The H₂O Supermaser Emission Region in Orion KL

Abstract

A b s t r a c t . We compare spatial structure of the H2O maser during an active period with that during a later quiescent period, received with VLBA in 1993.We have studied the fine structure of the H2O super maser emission region during a period of high activity (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)) using global VLBI and using the NRAO VLBA in 1993 during a comparatively quiescent period. In the active period flux density levels reached (0.5 -7) • 10 6 Jy, (Abraham et al. 1981;Matveenko 1981;Garay, Moran, & Hashick 1989) and the quiescent period F < 2.2-IQpJy.In this period maser emission is dominated by a chain of four groups of compact components, P.A. ~ 90° with a total extent of ~ 8.3A£7, (Matveenko, Graham, & Diamond 1988; Matveenko & Diamond 1993). The velocities of these groups range from 6.1 ('C-group) to 8.5 km/s ('D'-group). The mean velocity gradient across the chain is 0.14 km/s/mas or 0.29 km/s /AU. The main group, 'A' at V=7.5 km/s, consists of 7 components. The velocity gradient is 0.32 km/s/mas. The component sizes are 0.1-0.15 AU. The brightness temperatures of the components are T 0 = 10 1 6 -1 7 K. The compact components of the main group are linearly polarized with P > 85%. The position angle of polarization was measured for each component and correlated with the component location, yielding a gradient of polarization angle 6x/t>V -25°/km/s or S\/6L = U.5°/AU.The NRAO VLBA measurements of the super maser structure during the quiescent period of 1993 had high dynamic range. The maser emission was F < 2200Jy in the 'super-maser' velocity range . The compact structure can be summarized as 6 compact components lying along a line PA = -43° within ~ 6AU. The component velocities are VLSR= 5.5 -8.9 km/s. The velocity of the central component is 6.2 km/s corresponding to the velocity of the 'C' group seen during the active period. The component emission is F=1200 Jy and is linearly polarized. The position angle of polarization changes with velocity with a gradient equal to 6\/SV -13°/km/s.We assume the systemic velocity is that of the component at 6.1 km/s. The jet-like structure near this component is visible at V = (4.3 -5) km/s, where emission of the main component is weak. The jet size is 1 AU and orientation PA = 90°, F = 27 Jy/beam, Fig.l. Our results can be described by a proto-planetary disc-ring model. The compact maser components lie in a thin, edge-on, rotating and expanding discring, we suggest that the velocity center of the system is that of the strong component at VLSR '