Periodic methanol masers and colliding wind binaries

Abstract

The process by which massive stars form is not yet fully understood. Since massive stars are rare, and generally found at great distances from the Sun, it is difficult to study them and their influence on their environment during the earliest stages of their formation. Although huge strides have been made to better the resolution at which we can observe the environments of these massive stars, it is still very difficult to resolve small scale structures (AU scale) at these large distances where massive stars are located. Since the discovery of Microwave Amplification by Stimulated Emission of Radiation (MASERs) we have been provided with a valuable tool by which High-Mass Star Formation (HMSF) can be studied. Here we are especially interested in the class II methanol masers at 6.7 GHz, first observed by Menten (1991), and the 12.2 GHz methanol masers (Batrla et al., 1987). In the last few decades it has been firmly established that class II 6.7 GHz and 12.2 GHz methanol masers are exclusively associated with HMSF (Minier et al., 2002, Ellingsen, 2006, Breen et al., 2013). To date ' 1000 class II methanol masers have been detected in High-Mass Star Forming Regions (HMSFRs) (Caswell et al., 2010, 2011, Green et al., 2010, 2012). A number of these methanol masers have been observed, that show regular/periodic flaring behaviour. Several of these periodic/regular flaring methanol masers, including the first one discovered G9.62+0.20E (Goedhart et al., 2003) show similar light curves. Since the discovery of the periodic/regular flaring several proposals have been made to explain this behaviour. In this work we proposed that the periodicity of the methanol masers are caused by a Colliding Wind Binary (CWB) system. The framework is that the methanol masers are projected on the partially ionized gas of the ionization front of the background HII region, i.e. the masers amplify the radio free-free \seed" photons from the background HII region. The UV and X-ray photons produced in the very hot (106-108 K) shocked gas of the colliding stellar winds are modulated by the stars' orbital motion in an eccentric binary system. This results in a \pulse" of ionizing photons around periastron, which increases the electron density in the partially ionized gas at the ionization front. The increase in electron density causes an increase in the radio free-free emission from that part of the ionization front which the maser amplifies. This was investigated using a hydrodynamical model to simulate the colliding winds, from which SEDs for an entire orbit were calculated using a plasma emission model. The SEDs were used together with the radiation field of a black body (representing the star that maintains the HII region) to simulate whether the additional ionizing photons will be able to cause an increase in the electron density at the ionization front, i.e. change the position of the ionization front. It is shown that within the framework of the primary star, which maintains the HII region, the additional ionizing photons cause changes in the position of the ionization front. This suggests that the energy generated in the shocked gas is enough to change the position of the ionization front, and can thus change the radio free-free emission from that part in the partially ionized gas of the ionization front. With the proposition that the masers amplify the background radio freefree emission, n2e is solved time-dependently in the approximation that the HII region is optically thin for radio free-free emission from the ionization front towards the maser. The CWB model is compared to the observed maser light-curves, and the CWB model describes the periodic maser are profiles very well. Thus, it strongly suggests that the observed changes in the maser light curves are most likely due to changes in the free-free emission from the background HII region.