Three generations of stars: a possible case of triggered star formation

TL;DR

This study presents observational evidence linking three generations of stars through triggered star formation, involving an evolved massive star, an Hii region complex, and young stellar objects, supported by spectroscopic and spatial analysis.

Contribution

It provides a detailed case study demonstrating triggered star formation across multiple stellar generations using multi-scale observational data.

Findings

LS II +26 8 is a B0 III star at the center of a star-forming complex.

The Hii region Sh2-90 is likely triggered by LS II +26 8.

A molecular shell of about 13 pc radius surrounds the region, supporting the triggered formation scenario.

Abstract

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.