after correction for extinction with a single value C(Hbeta) = 0.225. This implies that the star-forming region observed in the optical range is the only source of ionisation and there is no additional source of ionisation that is seen in the NIR range but is hidden in the optical range. For the LBV star in PHL 293B we find broad emission with P Cygni profiles in several Balmer hydrogen emission lines and for the first time in several Paschen hydrogen lines and in several HeI emission lines, implying temporal evolution of the LBV on a time scale of 8 years. The Halpha luminosity of the LBV star is by one order of magnitude higher than the one obtained for the LBV star in NGC 2363=Mrk 71 which has a slightly higher metallicity 12+logO/H = 7.87. The terminal velocity of the stellar wind in the low-metallicity LBV of PHL293B is high, ~800 km/s, and is comparable to that seen in spectra of some extragalactic LBVs during outbursts. We find that the averaged terminal velocities derived from the Paschen and HeI emission lines are by some ~40-60 km/s lower than those derived from the Balmer emission lines. This probably indicates the presence of the wind accelerating outward.
Highlights
Critical evolutionary phases in the life of massive stars such as the stage of luminous blue variables (LBVs; Conti 1984) are hard to study due to the exceedingly short time scales involved
The differences in the terminal velocities for different transitions can be explained by a wind accelerating outward. Another important feature of the Very Large Telescope (VLT)/X-shooter spectrum is that our derived FWHMs of ∼1000 km s−1 are by a factor of ∼1.5 higher than those obtained by Izotov & Thuan (2009), but are consistent with FWHMs derived by Izotov & Thuan (2009) for another low-metallicity LBV in the blue compact dwarf (BCD) DDO 68
We have studied here the properties of the narrow-line spectrum of the blue compact dwarf (BCD) galaxy PHL 293B and the broad line emission of the luminous blue variable (LBV) star in this galaxy from the VLT/X-shooter spectroscopic observations in the wavelength range λλ3000–22 650 Å
Summary
Critical evolutionary phases in the life of massive stars such as the stage of luminous blue variables (LBVs; Conti 1984) are hard to study due to the exceedingly short time scales involved. The attention of Izotov et al (2007) was drawn to the spectrum of the BCD PHL 293B ≡ SDSS J2230–0006, with 12 + log O/H = 7.72, which shows broad Balmer hydrogen emission lines with P Cygni profiles, that are characteristic of a LBV star Several months after this finding, Pustilnik et al (2008) discovered another bright LBV in the BCD DDO 68 with interstellar 12 + log O/H = 7.14. We present here new VLT/X-shooter spectroscopic observations of PHL 293B over a larger wavelength range of ∼λ3000– 23 000 Å as compared to the SDSS (∼λ3800–9200 Å) and VLT/UVES (∼λ3000–7000 Å) optical data discussed by Izotov & Thuan (2009) These new observations allow us to derive element abundances in the H ii region, to study whether any hidden star formation is present in this galaxy, and to obtain emission line parameters of the LBV star for the epoch ∼8 years after previous observations.
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