The molecular complex associated with the Galactic H II region Sh2-90: a possible site of triggered star formation

Abstract

Aims. We investigate the star formation activity in the molecular complex associated with the Galactic H ii region Sh2-90. Methods. We obtain the distribution of the ionized and cold neutral gas using radio-continuum and Herschel observations. We use near-infrared and Spitzer data to investigate the stellar content of the complex. We discuss the evolutionary status of embedded massive young stellar objects (MYSOs) using their spectral energy distribution. Results. The Sh2-90 region presents a bubble morphology in the mid-infrared. Radio observations suggest it is an evolved H ii region with an electron density ∼144 cm⁻³ , emission measure ∼6.7 × 10⁴ cm⁻⁶ pc and an ionized mass ∼55 Mꙩ . From Herschel and CO (J = 3−2) observations we found that the H ii region is part of an elongated extended molecular cloud (H2 column density ≥3 × 10 ²¹ cm⁻² and dust temperature 18−27 K) of total mass ≥1 × 104 M . We identify the ionizing cluster of Sh2-90, the main exciting star being an O8–O9 V star. Five cold dust clumps, four mid-IR blobs around B stars, and a compact H ii region are found at the edge of the bubble. The velocity information derived from CO data cubes suggest that most of them are associated with the Sh2-90 region. One hundred and twenty-nine low mass (≤3 M ) YSOs have been identified, and they are found to be distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found. We suggest that multi-generation star formation is present in the complex. From evidence of interaction, time scales involved, and evolutionary status of stellar/protostellar sources, we argue that the star formation at the edges of Sh2-90 might have been triggered. However, several young sources in this complex are probably formed by some other processes.