Multiwavelength observations of southern hot molecular cores traced by methanol masers - I. Ammonia and 24-GHz continuum data

Abstract

We present observations of the (1,1), (2,2), (4,4) and (5,5) inversion transitions of para-ammonia (NH 3 ) and 24-GHz continuum, taken with the Australia Telescope Compact Array towards 21 southern Galactic hot molecular cores traced by 6.7-GHz methanol maser emission. We detect NH 3 (1,1) emission towards all 21 regions and 24-GHz continuum emission towards 12 of the regions, including six with no reported 8-GHz continuum counterparts. In total, we find the 21 regions contain 41 NH 3 (1,1) cores but around half of the regions only contain a single core. We extract characteristic spectra for every core at each of the NH 3 transitions and present both integrated intensity maps and channel maps for each region. NH 3 (2,2) emission was detected towards all NH 3 (1,1) cores. NH 3 (4,4) emission was detected in 13 of the NH 3 (1,1) cores with NH 3 (5,5) emission coincident with 11 of these. The NH 3 (4,4) and (5,5) emission is always unresolved and found at the methanol maser position. An analysis of the NH 3 (1,1) and (2,2) line ratios suggests that the cores with NH 3 (4,4) and (5,5) emission are warmer than the remaining cores rather than simply containing more ammonia. The coincidence of the maser emission with the higher spatial resolution NH 3 (4,4) and (5,5) emission indicates that the methanol masers are found at the warmest part of the core. In all cores detected at NH 3 (4,4) (with the exception of G12.68-0.18 core 4), the measured linewidth increases with transition energy. The NH 3 (1,1) spectra of several cores show an emission and absorption component slightly offset in velocity but it is unclear whether or not this is due to systematic motion of the gas. We observe large asymmetries in the NH 3 (1,1) hyperfine line profiles and conclude that this is due to non-local thermodynamic equilibrium conditions arising from a number of dense, small clumps within the beam, rather than systematic motions of gas in the cores. Assuming that the 24-GHz continuum emission is optically-thin bremsstrahlung, we derive properties of the ionized gas. The rate of Lyman continuum photons required to ionize the gas of 10 45 -10 48 s -1 suggests that the continuum emission is powered by stars of mass >8 M ⊙ . We investigate the nature of the 24-GHz continuum sources which were not detected at 8 GHz and find that these are always coincident with both ammonia and methanol maser emission. This is in contrast to those detected at both 8 and 24 GHz which are generally offset from the methanol maser emission. We investigate the possibility that these may be hypercompact H II regions. Finally, we separate the cores into five groups, based on their association with NH 3 , methanol maser and continuum emission. From the different physical properties of the cores in the groups, we discuss the possibility that these groups may represent cores at different evolutionary stages of the massive star formation process.