SOAR–OSIRIS observations of the Sh 2-307 H ii region: triggered star formation beyond the Solar Circle

This work aims to study the Sh 2-307 H ii region and related stellar population. Near-infrared imaging and spectroscopic observations in the direction of Sh 2-307 were performed using the Ohio State InfraRed Imager/Spectrometer (OSIRIS) at the Southern Observatory for Astrophysical Research (SOAR) Telescope. The photometric data were analysed from colour–colour and colour–magnitude diagrams, while the spectroscopic results were interpreted from the comparison of the science spectra with those obtained from known OB stars. From J-, H- and K-band spectra of the brightest source in the cluster, we conclude that it has a near-infrared spectrum compatible with those of O9V–O9.5V stars. Using the derived spectral type and the respective J-, H- and K-band photometry, we compute a heliocentric distance of 3.2 ± 0.5 kpc, which, for R₀= 8 kpc, puts this cluster at more than 10 kpc from the Galactic Centre. From the Brγ, H₂ and continuum narrow-band images we were able to detect both the near-infrared (NIR) counterpart of the associated H ii region and the interface between the ionized and cool molecular gas. Using the 5-GHz continuum flux density taken from the Parkes–MIT–NRAO (PMN) catalogue and the Brγ narrow-band image, we estimate that the H ii region has a mean diameter of 0.94 ± 0.15 pc, mean electron density of 550 cm⁻³ and a dynamical age of 1.6 × 10⁶ yr. The large fraction of sources presenting excess emission at 2 μm suggests that the stellar population is very young, with many sources still in the pre-main-sequence accreting phase. By the use of theoretical pre-main-sequence tracks we derived a cluster mean age of about 2.5 Myr, and from the analyses of the fraction of excess emission sources as a function of their spatial distribution we found evidence for an age spread for the embedded pre-main-sequence stellar population. Finally, from the study of the spatial distribution of the low-mass sources relative to the main cluster source and associated photodissociation zones, we conclude that the O-type star has probably been triggering the star formation process in the region.