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
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.
Contact - Pustilnik S.