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| EPoS Contribution |
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Periodic methanol masers and colliding-wind binaries
Fanie van den Heever NWU, Potchefstroom, SA | |
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It has been firmly established that class II methanol masers are
exclusively associated with high mass star forming regions. Recently a
number of these methanol masers associated with high mass star forming
regions have been found to show periodic flaring phenomenon. At present
nine of these periodic methanol masers associated with high mass star
forming regions are known. The periodic phenomenon of these sources can
be ascribed to either changes in the pumping of the masers or to
changes in the background seed photon flux. In this work we consider
the changes in the background seed photon flux from the HII region to
be the cause of the periodic phenomenon, and the source of the
periodicity to be a colliding-wind binary system (CWB). The sources of
interest is G9.62+0.20E, G188.89+0.95 and G322.357+0.066.
In this scenario there is not only a single star but a binary system in which the stellar winds of the two stars collide to produce very hot shocked gas, which emits ionizing photons. Since the HII region is maintained by the ionizing radiation of the massive star, the ionizing radiation from the shocked gas can propagate virtually unattenuated to the edge of the HII region where it can cause changes in the position of the ionization front. The changes in the ionization front leads to small changes in the free-free emission which the maser amplifies. The free-free emission from the ionization front is modulated by the orbital motion of the secondary star in an eccentric CWB system with maximum effect at periastron which then also leads to a maser "flare". When the secondary star passes periastron the decay part of the maser "flare" can be described as the recombination of a thermal plasma from a higher to a lower equilibrium state of ionization. The question is whether or not the abovementioned CWB hypothesis can explain the observed periodic behaviour of these methanol masers. It is also necessary to determine whether the CWB scenario is an energetically realistic scenario. To address this question we use a 2D-hydrodynamical model to simulate the shocked gas of the colliding stellar winds. The results from these simulations are then used in various calculations to establish whether or not the ionizing radiation from the shocked gas can infact cause additional ionization at the ionization front of the HII region. I will present and discuss some results of the numerical calculations to show that the colliding wind binary scenario might be a realistic explanation of the maser flaring in a number of sources. | |
| Collaborators: D.J. van der Walt, NWU, SA M.G. Hoare, UL, UK J.M. Pittard, UL, UK |
Suggested Session:
Massive Star Formation and Feedback |