2008MNRAS.390.1170P

The nature of the source G25.5+0.2 is open to considerable uncertainty, and it has variously been identified as being a young stellar outflow, an HII region, a young supernova remnant and a planetary nebula. We present further Two-Micron All-Sky Survey (2MASS), Mid-course Space Experiment (MSX) and Spitzer near- (NIR) and mid-infrared (MIR) photometry and mapping in an attempt to clarify the properties of the source. It is apparent that the morphology varies appreciably with MIR wavelength and indicates the presence of a bipolar structure and probable circum-nebular torus. It is also noted that the 2MASS stellar photometry is quite disparate from that of earlier NIR measures and implies an appreciable variation in the brightness and colour of the central star. These two factors, the nuclear variability and bipolarity, combine to suggest the G25.5+0.2 may be identifiable as a symbiotic outflow. The nebular continuum peaks at wavelengths λ ∼ 40µm, and may be fitted using a λ^–2 fall-off in grain emissivity, together with a model in which the grains have temperatures T_GR= 65 and 130K. An analysis of dust/gas mass ratios also implies that m_d/m_g ∼ 1.2x10^–2 (n_e/10⁴/cm3) ⟶ 1.27x10^–1 (n_e/10⁴/cm3), depending upon the types of grains and temperatures involved. Such a range is typical of what is observed in planetary nebulae and symbiotic outflows.