2010MNRAS.405..245R

We present an analysis of the structure and properties of the compact HII region Sh 2-89, and certain of the young stellar objects (YSOs) within this regime, using mid-infrared (MIR) mapping derived from the Spitzer Space Telescope (SST) and visual slit spectroscopy of the inner regions of the source. We show that the region has a bipolar structure, and contains a variety of Class I and II YSOs. Much of the MIR emission appears to be dominated by polycyclic aromatic hydrocarbon emission bands, which cause strong increases in flux in the 5.8 and 8.0 µm photometric channels, whilst the variation of Hα, [NII] λ6583 Å, [SII] λλ6716+6731 Å and MIR emission profiles confirms the presence of complex ionization fronts, and ionization stratification. One of the sources in this region has been previously identified as a likely symbiotic star. We show however that whilst it contains TiO absorption bands, it shows little evidence for higher excitation Heii λ4686 Å or [OIII] λ5007 Å emission. Such a result does not rule out the possibility of the source being a symbiotic star, although we note that the detection limits on these lines, and the observed visual/infrared continuum, would make it difficult to fit it into either the D- or S-type symbiotic classifications. An analysis and modelling of the infrared continuum, on the other hand, together with a consideration of the associated HII region, suggests that it is more likely to represent a Class II YSO in which most of the emission derives from a massive accretion disc. Where this is the case, then the star is likely to have a temperature T_EFF∼ 10⁴-2 x10⁴ K, mass ∼2-4 M_☉, luminosity 0.5-3x10² L_☉, and age in the range 10⁶-10⁷ yr. It also appears to be associated with strong Hα emission which, where it derives from radiatively driven stellar/disc winds, and collimated or spherically symmetric mass outflows, would likely imply mass-loss rates of the order of dM/dt ∼ 4.5x10^–8 to 4.5x10^–7 M_☉/yr (depending upon the outflow velocity V_W). Such a mass-loss rate is entirely plausible for this category of source, although it is also noted that the Hα may derive from other mechanisms as well, including chromospheric activity, and the ionization of circumstellar material - the result of interaction between local UV radiation fields and winds and the stellar accretion disc or envelope.