New LBV Discovered in DDO 68 Galaxy

    Massive stars, having 10-150 masses of the Sun, with metallicities Z tens to hundreds times less than that of the Sun (Zo), are the main regulators of the galaxy formation and evolution processes in the early Universe. Although the models of these stars are widely studied, it is extremely difficult to check these models against the observational data. In order to do that, we need to have nearby galaxies resolved into stars, with young regions of star formation still containing "live" massive stars, where the metallicities of gas and young stars are Z << Zo. However, most of the nearby galaxies have the Z of gas that equals to or higher than Zo/10.
    Dwarf galaxy DDO 68 is the closest to us galaxy (D=10Mpc) with the lowest metal content Z=Zo/36 known to date. In this galaxy, using the 6 m BTA telescope, a new massive blue variable star with high luminosity (>=10⁵ Sun's luminocities in visible light) was found. As far as its properties are concerned, this star falls into an LBV (Luminous Blue Variable) class of stars. LBVs are stars going through a short phase after the main sequence, before evolving to Wolf-Rayet stars and exploding as supernovae. It is characteristic of such stars to possess a shell and a very strong stellar wind, the manifestations of which are visible in their spectra. The stellar winds speed in this metal-poor star approximately equals to 800km/s, what is several times higher than in the known to date "normal" LBVs, giving us a chance to use this data for a critical check of the metal-poor massive stars models.
    On the top panel, you can see an image of DDO 68, obtained on the 6 m telescope, with an arrow pointing to the HII region No.3, where the LBV was detected. On the bottom panel the BTA spectrum of this star is shown superimosed on the spectrum of HII region No.3 (narrow red-colour lines), in which it is immersed. The wind lines of the star are much broader than those of the nebula. They show as well an absorption component in the blue wing. During the last year, the total luminosity of the region No.3 (star+nebula) increased by a factor of 1.5, what corresponds to brightening of the LBV by 1.5 magnitudes.
    To read about the discovery of this unique star in more detail, see our article:
Pustilnik S., Tepliakova A., Kniazev A., Burenkov A.
"Discovery of a massive variable star with Z=Zo/36 in the DDO 68 galaxy", 2008, MNRAS, vol.388, Is.1, L24-L28.
(http://www3.interscience.wiley.com/journal/120126383/abstract)
(http://arxiv.org/abs/0805.3551)