SOAR-OSIRIS observations of the Sh 2-307 HII region: Triggered star
  formation beyond the solar circle
  This work aims to the study of the Sh 2-307 HII region and related stellar
population. Near-infrared imaging and spectroscopic observations in the
direction of Sh 2-307 were performed using OSIRIS at SOAR Telescope. From J-,
H- and K-band spectra of the brightest source in the cluster, we conclude that
it has a near-infrared spectra compatible with that taken for 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 Brg, H2,
and continuum narrow-band images we were able to detect both the NIR
counterpart of the associated HII region, as well as, the interface between the
ionised and the cool molecular gas. Using the 5 Ghz continuum flux density
taken from the PMN catalogue and the Brg narrow band image we estimate that the
HII region has a mean diameter of 0.94(0.15) pc, mean electron density of 550
cm-3 and an estimated dynamical age of 1.6 Myears years. The large fraction of
sources presenting excess emission at 2micron 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 Myears, 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 photo-dissociation
zones, we conclude that the O-type star probably has been triggering the star
formation process in the region.
