Three generations of stars: a possible case of triggered star formation
  Evidence for triggered star formation linking three generations of stars is
difficult to assemble, as it requires convincingly associating evolved massive
stars with Hii regions that, in turn, would need to present signs of active
star formation. We present observational evidence for triggered star formation
relating three generations of stars in the neighbourhood of the star LS II +26
8. We carried out new spectroscopic observations of LS II +26 8, revealing that
it is a B0 III-type star. We note that LS II +26 8 is located exactly at the
geometric centre of a semi-shell-like Hii region complex. The most conspicuous
component of this complex is the Hii region Sh2-90, which is probably
triggering a new generation of stars. The distances to LS II +26 8 and to
Sh2-90 are in agreement (between 2.6 and 3 kpc). Analysis of the interstellar
medium on a larger spatial scale shows that Hii region complex lies on the
northwestern border of an extended H2 shell. The radius of this molecular shell
is about 13 pc, which is in agreement with what an O9V star (the probable
initial spectral type of LS II +26 8 as inferred from evolutive tracks) can
generate through its winds in the molecular environment. In conclusion, the
spatial and temporal correspondences derived in our analysis enable us to
propose a probable triggered star formation scenario initiated by the evolved
massive star LS II +26 8 during its main sequence stage, followed by stars
exciting the Hii region complex formed in the molecular shell, and culminating
in the birth of YSOs around Sh2-90.
