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

Abstract

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. © 2009 RAS.